Move-to-front algorithm
You are encouraged to solve this task according to the task description, using any language you may know.
Given a symbol table of a zero-indexed array of all possible input symbols
this algorithm reversibly transforms a sequence
of input symbols into an array of output numbers (indices).
The transform in many cases acts to give frequently repeated input symbols
lower indices which is useful in some compression algorithms.
- Encoding algorithm
for each symbol of the input sequence:
output the index of the symbol in the symbol table
move that symbol to the front of the symbol table
- Decoding algorithm
# Using the same starting symbol table
for each index of the input sequence:
output the symbol at that index of the symbol table
move that symbol to the front of the symbol table
- Example
Encoding the string of character symbols 'broood' using a symbol table of the characters 'a'-to-'z'
| Input | Output | SymbolTable |
|---|---|---|
| broood | 1 | 'abcdefghijklmnopqrstuvwxyz' |
| broood | 1 17 | 'bacdefghijklmnopqrstuvwxyz' |
| broood | 1 17 15 | 'rbacdefghijklmnopqstuvwxyz' |
| broood | 1 17 15 0 | 'orbacdefghijklmnpqstuvwxyz' |
| broood | 1 17 15 0 0 | 'orbacdefghijklmnpqstuvwxyz' |
| broood | 1 17 15 0 0 5 | 'orbacdefghijklmnpqstuvwxyz' |
Decoding the indices back to the original symbol order:
| Input | Output | SymbolTable |
|---|---|---|
| 1 17 15 0 0 5 | b | 'abcdefghijklmnopqrstuvwxyz' |
| 1 17 15 0 0 5 | br | 'bacdefghijklmnopqrstuvwxyz' |
| 1 17 15 0 0 5 | bro | 'rbacdefghijklmnopqstuvwxyz' |
| 1 17 15 0 0 5 | broo | 'orbacdefghijklmnpqstuvwxyz' |
| 1 17 15 0 0 5 | brooo | 'orbacdefghijklmnpqstuvwxyz' |
| 1 17 15 0 0 5 | broood | 'orbacdefghijklmnpqstuvwxyz' |
- Task
- Encode and decode the following three strings of characters using the symbol table of the characters 'a'-to-'z' as above.
- Show the strings and their encoding here.
- Add a check to ensure that the decoded string is the same as the original.
The strings are:
broood bananaaa hiphophiphop
(Note the spellings.)
Ada
<lang Ada>with Ada.Text_IO;
procedure Move_To_Front is
subtype Lower_Case is Character range 'a' .. 'z';
subtype Index is Integer range 0 .. 25;
type Table is array (Index) of Lower_Case;
Alphabet: constant Table := "abcdefghijklmnopqrstuvwxyz";
type Number_String is array(Positive range <>) of Natural;
function Encode(S: String) return Number_String is
Key: Table := Alphabet;
function Encode(S: String; Tab: in out Table) return Number_String is
procedure Look_Up(A: in out Table; Ch: Lower_Case; Pos: out Index) is begin for I in A'Range loop if A(I) = Ch then Pos := I; A := A(Pos) & A(A'First .. Pos-1) & A(Pos+1 .. A'Last); return; end if; end loop; raise Program_Error with "unknown character"; end Look_Up;
Empty: Number_String(1 .. 0); Result: Natural;
begin
if S'Length = 0 then return Empty; else Look_Up(Tab, S(S'First), Result); return Result & Encode(S(S'First+1 .. S'Last), Tab); end if;
end Encode;
begin
return Encode(S, Key);
end Encode;
function Decode(N: Number_String) return String is
Key: Table := Alphabet;
function Decode(N: Number_String; Tab: in out Table) return String is
procedure Look_Up(A: in out Table; Pos: Index; Ch: out Lower_Case) is begin Ch := A(Pos); A := A(Pos) & A(A'First .. Pos-1) & A(Pos+1 .. A'Last); end Look_Up;
Result: String(N'Range);
begin
for I in N'Range loop Look_Up(Tab, N(I), Result(I)); end loop; return Result;
end Decode;
begin
return Decode(N, Key);
end Decode;
procedure Encode_Write_Check(S: String) is
N: Number_String := Encode(S);
T: String := Decode(N);
Check: String := (if S=T then "Correct!" else "*WRONG*!");
begin
Ada.Text_IO.Put("'" & S & "' encodes to");
for Num of N loop
Ada.Text_IO.Put(Integer'Image(Num));
end loop;
Ada.Text_IO.Put_Line(". This decodes to '" & T & "'. " & Check);
end Encode_Write_Check;
begin
Encode_Write_Check("broood");
Encode_Write_Check("bananaaa");
Encode_Write_Check("hiphophiphop");
end Move_To_Front;</lang>
- Output:
'broood' encodes to 1 17 15 0 0 5. This decodes to 'broood'. Correct! 'bananaaa' encodes to 1 1 13 1 1 1 0 0. This decodes to 'bananaaa'. Correct! 'hiphophiphop' encodes to 7 8 15 2 15 2 2 3 2 2 3 2. This decodes to 'hiphophiphop'. Correct!
Aime
<lang aime>text decode(list l) {
integer c, e; data al, s;
al = "abcdefghijklmnopqrstuvwxyz";
for (, e in l) {
s.append(c = al[e]);
al.delete(e).insert(0, c);
}
s;
}
list encode(text s) {
integer c, e; data al; list l;
al = "abcdefghijklmnopqrstuvwxyz";
for (, c in b_draft(s)) {
l.append(e = al.place(c));
al.delete(e).insert(0, c);
}
l;
}
integer main(void) {
for (, text s in list("broood", "bananaaa", "hiphophiphop")) {
list l;
l = encode(s);
l.ucall(o_, 1, " ");
o_(": ", decode(l), "\n");
}
0;
}</lang>
- Output:
1 17 15 0 0 5: broood 1 1 13 1 1 1 0 0: bananaaa 7 8 15 2 15 2 2 3 2 2 3 2: hiphophiphop
ALGOL 68
<lang algol68># move the character at text pos to the front of text #
- note text pos is based from 0 #
PROC move to front = ( STRING text, INT text pos )STRING:
IF text pos < 1
THEN
# the character is already at the front (or not in the string) #
text
ELSE
# the character isn't already at the front - construct the new string #
INT char pos = LWB text + text pos;
( text[ char pos : char pos ]
+ text[ : char pos - 1 ]
+ text[ char pos + 1 : ]
)
FI;
- encode the string "text", using "initial table" as the starting symbol table#
PROC encode = ( STRING text, STRING initial table )[]INT: BEGIN
[ 1 : ( UPB text - LWB text ) + 1 ]INT result;
STRING symbol table := initial table;
FOR text pos FROM LWB text TO UPB text
DO
INT symbol pos := 0;
result[ text pos ]
:= IF char in string( text[ text pos ], symbol pos, symbol table )
THEN
# the character is in the symbol table at symbol pos #
# (indexed from LWB text) - we store the positions #
# indexed from 0 #
symbol pos - LWB text
ELSE
# the character isn't in the symbol table #
-1
FI;
# modify the symbol table so the latest character is at the front #
symbol table := move to front( symbol table, result[ text pos ] )
OD;
result
END; # encode #
- decode "encoded", using "initial table" as the starting symbol table #
PROC decode = ( []INT encoded, STRING initial table )STRING: BEGIN
STRING result := ""; STRING symbol table := initial table;
FOR text pos FROM LWB encoded TO UPB encoded
DO
result
+:= IF encoded[ text pos ] < 0
THEN
# the encoded character wasn't in the string #
"?"
