# Zhang-Suen thinning algorithm

Zhang-Suen thinning algorithm
You are encouraged to solve this task according to the task description, using any language you may know.

This is an algorithm used to thin a black and white i.e. one bit per pixel images.

For example, with an input image of:

```
#################                   #############
##################               ################
###################            ##################
########     #######          ###################
######     #######         #######       ######
######     #######        #######
#################         #######
################          #######
#################         #######
######     #######        #######
######     #######        #######
######     #######         #######       ######
########     #######          ###################
########     ####### ######    ################## ######
########     ####### ######      ################ ######
########     ####### ######         ############# ######
```

It produces the thinned output:

```

# ##########                       #######
##        #                   ####       #
#          #                 ##
#          #                #
#          #                #
#          #                #
############               #
#          #               #
#          #                #
#          #                #
#          #                #
#                            ##
#                             ############
###                          ###

```
Algorithm

Assume black pixels are one and white pixels zero, and that the input image is a rectangular N by M array of ones and zeroes.

The algorithm operates on all black pixels P1 that can have eight neighbours.

The neighbours are, in order, arranged as:

 P9 P2 P3 P8 P1 P4 P7 P6 P5

Obviously the boundary pixels of the image cannot have the full eight neighbours.

• Define ${\displaystyle A(P1)}$ = the number of transitions from white to black, (0 -> 1) in the sequence P2,P3,P4,P5,P6,P7,P8,P9,P2. (Note the extra P2 at the end - it is circular).
• Define ${\displaystyle B(P1)}$ = The number of black pixel neighbours of P1. ( = sum(P2 .. P9) )

Step 1

All pixels are tested and pixels satisfying all the following conditions (simultaneously) are just noted at this stage.

• (0) The pixel is black and has eight neighbours
• (1) ${\displaystyle 2<=B(P1)<=6}$
• (2) A(P1) = 1
• (3) At least one of P2 and P4 and P6 is white
• (4) At least one of P4 and P6 and P8 is white

After iterating over the image and collecting all the pixels satisfying all step 1 conditions, all these condition satisfying pixels are set to white.

Step 2

All pixels are again tested and pixels satisfying all the following conditions are just noted at this stage.

• (0) The pixel is black and has eight neighbours
• (1) ${\displaystyle 2<=B(P1)<=6}$
• (2) A(P1) = 1
• (3) At least one of P2 and P4 and P8 is white
• (4) At least one of P2 and P6 and P8 is white

After iterating over the image and collecting all the pixels satisfying all step 2 conditions, all these condition satisfying pixels are again set to white.

Iteration

If any pixels were set in this round of either step 1 or step 2 then all steps are repeated until no image pixels are so changed.

1. Write a routine to perform Zhang-Suen thinning on an image matrix of ones and zeroes.
2. Use the routine to thin the following image and show the output here on this page as either a matrix of ones and zeroes, an image, or an ASCII-art image of space/non-space characters.
```          00000000000000000000000000000000
01111111110000000111111110000000
01110001111000001111001111000000
01110000111000001110000111000000
01110001111000001110000000000000
01111111110000001110000000000000
01110111100000001110000111000000
01110011110011101111001111011100
01110001111011100111111110011100
00000000000000000000000000000000
```

Reference

## AutoHotkey

Works with: AutoHotkey_L

Reads input from a text file and writes output to a different text file (first creating the file, if necessary).

`FileIn  := A_ScriptDir "\Zhang-Suen.txt"FileOut := A_ScriptDir "\NewFile.txt" if (!FileExist(FileIn)) {	MsgBox, 48, File Not Found, % "File """ FileIn """ not found."	ExitApp}S := {}N := [2,3,4,5,6,7,8,9,2] Loop, Read, % FileIn{	LineNum := A_Index	Loop, Parse, A_LoopReadLine		S[LineNum, A_Index] := A_LoopField} Loop {	FlipCount := 0	Loop, 2 {		Noted := [], i := A_Index		for LineNum, Line in S {			for PixNum, Pix in Line {			; (0)				if (Pix = 0 || (P := GetNeighbors(LineNum, PixNum, S)) = 1)					continue			; (1)					BP := 0				for j, Val in P					BP += Val				if (BP < 2 || BP > 6)					continue			; (2)				AP := 0				Loop, 8					if (P[N[A_Index]] = "0" && P[N[A_Index + 1]] = "1")						AP++				if (AP != 1)					continue			; (3 and 4)				if (i = 1) {					if (P[2] + P[4] + P[6] = 3 || P[4] + P[6] + P[8] = 3)						continue				}				else if (P[2] + P[4] + P[8] = 3 || P[2] + P[6] + P[8] = 3)					continue 				Noted.Insert([LineNum, PixNum])				FlipCount++			}		}		for j, Coords in Noted			S[Coords[1], Coords[2]] := 0	}	if (!FlipCount)		break} for LineNum, Line in S {	for PixNum, Pix in Line		Out .= Pix ? "#" : " "	Out .= "`n"}FileAppend, % Out, % FileOut GetNeighbors(Y, X, S) {	Neighbors := []	if ((Neighbors[8] := S[Y, X - 1]) = "") 		return 1	if ((Neighbors[4] := S[Y, X + 1]) = "")		return 1	Loop, 3		if ((Neighbors[A_Index = 1 ? 9 : A_Index] := S[Y - 1, X - 2 + A_Index]) = "")			return 1	Loop, 3		if ((Neighbors[8 - A_Index] := S[Y + 1, X - 2 + A_Index]) = "")			return 1	return Neighbors}`

Output:

```
#######         ######
#     #        ##
#      #       #
#     #        #
##### #        #
##        #
#    #   ##    ##   #
#         ####
```

## C

Input and out images written from and to files. Format of input file is :

```<Rows> <Columns>
<Blank pixel character> <Image Pixel character>
<Image of specified rows and columns made up of the two pixel types specified in the second line.>
```

The images before and after thinning are also printed on the console.

` #include<stdlib.h>#include<stdio.h> char** imageMatrix; char blankPixel,imagePixel; typedef struct{	int row,col;}pixel; int getBlackNeighbours(int row,int col){ 	int i,j,sum = 0; 	for(i=-1;i<=1;i++){		for(j=-1;j<=1;j++){			if(i!=0 && j!=0)				sum+= (imageMatrix[row+i][col+i]==imagePixel);		}	} 	return sum;} int getBWTransitions(int row,int col){	return 	((imageMatrix[row-1][col]==blankPixel && imageMatrix[row-1][col+1]==imagePixel)			+(imageMatrix[row-1][col+1]==blankPixel && imageMatrix[row][col+1]==imagePixel)			+(imageMatrix[row][col+1]==blankPixel && imageMatrix[row+1][col+1]==imagePixel)			+(imageMatrix[row+1][col+1]==blankPixel && imageMatrix[row+1][col]==imagePixel)			+(imageMatrix[row+1][col]==blankPixel && imageMatrix[row+1][col-1]==imagePixel)			+(imageMatrix[row+1][col-1]==blankPixel && imageMatrix[row][col-1]==imagePixel)			+(imageMatrix[row][col-1]==blankPixel && imageMatrix[row-1][col-1]==imagePixel)			+(imageMatrix[row-1][col-1]==blankPixel && imageMatrix[row-1][col]==imagePixel));} int zhangSuenTest1(int row,int col){	int neighbours = getBlackNeighbours(row,col); 	return ((neighbours>=2 && neighbours<=6) 		&& (getBWTransitions(row,col)==1) 		&& (imageMatrix[row-1][col]==blankPixel||imageMatrix[row][col+1]==blankPixel||imageMatrix[row+1][col]==blankPixel) 		&& (imageMatrix[row][col+1]==blankPixel||imageMatrix[row+1][col]==blankPixel||imageMatrix[row][col-1]==blankPixel));} int zhangSuenTest2(int row,int col){	int neighbours = getBlackNeighbours(row,col); 	return ((neighbours>=2 && neighbours<=6) 		&& (getBWTransitions(row,col)==1) 		&& (imageMatrix[row-1][col]==blankPixel||imageMatrix[row][col+1]==blankPixel||imageMatrix[row][col-1]==blankPixel) 		&& (imageMatrix[row-1][col]==blankPixel||imageMatrix[row+1][col]==blankPixel||imageMatrix[row][col+1]==blankPixel));} void zhangSuen(char* inputFile, char* outputFile){ 	int startRow = 1,startCol = 1,endRow,endCol,i,j,count,rows,cols,processed; 	pixel* markers; 	FILE* inputP = fopen(inputFile,"r"); 	fscanf(inputP,"%d%d",&rows,&cols); 	fscanf(inputP,"%d%d",&blankPixel,&imagePixel); 	blankPixel<=9?blankPixel+='0':blankPixel;	imagePixel<=9?imagePixel+='0':imagePixel; 	printf("\nPrinting original image :\n"); 	imageMatrix = (char**)malloc(rows*sizeof(char*)); 	for(i=0;i<rows;i++){		imageMatrix[i] = (char*)malloc((cols+1)*sizeof(char));		fscanf(inputP,"%s\n",imageMatrix[i]);		printf("\n%s",imageMatrix[i]); 	} 	fclose(inputP); 	endRow = rows-2;	endCol = cols-2;	do{		markers = (pixel*)malloc((endRow-startRow+1)*(endCol-startCol+1)*sizeof(pixel));		count = 0; 		for(i=startRow;i<=endRow;i++){			for(j=startCol;j<=endCol;j++){				if(imageMatrix[i][j]==imagePixel && zhangSuenTest1(i,j)==1){					markers[count].row = i;					markers[count].col = j;					count++;				}			}		} 		processed = (count>0); 		for(i=0;i<count;i++){			imageMatrix[markers[i].row][markers[i].col] = blankPixel;		} 		free(markers);		markers = (pixel*)malloc((endRow-startRow+1)*(endCol-startCol+1)*sizeof(pixel));		count = 0; 		for(i=startRow;i<=endRow;i++){			for(j=startCol;j<=endCol;j++){				if(imageMatrix[i][j]==imagePixel && zhangSuenTest2(i,j)==1){					markers[count].row = i;					markers[count].col = j;					count++;				}			}		} 		if(processed==0)			processed = (count>0); 		for(i=0;i<count;i++){			imageMatrix[markers[i].row][markers[i].col] = blankPixel;		} 		free(markers);	}while(processed==1); 	FILE* outputP = fopen(outputFile,"w"); 	printf("\n\n\nPrinting image after applying Zhang Suen Thinning Algorithm : \n\n\n"); 	for(i=0;i<rows;i++){		for(j=0;j<cols;j++){			printf("%c",imageMatrix[i][j]);			fprintf(outputP,"%c",imageMatrix[i][j]);		}		printf("\n");		fprintf(outputP,"\n");	} 	fclose(outputP); 	printf("\nImage also written to : %s",outputFile);} int main(){	char inputFile[100],outputFile[100]; 	printf("Enter full path of input image file : ");	scanf("%s",inputFile); 	printf("Enter full path of output image file : ");	scanf("%s",outputFile); 	zhangSuen(inputFile,outputFile); 	return 0;} `

Contents of input file : zhImage.txt

```10 32
0 1
00000000000000000000000000000000
01111111110000000111111110000000
01110001111000001111001111000000
01110000111000001110000111000000
01110001111000001110000000000000
01111111110000001110000000000000
01110111100000001110000111000000
01110011110011101111001111011100
01110001111011100111111110011100
00000000000000000000000000000000
```

Console interaction :

```Enter full path of input image file : zhImage.txt
Enter full path of output image file : out.txt

Printing original image :

00000000000000000000000000000000
01111111110000000111111110000000
01110001111000001111001111000000
01110000111000001110000111000000
01110001111000001110000000000000
01111111110000001110000000000000
01110111100000001110000111000000
01110011110011101111001111011100
01110001111011100111111110011100
00000000000000000000000000000000

Printing image after applying Zhang Suen Thinning Algorithm :

00000000000000000000000000000000
00111111100000000011111100000000
00100000100000000110000000000000
01000000100000000100000000000000
01000000100000001000000000000000
01111111100000001000000000000000
01000001000000000100000011000000
01000001000001100110000110001100
01000000000010000001111000010000
00000000000000000000000000000000

Image also written to : out.txt
```

Contents of out.txt :

```00000000000000000000000000000000
00111111100000000011111100000000
00100000100000000110000000000000
01000000100000000100000000000000
01000000100000001000000000000000
01111111100000001000000000000000
01000001000000000100000011000000
01000001000001100110000110001100
01000000000010000001111000010000
00000000000000000000000000000000
```

## C++

Compiled with --std=c++14

`#include <iostream>#include <string>#include <sstream>#include <valarray>const std::string input {"................................"".#########.......########......."".###...####.....####..####......"".###....###.....###....###......"".###...####.....###............."".#########......###............."".###.####.......###....###......"".###..####..###.####..####.###.."".###...####.###..########..###..""................................"};const std::string input2 {".........................................................."".#################...................#############........"".##################...............################........"".###################............##################........"".########.....#######..........###################........""...######.....#######.........#######.......######........""...######.....#######........#######......................""...#################.........#######......................""...################..........#######......................""...#################.........#######......................""...######.....#######........#######......................""...######.....#######........#######......................""...######.....#######.........#######.......######........"".########.....#######..........###################........"".########.....#######.######....##################.######."".########.....#######.######......################.######."".########.....#######.######.........#############.######."".........................................................."}; class ZhangSuen; class Image {public:    friend class ZhangSuen;    using pixel_t = char;    static const pixel_t BLACK_PIX;    static const pixel_t WHITE_PIX;     Image(unsigned width = 1, unsigned height = 1)     : width_{width}, height_{height}, data_( '\0', width_ * height_)    {}    Image(const Image& i) : width_{ i.width_}, height_{i.height_}, data_{i.data_}    {}    Image(Image&& i) : width_{ i.width_}, height_{i.height_}, data_{std::move(i.data_)}    {}    ~Image() = default;    Image& operator=(const Image& i) {        if (this != &i) {            width_ = i.width_;            height_ = i.height_;            data_ = i.data_;        }        return *this;    }    Image& operator=(Image&& i) {        if (this != &i) {            width_ = i.width_;            height_ = i.height_;            data_ = std::move(i.data_);        }        return *this;    }    size_t idx(unsigned x, unsigned y) const noexcept { return y * width_ + x; }    bool operator()(unsigned x, unsigned y) {        return data_[idx(x, y)];    }    friend std::ostream& operator<<(std::ostream& o, const Image& i) {        o << i.width_ << " x " << i.height_ << std::endl;        size_t px = 0;        for(const auto& e : i.data_) {            o << (e?Image::BLACK_PIX:Image::WHITE_PIX);            if (++px % i.width_ == 0)                o << std::endl;        }        return o << std::endl;    }    friend std::istream& operator>>(std::istream& in, Image& img) {        auto it = std::begin(img.data_);        const auto end = std::end(img.data_);        Image::pixel_t tmp;        while(in && it != end) {            in >> tmp;            if (tmp != Image::BLACK_PIX && tmp != Image::WHITE_PIX)                throw "Bad character found in image";            *it = (tmp == Image::BLACK_PIX)?1:0;            ++it;        }        return in;    }    unsigned width() const noexcept { return width_; }    unsigned height() const noexcept { return height_; }    struct Neighbours {        // 9 2 3        // 8 1 4        // 7 6 5        Neighbours(const Image& img, unsigned p1_x, unsigned p1_y)        : img_{img}        , p1_{img.idx(p1_x, p1_y)}        , p2_{p1_ - img.width()}        , p3_{p2_ + 1}        , p4_{p1_ + 1}        , p5_{p4_ + img.width()}        , p6_{p5_ - 1}        , p7_{p6_ - 1}        , p8_{p1_ - 1}        , p9_{p2_ - 1}         {}        const Image& img_;        const Image::pixel_t& p1() const noexcept { return img_.data_[p1_]; }        const Image::pixel_t& p2() const noexcept { return img_.data_[p2_]; }        const Image::pixel_t& p3() const noexcept { return img_.data_[p3_]; }        const Image::pixel_t& p4() const noexcept { return img_.data_[p4_]; }        const Image::pixel_t& p5() const noexcept { return img_.data_[p5_]; }        const Image::pixel_t& p6() const noexcept { return img_.data_[p6_]; }        const Image::pixel_t& p7() const noexcept { return img_.data_[p7_]; }        const Image::pixel_t& p8() const noexcept { return img_.data_[p8_]; }        const Image::pixel_t& p9() const noexcept { return img_.data_[p9_]; }        const size_t p1_, p2_, p3_, p4_, p5_, p6_, p7_, p8_, p9_;    };    Neighbours neighbours(unsigned x, unsigned y) const { return Neighbours(*this, x, y); }private:    unsigned height_ { 0 };    unsigned width_ { 0 };    std::valarray<pixel_t> data_;}; constexpr const Image::pixel_t Image::BLACK_PIX = '#';constexpr const Image::pixel_t Image::WHITE_PIX = '.'; class ZhangSuen {public:     // the number of transitions from white to black, (0 -> 1) in the sequence P2,P3,P4,P5,P6,P7,P8,P9,P2    unsigned transitions_white_black(const Image::Neighbours& a) const {        unsigned sum = 0;        sum += (a.p9() == 0) && a.p2();        sum += (a.p2() == 0) && a.p3();        sum += (a.p3() == 0) && a.p4();        sum += (a.p8() == 0) && a.p9();        sum += (a.p4() == 0) && a.p5();        sum += (a.p7() == 0) && a.p8();        sum += (a.p6() == 0) && a.p7();        sum += (a.p5() == 0) && a.p6();        return sum;    }     // The number of black pixel neighbours of P1. ( = sum(P2 .. P9) )    unsigned black_pixels(const Image::Neighbours& a) const {        unsigned sum = 0;        sum += a.p9();        sum += a.p2();        sum += a.p3();        sum += a.p8();        sum += a.p4();        sum += a.p7();        sum += a.p6();        sum += a.p5();        return sum;    }    const Image& operator()(const Image& img) {        tmp_a_ = img;        size_t changed_pixels = 0;        do {            changed_pixels = 0;            // Step 1            tmp_b_ = tmp_a_;            for(size_t y = 1; y < tmp_a_.height() - 1; ++y) {                for(size_t x = 1; x < tmp_a_.width() - 1; ++x) {                    if (tmp_a_.data_[tmp_a_.idx(x, y)]) {                        auto n = tmp_a_.neighbours(x, y);                        auto bp = black_pixels(n);                        if (bp >= 2 && bp <= 6) {                            auto tr = transitions_white_black(n);                            if (    tr == 1                                 && (n.p2() * n.p4() * n.p6() == 0)                                && (n.p4() * n.p6() * n.p8() == 0)                                ) {                                tmp_b_.data_[n.p1_] = 0;                                ++changed_pixels;                            }                        }                    }                 }            }            // Step 2            tmp_a_ = tmp_b_;            for(size_t y = 1; y < tmp_b_.height() - 1; ++y) {                for(size_t x = 1; x < tmp_b_.width() - 1; ++x) {                    if (tmp_b_.data_[tmp_b_.idx(x, y)]) {                        auto n = tmp_b_.neighbours(x, y);                        auto bp = black_pixels(n);                        if (bp >= 2 && bp <= 6) {                            auto tr = transitions_white_black(n);                            if (    tr == 1                                 && (n.p2() * n.p4() * n.p8() == 0)                                && (n.p2() * n.p6() * n.p8() == 0)                                ) {                                tmp_a_.data_[n.p1_] = 0;                                ++changed_pixels;                            }                        }                    }                 }            }        } while(changed_pixels > 0);        return tmp_a_;    }private:    Image tmp_a_;    Image tmp_b_;}; int main(int argc, char const *argv[]){    using namespace std;    Image img(32, 10);    istringstream iss{input};    iss >> img;    cout << img;    cout << "ZhangSuen" << endl;    ZhangSuen zs;    Image res = std::move(zs(img));    cout << res << endl;     Image img2(58,18);    istringstream iss2{input2};    iss2 >> img2;    cout << img2;    cout << "ZhangSuen with big image" << endl;    Image res2 = std::move(zs(img2));    cout << res2 << endl;    return 0;} `

Output:

```32 x 10
................................
.#########.......########.......
.###...####.....####..####......
.###....###.....###....###......
.###...####.....###.............
.#########......###.............
.###.####.......###....###......
.###..####..###.####..####.###..
.###...####.###..########..###..
................................

ZhangSuen
32 x 10
................................
..#######.........######........
..#.....#........##.............
..#......#.......#..............
..#.....#........#..............
..#####.#........#..............
.......##........#..............
........#....#...##....##...#...
.........#.........####.........
................................

58 x 18
..........................................................
.#################...................#############........
.##################...............################........
.###################............##################........
.########.....#######..........###################........
...######.....#######.........#######.......######........
...######.....#######........#######......................
...#################.........#######......................
...################..........#######......................
...#################.........#######......................
...######.....#######........#######......................
...######.....#######........#######......................
...######.....#######.........#######.......######........
.########.....#######..........###################........
.########.....#######.######....##################.######.
.########.....#######.######......################.######.
.########.....#######.######.........#############.######.
..........................................................

ZhangSuen with big image
58 x 18
..........................................................
..........................................................
....#.##########.......................#######............
.....##........#...................####.......#...........
.....#..........#.................##......................
.....#..........#................#........................
.....#..........#................#........................
.....#..........#................#........................
.....############...............#.........................
.....#..........#...............#.........................
.....#..........#................#........................
.....#..........#................#........................
.....#..........#................#........................
.....#............................##......................
.....#.............................############...........
.......................###..........................###...
..........................................................
..........................................................
```

## D

This uses the module from the Bitmap Task. And it performs no heap allocations.

`import std.stdio, std.algorithm, std.string, std.functional,       std.typecons, std.typetuple, bitmap; struct BlackWhite {    ubyte c;    alias c this;    static immutable black = typeof(this)(0),                     white = typeof(this)(1);} alias Neighbours = BlackWhite[9];alias Img = Image!BlackWhite; /// Zhang-Suen thinning algorithm.Img zhangSuen(Img image1, Img image2) pure nothrow @safe @nogcin {    assert(image1.image.all!(x => x == Img.black || x == Img.white));    assert(image1.nx == image2.nx && image1.ny == image2.ny);} out(result) {    assert(result.nx == image1.nx && result.ny == image1.ny);    assert(result.image.all!(x => x == Img.black || x == Img.white));} body {    /// True if inf <= x <= sup.    static inInterval(T)(in T x, in T inf, in T sup) pure nothrow @safe @nogc {        return x >= inf && x <= sup;    }     /// Return 8-neighbours+1 of point (x,y) of given image, in order.    static void neighbours(in Img I, in size_t x, in size_t y,                           out Neighbours n) pure nothrow @safe @nogc {        n = [I[x,y-1], I[x+1,y-1], I[x+1,y], I[x+1,y+1], // P2,P3,P4,P5             I[x,y+1], I[x-1,y+1], I[x-1,y], I[x-1,y-1], // P6,P7,P8,P9             I[x,y-1]];    }     if (image1.nx < 3 || image1.ny < 3) {        image2.image[] = image1.image[];        return image2;    }     immutable static zeroOne = [0, 1]; //**    Neighbours n;    bool hasChanged;    do {        hasChanged = false;         foreach (immutable ab; TypeTuple!(tuple(2, 4), tuple(0, 6))) {            foreach (immutable y; 1 .. image1.ny - 1) {                foreach (immutable x; 1 .. image1.nx - 1) {                    neighbours(image1, x, y, n);                    if (image1[x, y] &&                    // Cond. 0                        (!n[ab[0]] || !n[4] || !n[6]) &&   // Cond. 4                        (!n[0] || !n[2] || !n[ab[1]]) &&   // Cond. 3                        //n[].count([0, 1]) == 1 &&                        n[].count(zeroOne) == 1 &&         // Cond. 2                        // n[0 .. 8].sum in iota(2, 7)) {                        inInterval(n[0 .. 8].sum, 2, 6)) { // Cond. 1                        hasChanged = true;                        image2[x, y] = Img.black;                    } else                        image2[x, y] = image1[x, y];                }            }            image1.swap(image2);        }    } while (hasChanged);     return image1;} void main() {    immutable before_txt = "    ##..###    ##..###    ##..###    ##..###    ##..##.    ##..##.    ##..##.    ##..##.    ##..##.    ##..##.    ##..##.    ##..##.    ######.    .......";     immutable small_rc = "    ................................    .#########.......########.......    .###...####.....####..####......    .###....###.....###....###......    .###...####.....###.............    .#########......###.............    .###.####.......###....###......    .###..####..###.####..####.###..    .###...####.###..########..###..    ................................";     immutable rc = "    ...........................................................    .#################...................#############.........    .##################...............################.........    .###################............##################.........    .########.....#######..........###################.........    ...######.....#######.........#######.......######.........    ...######.....#######........#######.......................    ...#################.........#######.......................    ...################..........#######.......................    ...#################.........#######.......................    ...######.....#######........#######.......................    ...######.....#######........#######.......................    ...######.....#######.........#######.......######.........    .########.....#######..........###################.........    .########.....#######.######....##################.######..    .########.....#######.######......################.######..    .########.....#######.######.........#############.######..    ...........................................................";     foreach (immutable txt; [before_txt, small_rc, rc]) {        auto img = Img.fromText(txt);        "From:".writeln;        img.textualShow(/*bl=*/ '.', /*wh=*/ '#');        "\nTo thinned:".writeln;        img.zhangSuen(img.dup).textualShow(/*bl=*/ '.', /*wh=*/ '#');        writeln;    }}`
Output:
```From:
##..###
##..###
##..###
##..###
##..##.
##..##.
##..##.
##..##.
##..##.
##..##.
##..##.
##..##.
######.
.......

To thinned:
##..###
#.....#
#.....#
#...###
#...#..
#...#..
#...#..
#...#..
#...#..
#...#..
#...#..
#...#..
#####..
.......

From:
................................
.#########.......########.......
.###...####.....####..####......
.###....###.....###....###......
.###...####.....###.............
.#########......###.............
.###.####.......###....###......
.###..####..###.####..####.###..
.###...####.###..########..###..
................................

To thinned:
................................
..#######.........######........
..#.....#........##.............
..#......#.......#..............
..#.....#........#..............
..#####.#........#..............
.......##........#..............
........#....#...##....##...#...
.........#.........####.........
................................

From:
...........................................................
.#################...................#############.........
.##################...............################.........
.###################............##################.........
.########.....#######..........###################.........
...######.....#######.........#######.......######.........
...######.....#######........#######.......................
...#################.........#######.......................
...################..........#######.......................
...#################.........#######.......................
...######.....#######........#######.......................
...######.....#######........#######.......................
...######.....#######.........#######.......######.........
.########.....#######..........###################.........
.########.....#######.######....##################.######..
.########.....#######.######......################.######..
.########.....#######.######.........#############.######..
...........................................................

To thinned:
...........................................................
...........................................................
....#.##########.......................#######.............
.....##........#...................####.......#............
.....#..........#.................##.......................
.....#..........#................#.........................
.....#..........#................#.........................
.....#..........#................#.........................
.....############...............#..........................
.....#..........#...............#..........................
.....#..........#................#.........................
.....#..........#................#.........................
.....#..........#................#.........................
.....#............................##.......................
.....#.............................############............
.......................###..........................###....
...........................................................
...........................................................```

## Elena

ELENA 3.4 :

Translation of: Java
`import system'collections.import system'routines.import extensions.import extensions'routines. const image = (        "                                                          ",        " #################                   #############        ",        " ##################               ################        ",        " ###################            ##################        ",        " ########     #######          ###################        ",        "   ######     #######         #######       ######        ",        "   ######     #######        #######                      ",        "   #################         #######                      ",        "   ################          #######                      ",        "   #################         #######                      ",        "   ######     #######        #######                      ",        "   ######     #######        #######                      ",        "   ######     #######         #######       ######        ",        " ########     #######          ###################        ",        " ########     ####### ######    ################## ###### ",        " ########     ####### ######      ################ ###### ",        " ########     ####### ######         ############# ###### ",        "                                                          "). nbrs = ((0, -1), (1, -1), (1, 0), (1, 1), (0, 1),        (-1, 1), (-1, 0), (-1, -1), (0, -1)). nbrGroups = (((0, 2, 4), (2, 4, 6)), ((0, 2, 6),        (0, 4, 6))). extension<Matrix<CharValue>> zhangsuenOp{    proceed(r, c, toWhite, firstStep)    [        if (self[r][c] != \$35)            [ ^ false ].         int nn := self numNeighbors(r,c).         if ((nn < 2) || (nn > 6))            [ ^ false ].         if(self numTransitions(r,c) != 1)            [ ^ false ].         ifnot (self atLeastOneIsWhite(r,c,firstStep iif(0,1)))            [ ^ false ].         toWhite append:{ x = c. y = r. }.         ^ true.          ]     numNeighbors(r,c)    [        int count := 0.         0 till(nbrs length - 1) do(:i)        [            if (self[r + nbrs[i][1]][c + nbrs[i][0]] == \$35)                [ count := count + 1. ].        ].         ^ count.    ]     numTransitions(r,c)    [        int count := 0.         0 till(nbrs length - 1) do(:i)        [            if (self[r + nbrs[i][1]][c + nbrs[i][0]] == \$32)            [                if (self[r + nbrs[i + 1][1]][c + nbrs[i + 1][0]] == \$35)                [                    count := count + 1.                ].            ].        ].         ^ count.    ]       atLeastOneIsWhite(r, c, step)    [        int count := 0.        var group := nbrGroups[step].        0 till:2 do(:i)        [                        0 till(group[i] length) seek(:j)            [                var nbr := nbrs[group[i][j]].                 if (self[r + nbr[1]][c + nbr[0]] == \$32)                    [ count := count + 1. ^ true ].                 ^ false.                                ].        ].         ^ count > 1.    ]         thinImage    [        bool firstStep := false.        bool hasChanged := true.        var toWhite := List new.         while (hasChanged || firstStep)        [            hasChanged := false.            firstStep := firstStep inverted.             1 till(self rows - 1) do(:r)             [                1 till(self columns - 1) do(:c)                [                    if(self proceed(r,c,toWhite,firstStep))                        [ hasChanged := true ].                ].            ].             toWhite forEach(:p)[ self[p y][p x] := \$32. ].                            toWhite clear.        ].    ]     print    [        var it := self enumerator.         it forEach(:ch) [ console print(ch," ") ].        while (it next)        [            console writeLine.             it forEach(:ch) [ console print(ch," ") ].        ].    ]} public program[    Matrix<CharValue> grid := MatrixSpace::    {        int rows = image length.         int columns = image[0] length.         getAt(int i, int j)            = image[i][j].         setAt(int i, int j, object o)        [            image[i][j] := o.        ]    }.     grid thinImage.     grid print.     console readChar]`
Output:
```