ELSE
# the character is in the symbol table #
symbol table[ encoded[ text pos ] + LWB symbol table ]
FI;
# modify the symbol table so the latest character is at the front #
symbol table := move to front( symbol table, encoded[ text pos ] )
OD;
result
END; # decode #
- routine to test the encode and decode routines #
PROC test encode and decode = ( STRING text )VOID: BEGIN
# initial value for the symbol table # []CHAR initial table = "abcdefghijklmnopqrstuvwxyz";
# procedure to format the encoded value #
PROC format encoded value = ( []INT values )STRING:
BEGIN
STRING result := "";
FOR value pos FROM LWB values TO UPB values
DO
result +:= ( " " + whole( values[ value pos ], 0 ) )
OD;
result END; # format encoded value #
[]INT encoded = encode( text, initial table ); STRING decoded = decode( encoded, initial table );
print( ( ( text
+ " encodes to ["
+ format encoded value( encoded )
+ " ] which "
+ IF text = decoded
THEN
"correctly"
ELSE
"INCORRECTLY"
FI
+ " decodes to """
+ decoded
+ """"
)
, newline
)
)
END; # test encode and decode #
main: (
test encode and decode( "broood" ) ; test encode and decode( "bananaaa" ) ; test encode and decode( "hiphophiphop" )
- ; test encode and decode( "abcdefghijklmnopqrstuvwxyz" ) #
- ; test encode and decode( "zyxwvutsrqponmlkjihgfedcba" ) #
)</lang>
- Output:
broood encodes to [ 1 17 15 0 0 5 ] which correctly decodes to "broood" bananaaa encodes to [ 1 1 13 1 1 1 0 0 ] which correctly decodes to "bananaaa" hiphophiphop encodes to [ 7 8 15 2 15 2 2 3 2 2 3 2 ] which correctly decodes to "hiphophiphop"
AutoHotkey
<lang AutoHotkey>MTF_Encode(string){ str := "abcdefghijklmnopqrstuvwxyz" loop, parse, string code .= (A_Index>1 ? ",":"") . InStr(str, A_LoopField) - 1 , str := A_LoopField . StrReplace(str, A_LoopField) return code }
MTF_Decode(code){ str := "abcdefghijklmnopqrstuvwxyz" loop, parse, code, `, string .= (letter := SubStr(str, A_LoopField+1, 1)) , str := letter . StrReplace(str, letter) return string } </lang> Examples:<lang AutoHotkey>testStrings = broood,bananaaa,hiphophiphop loop, parse, testStrings, `, Output .= A_LoopField "`t" MTF_Encode(A_LoopField) "`t" MTF_Decode(MTF_Encode(A_LoopField)) "`n" MsgBox % Output return</lang>
Outputs:
broood 1,17,15,0,0,5 broood bananaaa 1,1,13,1,1,1,0,0 bananaaa hiphophiphop 7,8,15,2,15,2,2,3,2,2,3,2 hiphophiphop
Bracmat
<lang bracmat> ( encode
= string symboltable
. !arg:(?string.?symboltable)
& vap
$ ( (
= A Z i
. !symboltable:?A [?i !arg ?Z
& !arg !A !Z:?symboltable
& !i
)
. !string
)
)
& ( decode
= indices symboltable
. !arg:(?indices.?symboltable)
& str
$ ( map
$ ( (
= A Z symbol
. !symboltable:?A [!arg %?symbol ?Z
& !symbol !A !Z:?symboltable
& !symbol
)
. !indices
)
)
)
& ( test
= string symboltable encoded decoded
. !arg:(?string.?symboltable)
& put$str$("input:" !string ", ")
& encode$(!string.!symboltable):?encoded
& put$("encoded:" !encoded ", ")
& decode$(!encoded.!symboltable):?decoded
& put$str$("decoded:" !decoded ", ")
& ( !string:!decoded
& out$OK
| out$WRONG
)
)
& a b c d e f g h i j k l m n o p q r s t y v w x y z
: ?symboltable
& test$(broood.!symboltable) & test$(bananaaa.!symboltable) & test$(hiphophiphop.!symboltable)</lang>
C
<lang c>#include<stdio.h>
- include<stdlib.h>
- include<string.h>
- define MAX_SIZE 100
int move_to_front(char *str,char c) {
char *q,*p;
int shift=0;
p=(char *)malloc(strlen(str)+1);
strcpy(p,str);
q=strchr(p,c); //returns pointer to location of char c in string str
shift=q-p; // no of characters from 0 to position of c in str
strncpy(str+1,p,shift);
str[0]=c;
free(p);
// printf("\n%s\n",str);
return shift;
}
void decode(int* pass,int size,char *sym) {
int i,index;
char c;
char table[]="abcdefghijklmnopqrstuvwxyz";
for(i=0;i<size;i++)
{
c=table[pass[i]];
index=move_to_front(table,c);
if(pass[i]!=index) printf("there is an error");
sym[i]=c;
}
sym[size]='\0';
}
void encode(char *sym,int size,int *pass) {
int i=0;
char c;
char table[]="abcdefghijklmnopqrstuvwxyz";
for(i=0;i<size;i++)
{
c=sym[i];
pass[i]=move_to_front(table,c);
}
}
int check(char *sym,int size,int *pass) {
int *pass2=malloc(sizeof(int)*size); char *sym2=malloc(sizeof(char)*size); int i,val=1;
encode(sym,size,pass2); i=0; while(i<size && pass[i]==pass2[i])i++; if(i!=size)val=0;
decode(pass,size,sym2); if(strcmp(sym,sym2)!=0)val=0;
free(sym2); free(pass2);
return val;
}
int main() {
char sym[3][MAX_SIZE]={"broood","bananaaa","hiphophiphop"};
int pass[MAX_SIZE]={0};
int i,len,j;
for(i=0;i<3;i++)
{
len=strlen(sym[i]);
encode(sym[i],len,pass);
printf("%s : [",sym[i]);
for(j=0;j<len;j++)
printf("%d ",pass[j]);
printf("]\n");
if(check(sym[i],len,pass))
printf("Correct :)\n");
else
printf("Incorrect :(\n");
}
return 0;
}</lang>
- Output:
broood : [1 17 15 0 0 5 ] Correct :) bananaaa : [1 1 13 1 1 1 0 0 ] Correct :) hiphophiphop : [7 8 15 2 15 2 2 3 2 2 3 2 ] Correct :)
C#
<lang csharp> using System; using System.Collections.Generic; using System.Text;
namespace MoveToFront {
class Program
{
private static char[] symbolTable;
private static void setSymbolTable()
{
symbolTable = "abcdefghijklmnopqrstuvwxyz".ToCharArray();
}
private static void moveToFront(int charIndex)
{
char toFront = symbolTable[charIndex];
for (int j = charIndex; j > 0; j--)
{
symbolTable[j] = symbolTable[j - 1];
}
symbolTable[0] = toFront;
}
public static int[] Encode(string input)
{
setSymbolTable();
var output = new List<int>();
foreach (char c in input)
{
for (int i = 0; i < 26; i++)
{
if (symbolTable[i] == c)
{
output.Add(i);
moveToFront(i);
break;
}
}
}
return output.ToArray();
}
public static string Decode(int[] input)
{
setSymbolTable();
var output = new StringBuilder(input.Length);
foreach (int n in input)
{
output.Append(symbolTable[n]);
moveToFront(n);
}
return output.ToString();
}
static void Main(string[] args)
{
string[] testInputs = new string[] { "broood", "bananaaa", "hiphophiphop" };
int[] encoding;
foreach (string s in testInputs)
{
Console.WriteLine($"Encoding for '{s}':");
encoding = Encode(s);
foreach (int i in encoding)
{
Console.Write($"{i} ");
}
Console.WriteLine($"\nDecoding for '{s}':");
Console.WriteLine($"{Decode(encoding)}\n");
}
}
}
} </lang>
C++
<lang Cpp>
- include <iostream>
- include <iterator>
- include <sstream>
- include <vector>
using namespace std;
class MTF { public:
string encode( string str )
{
fillSymbolTable(); vector<int> output; for( string::iterator it = str.begin(); it != str.end(); it++ ) { for( int i = 0; i < 26; i++ ) { if( *it == symbolTable[i] ) { output.push_back( i ); moveToFront( i ); break; } } } string r; for( vector<int>::iterator it = output.begin(); it != output.end(); it++ ) { ostringstream ss; ss << *it; r += ss.str() + " "; } return r;
}
string decode( string str )
{
fillSymbolTable(); istringstream iss( str ); vector<int> output; copy( istream_iterator<int>( iss ), istream_iterator<int>(), back_inserter<vector<int> >( output ) ); string r; for( vector<int>::iterator it = output.begin(); it != output.end(); it++ ) { r.append( 1, symbolTable[*it] ); moveToFront( *it ); } return r;
}
private:
void moveToFront( int i )
{
char t = symbolTable[i]; for( int z = i - 1; z >= 0; z-- ) symbolTable[z + 1] = symbolTable[z];
symbolTable[0] = t; }
void fillSymbolTable()
{
for( int x = 0; x < 26; x++ )
symbolTable[x] = x + 'a';
}
char symbolTable[26];
};
int main() {
MTF mtf;
string a, str[] = { "broood", "bananaaa", "hiphophiphop" };
for( int x = 0; x < 3; x++ )
{
a = str[x];
cout << a << " -> encoded = ";
a = mtf.encode( a );
cout << a << "; decoded = " << mtf.decode( a ) << endl;
}
return 0;
} </lang>
- Output:
broood -> encoded = 1 17 15 0 0 5 ; decoded = broood bananaaa -> encoded = 1 1 13 1 1 1 0 0 ; decoded = bananaaa hiphophiphop -> encoded = 7 8 15 2 15 2 2 3 2 2 3 2 ; decoded = hiphophiphop
Clojure
<lang clojure>(def lowercase (map char (range (int \a) (inc (int \z)))))
(defn move-to-front [x xs]
(cons x (remove #{x} xs)))
(defn encode [text table & {:keys [acc] :or {acc []}}]
(let [c (first text)
idx (.indexOf table c)]
(if (empty? text) acc (recur (drop 1 text) (move-to-front c table) {:acc (conj acc idx)}))))
(defn decode [indices table & {:keys [acc] :or {acc []}}]
(if (empty? indices) (apply str acc)
(let [n (first indices)
c (nth table n)]
(recur (drop 1 indices) (move-to-front c table) {:acc (conj acc c)}))))
(doseq [word ["broood" "bananaaa" "hiphophiphop"]]
(let [encoded (encode word lowercase)
decoded (decode encoded lowercase)]
(println (format "%s encodes to %s which decodes back to %s."
word encoded decoded))))</lang>
- Output:
broood encodes to [1 17 15 0 0 5] which decodes back to broood. bananaaa encodes to [1 1 13 1 1 1 0 0] which decodes back to bananaaa. hiphophiphop encodes to [7 8 15 2 15 2 2 3 2 2 3 2] which decodes back to hiphophiphop.
Common Lisp
<lang lisp>(defconstant +lower+ (coerce "abcdefghijklmnopqrstuvwxyz" 'list))
(defun move-to-front (x xs)
(cons x (remove x xs)))
(defun enc (text table)
(map 'list
(lambda (c)
(let ((idx (position c table)))
(setf table (move-to-front c table))
idx))
text))
(defun dec (indices table)
(coerce (mapcar (lambda (idx)
(let ((c (nth idx table)))
(setf table (move-to-front c table))
c))
indices)
'string))
(loop for word in '("broood" "bananaaa" "hiphophiphop")
do (let* ((encoded (enc word +lower+))
(decoded (dec encoded +lower+)))
(assert (string= word decoded))
(format T "~s encodes to ~a which decodes back to ~s.~%"
word encoded decoded)))</lang>
- Output:
"broood" encodes to (1 17 15 0 0 5) which decodes back to "broood". "bananaaa" encodes to (1 1 13 1 1 1 0 0) which decodes back to "bananaaa". "hiphophiphop" encodes to (7 8 15 2 15 2 2 3 2 2 3 2) which decodes back to "hiphophiphop".