#   # # # # # # # # # #                                               # # # # # # #
# #                 #                                       # # # #               #
#                     #                                   # #
#                     #                                 #
#                     #                                 #
#                     #                                 #
# # # # # # # # # # # #                               #
#                     #                               #
#                     #                                 #
#                     #                                 #
#                     #                                 #
#                                                         # #
#                                                           # # # # # # # # # # # #
# # #                                                     # # #

```

## Elixir

Translation of: Ruby
`defmodule ZhangSuen do  @neighbours  [{-1,0},{-1,1},{0,1},{1,1},{1,0},{1,-1},{0,-1},{-1,-1}]  # 8 neighbours   def thinning(str, black \\ ?#) do    s0 = for {line, i} <- (String.split(str, "\n") |> Enum.with_index),             {c, j}    <- (to_char_list(line) |> Enum.with_index),             into: Map.new,             do: {{i,j}, (if c==black, do: 1, else: 0)}    {xrange, yrange} = range(s0)    print(s0, xrange, yrange)    s1 = thinning_loop(s0, xrange, yrange)    print(s1, xrange, yrange)  end   defp thinning_loop(s0, xrange, yrange) do    s1 = step(s0, xrange, yrange, 1)            # Step 1    s2 = step(s1, xrange, yrange, 0)            # Step 2    if Map.equal?(s0, s2), do: s2, else: thinning_loop(s2, xrange, yrange)  end   defp step(s, xrange, yrange, g) do    for x <- xrange, y <- yrange, into: Map.new, do: {{x,y}, s[{x,y}] - zs(s,x,y,g)}  end   defp zs(s, x, y, g) do    if get(s,x,y) == 0 or                                       # P1      (get(s,x-1,y) + get(s,x,y+1) + get(s,x+g,y-1+g)) == 3 or  # P2, P4, P6/P8      (get(s,x-1+g,y+g) + get(s,x+1,y) + get(s,x,y-1)) == 3 do  # P4/P2, P6, P8      0    else      next = for {i,j} <- @neighbours, do: get(s, x+i, y+j)      bp1 = Enum.sum(next)                                      # B(P1)      if bp1 in 2..6 do        ap1 = (next++[hd(next)]) |> Enum.chunk(2,1) |> Enum.count(fn [a,b] -> a<b end)  # A(P1)        if ap1 == 1, do: 1, else: 0      else        0      end    end  end   defp get(map, x, y), do: Map.get(map, {x,y}, 0)   defp range(map), do: range(Map.keys(map), 0, 0)  defp range([], xmax, ymax), do: {0 .. xmax, 0 .. ymax}  defp range([{x,y} | t], xmax, ymax), do: range(t, max(x,xmax), max(y,ymax))   @display  %{0 => " ", 1 => "#"}  defp print(map, xrange, yrange) do    Enum.each(xrange, fn x ->      IO.puts (for y <- yrange, do: @display[map[{x,y}]])    end)  endend str = """............................................................#################...................#############..........##################...............################..........###################............##################..........########.....#######..........###################............######.....#######.........#######.......######............######.....#######........#######..........................#################.........#######..........................###############...........#######..........................#################.........#######..........................######....########........#######..........................######.....#######........#######..........................######.....#######.........#######.......######..........########.....#######..........###################..........########.....#######..#####....##################.######...########.....#######..#####......################.######...########.....#######..#####.........#############.######............................................................."""ZhangSuen.thinning(str) str = """00000000000000000000000000000000011111111100000001111111100000000111000111100000111100111100000001110000111000001110000111000000011100011110000011100000000000000111111111000000111000000000000001110111100000001110000111000000011100111100111011110011110111000111000111101110011111111001110000000000000000000000000000000000"""ZhangSuen.thinning(str, ?1)`
Output:
```
#################                   #############
##################               ################
###################            ##################
########     #######          ###################
######     #######         #######       ######
######     #######        #######
#################         #######
###############           #######
#################         #######
######    ########        #######
######     #######        #######
######     #######         #######       ######
########     #######          ###################
########     #######  #####    ################## ######
########     #######  #####      ################ ######
########     #######  #####         ############# ######

# ##########                       #######
##        #                   ####       #
#          #                 ##
#          #                #
#          #                #
#          #                #
############               #
#          #               #
#          #                #
#          #                #
#          #                #
#                            ##
#                             ############
##                          ###

#########       ########
###   ####     ####  ####
###    ###     ###    ###
###   ####     ###
#########      ###
### ####       ###    ###
###  ####  ### ####  #### ###
###   #### ###  ########  ###

#######         ######
#     #        ##
#      #       #
#     #        #
##### #        #
##        #
#    #   ##    ##   #
#         ####

```

## FreeBASIC

`' version 08-10-2016' compile with: fbc -s console Data "00000000000000000000000000000000"Data "01111111110000000111111110000000"Data "01110001111000001111001111000000"Data "01110000111000001110000111000000"Data "01110001111000001110000000000000"Data "01111111110000001110000000000000"Data "01110111100000001110000111000000"Data "01110011110011101111001111011100"Data "01110001111011100111111110011100"Data "00000000000000000000000000000000"Data "END" ' ------=< MAIN >=------ Dim As UInteger x, y, m, nDim As String input_str Do        ' find out how big it is  Read input_str  If input_str = "END" Then Exit Do  If x < Len(input_str) Then x = Len(input_str)  y = y + 1Loop m = x -1 : n = y -1ReDim As UByte old(m, n), new_(m, n) y = 0Restore   ' restore data pointerDo        ' put data in array  Read input_str  If input_str="END" Then Exit Do  For x = 0 To Len(input_str) -1    old(x,y) = input_str[x] - Asc("0")    ' print image    If old(x, y) = 0 Then Print "."; Else Print "#";  Next  Print  y = y + 1Loop 'corners and sides do not changeFor x = 0 To m  new_(x, 0) = old(x, 0)  new_(x, n) = old(x, n)Next For y = 0 To n  new_(0, y) = old(0, y)  new_(m, y) = old(m, y)Next Dim As UInteger tmp, change, stage = 1Do  change = 0  For y = 1 To n -1    For x = 1 To m -1      ' -1-      If old(x,y) = 0 Then ' first condition, p1 must be black        new_(x,y) = 0        Continue For      End If      ' -2-      tmp = old(x, y -1) + old(x +1, y -1)      tmp = tmp + old(x +1, y) + old(x +1, y +1) + old(x, y +1)      tmp = tmp + old(x -1, y +1) + old(x -1, y) + old(x -1, y -1)      If tmp < 2 OrElse tmp > 6 Then ' 2 <= B(p1) <= 6        new_(x, y) = 1        Continue For      End If      ' -3-      tmp = 0      If old(x   , y   ) = 0 And old(x   , y -1) = 1 Then tmp += 1  ' p1 > p2      If old(x   , y -1) = 0 And old(x +1, y -1) = 1 Then tmp += 1  ' p2 > p3      If old(x +1, y -1) = 0 And old(x +1, y   ) = 1 Then tmp += 1  ' p3 > p4      If old(x +1, y   ) = 0 And old(x +1, y +1) = 1 Then tmp += 1  ' p4 > p5      If old(x +1, y +1) = 0 And old(x   , y +1) = 1 Then tmp += 1  ' p5 > p6      If old(x   , y +1) = 0 And old(x -1, y +1) = 1 Then tmp += 1  ' p6 > p7      If old(x -1, y +1) = 0 And old(x -1, y   ) = 1 Then tmp += 1  ' p7 > p8      If old(x -1, y   ) = 0 And old(x -1, y -1) = 1 Then tmp += 1  ' p8 > p9      If old(x -1, y -1) = 0 And old(x   , y -1) = 1 Then tmp += 1  ' p9 > p2      ' tmp = 1 ==> A(P1) = 1      If tmp <> 1 Then        new_(x, y) = 1        Continue For      End If      If (stage And 1) = 1 Then        ' step 1 -4- -5-        If (old(x, y -1) + old(x +1, y) + old(x, y +1)) = 3 OrElse _           (old(x +1, y) + old(x, y +1) + old(x -1, y)) = 3 Then          new_(x, y) = 1          Continue For        End If      Else        ' step 2 -4- -5-        If (old(x, y -1) + old(x +1, y) + old(x -1, y)) = 3 OrElse _           (old(x, y -1) + old(x, y +1) + old(x -1, y)) = 3 Then          new_(x, y) = 1          Continue For        End If      End If      ' all condition are met, make p1 white (0)      new_(x, y) = 0      change = 1 ' flag change    Next  Next   ' copy new_() into old()  For y = 0 To n    For x = 0 To m      old(x, y) = new_(x, y)    Next  Next   stage += 1Loop Until change = 0 ' stop when there are no changes made Print ' print resultPrint "End result"For y = 0 To n  For x = 0 To m    If old(x, y) = 0 Then Print "."; Else Print "#";  Next  PrintNext  ' empty keyboard bufferWhile Inkey <> "" : WendPrint : Print "hit any key to end program"SleepEnd`
Output:
```................................
.#########.......########.......
.###...####.....####..####......
.###....###.....###....###......
.###...####.....###.............
.#########......###.............
.###.####.......###....###......
.###..####..###.####..####.###..
.###...####.###..########..###..
................................

End result
................................
..#######.........######........
..#.....#........##.............
..#......#.......#..............
..#.....#........#..............
..#####.#........#..............
.......##........#..............
........#....#...##....##...#...
.........#.........####.........
................................```

## Fortran

With F90 came standardisation of a variety of array manipulation facilities. Since the image array is to be inspected as a whole then adjusted rather than adjusted step-by-step as it is inspected, the first thought was to employ the special facility of the FOR ALL statement, which is that in an expression such as
`FOR ALL (i = 2:n - 1) A(i) = (A(i - 1) + A(i) + A(i + 1))/3`
all right-hand-side expressions will be evaluated with the original values of the array, while in the less special array assignment
`A(2:N - 1) = (A(1:N - 2) + A(2:N - 1) + A(3:N))/3`
as in the case of the equivalent DO-loop, the processing will be with a mixture of old and new values as the loop proceeds. So, that suggests something like
`      FOR ALL (I = 2:N - 1, J = 2:M - 1)       WHERE(DOT(I,J) .NE. 0) DOT(I,J) = ADJUST(DOT,I,J)`

This requires function ADJUST to be a "pure" function, and they are not supposed to perpetrate side effects, such as one reporting that any adjustment was made. Nor is it clear that array DOT must be presented as a parameter either as the entire array or as element DOT(i,j), or if not, that it can be global to function ADJUST - which would also be an impurity - and for that matter, variables I and J could be global also...

Instead, thought turned to more closely following the task specification, which involves producing a list of elements to be adjusted after an inspection pass. Given that array DOT is two-dimensional, it would be nice if an element could be indexed via an expression such as `DOT(INDEX)` where INDEX was an array of two elements with INDEX(1) = i, and INDEX(2) = j, so as to access DOT(i,j) If this were possible, then obviously one could hope that array INDEX could be extended so as to store the multiple elements of a list of such locations to access, with a view to `DOT(INDEX(1:n)) = 0` adjusting the image.

Alas, such a syntax form is not accommodated. However, F90 also introduced the ability to define and use compound data types, such as the type PLACE as used below. It is not possible to define a type of a special, recognised form, such as say "SUBSCRIPT LIST" that can be used as dreamt of above, so the components are just ordinary variables. Two ordinary arrays could be used, one for each of the two subscripts, or a compound type could be devised in a hint towards self-documentation. Thus,
`      DOT(WHACK(1:WHACKCOUNT).I,WHACK(1:WHACKCOUNT).J) = 0`

But it doesn't work... After a fair amount of head scratching, not at all assisted by the woolly generalities and inane examples of the compiler's "help" collection, it became apparent that the expression did not work through a list of indices as anticipated, but instead, for each value of the first index, all the values of the second index were selected. Thus, instead of the first change being DOT(WHACK(1).I,WHACK(1).J) only, it was DOT(WHACK(1).I,WHACK(1:WHACKCOUNT).J) that were being cleared. Accordingly, the fancy syntax has to be abandoned in favour of a specific DO-loop.

`      MODULE ZhangSuenThinning	!Image manipulation.       CONTAINS        SUBROUTINE ZST(DOT)	!Attempts to thin out thick lines.         INTEGER DOT(:,:)	!The image in an array, rows down the page.         TYPE PLACE		!This records an array location.          INTEGER I			!Via its          INTEGER J			!Indices.          END TYPE PLACE	!A lot of baggage.         TYPE(PLACE) WHACK(UBOUND(DOT,DIM = 1)*UBOUND(DOT,DIM = 2))	!Allow a whack for every dot.         INTEGER WHACKCOUNT	!Counts up those to be wiped out.         LOGICAL WHACKED	!Notes if any have been.         INTEGER STEP,I,N,J,M	!Assistants.         INTEGER D9(9)		!Holds a 3x3 portion.         INTEGER HIT1(3,2),HIT2(3,2)	!Lists of elements to inspect for certain tests.         PARAMETER (HIT1 = (/2,6,8, 4,2,6/))	!Two stages.         PARAMETER (HIT2 = (/4,8,6, 2,4,8/))	!Each with two hit lists.          N = UBOUND(DOT,DIM = 1)	!Number of rows.          M = UBOUND(DOT,DIM = 2)	!Number of columns.Commence a pass.   10     WHACKED = .FALSE.	!No damage so far.          DO STEP = 1,2		!Each pass is in two stages.            WHACKCOUNT = 0	!No dots have been selected for whitewashing.            DO I = 2,N - 1	!Scan down the rows.              DO J = 2,M - 1	!And the columns. Interior dots only.                IF (DOT(I,J).NE.0) THEN	!Rule 0: Is the dot black? Eight neighbours are present due to loop control.                  D9(1:3) = DOT(I - 1,J - 1:J + 1)	!Yes. Form a 3x3 mesh.	1 2 3  not  9 2 3                  D9(4:6) = DOT(I    ,J - 1:J + 1)	!As a 1-D array.	4 5 6       8 1 4                  D9(7:9) = DOT(I + 1,J - 1:J + 1)	!For eased access.	7 8 9       7 6 5                  CALL INSPECT(D9,HIT1(1,STEP),HIT2(1,STEP))	!Apply rules one to four, as specified.                END IF			!So much for a black dot.              END DO		!On to the next column.            END DO		!On to the next row.            IF (WHACKCOUNT.GT.0) THEN	!Are any to be wiped out?              DO I = 1,WHACKCOUNT		!Yes!                DOT(WHACK(I).I,WHACK(I).J) = 0		!One by one.              END DO				!On to the next victim.Can't use     DOT(WHACK(1:WHACKCOUNT).I,WHACK(1:WHACKCOUNT).J) = 0              WHACKED = .TRUE.			!There has been a change.            END IF			!So much for changes.          END DO		!On to the second stage.          IF (WHACKED) GO TO 10	!If there had been changes, perhaps there will be more.         CONTAINS	!Some helpers.          SUBROUTINE INSPECT(BLOB,HIT1,HIT2)	!Inspect a 3x3 piece according to the four levels of tests as specified.           INTEGER BLOB(9)		!The piece. BLOB(5) is DOT(I,J), and is expected to be 1.           INTEGER HIT1(3),HIT2(3)	!Two hit lists.           INTEGER TWIRL(9)		!traces the periphery of the piece.           PARAMETER (TWIRL = (/2,3,6,9,8,7,4,1,2/))	!Cycle around the periphery.           INTEGER B	!A counter.			!Rule:            B = SUM(BLOB) - BLOB(5)			!1: Count the neighbours having one, not zero.            IF (2 <= B .AND. B <= 6) THEN		!   The test. Can't have 2 <= B <= 6, alas.              IF (COUNT(BLOB(TWIRL(1:8))		!2: Counting transitions.     *              .LT.BLOB(TWIRL(2:9))) .EQ.1) THEN	!   The test of 0 --> positive.                IF (ANY(BLOB(HIT1).EQ.0)) THEN		!3: At least one must be white.                  IF (ANY(BLOB(HIT2).EQ.0)) THEN	!4: Of two sets of three.                    WHACKCOUNT = WHACKCOUNT + 1			!Another one down!                    WHACK(WHACKCOUNT) = PLACE(I,J)		!This is the place.                  END IF				!Now back out of the nested IF-statements.                END IF				!Since the tests must all be passed              END IF			!Rather than say three out of four.            END IF		!For the given method.          END SUBROUTINE INSPECT!That was weird.        END SUBROUTINE ZST	!But so it goes.         SUBROUTINE SHOW(A)	!Display an image array on the standard output.         INTEGER A(:,:)		!Values are expected to be zero and one.         CHARACTER*1 HIC(0:1)	!But I don't want to look at wads of digits.         PARAMETER (HIC = (/".","#"/))	!These offer better contrast.         INTEGER I		!A stepper.         DO I = 1,UBOUND(A,DIM = 1)	!Work down the given number of rows.           WRITE (6,"(666A1)") HIC(A(I,:))	!Roll a translated line.         END DO				!Hopefully, no more than 666 to a line.        END SUBROUTINE SHOW	!That was straightforward.      END MODULE ZhangSuenThinning       PROGRAM POKE	!Just set up the example.      USE ZhangSuenThinning      INTEGER N,M		!Parameters for the example.      PARAMETER (N = 10,M = 32)	!Rows and columns.      CHARACTER*(M) CANVAS(N)	!Rather than some monster DATA statement,      PARAMETER (CANVAS = (/	!It is easier to prepare a worksheet.     1 "                                ",     2 " 111111111       11111111       ",     3 " 111   1111     1111  1111      ",     4 " 111    111     111    111      ",     5 " 111   1111     111             ",     6 " 111111111      111             ",     7 " 111 1111       111    111      ",     8 " 111  1111  111 1111  1111 111  ",     9 " 111   1111 111  11111111  111  ",     o "                                "/))       INTEGER IMAGE(N,M)	!The image array. Exactly the required size.       INTEGER I		!A stepper.        DO I = 1,N		!Read the rows.         READ (CANVAS(I),"(666I1)") IMAGE(I,:)	!Presumably, 666 will suffice.       END DO			!A blank is taken as a zero with formatted input.        WRITE (6,*) "The initial image..."       CALL SHOW(IMAGE)       WRITE (6,*)        CALL ZST(IMAGE)       WRITE (6,*) "And after 'thinning'..."       CALL SHOW(IMAGE)       END PROGRAM POKE `