D
<lang d>import std.stdio, std.string, std.ascii, std.algorithm;
ptrdiff_t[] mtfEncoder(in string data) pure nothrow @safe in {
assert(data.countchars("a-z") == data.length);
} out(result) {
assert(result.length == data.length); assert(result.all!(e => e >= 0 && e < lowercase.length));
} body {
ubyte[lowercase.length] order = lowercase.representation; auto encoded = new typeof(return)(data.length);
size_t i = 0;
foreach (immutable b; data) {
immutable j = encoded[i++] = order[].countUntil(b);
bringToFront(order[0 .. j], order[j .. j + 1]);
}
return encoded;
}
string mtfDecoder(in ptrdiff_t[] encoded) pure nothrow @safe in {
assert(encoded.all!(e => e >= 0 && e < lowercase.length));
} out(result) {
assert(result.length == encoded.length);
assert(result.countchars("a-z") == result.length);
} body {
ubyte[lowercase.length] order = lowercase.representation; auto decoded = new char[encoded.length];
size_t i = 0;
foreach (immutable code; encoded) {
decoded[i++] = order[code];
bringToFront(order[0 .. code], order[code .. code + 1]);
}
return decoded;
}
void main() {
foreach (immutable word; ["broood", "bananaaa", "hiphophiphop"]) {
immutable encoded = word.mtfEncoder;
immutable decoded = encoded.mtfDecoder;
writefln("'%s' encodes to %s, which decodes back to '%s'",
word, encoded, decoded);
assert(word == decoded);
}
}</lang>
- Output:
'broood' encodes to [1, 17, 15, 0, 0, 5], which decodes back to 'broood' 'bananaaa' encodes to [1, 1, 13, 1, 1, 1, 0, 0], which decodes back to 'bananaaa' 'hiphophiphop' encodes to [7, 8, 15, 2, 15, 2, 2, 3, 2, 2, 3, 2], which decodes back to 'hiphophiphop'
Elixir
<lang elixir>defmodule MoveToFront do
@table Enum.to_list(?a..?z) def encode(text), do: encode(to_char_list(text), @table, []) defp encode([], _, output), do: Enum.reverse(output) defp encode([h|t], table, output) do i = Enum.find_index(table, &(&1 == h)) encode(t, move2front(table, i), [i | output]) end def decode(indices), do: decode(indices, @table, []) defp decode([], _, output), do: Enum.reverse(output) |> to_string defp decode([h|t], table, output) do decode(t, move2front(table, h), [Enum.at(table, h) | output]) end def move2front(table, i), do: [Enum.at(table,i) | List.delete_at(table, i)]
end
Enum.each(["broood", "bananaaa", "hiphophiphop"], fn word ->
IO.inspect word
IO.inspect enc = MoveToFront.encode(word)
IO.puts "#{word == MoveToFront.decode(enc)}\n"
end)</lang>
- Output:
"broood" [1, 17, 15, 0, 0, 5] true "bananaaa" [1, 1, 13, 1, 1, 1, 0, 0] true "hiphophiphop" [7, 8, 15, 2, 15, 2, 2, 3, 2, 2, 3, 2] true
Factor
<lang factor>USING: formatting kernel locals make sequences ;
- to-front ( elt seq -- seq' ) over [ remove ] dip prefix ;
- encode ( symbols msg -- indices )
[ [ swap 2dup index , to-front ] each ] { } make nip ;
- decode ( symbols indices -- msg )
[ [ swap [ nth ] keep over , to-front ] each ] "" make nip ;
- round-trip ( msg symbols -- )
symbols msg encode :> encoded symbols encoded decode :> decoded msg decoded assert= msg encoded decoded "%s -> %u -> %s\n" printf ;
"broood" "bananaaa" "hiphophiphop" [ "abcdefghijklmnopqrstuvwxyz" round-trip ] tri@</lang>
- Output:
broood -> { 1 17 15 0 0 5 } -> broood
bananaaa -> { 1 1 13 1 1 1 0 0 } -> bananaaa
hiphophiphop -> { 7 8 15 2 15 2 2 3 2 2 3 2 } -> hiphophiphop
Gambas
Click this link to run this code <lang gambas>Public Sub Main() Dim sToCode As String[] = ["broood", "bananaaa", "hiphophiphop"] 'Samples to process Dim sHold As New String[] 'To store results Dim siCount, siCounter, siPos As Short 'Various variables Dim sOutput, sCode, sWork, sEach As String 'Various variables
For siCounter = 0 To sToCode.Max 'To loop through each 'Sample'
sCode = "abcdefghijklmnopqrstuvwxyz" 'Set sCode to default setting For siCount = 1 To Len(sToCode[siCounter]) 'Loop through each letter in 'Sample' sWork = Mid(sToCode[siCounter], siCount, 1) 'sWork to store the Letter siPos = InStr(scode, sWork) - 1 'Find the position of the letter in sCode, -1 for '0' based array sOutput &= Str(siPos) & " " 'Add the postion to sOutput sCode = Mid(sCode, siPos + 1, 1) & Replace(sCode, sWork, "") 'sCode = the letter + the rest of sCode except the letter Next Print sToCode[siCounter] & " = " & sOutput 'Print the 'Sample' and the coded version sHold.Add(Trim(sOutput)) 'Add the code to the sHold array sOutput = "" 'Clear sOutput
Next
Print 'Print a blank line
For siCounter = 0 To sHold.Max 'To loop through each coded 'Sample'
sCode = "abcdefghijklmnopqrstuvwxyz" 'Set sCode to default setting For Each sEach In Split(sHold[siCounter], " ") 'For each 'code' in coded 'Sample' sWork = Mid(sCode, Val(sEach) + 1, 1) 'sWork = the decoded letter sOutput &= sWork 'Add the decoded letter to sOutput sCode = sWork & Replace(sCode, sWork, "") 'sCode = the decoded letter + the rest of sCode except the letter Next Print sHold[siCounter] & " = " & sOutput 'Print the coded and decoded result sOutput = "" 'Clear sOutput
Next
End</lang> Output:
broood = 1 17 15 0 0 5 bananaaa = 1 1 13 1 1 1 0 0 hiphophiphop = 7 8 15 2 15 2 2 3 2 2 3 2 1 17 15 0 0 5 = broood 1 1 13 1 1 1 0 0 = bananaaa 7 8 15 2 15 2 2 3 2 2 3 2 = hiphophiphop
Go
<lang go>package main
import ( "bytes" "fmt" )
type symbolTable string
func (symbols symbolTable) encode(s string) []byte { seq := make([]byte, len(s)) pad := []byte(symbols) for i, c := range []byte(s) { x := bytes.IndexByte(pad, c) seq[i] = byte(x) copy(pad[1:], pad[:x]) pad[0] = c } return seq }
func (symbols symbolTable) decode(seq []byte) string { chars := make([]byte, len(seq)) pad := []byte(symbols) for i, x := range seq { c := pad[x] chars[i] = c copy(pad[1:], pad[:x]) pad[0] = c } return string(chars) }
func main() { m := symbolTable("abcdefghijklmnopqrstuvwxyz") for _, s := range []string{"broood", "bananaaa", "hiphophiphop"} { enc := m.encode(s) dec := m.decode(enc) fmt.Println(s, enc, dec) if dec != s { panic("Whoops!") } } }</lang>
- Output:
broood [1 17 15 0 0 5] broood bananaaa [1 1 13 1 1 1 0 0] bananaaa hiphophiphop [7 8 15 2 15 2 2 3 2 2 3 2] hiphophiphop
Haskell
<lang Haskell>import Data.List (delete, elemIndex, mapAccumL)
import Data.Maybe (fromJust)
table :: String table = ['a' .. 'z']
encode :: String -> [Int] encode =
let f t s = (s : delete s t, fromJust (elemIndex s t)) in snd . mapAccumL f table
decode :: [Int] -> String decode = snd . mapAccumL f table
where
f t i =
let s = (t !! i)
in (s : delete s t, s)
main :: IO () main =
mapM_ print $ (,) <*> uncurry ((==) . fst) <$> -- Test that ((fst . fst) x) == snd x) ((,) <*> (decode . snd) <$> ((,) <*> encode <$> ["broood", "bananaaa", "hiphophiphop"]))</lang>
- Output:
((("broood",[1,17,15,0,0,5]),"broood"),True)
((("bananaaa",[1,1,13,1,1,1,0,0]),"bananaaa"),True)
((("hiphophiphop",[7,8,15,2,15,2,2,3,2,2,3,2]),"hiphophiphop"),True)
Icon and Unicon
Works in both languages: <lang unicon>procedure main(A)
every writes(s := !A, " -> [") do {
every writes(!(enc := encode(&lcase,s))," ")
writes("] -> ",s2 := decode(&lcase,enc))
write((s == s2, " (Correct)") | " (Incorrect)")
}
end
procedure encode(m,s)
enc := []
every c := !s do {
m ?:= reorder(tab(i := upto(c)),move(1),tab(0))
put(enc,i-1) # Strings are 1-based
}
return enc
end
procedure decode(m,enc)
dec := ""
every i := 1 + !enc do { # Lists are 1-based
dec ||:= m[i]
m ?:= reorder(tab(i),move(1),tab(0))
}
return dec
end
procedure reorder(s1,s2,s3)
return s2||s1||s3
end</lang>
Sample run:
->mtfa broood bananaaa hiphophiphop broood -> [1 17 15 0 0 5 ] -> broood (Correct) bananaaa -> [1 1 13 1 1 1 0 0 ] -> bananaaa (Correct) hiphophiphop -> [7 8 15 2 15 2 2 3 2 2 3 2 ] -> hiphophiphop (Correct) ->
J
<lang J>spindizzy=:3 :0
'seq table'=. y ndx=.$0 orig=. table for_sym. seq do. ndx=.ndx,table i.sym table=. sym,table-.sym end. ndx assert. seq-:yzzidnips ndx;orig
)
yzzidnips=:3 :0
'ndx table'=. y
seq=.
for_n. ndx do.
seq=.seq,sym=. n{table
table=. sym,table-.sym
end.
seq
)</lang>
Required examples:
<lang J> spindizzy 'broood';'abcdefghijklmnopqrstuvwxyz' 1 17 15 0 0 5
spindizzy 'bananaaa';'abcdefghijklmnopqrstuvwxyz'
1 1 13 1 1 1 0 0
spindizzy 'hiphophiphop';'abcdefghijklmnopqrstuvwxyz'
7 8 15 2 15 2 2 3 2 2 3 2</lang>
It's not clear, though, why anyone would think that this is any better than lookups against an unmodified symbol table.