Output:

``` The initial image...
................................
.#########.......########.......
.###...####.....####..####......
.###....###.....###....###......
.###...####.....###.............
.#########......###.............
.###.####.......###....###......
.###..####..###.####..####.###..
.###...####.###..########..###..
................................

And after 'thinning'...
................................
..#######.........######........
..#.....#........##.............
..#......#.......#..............
..#.....#........#..............
..#####.#........#..............
.......##........#..............
........#....#...##....##...#...
.........#.........####.........
................................
```

## Go

`package main import (    "bytes"    "fmt"    "strings") var in = `00000000000000000000000000000000011111111100000001111111100000000111000111100000111100111100000001110000111000001110000111000000011100011110000011100000000000000111111111000000111000000000000001110111100000001110000111000000011100111100111011110011110111000111000111101110011111111001110000000000000000000000000000000000` func main() {    b := wbFromString(in, '1')    b.zhangSuen()    fmt.Println(b)} const (    white = 0    black = 1) type wbArray [][]byte // elements are white or black. // parameter blk is character to read as black.  otherwise kinda rigid,// expects ascii, leading newline, no trailing newline,// takes color from low bit of character.func wbFromString(s string, blk byte) wbArray {    lines := strings.Split(s, "\n")[1:]    b := make(wbArray, len(lines))    for i, sl := range lines {        bl := make([]byte, len(sl))        for j := 0; j < len(sl); j++ {            bl[j] = sl[j] & 1        }        b[i] = bl    }    return b} // rigid again, hard coded to output space for white, # for black,// no leading or trailing newline.var sym = [2]byte{    white: ' ',    black: '#',} func (b wbArray) String() string {    b2 := bytes.Join(b, []byte{'\n'})    for i, b1 := range b2 {        if b1 > 1 {            continue        }        b2[i] = sym[b1]    }    return string(b2)} // neighbor offsetsvar nb = [...][2]int{    2: {-1, 0}, // p2 offsets    3: {-1, 1}, // ...    4: {0, 1},    5: {1, 1},    6: {1, 0},    7: {1, -1},    8: {0, -1},    9: {-1, -1}, // p9 offsets} func (b wbArray) reset(en []int) (rs bool) {    var r, c int    var p [10]byte     readP := func() {        for nx := 1; nx <= 9; nx++ {            n := nb[nx]            p[nx] = b[r+n[0]][c+n[1]]        }    }     shiftRead := func() {        n := nb[3]        p[9], p[2], p[3] = p[2], p[3], b[r+n[0]][c+n[1]]        n = nb[4]        p[8], p[1], p[4] = p[1], p[4], b[r+n[0]][c+n[1]]        n = nb[5]        p[7], p[6], p[5] = p[6], p[5], b[r+n[0]][c+n[1]]    }     // returns "A", count of white->black transitions in circuit of neighbors    // of an interior pixel b[r][c]    countA := func() (ct byte) {        bit := p[9]        for nx := 2; nx <= 9; nx++ {            last := bit            bit = p[nx]            if last == white {                ct += bit            }        }        return ct    }     // returns "B", count of black pixels neighboring interior pixel b[r][c].    countB := func() (ct byte) {        for nx := 2; nx <= 9; nx++ {            ct += p[nx]        }        return ct    }     lastRow := len(b) - 1    lastCol := len(b[0]) - 1     mark := make([][]bool, lastRow)    for r = range mark {        mark[r] = make([]bool, lastCol)    }     for r = 1; r < lastRow; r++ {        c = 1        readP()        for { // column loop            m := false            // test for failure of any of the five conditions,            if !(p[1] == black) {                goto markDone            }            if b1 := countB(); !(2 <= b1 && b1 <= 6) {                goto markDone            }            if !(countA() == 1) {                goto markDone            }            {                e1, e2 := p[en[1]], p[en[2]]                if !(p[en[0]]&e1&e2 == 0) {                    goto markDone                }                if !(e1&e2&p[en[3]] == 0) {                    goto markDone                }            }            // no conditions failed, mark this pixel for reset            m = true            rs = true // and mark that image changes        markDone:            mark[r][c] = m            c++            if c == lastCol {                break            }            shiftRead()        }    }    if rs {        for r = 1; r < lastRow; r++ {            for c = 1; c < lastCol; c++ {                if mark[r][c] {                    b[r][c] = white                }            }        }    }    return rs} var step1 = []int{2, 4, 6, 8}var step2 = []int{4, 2, 8, 6} func (b wbArray) zhangSuen() {    for {        rs1 := b.reset(step1)        rs2 := b.reset(step2)        if !rs1 && !rs2 {            break        }    }}`
Output:
```
#######         ######
#     #        ##
#      #       #
#     #        #
##### #        #
##        #
#    #   ##    ##   #
#         ####

```

## Groovy

`def zhangSuen(text) {    def image = text.split('\n').collect { line -> line.collect { it == '#' ? 1 : 0} }    def p2, p3, p4, p5, p6, p7, p8, p9    def step1 = { (p2 * p4 * p6 == 0) && (p4 * p6 * p8 == 0) }    def step2 = { (p2 * p4 * p8 == 0) && (p2 * p6 * p8 == 0) }    def reduce = { step ->        def toWhite = []        image.eachWithIndex{ line, y ->            line.eachWithIndex{ pixel, x ->                if (!pixel) return                (p2, p3, p4, p5, p6, p7, p8, p9) = [image[y-1][x], image[y-1][x+1], image[y][x+1], image[y+1][x+1], image[y+1][x], image[y+1][x-1], image[y][x-1], image[y-1][x-1]]                def a = [[p2,p3],[p3,p4],[p4,p5],[p5,p6],[p6,p7],[p7,p8],[p8,p9],[p9,p2]].collect { a1, a2 -> (a1 == 0 && a2 ==1) ? 1 : 0 }.sum()                def b = [p2, p3, p4, p5, p6, p7, p8, p9].sum()                if (a != 1 || b < 2 || b > 6) return                 if (step.call()) toWhite << [y,x]            }        }        toWhite.each { y, x -> image[y][x] = 0 }        !toWhite.isEmpty()    }     while (reduce(step1) | reduce(step2));    image.collect { line -> line.collect { it ? '#' : '.' }.join('') }.join('\n')}`

Testing:

`def small = """\    ................................    .#########.......########.......    .###...####.....####..####......    .###....###.....###....###......    .###...####.....###.............    .#########......###.............    .###.####.......###....###......    .###..####..###.####..####.###..    .###...####.###..########..###..    ................................""".stripIndent() def large = """\    ...........................................................    .#################...................#############.........    .##################...............################.........    .###################............##################.........    .########.....#######..........###################.........    ...######.....#######.........#######.......######.........    ...######.....#######........#######.......................    ...#################.........#######.......................    ...################..........#######.......................    ...#################.........#######.......................    ...######.....#######........#######.......................    ...######.....#######........#######.......................    ...######.....#######.........#######.......######.........    .########.....#######..........###################.........    .########.....#######.######....##################.######..    .########.....#######.######......################.######..    .########.....#######.######.........#############.######..    ...........................................................""".stripIndent() [small, large].each {    println "From:"    println it    println "To:"    println zhangSuen(it)    println()}`

Output:

```From:
................................
.#########.......########.......
.###...####.....####..####......
.###....###.....###....###......
.###...####.....###.............
.#########......###.............
.###.####.......###....###......
.###..####..###.####..####.###..
.###...####.###..########..###..
................................
To:
................................
..#######.........######........
..#.....#........##.............
..#......#.......#..............
..#.....#........#..............
..#####.#........#..............
.......##........#..............
........#....#...##....##...#...
.........#.........####.........
................................

From:
...........................................................
.#################...................#############.........
.##################...............################.........
.###################............##################.........
.########.....#######..........###################.........
...######.....#######.........#######.......######.........
...######.....#######........#######.......................
...#################.........#######.......................
...################..........#######.......................
...#################.........#######.......................
...######.....#######........#######.......................
...######.....#######........#######.......................
...######.....#######.........#######.......######.........
.########.....#######..........###################.........
.########.....#######.######....##################.######..
.########.....#######.######......################.######..
.########.....#######.######.........#############.######..
...........................................................
To:
...........................................................
...........................................................
....#.##########.......................#######.............
.....##........#...................####.......#............
.....#..........#.................##.......................
.....#..........#................#.........................
.....#..........#................#.........................
.....#..........#................#.........................
.....############...............#..........................
.....#..........#...............#..........................
.....#..........#................#.........................
.....#..........#................#.........................
.....#..........#................#.........................
.....#............................##.......................
.....#.............................############............
.......................###..........................###....
...........................................................
...........................................................```

`import Data.Arrayimport qualified Data.List as List data BW = Black | White        deriving (Eq, Show) type Index = (Int, Int)type BWArray = Array Index BW toBW :: Char -> BWtoBW '0' = WhitetoBW '1' = BlacktoBW ' ' = WhitetoBW '#' = BlacktoBW _   = error "toBW: illegal char" toBWArray :: [String] -> BWArraytoBWArray strings = arr  where    height = length strings    width  = minimum \$ map length strings    arr    = listArray ((0, 0), (width - 1, height - 1))             . map toBW . concat . List.transpose \$ map (take width) strings toChar :: BW -> ChartoChar White = ' 'toChar Black = '#' chunksOf :: Int -> [a] -> [[a]]chunksOf _ [] = []chunksOf n xs = take n xs : (chunksOf n \$ drop n xs) showBWArray :: BWArray -> StringshowBWArray arr =  List.intercalate "\n" . List.transpose  . chunksOf (height + 1) . map toChar \$ elems arr  where    (_, (_, height)) = bounds arr add :: Num a => (a, a) -> (a, a) -> (a, a)add (a, b) (x, y) = (a + x, b + y) within :: Ord a => ((a, a), (a, a)) -> (a, a) -> Boolwithin ((a, b), (c, d)) (x, y) =  a <= x && x <= c &&  b <= y && y <= d p2, p3, p4, p5, p6, p7, p8, p9 :: Indexp2 = ( 0, -1)p3 = ( 1, -1)p4 = ( 1,  0)p5 = ( 1,  1)p6 = ( 0,  1)p7 = (-1,  1)p8 = (-1,  0)p9 = (-1, -1) ixamap :: Ix i => ((i, a) -> b) -> Array i a -> Array i bixamap f a = listArray (bounds a) \$ map f \$ assocs a thin :: BWArray -> BWArraythin arr =  if pass2 == arr then pass2 else thin pass2  where    (low, high)     = bounds arr    lowB            = low `add` (1, 1)    highB           = high `add` (-1, -1)    isInner         = within (lowB, highB)    offs p          = map (add p) [p2, p3, p4, p5, p6, p7, p8, p9]    trans c (a, b)  = if a == White && b == Black then c + 1 else c    zipshift xs     = zip xs (drop 1 xs ++ xs)    transitions a   = (== (1 :: Int)) . foldl trans 0 . zipshift . map (a !) . offs    within2to6 n    = 2 <= n && n <= 6    blacks a p      = within2to6 . length . filter ((== Black) . (a !)) \$ offs p    oneWhite xs a p = any ((== White) . (a !) . add p) xs    oneRight        = oneWhite [p2, p4, p6]    oneDown         = oneWhite [p4, p6, p8]    oneUp           = oneWhite [p2, p4, p8]    oneLeft         = oneWhite [p2, p6, p8]    precond a p     = (a ! p == Black) && isInner p && blacks a p && transitions a p    stage1 a p      = precond a p && oneRight a p && oneDown a p    stage2 a p      = precond a p && oneUp a p && oneLeft a p    stager f (p, d) = if f p then White else d    pass1           = ixamap (stager \$ stage1 arr) arr    pass2           = ixamap (stager \$ stage2 pass1) pass1 sampleExA :: [String]sampleExA =  ["00000000000000000000000000000000"  ,"01111111110000000111111110000000"  ,"01110001111000001111001111000000"  ,"01110000111000001110000111000000"  ,"01110001111000001110000000000000"  ,"01111111110000001110000000000000"  ,"01110111100000001110000111000000"  ,"01110011110011101111001111011100"  ,"01110001111011100111111110011100"  ,"00000000000000000000000000000000"] sampleExB :: [String]sampleExB =  ["                                                          "  ," #################                   #############        "  ," ##################               ################        "  ," ###################            ##################        "  ," ########     #######          ###################        "  ,"   ######     #######         #######       ######        "  ,"   ######     #######        #######                      "  ,"   #################         #######                      "  ,"   ################          #######                      "  ,"   #################         #######                      "  ,"   ######     #######        #######                      "  ,"   ######     #######        #######                      "  ,"   ######     #######         #######       ######        "  ," ########     #######          ###################        "  ," ########     ####### ######    ################## ###### "  ," ########     ####### ######      ################ ###### "  ," ########     ####### ######         ############# ###### "  ,"                                                          "] main :: IO ()main = mapM_ (putStrLn . showBWArray . thin . toBWArray) [sampleExA, sampleExB]`
Output:
```  #######         ######
#     #        ##
#      #       #
#     #        #
##### #        #
##        #
#    #   ##    ##   #
#         ####