Java
<lang java5>import java.util.LinkedList; import java.util.List;
public class MTF{ public static List<Integer> encode(String msg, String symTable){ List<Integer> output = new LinkedList<Integer>(); StringBuilder s = new StringBuilder(symTable); for(char c : msg.toCharArray()){ int idx = s.indexOf("" + c); output.add(idx); s = s.deleteCharAt(idx).insert(0, c); } return output; }
public static String decode(List<Integer> idxs, String symTable){ StringBuilder output = new StringBuilder(); StringBuilder s = new StringBuilder(symTable); for(int idx : idxs){ char c = s.charAt(idx); output = output.append(c); s = s.deleteCharAt(idx).insert(0, c); } return output.toString(); }
private static void test(String toEncode, String symTable){ List<Integer> encoded = encode(toEncode, symTable); System.out.println(toEncode + ": " + encoded); String decoded = decode(encoded, symTable); System.out.println((toEncode.equals(decoded) ? "" : "in") + "correctly decoded to " + decoded); }
public static void main(String[] args){ String symTable = "abcdefghijklmnopqrstuvwxyz"; test("broood", symTable); test("bananaaa", symTable); test("hiphophiphop", symTable); } }</lang>
- Output:
broood: [1, 17, 15, 0, 0, 5] correctly decoded to broood bananaaa: [1, 1, 13, 1, 1, 1, 0, 0] correctly decoded to bananaaa hiphophiphop: [7, 8, 15, 2, 15, 2, 2, 3, 2, 2, 3, 2] correctly decoded to hiphophiphop
JavaScript
<lang javascript>var encodeMTF = function (word) {
var init = {wordAsNumbers: [], charList: 'abcdefghijklmnopqrstuvwxyz'.split()};
return word.split().reduce(function (acc, char) {
var charNum = acc.charList.indexOf(char); //get index of char
acc.wordAsNumbers.push(charNum); //add original index to acc
acc.charList.unshift(acc.charList.splice(charNum, 1)[0]); //put at beginning of list
return acc;
}, init).wordAsNumbers; //return number list
};
var decodeMTF = function (numList) {
var init = {word: , charList: 'abcdefghijklmnopqrstuvwxyz'.split()};
return numList.reduce(function (acc, num) {
acc.word += acc.charList[num];
acc.charList.unshift(acc.charList.splice(num, 1)[0]); //put at beginning of list
return acc;
}, init).word;
};
//test our algorithms var words = ['broood', 'bananaaa', 'hiphophiphop']; var encoded = words.map(encodeMTF); var decoded = encoded.map(decodeMTF);
//print results console.log("from encoded:"); console.log(encoded); console.log("from decoded:"); console.log(decoded);</lang>
- Output:
from encoded: [ [1, 17, 15, 0, 0, 5], [1, 1, 13, 1, 1, 1, 0, 0], [7, 8, 15, 2, 15, 2, 2, 3, 2, 2, 3, 2] ] from decoded: ['broood', 'bananaaa', 'hiphophiphop']
jq
<lang jq># Input is the string to be encoded, st is the initial symbol table (an array)
- Output: the encoded string (an array)
def m2f_encode(st):
reduce explode[] as $ch
( [ [], st]; # state: [ans, st]
(.[1]|index($ch)) as $ix
| .[1] as $st
| [ (.[0] + [ $ix ]), [$st[$ix]] + $st[0:$ix] + $st[$ix+1:] ] )
| .[0];
- Input should be the encoded string (an array)
- and st should be the initial symbol table (an array)
def m2f_decode(st):
reduce .[] as $ix
( [ [], st]; # state: [ans, st]
.[1] as $st
| [ (.[0] + [ $st[$ix] ]), [$st[$ix]] + $st[0:$ix] + $st[$ix+1:] ] )
| .[0]
| implode;</lang>
Example: <lang jq>("abcdefghijklmnopqrstuvwxyz" | explode) as $ST
| ("broood", "bananaaa", "hiphophiphop")
| . as $string
| m2f_encode($ST)
| . as $encoded
| m2f_decode($ST) as $decoded
| if $string == $decoded then "\($string) => \($encoded) => \($decoded)"
else "INTERNAL ERROR: encoding of \($string) => \($encoded) => \($decoded)"
end</lang>
- Output:
<lang sh>$ jq -r -n -f move_to_front.jq broood => [1,17,15,0,0,5] => broood bananaaa => [1,1,13,1,1,1,0,0] => bananaaa hiphophiphop => [7,8,15,2,15,2,2,3,2,2,3,2] => hiphophiphop</lang>
Julia
<lang julia>function encodeMTF(str::AbstractString, symtable::Vector{Char}=collect('a':'z'))
function encode(ch::Char)
r = findfirst(symtable, ch)
deleteat!(symtable, r)
prepend!(symtable, ch)
return r
end
collect(encode(ch) for ch in str)
end
function decodeMTF(arr::Vector{Int}, symtable::Vector{Char}=collect('a':'z'))
function decode(i::Int)
r = symtable[i]
deleteat!(symtable, i)
prepend!(symtable, r)
return r
end
join(decode(i) for i in arr)
end
testset = ["broood", "bananaaa", "hiphophiphop"] encoded = encodeMTF.(testset) decoded = decodeMTF.(encoded) for (str, enc, dec) in zip(testset, encoded, decoded)
println("Test string: $str\n -> Encoded: $enc\n -> Decoded: $dec")
end
using Base.Test @testset "Decoded string equal to original" begin
for (str, dec) in zip(testset, decoded)
@test str == dec
end
end</lang>
- Output:
Test string: broood -> Encoded: [2, 18, 16, 1, 1, 6] -> Decoded: broood Test string: bananaaa -> Encoded: [2, 2, 14, 2, 2, 2, 1, 1] -> Decoded: bananaaa Test string: hiphophiphop -> Encoded: [8, 9, 16, 3, 16, 3, 3, 4, 3, 3, 4, 3] -> Decoded: hiphophiphop Test Summary: | Pass Total Decoded string equal to original | 3 3
Kotlin
<lang scala>// version 1.1.2
fun encode(s: String): IntArray {
if (s.isEmpty()) return intArrayOf()
val symbols = "abcdefghijklmnopqrstuvwxyz".toCharArray()
val result = IntArray(s.length)
for ((i, c) in s.withIndex()) {
val index = symbols.indexOf(c)
if (index == -1)
throw IllegalArgumentException("$s contains a non-alphabetic character")
result[i] = index
if (index == 0) continue
for (j in index - 1 downTo 0) symbols[j + 1] = symbols[j]
symbols[0] = c
}
return result
}
fun decode(a: IntArray): String {
if (a.isEmpty()) return ""
val symbols = "abcdefghijklmnopqrstuvwxyz".toCharArray()
val result = CharArray(a.size)
for ((i, n) in a.withIndex()) {
if (n !in 0..25)
throw IllegalArgumentException("${a.contentToString()} contains an invalid number")
result[i] = symbols[n]
if (n == 0) continue
for (j in n - 1 downTo 0) symbols[j + 1] = symbols[j]
symbols[0] = result[i]
}
return result.joinToString("")
}
fun main(args: Array<String>) {
val strings = arrayOf("broood", "bananaaa", "hiphophiphop")
val encoded = Array<IntArray?>(strings.size) { null }
for ((i, s) in strings.withIndex()) {
encoded[i] = encode(s)
println("${s.padEnd(12)} -> ${encoded[i]!!.contentToString()}")
}
println()
val decoded = Array<String?>(encoded.size) { null }
for ((i, a) in encoded.withIndex()) {
decoded[i] = decode(a!!)
print("${a.contentToString().padEnd(38)} -> ${decoded[i]!!.padEnd(12)}")
println(" -> ${if (decoded[i] == strings[i]) "correct" else "incorrect"}")
}
}</lang>
- Output:
broood -> [1, 17, 15, 0, 0, 5] bananaaa -> [1, 1, 13, 1, 1, 1, 0, 0] hiphophiphop -> [7, 8, 15, 2, 15, 2, 2, 3, 2, 2, 3, 2] [1, 17, 15, 0, 0, 5] -> broood -> correct [1, 1, 13, 1, 1, 1, 0, 0] -> bananaaa -> correct [7, 8, 15, 2, 15, 2, 2, 3, 2, 2, 3, 2] -> hiphophiphop -> correct
Lua
<lang lua>-- Return table of the alphabet in lower case function getAlphabet ()
local letters = {}
for ascii = 97, 122 do table.insert(letters, string.char(ascii)) end
return letters
end
-- Move the table value at ind to the front of tab function moveToFront (tab, ind)
local toMove = tab[ind] for i = ind - 1, 1, -1 do tab[i + 1] = tab[i] end tab[1] = toMove
end
-- Perform move-to-front encoding on input function encode (input)
local symbolTable, output, index = getAlphabet(), {}
for pos = 1, #input do
for k, v in pairs(symbolTable) do
if v == input:sub(pos, pos) then index = k end
end
moveToFront(symbolTable, index)
table.insert(output, index - 1)
end
return table.concat(output, " ")
end
-- Perform move-to-front decoding on input function decode (input)
local symbolTable, output = getAlphabet(), ""
for num in input:gmatch("%d+") do
output = output .. symbolTable[num + 1]
moveToFront(symbolTable, num + 1)
end
return output
end
-- Main procedure local testCases, output = {"broood", "bananaaa", "hiphophiphop"} for _, case in pairs(testCases) do
output = encode(case)
print("Original string: " .. case)
print("Encoded: " .. output)
print("Decoded: " .. decode(output))
print()
end</lang>
- Output:
Original string: broood Encoded: 1 17 15 0 0 5 Decoded: broood Original string: bananaaa Encoded: 1 1 13 1 1 1 0 0 Decoded: bananaaa Original string: hiphophiphop Encoded: 7 8 15 2 15 2 2 3 2 2 3 2 Decoded: hiphophiphop
edit
M2000 Interpreter
In Encode$() we use a string of parameters (function Quote$() make this)
Function Param() get the string with parameters and place it in code (inline).
In Decode$ we use current stack (first we flush to ensure that isn't some other arguments on it, M2000 has no signature control over the call of user functions, we can place anything)
For deleting and inserting characters in a string we use Insert. Insert of "" in a place of 1 char means delete it. So Insert k, 1 SymbolTable$="" means that in k position, for one char we place empty string.
To extract information to clipboard we use global variables, is handy;
Number pop a number from stack of values, if no number found then raise error.