# ##########                       #######
##        #                   ####       #
#          #                 ##
#          #                #
#          #                #
#          #                #
############               #
#          #               #
#          #                #
#          #                #
#          #                #
#                            ##
#                             ############
###                          ###
```

## J

Solution:

`isBlackPx=: '1'&=;._2             NB. boolean array of black pixelstoImage=: [: , LF ,.~ '01' {~ ]   NB. convert to original representationframeImg=: 0 ,. 0 , >:@\$ {. ]     NB. adds border of 0's to image neighbrs=: 1 :'(1 1 ,: 3 3)&(u;._3)'  NB. applies verb u to neighbourhoods Bdry=: 1 2 5 8 7 6 3 0 1          NB. map pixel index to neighbour order getPx=: { ,                       NB. get desired pixels from neighbourhoodAp1=: [: +/ 2 </\ Bdry&getPx      NB. count 0->1 transitionsBp1=: [: +/ [: }. Bdry&getPx      NB. count black neighbours c11=: (2&<: *. <:&6)@Bp1          NB. step 1, condition 1c12=: 1 = Ap1                     NB. ...c13=: 0 e. 1 5 7&getPxc14=: 0 e. 5 7 3&getPxc23=: 0 e. 1 5 3&getPx            NB. step2, condition 3c24=: 0 e. 1 7 3&getPx cond1=: c11 *. c12 *. c13 *. c14  NB. step1 conditionscond2=: c11 *. c12 *. c23 *. c24  NB. step2 conditionswhiten=: [ * [email protected]:*.               NB. make black pixels whitestep1=: whiten [email protected](cond1 neighbrs)step2=: whiten [email protected](cond2 neighbrs) zhangSuen=: [: toImage [: [email protected]^:_ isBlackPx`

Alternative, explicit representation of last verb above

`zhangSuenX=: verb define  img=. isBlackPx y  whilst. 0 < +/ , msk1 +.&-. msk2 do.    msk1=. ([email protected]:*. [: frameImg cond1 neighbrs) img    img=. msk1 * img    msk2=. ([email protected]:*. [: frameImg cond2 neighbrs) img    img=. msk2 * img  end.  toImage img)`

Example Use:

`toASCII=: ' #' {~ '1'&=;._2       NB. convert to ASCII representation ExampleImg=: noun define00000000000000000000000000000000011111111100000001111111100000000111000111100000111100111100000001110000111000001110000111000000011100011110000011100000000000000111111111000000111000000000000001110111100000001110000111000000011100111100111011110011110111000111000111101110011111111001110000000000000000000000000000000000)    toASCII zhangSuen ExampleImg   #######         ######          #     #        ##               #      #       #                #     #        #                ##### #        #                     ##        #                      #    #   ##    ##   #            #         ####          `

## Java

Works with: Java version 7
`import java.awt.Point;import java.util.*; public class ZhangSuen {     final static String[] image = {        "                                                          ",        " #################                   #############        ",        " ##################               ################        ",        " ###################            ##################        ",        " ########     #######          ###################        ",        "   ######     #######         #######       ######        ",        "   ######     #######        #######                      ",        "   #################         #######                      ",        "   ################          #######                      ",        "   #################         #######                      ",        "   ######     #######        #######                      ",        "   ######     #######        #######                      ",        "   ######     #######         #######       ######        ",        " ########     #######          ###################        ",        " ########     ####### ######    ################## ###### ",        " ########     ####### ######      ################ ###### ",        " ########     ####### ######         ############# ###### ",        "                                                          "};     final static int[][] nbrs = {{0, -1}, {1, -1}, {1, 0}, {1, 1}, {0, 1},        {-1, 1}, {-1, 0}, {-1, -1}, {0, -1}};     final static int[][][] nbrGroups = {{{0, 2, 4}, {2, 4, 6}}, {{0, 2, 6},        {0, 4, 6}}};     static List<Point> toWhite = new ArrayList<>();    static char[][] grid;     public static void main(String[] args) {        grid = new char[image.length][];        for (int r = 0; r < image.length; r++)            grid[r] = image[r].toCharArray();         thinImage();    }     static void thinImage() {        boolean firstStep = false;        boolean hasChanged;         do {            hasChanged = false;            firstStep = !firstStep;             for (int r = 1; r < grid.length - 1; r++) {                for (int c = 1; c < grid[0].length - 1; c++) {                     if (grid[r][c] != '#')                        continue;                     int nn = numNeighbors(r, c);                    if (nn < 2 || nn > 6)                        continue;                     if (numTransitions(r, c) != 1)                        continue;                     if (!atLeastOneIsWhite(r, c, firstStep ? 0 : 1))                        continue;                     toWhite.add(new Point(c, r));                    hasChanged = true;                }            }             for (Point p : toWhite)                grid[p.y][p.x] = ' ';            toWhite.clear();         } while (firstStep || hasChanged);         printResult();    }     static int numNeighbors(int r, int c) {        int count = 0;        for (int i = 0; i < nbrs.length - 1; i++)            if (grid[r + nbrs[i][1]][c + nbrs[i][0]] == '#')                count++;        return count;    }     static int numTransitions(int r, int c) {        int count = 0;        for (int i = 0; i < nbrs.length - 1; i++)            if (grid[r + nbrs[i][1]][c + nbrs[i][0]] == ' ') {                if (grid[r + nbrs[i + 1][1]][c + nbrs[i + 1][0]] == '#')                    count++;            }        return count;    }     static boolean atLeastOneIsWhite(int r, int c, int step) {        int count = 0;        int[][] group = nbrGroups[step];        for (int i = 0; i < 2; i++)            for (int j = 0; j < group[i].length; j++) {                int[] nbr = nbrs[group[i][j]];                if (grid[r + nbr[1]][c + nbr[0]] == ' ') {                    count++;                    break;                }            }        return count > 1;    }     static void printResult() {        for (char[] row : grid)            System.out.println(row);    }}`

Output:

```
# ##########                       #######
##        #                   ####       #
#          #                 ##
#          #                #
#          #                #
#          #                #
############               #
#          #               #
#          #                #
#          #                #
#          #                #
#                            ##
#                             ############
###                          ###   ```

## JavaScript

Translation of: Java
`function Point(x, y) {    this.x = x;    this.y = y;}var ZhangSuen = (function () {    function ZhangSuen() {    }    ZhangSuen.image =     ["                                                          ",     " #################                   #############        ",     " ##################               ################        ",     " ###################            ##################        ",     " ########     #######          ###################        ",     "   ######     #######         #######       ######        ",     "   ######     #######        #######                      ",     "   #################         #######                      ",     "   ################          #######                      ",     "   #################         #######                      ",     "   ######     #######        #######                      ",     "   ######     #######        #######                      ",     "   ######     #######         #######       ######        ",     " ########     #######          ###################        ",     " ########     ####### ######    ################## ###### ",     " ########     ####### ######      ################ ###### ",     " ########     ####### ######         ############# ###### ",     "                                                          "];     ZhangSuen.nbrs = [[0, -1], [1, -1], [1, 0], [1, 1], [0, 1], [-1, 1], [-1, 0], [-1, -1], [0, -1]];     ZhangSuen.nbrGroups = [[[0, 2, 4], [2, 4, 6]], [[0, 2, 6], [0, 4, 6]]];     ZhangSuen.toWhite = new Array();    ;    ZhangSuen.main = function (args) {        ZhangSuen.grid = new Array(ZhangSuen.image.length);        for (var r = 0; r < ZhangSuen.image.length; r++)            ZhangSuen.grid[r] = (ZhangSuen.image[r]).split('');        ZhangSuen.thinImage();    };    ZhangSuen.thinImage = function () {        var firstStep = false;        var hasChanged;        do {            hasChanged = false;            firstStep = !firstStep;            for (var r = 1; r < ZhangSuen.grid.length - 1; r++) {                for (var c = 1; c < ZhangSuen.grid[0].length - 1; c++) {                    if (ZhangSuen.grid[r][c] !== '#')                        continue;                    var nn = ZhangSuen.numNeighbors(r, c);                    if (nn < 2 || nn > 6)                        continue;                    if (ZhangSuen.numTransitions(r, c) !== 1)                        continue;                    if (!ZhangSuen.atLeastOneIsWhite(r, c, firstStep ? 0 : 1))                        continue;                    ZhangSuen.toWhite.push(new Point(c, r));                    hasChanged = true;                }            }            for (let i = 0; i < ZhangSuen.toWhite.length; i++) {                var p = ZhangSuen.toWhite[i];                ZhangSuen.grid[p.y][p.x] = ' ';            }            ZhangSuen.toWhite = new Array();        } while ((firstStep || hasChanged));        ZhangSuen.printResult();    };    ZhangSuen.numNeighbors = function (r, c) {        var count = 0;        for (var i = 0; i < ZhangSuen.nbrs.length - 1; i++)            if (ZhangSuen.grid[r + ZhangSuen.nbrs[i][1]][c + ZhangSuen.nbrs[i][0]] === '#')                count++;        return count;    };    ZhangSuen.numTransitions = function (r, c) {        var count = 0;        for (var i = 0; i < ZhangSuen.nbrs.length - 1; i++)            if (ZhangSuen.grid[r + ZhangSuen.nbrs[i][1]][c + ZhangSuen.nbrs[i][0]] === ' ') {                if (ZhangSuen.grid[r + ZhangSuen.nbrs[i + 1][1]][c + ZhangSuen.nbrs[i + 1][0]] === '#')                    count++;            }        return count;    };    ZhangSuen.atLeastOneIsWhite = function (r, c, step) {        var count = 0;        var group = ZhangSuen.nbrGroups[step];        for (var i = 0; i < 2; i++)            for (var j = 0; j < group[i].length; j++) {                var nbr = ZhangSuen.nbrs[group[i][j]];                if (ZhangSuen.grid[r + nbr[1]][c + nbr[0]] === ' ') {                    count++;                    break;                }            }        return count > 1;    };    ZhangSuen.printResult = function () {        for (var i = 0; i < ZhangSuen.grid.length; i++) {            var row = ZhangSuen.grid[i];            console.log(row.join(''));        }    };    return ZhangSuen;}());ZhangSuen.main(null);`

Output:

```
# ##########                       #######
##        #                   ####       #
#          #                 ##
#          #                #
#          #                #
#          #                #
############               #
#          #               #
#          #                #
#          #                #
#          #                #
#                            ##
#                             ############
###                          ###    ```

## Julia

` const pixelstring ="00000000000000000000000000000000" *"01111111110000000111111110000000" *"01110001111000001111001111000000" *"01110000111000001110000111000000" *"01110001111000001110000000000000" *"01111111110000001110000000000000" *"01110111100000001110000111000000" *"01110011110011101111001111011100" *"01110001111011100111111110011100" *"00000000000000000000000000000000"const pixels = reshape([UInt8(c- 48) for c in pixelstring], (32,10))'  function surroundtesting(px, i, j, step)    if px[i,j] == 0        return false    end    isize, jsize = size(px)    if i < 1 || j < 1 || i == isize || j == jsize                         # criteria 0.both            return false    end    s = Array{Int,1}(9)    s[1] = s[9] = px[i-1,j]; s[2] = px[i-1,j+1]; s[3] = px[i,j+1]; s[4] = px[i+1,j+1]    s[5] = px[i+1,j]; s[6] = px[i+1,j-1]; s[7] = px[i,j-1]; s[8] = px[i-1,j-1]    b = sum(s[1:8])    if b < 2 || b > 6                                                     # criteria 1.both        return false    end    if sum([(s[i] == 0 && s[i+1] == 1) for i in 1:length(s)-1]) != 1      # criteria 2.both        return false    end    if step == 1        rightwhite = s[1] == 0 || s[3] == 0 || s[5] == 0                  # 1.3        downwhite = s[3] == 0 || s[5] == 0 || s[7] == 0                   # 1.4        return rightwhite && downwhite    end    upwhite = s[1] == 0 || s[3] == 0 || s[7] == 0                         # 2.3    leftwhite = s[1] == 0 || s[5] == 0 || s[7] == 0                       # 2.4    return upwhite && leftwhiteend  function zsthinning(mat)    retmat = copy(mat)    testmat = zeros(Int, size(mat))    isize, jsize = size(testmat)    needredo = true    loops = 0    while(needredo)        loops += 1        println("loop number \$loops")        needredo = false        for n in 1:2            for i in 1:isize, j in 1:jsize                testmat[i,j] = surroundtesting(retmat, i, j, n) ? 1 : 0            end            for i in 1:isize, j in 1:jsize                if testmat[i,j] == 1                    retmat[i,j] = 0                    needredo = true                end            end        end    end    retmatend  function asciiprint(mat)    for i in 1:size(mat)[1]        println(join(map(i -> i == 1 ? '#' : ' ', mat[i,:])))    endend  asciiprint(zsthinning(pixels))`
Output:
```
loop number 1
loop number 2
loop number 3

#######         ######
#     #        ##
#      #       #
#     #        #
##### #        #
##        #
#    #   ##    ##   #
#         ####

```

## Kotlin

Translation of: Java
`// version 1.1.2 class Point(val x: Int, val y: Int) val image = arrayOf(    "                                                          ",    " #################                   #############        ",    " ##################               ################        ",    " ###################            ##################        ",    " ########     #######          ###################        ",    "   ######     #######         #######       ######        ",    "   ######     #######        #######                      ",    "   #################         #######                      ",    "   ################          #######                      ",    "   #################         #######                      ",    "   ######     #######        #######                      ",    "   ######     #######        #######                      ",    "   ######     #######         #######       ######        ",    " ########     #######          ###################        ",    " ########     ####### ######    ################## ###### ",    " ########     ####### ######      ################ ###### ",    " ########     ####### ######         ############# ###### ",    "                                                          " ) val nbrs = arrayOf(    intArrayOf( 0, -1), intArrayOf( 1, -1), intArrayOf( 1,  0),     intArrayOf( 1,  1), intArrayOf( 0,  1), intArrayOf(-1,  1),     intArrayOf(-1,  0), intArrayOf(-1, -1), intArrayOf( 0, -1)) val nbrGroups = arrayOf(    arrayOf(intArrayOf(0, 2, 4), intArrayOf(2, 4, 6)),    arrayOf(intArrayOf(0, 2, 6), intArrayOf(0, 4, 6))) val toWhite = mutableListOf<Point>()val grid = Array(image.size) { image[it].toCharArray() } fun thinImage() {    var firstStep = false    var hasChanged: Boolean    do {        hasChanged = false        firstStep = !firstStep        for (r in 1 until grid.size - 1) {            for (c in 1 until grid[0].size - 1) {                if (grid[r][c] != '#') continue                val nn = numNeighbors(r, c)                if (nn !in 2..6) continue                 if (numTransitions(r, c) != 1) continue                val step = if (firstStep) 0 else 1                if (!atLeastOneIsWhite(r, c, step)) continue                toWhite.add(Point(c, r))                hasChanged = true            }        }        for (p in toWhite) grid[p.y][p.x] = ' '        toWhite.clear()    }    while (firstStep || hasChanged)    for (row in grid) println(row)} fun numNeighbors(r: Int, c: Int): Int {    var count = 0    for (i in 0 until nbrs.size - 1) {        if (grid[r + nbrs[i][1]][c + nbrs[i][0]] == '#') count++    }    return count} fun numTransitions(r: Int, c: Int): Int {    var count = 0    for (i in 0 until nbrs.size - 1) {        if (grid[r + nbrs[i][1]][c + nbrs[i][0]] == ' ') {            if (grid[r + nbrs[i + 1][1]][c + nbrs[i + 1][0]] == '#') count++        }    }    return count} fun atLeastOneIsWhite(r: Int, c: Int, step: Int): Boolean {    var count = 0;    val group = nbrGroups[step]    for (i in 0..1) {        for (j in 0 until group[i].size) {            val nbr = nbrs[group[i][j]]            if (grid[r + nbr[1]][c + nbr[0]] == ' ') {                count++                break            }        }    }    return count > 1} fun main(args: Array<String>) {    thinImage()}`
Output:
```
# ##########                       #######
##        #                   ####       #
#          #                 ##
#          #                #
#          #                #
#          #                #
############               #
#          #               #
#          #                #
#          #                #
#          #                #
#                            ##
#                             ############
###                          ###

```

## Lua

`function zhangSuenThin(img)    local dirs={        { 0,-1},        { 1,-1},        { 1, 0},        { 1, 1},        { 0, 1},        {-1, 1},        {-1, 0},        {-1,-1},        { 0,-1},    }     local black=1    local white=0     function A(x, y)        local c=0        local current=img[y+dirs[1][2]][x+dirs[1][1]]        for i=2,#dirs do            local to_compare=img[y+dirs[i][2]][x+dirs[i][1]]            if current==white and to_compare==black then                c=c+1            end            current=to_compare        end        return c    end     function B(x, y)        local c=0        for i=2,#dirs do            local value=img[y+dirs[i][2]][x+dirs[i][1]]            if value==black then                c=c+1            end        end        return c    end     function common_step(x, y)        if img[y][x]~=black or x<=1 or x>=#img[y] or y<=1 or y>=#img then            return false        end         local b_value=B(x, y)        if b_value<2 or b_value>6 then            return false        end         local a_value=A(x, y)        if a_value~=1 then            return false        end        return true    end     function step_one(x, y)        if not common_step(x, y) then            return false        end        local p2=img[y+dirs[1][2]][x+dirs[1][1]]        local p4=img[y+dirs[3][2]][x+dirs[3][1]]        local p6=img[y+dirs[5][2]][x+dirs[5][1]]        local p8=img[y+dirs[7][2]][x+dirs[7][1]]         if p4==white or p6==white or p2==white and p8==white then            return true        end        return false    end     function step_two(x, y)        if not common_step(x, y) then            return false        end        local p2=img[y+dirs[1][2]][x+dirs[1][1]]        local p4=img[y+dirs[3][2]][x+dirs[3][1]]        local p6=img[y+dirs[5][2]][x+dirs[5][1]]        local p8=img[y+dirs[7][2]][x+dirs[7][1]]         if p2==white or p8==white or p4==white and p6==white then            return true        end        return false    end     function convert(to_do)        for k,v in pairs(to_do) do            img[v[2]][v[1]]=white        end    end     function do_step_on_all(step)        local to_convert={}        for y=1,#img do            for x=1,#img[y] do                if step(x, y) then                    table.insert(to_convert, {x,y})                end            end        end        convert(to_convert)        return #to_convert>0    end     local continue=true    while continue do        continue=false        if do_step_on_all(step_one) then            continue=true        end         if do_step_on_all(step_two) then            continue=true        end    end     for y=1,#img do        for x=1,#img[y] do            io.write(img[y][x]==black and '#' or ' ')        end        io.write('\n')    endend local image = {    {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0},    {0,1,1,1,1,1,1,1,1,1,0,0,0,0,0,0,0,1,1,1,1,1,1,1,1,0,0,0,0,0,0,0},    {0,1,1,1,0,0,0,1,1,1,1,0,0,0,0,0,1,1,1,1,0,0,1,1,1,1,0,0,0,0,0,0},    {0,1,1,1,0,0,0,0,1,1,1,0,0,0,0,0,1,1,1,0,0,0,0,1,1,1,0,0,0,0,0,0},    {0,1,1,1,0,0,0,1,1,1,1,0,0,0,0,0,1,1,1,0,0,0,0,0,0,0,0,0,0,0,0,0},    {0,1,1,1,1,1,1,1,1,1,0,0,0,0,0,0,1,1,1,0,0,0,0,0,0,0,0,0,0,0,0,0},    {0,1,1,1,0,1,1,1,1,0,0,0,0,0,0,0,1,1,1,0,0,0,0,1,1,1,0,0,0,0,0,0},    {0,1,1,1,0,0,1,1,1,1,0,0,1,1,1,0,1,1,1,1,0,0,1,1,1,1,0,1,1,1,0,0},    {0,1,1,1,0,0,0,1,1,1,1,0,1,1,1,0,0,1,1,1,1,1,1,1,1,0,0,1,1,1,0,0},    {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0},} zhangSuenThin(image) `

Output:

```
#######         ######
#     #        ##
#      #       #
#     #        #
##### #        #
##        #
#    #   ##    ##   #
#         ####

```

## Mathematica

Mathematica supports directly the Thinning methods "Morphological" and "MedialAxis". The Zhang-Suen algorithm implementation could be done with:

`nB[mat_] := Delete[mat // Flatten, 5] // Total; nA[mat_] := Module[{l},   l = Flatten[mat][[{2, 3, 6, 9, 8, 7, 4, 1, 2}]];   Total[Map[If[#[[1]] == 0 && #[[2]] == 1, 1, 0] &,      Partition[l, 2, 1]]]   ]; iW1[mat_] :=   Module[{l = Flatten[mat]},    If[Apply[Times, l[[{2, 6, 8}]]] + Apply[Times, l[[{4, 6, 8}]]] ==      0, 0, 1]];iW2[mat_] :=   Module[{l = Flatten[mat]},    If[Apply[Times, l[[{2, 6, 4}]]] + Apply[Times, l[[{4, 2, 8}]]] ==      0, 0, 1]]; check[i_, j_, dat_, t_] := Module[{mat, d = Dimensions[dat], r, c},   r = d[[1]];   c = d[[2]];   If[i > 1 && i < r && j > 1 && j < c,    mat = dat[[i - 1 ;; i + 1, j - 1 ;; j + 1]];    If[dat[[i, j]] == 1 && nA[mat] == 1 && 2 <= nB[mat] <= 6 &&       If[t == 1, iW1[mat], iW2[mat]] == 0, 0, dat[[i, j]]],    dat[[i, j]]    ]]; iter[dat_] :=   Module[{i =      Flatten[Outer[List, Range[Dimensions[dat][[1]]],        Range[Dimensions[dat][[2]]]], 1], tmp},   tmp = Partition[check[#[[1]], #[[2]], dat, 1] & /@ i,      Dimensions[dat][[2]]];   Partition[check[#[[1]], #[[2]], tmp, 2] & /@ i,     Dimensions[tmp][[2]]]];  FixedPoint[iter, dat]`

Which results in: (printMat is only defined to print an text output - the natural Mathemaica way would be to use ArrayPlot function, which create a graphic object which we can't paste into this wiki)

```printMat[mat_] :=
StringReplace[
Riffle[Map[StringJoin, Map[ToString, mat, {2}]], "\n"] //
StringJoin, {"1" -> "#", "0" -> "."}];

dat1 = {{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}, {0, 1, 1, 1, 1, 1, 1, 1, 1,
1, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0,
0}, {0, 1, 1, 1, 0, 0, 0, 1, 1, 1, 1, 0, 0, 0, 0, 0, 1, 1, 1, 1,
0, 0, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0}, {0, 1, 1, 1, 0, 0, 0, 0, 1,
1, 1, 0, 0, 0, 0, 0, 1, 1, 1, 0, 0, 0, 0, 1, 1, 1, 0, 0, 0, 0, 0,
0}, {0, 1, 1, 1, 0, 0, 0, 1, 1, 1, 1, 0, 0, 0, 0, 0, 1, 1, 1, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}, {0, 1, 1, 1, 1, 1, 1, 1, 1,
1, 0, 0, 0, 0, 0, 0, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0}, {0, 1, 1, 1, 0, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 0,
0, 0, 0, 1, 1, 1, 0, 0, 0, 0, 0, 0}, {0, 1, 1, 1, 0, 0, 1, 1, 1,
1, 0, 0, 1, 1, 1, 0, 1, 1, 1, 1, 0, 0, 1, 1, 1, 1, 0, 1, 1, 1, 0,
0}, {0, 1, 1, 1, 0, 0, 0, 1, 1, 1, 1, 0, 1, 1, 1, 0, 0, 1, 1, 1,
1, 1, 1, 1, 1, 0, 0, 1, 1, 1, 0, 0}, {0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0}};

printMat[dat1]
printMat[FixedPoint[iter, dat1]]

................................
.#########.......########.......
.###...####.....####..####......
.###....###.....###....###......
.###...####.....###.............
.#########......###.............
.###.####.......###....###......
.###..####..###.####..####.###..
.###...####.###..########..###..
................................
................................
..#######.........######........
..#.....#........##.............
..#......#.......#..............
..#.....#........#..............
..#####.#........#..............
.......##........#..............
........#....#...##....##...#...
.........#.........####.........
................................

dat2 = {{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}, {0, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0}, {0, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0,
0, 0, 0, 0, 0}, {0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0,
0}, {0, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1,
1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0}, {0, 0, 0, 1,
1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0,
0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1,
1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0}, {0, 0, 0, 1, 1, 1, 1, 1, 1, 0,
0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1,
1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0}, {0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0}, {0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}, {0, 0, 0, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0,
0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}, {0, 0, 0, 1, 1, 1, 1, 1, 1, 0,
0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1,
1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0}, {0, 0, 0, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 1, 1,
1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0}, {0, 0, 0, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1,
1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0,
0, 0, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0}, {0, 1, 1, 1,
1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0}, {0, 1, 1, 1, 1, 1, 1, 1, 1, 0,
0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 0, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 1, 1, 1,
1, 1, 1, 0, 0}, {0, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 1, 1,
1, 1, 1, 1, 1, 0, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 1, 1, 1, 1, 1, 1, 0,
0}, {0, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1,
1, 0, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 0, 1, 1, 1, 1, 1, 1, 0, 0}, {0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}};

printMat[dat2]
printMat[FixedPoint[iter, dat2]]

...........................................................
.#################...................#############.........
.##################...............################.........
.###################............##################.........
.########.....#######..........###################.........
...######.....#######.........#######.......######.........
...######.....#######........#######.......................
...#################.........#######.......................
...################..........#######.......................
...#################.........#######.......................
...######.....#######........#######.......................
...######.....#######........#######.......................
...######.....#######.........#######.......######.........
.########.....#######..........###################.........
.########.....#######.######....##################.######..
.########.....#######.######......################.######..
.########.....#######.######.........#############.######..
...........................................................
...........................................................
...........................................................
....#.##########.......................#######.............
.....##........#...................####.......#............
.....#..........#.................##.......................
.....#..........#................#.........................
.....#..........#................#.........................
.....#..........#................#.........................
.....############...............#..........................
.....#..........#...............#..........................
.....#..........#................#.........................
.....#..........#................#.........................
.....#..........#................#.........................
.....#............................##.......................
.....#.............................############............
.......................###..........................###....
...........................................................
...........................................................```

## Perl

Translation of: Perl 6
`use List::Util qw(sum min); \$source = <<'END';..............................................................#################...................#############...........##################...............################...........###################............##################...........########.....#######..........###################.............######.....#######.........#######.......######.............######.....#######........#######...........................#################.........#######...........................################..........#######...........................#################.........#######...........................######.....#######........#######...........................######.....#######........#######...........................######.....#######.........#######.......######...........########.....#######..........###################...........########.....#######.######....##################.######....########.....#######.######......################.######....########.....#######.######.........#############.######..............................................................END for \$line (split "\n", \$source) {    push @lines, [map { 1 & ord \$_ } split '', \$line]} \$v = @lines;\$h = @{\$lines[0]};push @black, @\$_ for @lines;@p8 = ((-\$h-1), (-\$h+0), (-\$h+1),                    # flatland distances to 8 neighbors.          0-1,              0+1,         \$h-1,    \$h+0,    \$h+1)[1,2,4,7,6,5,3,0];   # (in cycle order) # Candidates have 8 neighbors and are known black@cand = grep { \$black[\$_] } map { my \$x = \$_; map \$_*\$h + \$x, 1..\$v-2 } 1..\$h-2; do {    sub seewhite {        my(\$w1,\$w2) = @_;        my(@results);        sub cycles { my(@neighbors)=@_; my \$c; \$c += \$neighbors[\$_] < \$neighbors[(\$_+1)%8] for 0..\$#neighbors; return \$c }        sub blacks { my(@neighbors)=@_; sum @neighbors }         @prior = @cand; @cand = ();        for \$p (@prior) {            @n = @black[map { \$_+\$p } @p8];            if (cycles(@n) == 1 and 2 <= sum(blacks(@n)) and sum(blacks(@n)) <= 6 and min(@n[@\$w1]) == 0 and min(@n[@\$w2]) == 0) {                push @results, \$p;            } else {                push @cand, \$p            }        }        return @results;    }     @goners1 = seewhite [0,2,4], [2,4,6]; @black[@goners1] = 0 x @goners1;    @goners2 = seewhite [0,2,6], [0,4,6]; @black[@goners2] = 0 x @goners2;} until @goners1 == 0 and @goners2 == 0; while (@black) { push @thinned, join '', qw<. #>[splice(@black,0,\$h)] } print join "\n", @thinned;`
Output:
```............................................................
............................................................
.....#.##########.......................#######.............
......##........#...................####.......#............
......#..........#.................##.......................
......#..........#................#.........................
......#..........#................#.........................
......#..........#................#.........................
......############...............#..........................
......#..........#...............#..........................
......#..........#................#.........................
......#..........#................#.........................
......#..........#................#.........................
......#............................##.......................
......#.............................############............
........................###..........................###....
............................................................
............................................................```

## Perl 6

Source image may be based on any characters whose low bits are 0 or 1 (which conveniently includes . and #).

`my \$source = qq:to/EOD/;.................................#########.......########........###...####.....####..####.......###....###.....###....###.......###...####.....###..............#########......###..............###.####.......###....###.......###..####..###.####..####.###...###...####.###..########..###..................................EOD my @lines = ([.ords X+& 1] for \$source.split("\n")); # The low bits Just Work.my \v = +@lines;my \h = +@lines[0];my @black = flat @lines.map: *.values;   # Flatten to 1-dimensional. my \p8 = [-h-1, -h+0, -h+1,         # Flatland distances to 8 neighbors.           0-1,        0+1,           h-1,  h+0,  h+1].[1,2,4,7,6,5,3,0];   # (in cycle order) # Candidates have 8 neighbors and are known blackmy @cand = grep { @black[\$_] }, do    for 1..v-2 X 1..h-2 -> (\y,\x) { y*h + x } repeat while my @goners1 or my @goners2 {    sub seewhite (\w1,\w2) {        sub cycles (@neighbors) { [+] @neighbors Z< @neighbors[].rotate }        sub blacks (@neighbors) { [+] @neighbors }         my @prior = @cand; @cand = ();         gather for @prior -> \p {            my \n = @black[p8 X+ p];            if cycles(n) == 1 and 2 <= blacks(n) <= 6 and n[w1].any == 0 and n[w2].any == 0                 { take p }            else { @cand.push: p }        }    }     @goners1 = seewhite (0,2,4), (2,4,6);    @black[@goners1] = 0 xx *;    say "Ping: {[+] @black} remaining after removing ", @goners1;     @goners2 = seewhite (0,2,6), (0,4,6);    @black[@goners2] = 0 xx *;    say "Pong: {[+] @black} remaining after removing ", @goners2;} say @black.splice(0,h).join.trans('01' => '.#') while @black;`
Output:
```Ping: 66 remaining after removing 33 41 49 56 67 71 74 80 83 86 89 99 106 114 119 120 121 131 135 138 146 169 178 195 197 210 215 217 227 230 233 236 238 240 243 246 249 251 253 257 258 259 263 264 266 268 269 270 273 274 279 280 283 284 285
Pong: 47 remaining after removing 65 73 88 97 104 112 129 137 144 161 167 176 193 198 208 216 225 226 231
Ping: 45 remaining after removing 87 194
Pong: 45 remaining after removing
Ping: 45 remaining after removing
Pong: 45 remaining after removing
................................
..#######.........######........
..#.....#........##.............
..#......#.......#..............
..#.....#........#..............
..#####.#........#..............
.......##........#..............
........#....#...##....##...#...
.........#.........####.........
................................```