<lang M2000 Interpreter> Module CheckIt {
Global All$, nl$
\\ upgrade to document
Document All$
Nl$<={
}
Function Encode$(Inp$) {
Def SymbolTable$="abcdefghijklmnopqrstuvwxyz", Out$=""
For i=1 to Len(Inp$)
c$=Mid$(Inp$, i, 1)
k=Instr(SymbolTable$, c$)
Insert k, 1 SymbolTable$=""
Out$=If$(Out$="" -> Quote$(k-1),Quote$(Param(Out$), k-1))
Insert 1 SymbolTable$=c$
\\ we use <= for globals
All$<=Format$(" {0} {1:30} {2}", c$, Out$, SymbolTable$)+nl$
Next i
=Out$
}
Function Decode$(Inp$) {
Def SymbolTable$="abcdefghijklmnopqrstuvwxyz", Out$=""
flush
Data Param(Inp$)
While not empty {
k=Number+1
c$=Mid$(SymbolTable$, k, 1)
Out$+=c$
Insert k, 1 SymbolTable$="" ' erase at position k
Insert 1 SymbolTable$=c$
All$<=Format$("{0::-2} {1} {2:30} {3}", k-1, c$, Out$, SymbolTable$)+nl$
}
=Out$
}
TryThis("broood")
TryThis("bananaaa")
TryThis("hiphophiphop")
ClipBoard All$
Sub TryThis(a$)
Local Out$=Encode$(a$)
Local final$=Decode$(Out$)
Print final$, final$=a$
End Sub
} CheckIt </lang>
- Output:
broood True bananaaa True hiphophiphop True
From clipboard
b 1 bacdefghijklmnopqrstuvwxyz r 1,17 rbacdefghijklmnopqstuvwxyz o 1,17,15 orbacdefghijklmnpqstuvwxyz o 1,17,15,0 orbacdefghijklmnpqstuvwxyz o 1,17,15,0,0 orbacdefghijklmnpqstuvwxyz d 1,17,15,0,0,5 dorbacefghijklmnpqstuvwxyz 1 b b bacdefghijklmnopqrstuvwxyz 17 r br rbacdefghijklmnopqstuvwxyz 15 o bro orbacdefghijklmnpqstuvwxyz 0 o broo orbacdefghijklmnpqstuvwxyz 0 o brooo orbacdefghijklmnpqstuvwxyz 5 d broood dorbacefghijklmnpqstuvwxyz b 1 bacdefghijklmnopqrstuvwxyz a 1,1 abcdefghijklmnopqrstuvwxyz n 1,1,13 nabcdefghijklmopqrstuvwxyz a 1,1,13,1 anbcdefghijklmopqrstuvwxyz n 1,1,13,1,1 nabcdefghijklmopqrstuvwxyz a 1,1,13,1,1,1 anbcdefghijklmopqrstuvwxyz a 1,1,13,1,1,1,0 anbcdefghijklmopqrstuvwxyz a 1,1,13,1,1,1,0,0 anbcdefghijklmopqrstuvwxyz 1 b b bacdefghijklmnopqrstuvwxyz 1 a ba abcdefghijklmnopqrstuvwxyz 13 n ban nabcdefghijklmopqrstuvwxyz 1 a bana anbcdefghijklmopqrstuvwxyz 1 n banan nabcdefghijklmopqrstuvwxyz 1 a banana anbcdefghijklmopqrstuvwxyz 0 a bananaa anbcdefghijklmopqrstuvwxyz 0 a bananaaa anbcdefghijklmopqrstuvwxyz h 7 habcdefgijklmnopqrstuvwxyz i 7,8 ihabcdefgjklmnopqrstuvwxyz p 7,8,15 pihabcdefgjklmnoqrstuvwxyz h 7,8,15,2 hpiabcdefgjklmnoqrstuvwxyz o 7,8,15,2,15 ohpiabcdefgjklmnqrstuvwxyz p 7,8,15,2,15,2 pohiabcdefgjklmnqrstuvwxyz h 7,8,15,2,15,2,2 hpoiabcdefgjklmnqrstuvwxyz i 7,8,15,2,15,2,2,3 ihpoabcdefgjklmnqrstuvwxyz p 7,8,15,2,15,2,2,3,2 pihoabcdefgjklmnqrstuvwxyz h 7,8,15,2,15,2,2,3,2,2 hpioabcdefgjklmnqrstuvwxyz o 7,8,15,2,15,2,2,3,2,2,3 ohpiabcdefgjklmnqrstuvwxyz p 7,8,15,2,15,2,2,3,2,2,3,2 pohiabcdefgjklmnqrstuvwxyz 7 h h habcdefgijklmnopqrstuvwxyz 8 i hi ihabcdefgjklmnopqrstuvwxyz 15 p hip pihabcdefgjklmnoqrstuvwxyz 2 h hiph hpiabcdefgjklmnoqrstuvwxyz 15 o hipho ohpiabcdefgjklmnqrstuvwxyz 2 p hiphop pohiabcdefgjklmnqrstuvwxyz 2 h hiphoph hpoiabcdefgjklmnqrstuvwxyz 3 i hiphophi ihpoabcdefgjklmnqrstuvwxyz 2 p hiphophip pihoabcdefgjklmnqrstuvwxyz 2 h hiphophiph hpioabcdefgjklmnqrstuvwxyz 3 o hiphophipho ohpiabcdefgjklmnqrstuvwxyz 2 p hiphophiphop pohiabcdefgjklmnqrstuvwxyz
Mathematica
<lang Mathematica>mtf[word_]:=Module[{f,f2,p,q},
f[{output_,symList_},next_]:=Module[{index},index=Position[symList,next]1,1-1;
{output~Append~index,Prepend[Delete[symList,index+1],next]}];
p=Fold[f,{{},CharacterRange["a","z"]},Characters[ToString[word]]]1;
f2[{output_,symList_},next_]:=Module[{index},index=symListnext+1;
{output~Append~index,Prepend[DeleteCases[symList,ToString[index]],index]}];
q=Fold[f2,{{},CharacterRange["a","z"]},p]1;
Print["'", word,"' encodes to: ",p, " - " ,p," decodes to: '",StringJoin@q,"' - Input equals Output: " ,word===StringJoin@q];]</lang>
Testing out the function: <lang Mathematica>mtf /@ {"broood", "bananaaa", "hiphophiphop"}</lang>
- Output:
'broood' encodes to: {1,17,15,0,0,5} - {1,17,15,0,0,5} decodes to: 'broood' - Input equals Output: True
'bananaaa' encodes to: {1,1,13,1,1,1,0,0} - {1,1,13,1,1,1,0,0} decodes to: 'bananaaa' - Input equals Output: True
'hiphophiphop' encodes to: {7,8,15,2,15,2,2,3,2,2,3,2} - {7,8,15,2,15,2,2,3,2,2,3,2} decodes to: 'hiphophiphop' - Input equals Output: True
MATLAB
<lang MATLAB>function testMTF
symTable = 'abcdefghijklmnopqrstuvwxyz';
inStr = {'broood' 'bananaaa' 'hiphophiphop'};
for k = 1:length(inStr)
outArr = encodeMTF(inStr{k}, symTable);
outStr = decodeMTF(outArr, symTable);
fprintf('%s: [ %s]\n', inStr{k}, sprintf('%d ', outArr))
fprintf('%scorrectly decoded to %s\n', char('in'.*~strcmp(outStr, inStr{k})), outStr)
end
end
function arr = encodeMTF(str, symTable)
n = length(str);
arr = zeros(1, n);
for k = 1:n
arr(k) = find(str(k) == symTable, 1);
symTable = [symTable(arr(k)) symTable(1:arr(k)-1) symTable(arr(k)+1:end)];
end
arr = arr-1; % Change to zero-indexed array
end
function str = decodeMTF(arr, symTable)
arr = arr+1; % Change to one-indexed array
n = length(arr);
str = char(zeros(1, n));
for k = 1:n
str(k) = symTable(arr(k));
symTable = [symTable(arr(k)) symTable(1:arr(k)-1) symTable(arr(k)+1:end)];
end
end</lang>
- Output:
broood: [ 1 17 15 0 0 5 ] correctly decoded to broood bananaaa: [ 1 1 13 1 1 1 0 0 ] correctly decoded to bananaaa hiphophiphop: [ 7 8 15 2 15 2 2 3 2 2 3 2 ] correctly decoded to hiphophiphop
Perl
<lang perl>use strict; use warnings; sub encode { my ($str) = @_; my $table = join , 'a' .. 'z'; map { $table =~ s/(.*?)$_/$_$1/ or die; length($1); } split //, $str; }
sub decode { my $table = join , 'a' .. 'z'; join "", map { $table =~ s/(.{$_})(.)/$2$1/ or die; $2; } @_; }
for my $test ( qw(broood bananaaa hiphophiphop) ) { my @encoded = encode($test); print "$test: @encoded\n"; my $decoded = decode(@encoded); print "in" x ( $decoded ne $test ); print "correctly decoded to $decoded\n"; } </lang>
- Output:
broood: 1 17 15 0 0 5 correctly decoded to broood bananaaa: 1 1 13 1 1 1 0 0 correctly decoded to bananaaa hiphophiphop: 7 8 15 2 15 2 2 3 2 2 3 2 correctly decoded to hiphophiphop
Phix
<lang Phix>function encode(string s)
string symtab = "abcdefghijklmnopqrstuvwxyz"
sequence res = {}
for i=1 to length(s) do
integer ch = s[i]
integer k = find(ch,symtab)
res &= k-1
for j=k to 2 by -1 do
symtab[j] = symtab[j-1]
end for
symtab[1] = ch
end for
return res
end function
function decode(sequence s)
string symtab = "abcdefghijklmnopqrstuvwxyz"
string res = ""
for i=1 to length(s) do
integer k = s[i]+1
integer ch = symtab[k]
res &= ch
for j=k to 2 by -1 do
symtab[j] = symtab[j-1]
end for
symtab[1] = ch
end for
return res
end function
procedure test(string s)
sequence e = encode(s)
string d = decode(e)
?{s,e,d,{"**ERROR**","ok"}[(s=d)+1]}
end procedure test("broood") test("bananaaa") test("hiphophiphop")</lang>
- Output:
{"broood",{1,17,15,0,0,5},"broood","ok"}
{"bananaaa",{1,1,13,1,1,1,0,0},"bananaaa","ok"}
{"hiphophiphop",{7,8,15,2,15,2,2,3,2,2,3,2},"hiphophiphop","ok"}
PHP
<lang PHP><?php
function symbolTable() {
$symbol = array();
for ($c = ord('a') ; $c <= ord('z') ; $c++) {
$symbol[$c - ord('a')] = chr($c);
}
return $symbol;
}
function mtfEncode($original, $symbol) {
$encoded = array();
for ($i = 0 ; $i < strlen($original) ; $i++) {
$char = $original[$i];
$position = array_search($char, $symbol);
$encoded[] = $position;
$mtf = $symbol[$position];
unset($symbol[$position]);
array_unshift($symbol, $mtf);
}
return $encoded;
}
function mtfDecode($encoded, $symbol) {
$decoded = ;
foreach ($encoded AS $position) {
$char = $symbol[$position];
$decoded .= $char;
unset($symbol[$position]);
array_unshift($symbol, $char);
}
return $decoded;
}
foreach (array('broood', 'bananaaa', 'hiphophiphop') AS $original) {
$encoded = mtfEncode($original, symbolTable());
$decoded = mtfDecode($encoded, symbolTable());
echo
$original,
' -> [', implode(',', $encoded), ']',
' -> ', $decoded,
' : ', ($original === $decoded ? 'OK' : 'Error'),
PHP_EOL;
}</lang>
- Output:
broood -> [1,17,15,0,0,5] -> broood : OK bananaaa -> [1,1,13,1,1,1,0,0] -> bananaaa : OK hiphophiphop -> [7,8,15,2,15,2,2,3,2,2,3,2] -> hiphophiphop : OK