## Phix

`constant n = {{-1,0},{-1,1},{0,1},{1,1},{1,0},{1,-1},{0,-1},{-1,-1},{-1,0}}; function AB(sequence text, integer y, x, step)integer wtb = 0, bn = 0integer prev = '#', nextstring p2468 = ""    for i=1 to length(n) do        next = text[y+n[i][1]][x+n[i][2]]        wtb += (prev='.' and next<='#')        bn += (i>1 and next<='#')           if and_bits(i,1)=0 then p2468 = append(p2468,prev) end if        prev = next    end for    if step=2 then -- make it p6842        p2468 = p2468[3..4]&p2468[1..2]    end if    return {wtb,bn,p2468}end function procedure Zhang_Suen(sequence text)integer wtb, bn, changed, changesstring p2468    -- (p6842 for step 2)    text = split(text,'\n')    while 1 do        changed = 0        for step=1 to 2 do            changes = 0            for y=2 to length(text)-1 do                for x=2 to length(text[y])-1 do                    if text[y][x]='#' then                        {wtb,bn,p2468} = AB(text,y,x,step)                        if wtb=1                         and bn>=2 and bn<=6                         and find('.',p2468[1..3])                         and find('.',p2468[2..4])then                            changes = 1                            text[y][x] = '!'    -- (logically still black)                        end if                    end if                end for            end for            if changes then                for y=2 to length(text)-1 do                    text[y] = substitute(text[y],"!",".")                end for                changed = 1            end if        end for        if not changed then exit end if    end while    puts(1,join(text,"\n"))end procedure string small_rc = """.................................#########.......########........###...####.....####..####.......###....###.....###....###.......###...####.....###..............#########......###..............###.####.......###....###.......###..####..###.####..####.###...###...####.###..########..###.................................."""Zhang_Suen(small_rc)`
Output:
```................................
..#######.........######........
..#.....#........##.............
..#......#.......#..............
..#.....#........#..............
..#####.#........#..............
.......##........#..............
........#....#...##....##...#...
.........#.........####.........
................................
```

## PL/I

`zhang: procedure options (main);        /* 8 July 2014 */    declare pic(10) bit(32) initial (      '00000000000000000000000000000000'b,      '01111111110000000111111110000000'b,      '01110001111000001111001111000000'b,      '01110000111000001110000111000000'b,      '01110001111000001110000000000000'b,      '01111111110000001110000000000000'b,      '01110111100000001110000111000000'b,      '01110011110011101111001111011100'b,      '01110001111011100111111110011100'b,      '00000000000000000000000000000000'b );   declare image  (10,32) bit(1) defined pic;   declare status (10,32) fixed decimal (1);   declare changes bit(1);   declare (i, j, k, m, n) fixed binary;    m = hbound(image,1); n = hbound(image,2);    call display;     /* Pixel labelling for pixels surrounding P1, co-ordinates (i,j). */   /* P9 P2 P3 */   /* P8 P1 P4 */   /* P7 P6 P5 */    do k = 1 to 10 until (^changes);      changes = '0'b;      /* Set conditions as follows: */      /*   (0) The pixel is black and has eight neighbours */      /*   (1) 2 < = B(P1) < = 6                           */      /*   (2) A(P1) = 1                                   */      /*   (3) At least one of P2 and P4 and P6 is white   */      /*   (4) At least one of P4 and P6 and P8 is white   */      status = -1;      do i = 2 to m-1;         do j = 2 to n-1;            if image(i,j) then               if B(i,j) >= 2 & B(i,j) <= 6 then                  if A(i,j) = 1 then                     if ^image(i-1,j) | ^image(i,j+1) | ^image(i+1,j) then                        if ^image(i,j+1) | ^image(i+1,j) | ^image(i,j-1) then                           status(i,j) = 4;         end;      end;      /* Having determined a status for every bit in the image,   */      /* change those bits to white.                              */      do i = 2 to m-1;         do j = 2 to n-1;            if status(i,j) ^= -1 then do; image(i,j) = '0'b; changes = '1'b; end;         end;      end;       /* Set conditions as follows: */      /*   (0) The pixel is black and has eight neighbours */      /*   (1) 2 < = B(P1) < = 6                           */      /*   (2) A(P1) = 1                                   */      /*   (3) At least one of P2 and P4 and P8 is white   */      /*   (4) At least one of P2 and P6 and P8 is white   */      status = -1;      do i = 2 to m-1;         do j = 2 to n-1;            if image(i,j) then               if B(i,j) >= 2 & B(i,j) <= 6 then                  if A(i,j) = 1 then                     if ^image(i-1,j) | ^image(i,j+1) | ^image(i,j-1) then                        if ^image(i-1,j) | ^image(i+1,j) | ^image(i,j-1) then                           status(i,j) = 4;         end;      end;      /* Having determined a status for every bit in the image,   */      /* change those bits to white.                              */      do i = 2 to m-1;         do j = 2 to n-1;            if status(i,j) ^= -1 then do; image(i,j) = '0'b; changes = '1'b; end;         end;      end;    end; /* of the "until" loop */    put skip list ('Final image after ' || trim(k) || ' iterations:');   call display; display: procedure;   declare (i, j) fixed binary;   declare c character (1);    do i = 1 to m;      put skip edit ('row:', i) (A, F(3));      do j = 1 to n;         if image(i,j) then c = '.'; else c = ' ';         put edit (c) (A);      end;   end;   put skip;end; /* Returns the number of transitions from white to black from P2 through P9 and P2. */A: procedure (i,j) returns (fixed binary);   declare (i,j) fixed binary nonassignable;   declare n(2:10) bit(1);    n(2)  = image(i-1,j);  n(3) = image(i-1,j+1);   n(4)  = image(i, j+1); n(5) = image(i+1,j+1);   n(6)  = image(i+1,j);  n(7) = image(i+1,j-1);   n(8)  = image(i,j-1);  n(9) = image(i-1,j-1);   n(10) = image(i-1,j);    return ( tally(string(n), '01'b) );end A; /* Count the pixel neighbors of P1 that are black */B: procedure (i, j) returns (fixed binary);   declare (i,j) fixed binary nonassignable;   declare s fixed binary;    s = image(i-1,j-1) + image(i-1,j) + image(i-1,j+1);   s = s + image(i,j-1) + image(i,j+1);   return ( s + image(i+1,j-1) + image(i+1,j) + image(i+1,j+1) );end B; end zhang;`
```[Initial configuration:]
row:  1
row:  2 .........       ........
row:  3 ...   ....     ....  ....
row:  4 ...    ...     ...    ...
row:  5 ...   ....     ...
row:  6 .........      ...
row:  7 ... ....       ...    ...
row:  8 ...  ....  ... ....  .... ...
row:  9 ...   .... ...  ........  ...
row: 10

[Intermeduiate "images" omitted]

Final image after 3 iterations:
row:  1
row:  2  .......         ......
row:  3  .     .        ..
row:  4  .      .       .
row:  5  .     .        .
row:  6  ..... .        .
row:  7       ..        .
row:  8        .    .   ..    ..   .
row:  9         .         ....
row: 10

Second image:
Image to be thinned:
row  1:
row  2:  ...............
row  3: ..................
row  4:  ..................
row  5:     ....       .....
row  6:     ....        .....
row  7:     ....         .....
row  8:     ....         .....
row  9:     ....        ......
row 10:     ....        .....
row 11:     ....       .....
row 12:     ....      .....
row 13:     ....     .....
row 14:     .............
row 15:     ..............
row 16:     ...............
row 17:     ....      ......
row 18:     ....       ......
row 19:     ....        .....
row 20:     ....        ......
row 21:     ....         .....
row 22:     ....         .....
row 23:     ....        ......
row 24:     ....       ......
row 25:  ...................
row 26: ...................
row 27:  .................
row 28:

Final image after 3 iterations:
row  1:
row  2:
row  3:   ..............
row  4:      .          .
row  5:      .           .
row  6:      .            .
row  7:      .            .
row  8:      .            .
row  9:      .            .
row 10:      .           ..
row 11:      .           .
row 12:      .          .
row 13:      .          .
row 14:      .         .
row 15:      ...........
row 16:      .          .
row 17:      .          ..
row 18:      .           .
row 19:      .            .
row 20:      .            .
row 21:      .            .
row 22:      .            .
row 23:      .            .
row 24:      .           ..
row 25:      .          ..
row 26:   ... ...........
row 27:
row 28:```

## Python

Several input images are converted.

`# -*- coding: utf-8 -*- # Example from [http://nayefreza.wordpress.com/2013/05/11/zhang-suen-thinning-algorithm-java-implementation/ this blog post].beforeTxt = '''\11001111100111110011111001111100110110011011001101100110110011011001101100110110011011111100000000\''' # Thanks to [http://www.network-science.de/ascii/ this site] and vim for these next two examplessmallrc01 = '''\00000000000000000000000000000000011111111100000001111111100000000111000111100000111100111100000001110000111000001110000111000000011100011110000011100000000000000111111111000000111000000000000001110111100000001110000111000000011100111100111011110011110111000111000111101110011111111001110000000000000000000000000000000000\''' rc01 = '''\000000000000000000000000000000000000000000000000000000000000111111111111111110000000000000000000111111111111100000000001111111111111111110000000000000001111111111111111000000000011111111111111111110000000000001111111111111111110000000000111111110000011111110000000000111111111111111111100000000000011111100000111111100000000011111110000000111111000000000000111111000001111111000000001111111000000000000000000000000001111111111111111100000000011111110000000000000000000000000011111111111111110000000000111111100000000000000000000000000111111111111111110000000001111111000000000000000000000000001111110000011111110000000011111110000000000000000000000000011111100000111111100000000111111100000000000000000000000000111111000001111111000000000111111100000001111110000000000111111110000011111110000000000111111111111111111100000000001111111100000111111101111110000111111111111111111011111100011111111000001111111011111100000011111111111111110111111000111111110000011111110111111000000000111111111111101111110000000000000000000000000000000000000000000000000000000000000\''' def intarray(binstring):    '''Change a 2D matrix of 01 chars into a list of lists of ints'''    return [[1 if ch == '1' else 0 for ch in line]             for line in binstring.strip().split()] def chararray(intmatrix):    '''Change a 2d list of lists of 1/0 ints into lines of 1/0 chars'''    return '\n'.join(''.join(str(p) for p in row) for row in intmatrix) def toTxt(intmatrix):    '''Change a 2d list of lists of 1/0 ints into lines of '#' and '.' chars'''    return '\n'.join(''.join(('#' if p else '.') for p in row) for row in intmatrix) def neighbours(x, y, image):    '''Return 8-neighbours of point p1 of picture, in order'''    i = image    x1, y1, x_1, y_1 = x+1, y-1, x-1, y+1    #print ((x,y))    return [i[y1][x],  i[y1][x1],   i[y][x1],  i[y_1][x1],  # P2,P3,P4,P5            i[y_1][x], i[y_1][x_1], i[y][x_1], i[y1][x_1]]  # P6,P7,P8,P9 def transitions(neighbours):    n = neighbours + neighbours[0:1]    # P2, ... P9, P2    return sum((n1, n2) == (0, 1) for n1, n2 in zip(n, n[1:])) def zhangSuen(image):    changing1 = changing2 = [(-1, -1)]    while changing1 or changing2:        # Step 1        changing1 = []        for y in range(1, len(image) - 1):            for x in range(1, len(image[0]) - 1):                P2,P3,P4,P5,P6,P7,P8,P9 = n = neighbours(x, y, image)                if (image[y][x] == 1 and    # (Condition 0)                    P4 * P6 * P8 == 0 and   # Condition 4                    P2 * P4 * P6 == 0 and   # Condition 3                    transitions(n) == 1 and # Condition 2                    2 <= sum(n) <= 6):      # Condition 1                    changing1.append((x,y))        for x, y in changing1: image[y][x] = 0        # Step 2        changing2 = []        for y in range(1, len(image) - 1):            for x in range(1, len(image[0]) - 1):                P2,P3,P4,P5,P6,P7,P8,P9 = n = neighbours(x, y, image)                if (image[y][x] == 1 and    # (Condition 0)                    P2 * P6 * P8 == 0 and   # Condition 4                    P2 * P4 * P8 == 0 and   # Condition 3                    transitions(n) == 1 and # Condition 2                    2 <= sum(n) <= 6):      # Condition 1                    changing2.append((x,y))        for x, y in changing2: image[y][x] = 0        #print changing1        #print changing2    return image  if __name__ == '__main__':    for picture in (beforeTxt, smallrc01, rc01):        image = intarray(picture)        print('\nFrom:\n%s' % toTxt(image))        after = zhangSuen(image)        print('\nTo thinned:\n%s' % toTxt(after))`
Output:

```From:
................................
.#########.......########.......
.###...####.....####..####......
.###....###.....###....###......
.###...####.....###.............
.#########......###.............
.###.####.......###....###......
.###..####..###.####..####.###..
.###...####.###..########..###..
................................

To thinned:
................................
..#######.........######........
..#.....#........##.............
..#......#.......#..............
..#.....#........#..............
..#####.#........#..............
.......##........#..............
........#....#...##....##...#...
.........#.........####.........
................................```

## Racket

`#lang racket(define (img-01string->vector str)  (define lines (regexp-split "\n" str))  (define h (length lines))  (define w (if (zero? h) 0 (string-length (car lines))))  (define v (for*/vector #:length (* w h)              ((l (in-list lines)) (p (in-string l)))              (match p (#\0 0) (#\1 1) (#\# 1) (#\. 0))))  (values v h w)) ; Task (2) asks for "or an ASCII-art image of space/non-space characters."; Spaces don't really impress where the borders are, so we'll use a dot.(define cell->display-char (match-lambda (0 ".") (1 "#") (else "?"))) (define (display-img v w)  (for ((p (in-vector v)) (col (in-naturals)))    (printf "~a" (cell->display-char p))    (when (= (modulo col w) (sub1 w)) (newline)))) ; returns vector of ([P1's idx] P1 P2 ... P9)(define (Pns v w r c)  (define i (+ c (* r w)))  (define-syntax-rule (vi+ x) (vector-ref v (+ i x)))  (define-syntax-rule (vi- x) (vector-ref v (- i x)))  (vector i (vi+ 0) (vi- w) (vi+ (- 1 w))          (vi+ 1) (vi+ (+ w 1)) (vi+ w)          (vi+ (- w 1)) (vi- 1) (vi- (+ w 1)))) ; Second argument to in-vector is the start offset;; We skip offset 0 (idx) and 1 (P1)(define (B Ps) (for/sum ((Pn (in-vector Ps 2))) Pn)) (define (A Ps)  (define P2 (vector-ref Ps 2))  (define-values (rv _)    (for/fold ((acc 0) (Pn-1 P2))      ((Pn (in-sequences (in-vector Ps 3) (in-value P2))))      (values (+ acc (if (and (= 0 Pn-1) (= 1 Pn)) 1 0)) Pn)))  rv) (define-syntax-rule (not-all-black? Pa Pb Pc) (zero? (* Pa Pb Pc)))(define (z-s-thin v h w)  ; return idx when thin necessary, #f otherwise  (define (thin? Ps n/bour-check-1 n/bour-check-2)    (match-define (vector idx P1 P2 _ P4 _ P6 _ P8 _) Ps)    (and (= P1 1) (<= 2 (B Ps) 6) (= (A Ps) 1)         (n/bour-check-1 P2 P4 P6 P8)         (n/bour-check-2 P2 P4 P6 P8)         idx))   (define (has-white?-246 P2 P4 P6 P8) (not-all-black? P2 P4 P6))  (define (has-white?-468 P2 P4 P6 P8) (not-all-black? P4 P6 P8))  (define (has-white?-248 P2 P4 P6 P8) (not-all-black? P2 P4 P8))  (define (has-white?-268 P2 P4 P6 P8) (not-all-black? P2 P6 P8))  (define (step-n even-Pn-check-1 even-Pn-check-2)    (for*/list ((r (in-range 1 (- h 1)))                (c (in-range 1 (- w 1)))                (idx (in-value (thin? (Pns v w r c)                                      even-Pn-check-1                                      even-Pn-check-2)))                #:when idx) idx))   (define (step-1) (step-n has-white?-246 has-white?-468))  (define (step-2) (step-n has-white?-248 has-white?-268))    (define (inner-z-s-thin)    (define changed-list-1 (step-1))    (for ((idx (in-list changed-list-1))) (vector-set! v idx 0))    (define changed-list-2 (step-2))    (for ((idx (in-list changed-list-2))) (vector-set! v idx 0))    (unless (and (null? changed-list-1) (null? changed-list-2)) (inner-z-s-thin)))    (inner-z-s-thin)) (define (read-display-thin-display-image img-str)  (define-values (v h w) (img-01string->vector img-str))  (printf "Original image:~%") (display-img v w)  (z-s-thin v h w)  (printf "Thinned image:~%") (display-img v w)) (define e.g.-image #<<EOS00000000000000000000000000000000011111111100000001111111100000000111000111100000111100111100000001110000111000001110000111000000011100011110000011100000000000000111111111000000111000000000000001110111100000001110000111000000011100111100111011110011110111000111000111101110011111111001110000000000000000000000000000000000EOS  ) (define e.g.-image/2 #<<EOS##..#####..#####..#####..#####..##.##..##.##..##.##..##.##..##.##..##.##..##.##..##.######........EOS  ) (module+ main  ; (read-display-thin-display-image e.g.-image/2)  ; (newline)  (read-display-thin-display-image e.g.-image))`
Output:

Only the requested image is output:

```Original image:
................................
.#########.......########.......
.###...####.....####..####......
.###....###.....###....###......
.###...####.....###.............
.#########......###.............
.###.####.......###....###......
.###..####..###.####..####.###..
.###...####.###..########..###..
................................
Thinned image:
................................
..#######.........######........
..#.....#........##.............
..#......#.......#..............
..#.....#........#..............
..#####.#........#..............
.......##........#..............
........#....#...##....##...#...
.........#.........####.........
................................```

## REXX

`/*REXX program thins a  NxM  character grid  using  the  Zhang-Suen thinning  algorithm.*/parse arg iFID .;  if iFID==''  then iFID='ZHANG_SUEN.DAT'white=' ';         @.=white                      /* [↓]  read the input character grid. */           do row=1  while lines(iFID)\==0;  _=linein(iFID)           _=translate(_,,.0);               cols.row=length(_)               do col=1  for cols.row;  @.row.col=substr(_,col,1)               end   /*col*/                     /* [↑]  assign whole row of characters.*/           end       /*row*/rows=row-1                                       /*adjust ROWS because of the  DO loop. */call [email protected] 'input file ' iFID  " contents:"      /*display show the input character grid*/   do  until  changed==0;    changed=0            /*keep slimming until we're finished.  */       do step=1  for 2                          /*keep track of  step one  or step two.*/         do     r=1  for rows                    /*process all the  rows  and  columns. */             do c=1  for cols.r;  !.r.[email protected].r.c    /*assign an alternate grid.            */             if r==1|r==rows|c==1|c==cols.r  then iterate             /*is this an edge?*/             if @.r.c==white  then iterate       /*Is the character white?  Then skip it*/             call Ps; b=b()                      /*define   Ps   and also   "b".        */             if b<2 | b>6     then iterate       /*is   B   within the range ?          */             if a()\==1       then iterate       /*count the number of transitions.     */    /*  ╔══╦══╦══╗  */             if step==1       then if (p2 & p4 & p6)  |  p4 & p6 & p8  then iterate           /*  ║p9║p2║p3║  */             if step==2       then if (p2 & p4 & p8)  |  p2 & p6 & p8  then iterate           /*  ╠══╬══╬══╣  */             !.r.c=white                         /*set a grid character to  white.      */    /*  ║p8║p1║p4║  */             changed=1                           /*indicate a character was changed.    */    /*  ╠══╬══╬══╣  */             end   /*c*/                                                                      /*  ║p7║p6║p5║  */         end       /*r*/                                                                      /*  ╚══╩══╩══╝  */       call copy!                                /*copy the alternate to working grid.  */       end         /*step*/  end              /*until changed==0*/ call [email protected]  'slimmed output:'                    /*display the slimmed character grid.  */exit                                             /*stick a fork in it,  we're all done. *//*─────────────────────────────────────────────────────────────────────────────────────────────────────────────*/a: return (\p2==p3&p3)+(\p3==p4&p4)+(\p4==p5&p5)+(\p5==p6&p6)+(\p6==p7&p7)+(\p7==p8&p8)+(\p8==p9&p9)+(\p9==p2&p2)b: return p2 + p3 + p4 + p5 + p6 + p7 + p8 + p9copy!:                        do r=1  for rows;       do c=1  for cols.r;        @.r.c=!.r.c;  end;  end;  return[email protected]: say; say arg(1); say;  do r=1  for rows; _=;   do c=1  for cols.r; _=_ || @.r.c; end;  say _; end;  return/*──────────────────────────────────────────────────────────────────────────────────────*/Ps:  rm=r-1;  rp=r+1;  cm=c-1;  cp=c+1                       /*calculate some shortcuts.*/     [email protected].rm.c\==white; [email protected].rm.cp\==white; [email protected].r.cp\==white; [email protected].rp.cp\==white     [email protected].rp.c\==white; [email protected].rp.cm\==white; [email protected].r.cm\==white; [email protected].rm.cm\==white; return`

output   when using the default input:

```input file  ZHANG_SUEN.DAT  contents:

#################                   #############
##################               ################
###################            ##################
########     #######          ###################
######     #######         #######       ######
######     #######        #######
#################         #######
################          #######
#################         #######
######     #######        #######
######     #######        #######
######     #######         #######       ######
########     #######          ###################
########     ####### ######    ################## ######
########     ####### ######      ################ ######
########     ####### ######         ############# ######

slimmed output:

# ##########                       #######
##        #                   ####       #
#          #                 ##
#          #                #
#          #                #
#          #                #
############               #
#          #               #
#          #                #
#          #                #
#          #                #
#                            ##
#                             ############
###                          ###
```

output   when using the default input:   zhang_suen2.dat

```input file  zhang_suen2.dat  contents:

111111111       11111111
111   1111     1111  1111
111    111     111    111
111   1111     111
111111111      111
111 1111       111    111
111  1111  111 1111  1111 111
111   1111 111  11111111  111

slimmed output:

1111111         111111
1     1        11
1      1       1
1     1        1
11111 1        1
11        1
1    1   11    11   1
1         1111
```

## Ruby

First I define a function zs which given a point and its eight neighbours returns 1 if the point may be culled, 0 otherwise. g indicates if this is step 1 or step 2 in the task description. zs may be changed to remember the step independently if the reader does not wish to explore the algorithm.

`class ZhangSuen  NEIGHBOUR8 = [[-1,0],[-1,1],[0,1],[1,1],[1,0],[1,-1],[0,-1],[-1,-1]]  # 8 neighbors  CIRCULARS = NEIGHBOUR8 + [NEIGHBOUR8.first]                       # P2, ... P9, P2  def initialize(str, black="#")    s1 = str.each_line.map{|line| line.chomp.each_char.map{|c| c==black ? 1 : 0}}    s2 = s1.map{|line| line.map{0}}    xrange = 1 ... s1.size-1    yrange = 1 ... s1[0].size-1    printout(s1)    begin      @r = 0      xrange.each{|x| yrange.each{|y| s2[x][y] = s1[x][y] - zs(s1,x,y,1)}}  # Step 1      xrange.each{|x| yrange.each{|y| s1[x][y] = s2[x][y] - zs(s2,x,y,0)}}  # Step 2    end until @r == 0    printout(s1)  end  def zs(ng,x,y,g)    return 0 if ng[x][y] == 0 or                                    # P1               (ng[x-1][y] + ng[x][y+1] + ng[x+g][y-1+g]) == 3 or   # P2, P4, P6/P8               (ng[x-1+g][y+g] + ng[x+1][y] + ng[x][y-1]) == 3      # P4/P2, P6, P8    bp1 = NEIGHBOUR8.inject(0){|res,(i,j)| res += ng[x+i][y+j]}     # B(P1)    return 0 if bp1 < 2 or 6 < bp1    ap1 = CIRCULARS.map{|i,j| ng[x+i][y+j]}.each_cons(2).count{|a,b| a<b}   # A(P1)    return 0 if ap1 != 1    @r = 1  end  def printout(image)    puts image.map{|row| row.map{|col| " #"[col]}.join}  endend str = <<EOS............................................................#################...................#############..........##################...............################..........###################............##################..........########.....#######..........###################............######.....#######.........#######.......######............######.....#######........#######..........................#################.........#######..........................################..........#######..........................#################.........#######..........................######.....#######........#######..........................######.....#######........#######..........................######.....#######.........#######.......######..........########.....#######..........###################..........########.....#######.######....##################.######...########.....#######.######......################.######...########.....#######.######.........#############.######.............................................................EOS ZhangSuen.new(str) task_example = <<EOS00000000000000000000000000000000011111111100000001111111100000000111000111100000111100111100000001110000111000001110000111000000011100011110000011100000000000000111111111000000111000000000000001110111100000001110000111000000011100111100111011110011110111000111000111101110011111111001110000000000000000000000000000000000EOS ZhangSuen.new(task_example, "1")`
Output:

(only the requested result is shown here)

```
#######         ######
#     #        ##
#      #       #
#     #        #
##### #        #
##        #
#    #   ##    ##   #
#         ####

```

## Sidef

Translation of: Ruby
`class ZhangSuen(str, black="1") {  const NEIGHBOURS = [[-1,0],[-1,1],[0,1],[1,1],[1,0],[1,-1],[0,-1],[-1,-1]]  # 8 neighbors  const CIRCULARS = (NEIGHBOURS + [NEIGHBOURS.first])                         # P2, ... P9, P2   has r = 0  has image = [[]]   method init {    var s1 = str.lines.map{|line| line.chars.map{|c| c==black ? 1 : 0 }}    var s2 = s1.len.of { s1[0].len.of(0) }    var xr = range(1, s1.end-1)    var yr = range(1, s1[0].end-1)    do {        r = 0        xr.each{|x| yr.each{|y| s2[x][y] = (s1[x][y] - self.zs(s1,x,y,1)) }}  # Step 1        xr.each{|x| yr.each{|y| s1[x][y] = (s2[x][y] - self.zs(s2,x,y,0)) }}  # Step 2    } while !r.is_zero    image = s1  }   method zs(ng,x,y,g) {       (ng[x][y] == 0)                                   ->    || (ng[x-1][y] + ng[x][y+1] + ng[x+g][y+g - 1] == 3) ->    || (ng[x+g - 1][y+g] + ng[x+1][y] + ng[x][y-1] == 3) ->    && return 0     var bp1 = NEIGHBOURS.map {|p| ng[x+p[0]][y+p[1]] }.sum  # B(P1)    return 0 if ((bp1 < 2) || (6 < bp1))     var ap1 = 0    CIRCULARS.map {|p| ng[x+p[0]][y+p[1]] }.each_cons(2, {|a,b|        ++ap1 if (a < b)                                    # A(P1)    })     return 0 if (ap1 != 1)    r = 1  }   method display {    image.each{|row| say row.map{|col| col ? '#' : ' ' }.join }  }} var text = <<EOS00000000000000000000000000000000011111111100000001111111100000000111000111100000111100111100000001110000111000001110000111000000011100011110000011100000000000000111111111000000111000000000000001110111100000001110000111000000011100111100111011110011110111000111000111101110011111111001110000000000000000000000000000000000EOS ZhangSuen.new(text, black: "1").display`
Output:
```
#######         ######
#     #        ##
#      #       #
#     #        #
##### #        #
##        #
#    #   ##    ##   #
#         ####

```

## Tcl

Only the single image is converted.

`# -*- coding: utf-8 -*- set data {00000000000000000000000000000000011111111100000001111111100000000111000111100000111100111100000001110000111000001110000111000000011100011110000011100000000000000111111111000000111000000000000001110111100000001110000111000000011100111100111011110011110111000111000111101110011111111001110000000000000000000000000000000000}proc zhang-suen data {    set data [string trim \$data]    while 1 {	set n 0	incr n [step 1 data]	incr n [step 2 data]	if !\$n break    }    return \$data}proc step {number _data} {    upvar 1 \$_data data    set xmax [string length [lindex \$data 0]]    set ymax [llength \$data]    switch -- \$number {	1 {set cond {(!\$P2 || !\$P4 || !\$P6) && (!\$P4 || !\$P6 || !\$P8)}}	2 {set cond {(!\$P2 || !\$P4 || !\$P8) && (!\$P2 || !\$P6 || !\$P8)}}    }    set hits {}    for {set x 1} {\$x < \$xmax-1} {incr x} {	for {set y 1} {\$y < \$ymax-1} {incr y} {	    if {[getpix \$data \$x \$y] == 1} {		set b [B \$data \$x \$y]		if {2 <= \$b && \$b <= 6} {		    if {[A \$data \$x \$y] == 1} {			set P2 [getpix \$data \$x [expr \$y-1]]			set P4 [getpix \$data [expr \$x+1] \$y]			set P6 [getpix \$data \$x [expr \$y+1]]			set P8 [getpix \$data [expr \$x-1] \$y]			if \$cond {lappend hits \$x \$y} 		    }		}	    }	}    }    foreach {x y} \$hits {set data [setpix \$data \$x \$y 0]}    return [llength \$hits]}proc A {data x y} {    set res 0    set last [getpix \$data \$x [expr \$y-1]]    foreach {dx dy} {1 -1  1 0  1 1  0 1  -1 1  -1 0  -1 -1  0 -1} {	set this [getpix \$data [expr \$x+\$dx] [expr \$y+\$dy]]	if {\$this > \$last} {incr res}	set last \$this    }    return \$res}proc B {data x y} {    set res 0    foreach {dx dy} {1 -1  1 0  1 1  0 1  -1 1  -1 0  -1 -1  0 -1} {	incr res [getpix \$data [expr \$x+\$dx] [expr \$y+\$dy]]    }    return \$res}proc getpix {data x y} {    string index [lindex \$data \$y] \$x}proc setpix {data x y val} {    set row [lindex \$data \$y]    lset data \$y [string replace \$row \$x \$x \$val]    return \$data}puts [string map {1 @ 0 .} [join [zhang-suen \$data] \n]]`
Output:
```................................
[email protected]@@@@@@[email protected]@@@@@........
[email protected]@[email protected]@.............
[email protected]@[email protected]
[email protected]@[email protected]
[email protected]@@@@[email protected]@..............
[email protected]@[email protected]
[email protected]@[email protected]@[email protected]@[email protected]
[email protected]@@@@.........
................................
```