PicoLisp
<lang PicoLisp>(de encode (Str)
(let Table (chop "abcdefghijklmnopqrstuvwxyz")
(mapcar
'((C)
(dec
(prog1
(index C Table)
(rot Table @) ) ) )
(chop Str) ) ) )
(de decode (Lst)
(let Table (chop "abcdefghijklmnopqrstuvwxyz")
(pack
(mapcar
'((N)
(prog1
(get Table (inc 'N))
(rot Table N) ) )
Lst ) ) ) )</lang>
Test: <lang PicoLisp>(test (1 17 15 0 0 5)
(encode "broood") )
(test "broood"
(decode (1 17 15 0 0 5)) )
(test (1 1 13 1 1 1 0 0)
(encode "bananaaa") )
(test "bananaaa"
(decode (1 1 13 1 1 1 0 0)) )
(test (7 8 15 2 15 2 2 3 2 2 3 2)
(encode "hiphophiphop") )
(test "hiphophiphop"
(decode (7 8 15 2 15 2 2 3 2 2 3 2)) )</lang>
PL/I
<lang pli>*process source attributes xref or(!);
/*********************************************************************
* 25.5.2014 Walter Pachl translated from REXX
*********************************************************************/
ed: Proc Options(main);
Call enc_dec('broood');
Call enc_dec('bananaaa');
Call enc_dec('hiphophiphop');
enc_dec: Proc(in);
Dcl in Char(*);
Dcl out Char(20) Var Init();
Dcl st Char(26) Init('abcdefghijklmnopqrstuvwxyz');
Dcl sta Char(26) Init((st));
Dcl enc(20) Bin Fixed(31);
Dcl encn Bin Fixed(31) Init(0);
Dcl (i,p.k) Bin Fixed(31);
Dcl c Char(1);
Do i=1 To length(in);
c=substr(in,i,1);
p=index(st,c);
encn+=1;
enc(encn)=p-1;
st=c!!left(st,p-1)!!substr(st,p+1);
End;
Put Skip List(' in='!!in);
Put Skip List('sta='!!sta!!' original symbol table');
Put Skip Edit('enc=',(enc(i) do i=1 To encn))(a,20(F(3)));
Put Skip List(' st='!!st!!' symbol table after encoding');
Do i=1 To encn;
k=enc(i)+1;
out=out!!substr(sta,k,1);
sta=substr(sta,k,1)!!left(sta,k-1)!!substr(sta,k+1);
End;
Put Skip List('out='!!out);
Put Skip List(' ');
If out=in Then;
Else
Put Skip List('all wrong!!');
End;</lang>
- Output:
in=broood sta=abcdefghijklmnopqrstuvwxyz original symbol table enc= 1 17 15 0 0 5 st=dorbacefghijklmnpqstuvwxyz symbol table after encoding out=broood in=bananaaa sta=abcdefghijklmnopqrstuvwxyz original symbol table enc= 1 1 13 1 1 1 0 0 st=anbcdefghijklmopqrstuvwxyz symbol table after encoding out=bananaaa in=hiphophiphop sta=abcdefghijklmnopqrstuvwxyz original symbol table enc= 7 8 15 2 15 2 2 3 2 2 3 2 st=pohiabcdefgjklmnqrstuvwxyz symbol table after encoding out=hiphophiphop
PowerShell
<lang PowerShell>Function Test-MTF {
[CmdletBinding()]
Param
(
[Parameter(Mandatory=$true,Position=0)]
[string]$word,
[Parameter(Mandatory=$false)]
[string]$SymbolTable = 'abcdefghijklmnopqrstuvwxyz'
)
Begin
{
Function Encode
{
Param
(
[Parameter(Mandatory=$true,Position=0)]
[string]$word,
[Parameter(Mandatory=$false)]
[string]$SymbolTable = 'abcdefghijklmnopqrstuvwxyz'
)
foreach ($letter in $word.ToCharArray())
{
$index = $SymbolTable.IndexOf($letter)
$prop = [ordered]@{
Input = $letter
Output = [int]$index
SymbolTable = $SymbolTable
}
New-Object PSobject -Property $prop
$SymbolTable = $SymbolTable.Remove($index,1).Insert(0,$letter)
}
}
Function Decode
{
Param
(
[Parameter(Mandatory=$true,Position=0)]
[int[]]$index,
[Parameter(Mandatory=$false)]
[string]$SymbolTable = 'abcdefghijklmnopqrstuvwxyz'
)
foreach ($i in $index)
{
#Write-host $i -ForegroundColor Red
$letter = $SymbolTable.Chars($i)
$prop = [ordered]@{
Input = $i
Output = $letter
SymbolTable = $SymbolTable
}
New-Object PSObject -Property $prop
$SymbolTable = $SymbolTable.Remove($i,1).Insert(0,$letter)
}
}
}
Process
{
#Encoding
Write-Host "Encoding $word" -NoNewline
$Encoded = (Encode -word $word).output
Write-Host -NoNewline ": $($Encoded -join ',')"
#Decoding
Write-Host "`nDecoding $($Encoded -join ',')" -NoNewline
$Decoded = (Decode -index $Encoded).output -join
Write-Host -NoNewline ": $Decoded`n"
}
End{}
}</lang>
- Output:
Test-MTF -word broood Test-MTF -word bananaaa Test-MTF -word hiphophiphop Encoding broood: 1,17,15,0,0,5 Decoding 1,17,15,0,0,5: broood Encoding bananaaa: 1,1,13,1,1,1,0,0 Decoding 1,1,13,1,1,1,0,0: bananaaa Encoding hiphophiphop: 7,8,15,2,15,2,2,3,2,2,3,2 Decoding 7,8,15,2,15,2,2,3,2,2,3,2: hiphophiphop
Python
Python: Procedural
<lang python>from __future__ import print_function from string import ascii_lowercase
SYMBOLTABLE = list(ascii_lowercase)
def move2front_encode(strng, symboltable):
sequence, pad = [], symboltable[::]
for char in strng:
indx = pad.index(char)
sequence.append(indx)
pad = [pad.pop(indx)] + pad
return sequence
def move2front_decode(sequence, symboltable):
chars, pad = [], symboltable[::]
for indx in sequence:
char = pad[indx]
chars.append(char)
pad = [pad.pop(indx)] + pad
return .join(chars)
if __name__ == '__main__':
for s in ['broood', 'bananaaa', 'hiphophiphop']:
encode = move2front_encode(s, SYMBOLTABLE)
print('%14r encodes to %r' % (s, encode), end=', ')
decode = move2front_decode(encode, SYMBOLTABLE)
print('which decodes back to %r' % decode)
assert s == decode, 'Whoops!'</lang>
- Output:
'broood' encodes to [1, 17, 15, 0, 0, 5], which decodes back to 'broood'
'bananaaa' encodes to [1, 1, 13, 1, 1, 1, 0, 0], which decodes back to 'bananaaa'
'hiphophiphop' encodes to [7, 8, 15, 2, 15, 2, 2, 3, 2, 2, 3, 2], which decodes back to 'hiphophiphop'
Python: Functional
From the procedural version note that through the encoding or decoding loops both the final output is accumulated as well as the symbol table being rearranged.
For the functional forms a 2-item list of output to be accumulated and symboltable manipulation is calculated then only the former accumulated as the later works to transform the symbol table in-place.
<lang python>def m2f_e(s, st):
return [[st.index(ch), st.insert(0, st.pop(st.index(ch)))][0] for ch in s]
def m2f_d(sq, st):
return .join([st[i], st.insert(0, st.pop(i))][0] for i in sq)
ST = list('abcdefghijklmnopqrstuvwxyz') for s in ['broood', 'bananaaa', 'hiphophiphop']:
encode = m2f_e(s, ST[::])
print('%14r encodes to %r' % (s, encode), end=', ')
decode = m2f_d(encode, ST[::])
print('decodes back to %r' % decode)
assert s == decode, 'Whoops!'</lang>
- Output:
Similar to that of the procedural version above.
Racket
<lang racket>#lang racket (define default-symtab "abcdefghijklmnopqrstuvwxyz")
(define (move-to-front:encode in (symtab default-symtab))
(define inner-encode
(match-lambda**
[((? string? (app string->list in)) st acc) ; make input all listy
(inner-encode in st acc)]
[(in (? string? (app string->list st)) acc) ; make symtab all listy
(inner-encode in st acc)]
[((list) _ (app reverse rv)) ; nothing more to encode
rv]
[((list a tail ...) (list left ... a right ...) acc) ; encode and recur
(inner-encode tail `(,a ,@left ,@right) (cons (length left) acc))]))
(inner-encode in symtab null))
(define (move-to-front:decode in (symtab default-symtab))
(define inner-decode
(match-lambda**
[(in (? string? (app string->list st)) acc) ; make symtab all listy
(inner-decode in st acc)]
[((list) _ (app (compose list->string reverse) rv)) ; nothing more to encode
rv]
[((list a tail ...) symbols acc) ; decode and recur
(match/values
(split-at symbols a)
[(l (cons ra rd))
(inner-decode tail (cons ra (append l rd)) (cons ra acc))])]))
(inner-decode in symtab null))
(module+ test
;; Test against the example in the task (require rackunit) (check-equal? (move-to-front:encode "broood") '(1 17 15 0 0 5)) (check-equal? (move-to-front:decode '(1 17 15 0 0 5)) "broood") (check-equal? (move-to-front:decode (move-to-front:encode "broood")) "broood"))
(module+ main
(define (encode+decode-string str)
(define enc (move-to-front:encode str))
(define dec (move-to-front:decode enc))
(define crt (if (equal? dec str) "correctly" "incorrectly"))
(printf "~s encodes to ~s, which decodes ~s to ~s.~%" str enc crt dec))
(for-each encode+decode-string '("broood" "bananaaa" "hiphophiphop")))</lang>
- Output:
"broood" encodes to (1 17 15 0 0 5), which decodes "correctly" to "broood". "bananaaa" encodes to (1 1 13 1 1 1 0 0), which decodes "correctly" to "bananaaa". "hiphophiphop" encodes to (7 8 15 2 15 2 2 3 2 2 3 2), which decodes "correctly" to "hiphophiphop".
Raku
(formerly Perl 6)
<lang perl6>sub encode ( Str $word ) {
my @sym = 'a' .. 'z';
gather for $word.comb -> $c {
die "Symbol '$c' not found in @sym" if $c eq @sym.none;
@sym[0 .. take (@sym ... $c).end] .= rotate(-1); }
}
sub decode ( @enc ) {
my @sym = 'a' .. 'z';
[~] gather for @enc -> $pos {
take @sym[$pos];
@sym[0..$pos] .= rotate(-1);
}
}
use Test; plan 3; for <broood bananaaa hiphophiphop> -> $word {
my $enc = encode($word);
my $dec = decode($enc);
is $word, $dec, "$word.fmt('%-12s') ($enc[])";
}</lang>
- Output:
1..3 ok 1 - broood (1 17 15 0 0 5) ok 2 - bananaaa (1 1 13 1 1 1 0 0) ok 3 - hiphophiphop (7 8 15 2 15 2 2 3 2 2 3 2)
REXX
version 1
<lang rexx>/* REXX ***************************************************************
- 25.05.2014 Walter Pachl
- REXX strings start with position 1
- /
Call enc_dec 'broood' Call enc_dec 'bananaaa' Call enc_dec 'hiphophiphop' Exit enc_dec: Procedure Parse Arg in st='abcdefghijklmnopqrstuvwxyz' sta=st /* remember this for decoding */ enc= Do i=1 To length(in)
c=substr(in,i,1) p=pos(c,st) enc=enc (p-1) st=c||left(st,p-1)substr(st,p+1) End
Say ' in='in Say 'sta='sta 'original symbol table' Say 'enc='enc Say ' st='st 'symbol table after encoding' out= Do i=1 To words(enc)
k=word(enc,i)+1 out=out||substr(sta,k,1) sta=substr(sta,k,1)left(sta,k-1)substr(sta,k+1) End
Say 'out='out Say ' ' If out==in Then Nop Else
Say 'all wrong!!'
Return </lang>
- Output:
in=broood sta=abcdefghijklmnopqrstuvwxyz original symbol table enc= 1 17 15 0 0 5 st=dorbacefghijklmnpqstuvwxyz symbol table after encoding out=broood in=bananaaa sta=abcdefghijklmnopqrstuvwxyz original symbol table enc= 1 1 13 1 1 1 0 0 st=anbcdefghijklmopqrstuvwxyz symbol table after encoding out=bananaaa in=hiphophiphop sta=abcdefghijklmnopqrstuvwxyz original symbol table enc= 7 8 15 2 15 2 2 3 2 2 3 2 st=pohiabcdefgjklmnqrstuvwxyz symbol table after encoding out=hiphophiphop
version 2
Programming note: the two REXX statements that add/subtract one deal with the task's requirement that the symbol table be zero-indexed (the REXX language uses unity-based strings). <lang rexx>/*REXX program demonstrates the move─to─front algorithm encode/decode symbol table. */ parse arg xxx; if xxx= then xxx= 'broood bananaaa hiphophiphop' /*use the default?*/
one=1 /*(offset) for this task's requirement.*/
do j=1 for words(xxx); x=word(xxx, j) /*process a single word at a time. */
@= 'abcdefghijklmnopqrstuvwxyz'; @@=@ /*symbol table: the lowercase alphabet */
$= /*set the decode string to a null. */
do k=1 for length(x); z=substr(x, k, 1) /*encrypt a symbol in the word. */
_=pos(z, @); if _==0 then iterate /*the symbol position in symbol table. */
$=$ (_ - one); @=z || delstr(@, _, 1) /*(re─)adjust the symbol table string. */
end /*k*/ /* [↑] the move─to─front encoding. */
!= /*set the encode string to a null. */
do m=1 for words($); n=word($, m) + one /*decode the sequence table string. */
y=substr(@@, n, 1); !=! || y /*the decode symbol for the N word.*/
@@=y || delstr(@@, n, 1) /*rebuild the symbol table string. */
end /*m*/ /* [↑] the move─to─front decoding. */
say ' word: ' left(x, 20) "encoding:" left($, 35) word('wrong OK', 1 + (!==x) )
end /*j*/ /*stick a fork in it, we're all done. */</lang>
- output when using the default input:
word: broood encoding: 1 17 15 0 0 5 OK word: bananaaa encoding: 1 1 13 1 1 1 0 0 OK word: hiphophiphop encoding: 7 8 15 2 15 2 2 3 2 2 3 2 OK
Ring
<lang ring>
- Project : Move-to-front algorithm
test("broood") test("bananaaa") test("hiphophiphop")
func encode(s)
symtab = "abcdefghijklmnopqrstuvwxyz"
res = ""
for i=1 to len(s)
ch = s[i]
k = substr(symtab, ch)
res = res + " " + (k-1)
for j=k to 2 step -1
symtab[j] = symtab[j-1]
next
symtab[1] = ch
next
return res
func decode(s)
s = str2list( substr(s, " ", nl) )
symtab = "abcdefghijklmnopqrstuvwxyz"
res = ""
for i=1 to len(s)
k = number(s[i]) + 1
ch = symtab[k]
res = res + " " + ch
for j=k to 2 step -1
symtab[j] = symtab[j-1]
next
symtab[1] = ch
next
return right(res, len(res)-2)
func test(s)
e = encode(s)
d = decode(e)
see "" + s + " => " + "(" + right(e, len(e) - 1) + ") " + " => " + substr(d, " ", "") + nl
</lang> Output:
broood => (1 17 15 0 0 5) => broood bananaaa => (1 1 13 1 1 1 0 0) => bananaaa hiphophiphop => (7 8 15 2 15 2 2 3 2 2 3 2) => hiphophiphop
Ruby
Use a module as namespace: <lang ruby>module MoveToFront
ABC = ("a".."z").to_a.freeze
def self.encode(str)
ar = ABC.dup
str.chars.each_with_object([]) do |char, memo|
memo << (i = ar.index(char))
ar = m2f(ar,i)
end
end
def self.decode(indices)
ar = ABC.dup
indices.each_with_object("") do |i, str|
str << ar[i]
ar = m2f(ar,i)
end
end
private
def self.m2f(ar,i)
[ar.delete_at(i)] + ar
end
end
['broood', 'bananaaa', 'hiphophiphop'].each do |word|
p word == MoveToFront.decode(p MoveToFront.encode(p word))
end</lang>
- Output:
"broood" [1, 17, 15, 0, 0, 5] true "bananaaa" [1, 1, 13, 1, 1, 1, 0, 0] true "hiphophiphop" [7, 8, 15, 2, 15, 2, 2, 3, 2, 2, 3, 2] true
Rust
<lang rust>fn main() {
let examples = vec!["broood", "bananaaa", "hiphophiphop"];
for example in examples {
let encoded = encode(example);
let decoded = decode(&encoded);
println!(
"{} encodes to {:?} decodes to {}",
example, encoded, decoded
);
}
}
fn get_symbols() -> Vec<u8> {
(b'a'..b'z').collect()
}
fn encode(input: &str) -> Vec<usize> {
input
.as_bytes()
.iter()
.fold((Vec::new(), get_symbols()), |(mut o, mut s), x| {
let i = s.iter().position(|c| c == x).unwrap();
let c = s.remove(i);
s.insert(0, c);
o.push(i);
(o, s)
})
.0
}
fn decode(input: &[usize]) -> String {
input
.iter()
.fold((Vec::new(), get_symbols()), |(mut o, mut s), x| {
o.push(s[*x]);
let c = s.remove(*x);
s.insert(0, c);
(o, s)
})
.0
.into_iter()
.map(|c| c as char)
.collect()
}</lang>
- Output:
broood encodes to [1, 17, 15, 0, 0, 5] decodes to broood bananaaa encodes to [1, 1, 13, 1, 1, 1, 0, 0] decodes to bananaaa hiphophiphop encodes to [7, 8, 15, 2, 15, 2, 2, 3, 2, 2, 3, 2] decodes to hiphophiphop
Scala
<lang scala>package rosetta
import scala.annotation.tailrec
object MoveToFront {
/** * Default radix */ private val R = 256 /** * Default symbol table */ private def symbolTable = (0 until R).map(_.toChar).mkString
/**
* Apply move-to-front encoding using default symbol table.
*/
def encode(s: String): List[Int] = {
encode(s, symbolTable)
}
/**
* Apply move-to-front encoding using symbol table symTable.
*/
def encode(s: String, symTable: String): List[Int] = {
val table = symTable.toCharArray
@inline @tailrec def moveToFront(ch: Char, index: Int, tmpout: Char): Int = {
val tmpin = table(index)
table(index) = tmpout
if (ch != tmpin)
moveToFront(ch, index + 1, tmpin)
else {
table(0) = ch
index
}
}
@tailrec def encodeString(output: List[Int], s: List[Char]): List[Int] = s match {
case Nil => output
case x :: xs => {
encodeString(moveToFront(x, 0, table(0)) :: output, s.tail)
}
}
encodeString(Nil, s.toList).reverse
}
/**
* Apply move-to-front decoding using default symbol table.
*/
def decode(ints: List[Int]): String = {
decode(ints, symbolTable)
}
/**
* Apply move-to-front decoding using symbol table symTable.
*/
def decode(lst: List[Int], symTable: String): String = {
val table = symTable.toCharArray
@inline def moveToFront(c: Char, index: Int) {
for (i <- index-1 to 0 by -1)
table(i+1) = table(i)
table(0) = c
}
@tailrec def decodeList(output: List[Char], lst: List[Int]): List[Char] = lst match {
case Nil => output
case x :: xs => {
val c = table(x)
moveToFront(c, x)
decodeList(c :: output, xs)
}
}
decodeList(Nil, lst).reverse.mkString
}
def test(toEncode: String, symTable: String) {
val encoded = encode(toEncode, symTable) println(toEncode + ": " + encoded) val decoded = decode(encoded, symTable) if (toEncode != decoded) print("in") println("correctly decoded to " + decoded) } }
/**
* Unit tests the MoveToFront data type. */
object RosettaCodeMTF extends App { val symTable = "abcdefghijklmnopqrstuvwxyz" MoveToFront.test("broood", symTable) MoveToFront.test("bananaaa", symTable) MoveToFront.test("hiphophiphop", symTable) }</lang>
- Output:
broood: List(1, 17, 15, 0, 0, 5) correctly decoded to broood bananaaa: List(1, 1, 13, 1, 1, 1, 0, 0) correctly decoded to bananaaa hiphophiphop: List(7, 8, 15, 2, 15, 2, 2, 3, 2, 2, 3, 2) correctly decoded to hiphophiphop
Sidef
Implemented using regular expressions: <lang ruby>func encode(str) {
var table = ('a'..'z' -> join);
str.chars.map { |c|
var s = ;
table.sub!(Regex('(.*?)' + c), {|s1| s=s1; c + s1});
s.len;
}
}
func decode(nums) {
var table = ('a'..'z' -> join);
nums.map { |n|
var s = ;
table.sub!(Regex('(.{' + n + '})(.)'), {|s1, s2| s=s2; s2 + s1});
s;
}.join;
}
%w(broood bananaaa hiphophiphop).each { |test|
var encoded = encode(test);
say "#{test}: #{encoded}";
var decoded = decode(encoded);
print "in" if (decoded != test);
say "correctly decoded to #{decoded}";
}</lang>
Alternatively, implemented as a module, using arrays: <lang ruby>module MoveToFront {
define ABC = @("a".."z")
func m2f(ar,i) {
[ar.delete_index(i)] + ar
}
func encode(str) {
var ar = ABC+[]
gather {
str.each_char { |char|
take(var i = ar.index(char))
ar = m2f(ar, i);
}
}
}
func decode(indices) {
var ar = ABC+[]
gather {
indices.each { |i|
take ar[i];
ar = m2f(ar, i)
}
}.join
}
}
%w(broood bananaaa hiphophiphop).each { |test|
var encoded = MoveToFront::encode(test);
say "#{test}: #{encoded}";
var decoded = MoveToFront::decode(encoded);
print "in" if (decoded != test);
say "correctly decoded to #{decoded}";
}</lang>
- Output:
broood: 1 17 15 0 0 5 correctly decoded to broood bananaaa: 1 1 13 1 1 1 0 0 correctly decoded to bananaaa hiphophiphop: 7 8 15 2 15 2 2 3 2 2 3 2 correctly decoded to hiphophiphop
Swift
<lang swift>
var str="broood" var number:[Int]=[1,17,15,0,0,5]
//function to encode the string func encode(st:String)->[Int] {
var array:[Character]=["a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"]
var num:[Int]=[] var temp:Character="a" var i1:Int=0 for i in st.characters { for j in 0...25 { if i==array[j] { num.append(j) temp=array[j] i1=j while(i1>0) { array[i1]=array[i1-1] i1=i1-1
} array[0]=temp }
}
}
return num
}
func decode(s:[Int])->[Character] {
var st1:[Character]=[] var alph:[Character]=["a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"] var temp1:Character="a" var i2:Int=0 for i in 0...s.character.count-1 { i2=s[i] st1.append(alph[i2]) temp1=alph[i2]
while(i2>0) { alph[i2]=alph[i2-1] i2=i2-1
} alph[0]=temp1
} return st1
}
var encarr:[Int]=encode(st:str) var decarr:[Character]=decode(s:number) print(encarr) print(decarr) </lang>
Tcl
<lang tcl>package require Tcl 8.6
oo::class create MoveToFront {
variable symbolTable
constructor {symbols} {
set symbolTable [split $symbols ""]
}
method MoveToFront {table index} {
list [lindex $table $index] {*}[lreplace $table $index $index]
}
method encode {text} {
set t $symbolTable set r {} foreach c [split $text ""] { set i [lsearch -exact $t $c] lappend r $i set t [my MoveToFront $t $i] } return $r
}
method decode {numbers} {
set t $symbolTable set r "" foreach n $numbers { append r [lindex $t $n] set t [my MoveToFront $t $n] } return $r
}
}
MoveToFront create mtf "abcdefghijklmnopqrstuvwxyz" foreach tester {"broood" "bananaaa" "hiphophiphop"} {
set enc [mtf encode $tester] set dec [mtf decode $enc] puts [format "'%s' encodes to %s. This decodes to '%s'. %s" \
$tester $enc $dec [expr {$tester eq $dec ? "Correct!" : "WRONG!"}]] }</lang>
- Output:
'broood' encodes to 1 17 15 0 0 5. This decodes to 'broood'. Correct! 'bananaaa' encodes to 1 1 13 1 1 1 0 0. This decodes to 'bananaaa'. Correct! 'hiphophiphop' encodes to 7 8 15 2 15 2 2 3 2 2 3 2. This decodes to 'hiphophiphop'. Correct!
VBScript
<lang vb>Function mtf_encode(s) 'create the array list and populate it with the initial symbol position Set symbol_table = CreateObject("System.Collections.ArrayList") For j = 97 To 122 'a to z in decimal. symbol_table.Add Chr(j) Next output = "" For i = 1 To Len(s) char = Mid(s,i,1) If i = Len(s) Then output = output & symbol_table.IndexOf(char,0) symbol_table.RemoveAt(symbol_table.LastIndexOf(char)) symbol_table.Insert 0,char Else output = output & symbol_table.IndexOf(char,0) & " " symbol_table.RemoveAt(symbol_table.LastIndexOf(char)) symbol_table.Insert 0,char End If Next mtf_encode = output End Function
Function mtf_decode(s) 'break the function argument into an array code = Split(s," ") 'create the array list and populate it with the initial symbol position Set symbol_table = CreateObject("System.Collections.ArrayList") For j = 97 To 122 'a to z in decimal. symbol_table.Add Chr(j) Next output = "" For i = 0 To UBound(code) char = symbol_table(code(i)) output = output & char If code(i) <> 0 Then symbol_table.RemoveAt(symbol_table.LastIndexOf(char)) symbol_table.Insert 0,char End If Next mtf_decode = output End Function
'Testing the functions wordlist = Array("broood","bananaaa","hiphophiphop") For Each word In wordlist WScript.StdOut.Write word & " encodes as " & mtf_encode(word) & " and decodes as " &_ mtf_decode(mtf_encode(word)) & "." WScript.StdOut.WriteBlankLines(1) Next</lang>
- Output:
broood encodes as 1 17 15 0 0 5 and decodes as broood. bananaaa encodes as 1 1 13 1 1 1 0 0 and decodes as bananaaa. hiphophiphop encodes as 7 8 15 2 15 2 2 3 2 2 3 2 and decodes as hiphophiphop.
Wren
<lang ecmascript>import "/fmt" for Fmt import "/seq" for Lst
var encode = Fn.new { |s|
if (s.isEmpty) return []
var symbols = "abcdefghijklmnopqrstuvwxyz".toList
var result = List.filled(s.count, 0)
var i = 0
for (c in s) {
var index = Lst.indexOf(symbols, c)
if (index == -1) Fiber.abort("%(s) contains a non-alphabetic character")
result[i] = index
if (index > 0) {
for (j in index-1..0) symbols[j + 1] = symbols[j]
symbols[0] = c
}
i = i + 1
}
return result
}
var decode = Fn.new { |a|
if (a.isEmpty) return ""
var symbols = "abcdefghijklmnopqrstuvwxyz".toList
var result = List.filled(a.count, "")
var i = 0
for (n in a) {
if (n < 0 || n > 25) Fiber.abort("%(a) contains an invalid number")
result[i] = symbols[n]
if (n > 0) {
for (j in n-1..0) symbols[j + 1] = symbols[j]
symbols[0] = result[i]
}
i = i + 1
}
return result.join()
}
var strings = ["broood", "bananaaa", "hiphophiphop"] var encoded = List.filled(strings.count, null) var i = 0 for (s in strings) {
encoded[i] = encode.call(s)
Fmt.print("$-12s -> $n", s, encoded[i])
i = i + 1
} System.print() var decoded = List.filled(encoded.count, null) i = 0 for (a in encoded) {
decoded[i] = decode.call(a)
var correct = (decoded[i] == strings[i]) ? "correct" : "incorrect"
Fmt.print("$-38n -> $-12s -> $s", a, decoded[i], correct)
i = i + 1
}</lang>
- Output:
broood -> [1, 17, 15, 0, 0, 5] bananaaa -> [1, 1, 13, 1, 1, 1, 0, 0] hiphophiphop -> [7, 8, 15, 2, 15, 2, 2, 3, 2, 2, 3, 2] [1, 17, 15, 0, 0, 5] -> broood -> correct [1, 1, 13, 1, 1, 1, 0, 0] -> bananaaa -> correct [7, 8, 15, 2, 15, 2, 2, 3, 2, 2, 3, 2] -> hiphophiphop -> correct
zkl
<lang zkl>fcn encode(text){ //-->List
st:=["a".."z"].pump(Data); //"abcd..z" as byte array
text.reduce(fcn(st,c,sink){
n:=st.index(c); sink.write(n); st.del(n).insert(0,c); },st,sink:=L());
sink;
}</lang> Strings are immutable so we create a bit bucket (which is mutable) to hold the symbol table which can then be modified in place. <lang zkl>fcn decode(list){ //-->String
st:=["a".."z"].pump(String); //"abcd..z"
sink:=Sink(String);
list.reduce('wrap(st,n){ c:=st[n]; sink.write(c); c+st.del(n); },st);
sink.close();
}</lang> Here, we create a new symbol table each round as we would have to convert the byte we got from the bit bucket to string (so it is a wash garbage wise). <lang zkl>texts:=T("broood","bananaaa","hiphophiphop"); out:=texts.apply(encode); texts.zipWith(fcn(t,e){ println(t,"-->",e) },out);
out.apply(decode).println(); texts.zipWith('==,out.apply(decode)).println();</lang>
- Output:
broood-->L(1,17,15,0,0,5)
bananaaa-->L(1,1,13,1,1,1,0,0)
hiphophiphop-->L(7,8,15,2,15,2,2,3,2,2,3,2)
L("broood","bananaaa","hiphophiphop")
L(True,True,True)
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