Straddling checkerboard

Revision as of 04:52, 25 November 2016 by rosettacode>Craigd (→‎{{header|zkl}}: udpate)

Implement functions to encrypt and decrypt a message using the straddling checkerboard method. The checkerboard should take a 28 character alphabet (A-Z plus a full stop and an escape character) and two different numbers representing the blanks in the first row. The output will be a series of decimal digits.

Straddling checkerboard is a draft programming task. It is not yet considered ready to be promoted as a complete task, for reasons that should be found in its talk page.

Numbers should be encrypted by inserting the escape character before each digit, then including the digit unencrypted. This should be reversed for decryption.

ALGOL 68

Translation of: C++

Note: This specimen retains the original C++ coding style.

Works with: ALGOL 68 version Revision 1 - no extensions to language used.
Works with: ALGOL 68G version Any - tested with release 1.18.0-9h.tiny.

<lang algol68>#!/usr/local/bin/a68g --script #

PRIO MIN=5, MAX=5; OP MIN = (INT a, b)INT: (a<b|a|b),

  MAX = (INT a, b)INT: (a>b|a|b);

MODE STRADDLINGCHECKERBOARD=STRUCT(

 [0:9]CHAR first, second, third,
 [ABS ".": ABS "Z"]STRING table,
 INT row u, row v,
 CHAR esc, skip

);

STRUCT(

 PROC (REF STRADDLINGCHECKERBOARD #self#, STRING #alphabet#, INT #u#, INT #v#)VOID init,
 PROC (REF STRADDLINGCHECKERBOARD #self#, STRING #plain#)STRING encode,
 PROC (REF STRADDLINGCHECKERBOARD #self#, STRING #plain#)STRING decode

) sc class = (

  1. PROC init = # (REF STRADDLINGCHECKERBOARD self, STRING in alphabet, INT u, v)VOID:
 (
   STRING alphabet = in alphabet[@0];
   esc OF self := alphabet[UPB alphabet]; # use the last CHAR as the escape #
   skip OF self := alphabet[UPB alphabet-1];
   row u OF self := u MIN v;
   row v OF self := u MAX v;
   OP DIGIT = (INT i)CHAR: REPR(ABS "0" + i );
   INT j := LWB alphabet;
 # (first OF self)[u] := (first OF self)[v] := skip; #
   FOR i FROM LWB first OF self TO UPB first OF self  DO
     IF i NE u AND i NE v THEN
       (first OF self)[i] := alphabet[j];
       (table OF self)[ABS alphabet[j]] := DIGIT i;
       j+:=1
     FI;
     (second OF self)[i] := alphabet[i+8];
     (table OF self)[ABS alphabet[i+8]] := DIGIT (row u OF self) + DIGIT i;
     (third OF self)[i] := alphabet[i+18];
     (table OF self)[ABS alphabet[i+18]] := DIGIT (row v OF self) + DIGIT i
   OD
 ),
  1. PROC encode = # (REF STRADDLINGCHECKERBOARD self, STRING plain)STRING:
 (
   STRING esc = (table OF self)[ABS (esc OF self)];
   INT l2u = ABS "A" - ABS "a";
   STRING out := "";
   FOR i FROM LWB plain TO UPB plain DO
     CHAR c := plain[i];
     IF "a" <= c AND c <= "z" THEN
       c := REPR ( ABS c + l2u) FI;
     IF "A" <= c AND c <= "Z" THEN
       out +:= (table OF self)[ABS c]
     ELIF "0" <= c AND c <= "9" THEN
       out +:= esc + c
     FI
   OD;
   out # EXIT #
 ),
  1. PROC decode = # (REF STRADDLINGCHECKERBOARD self, STRING cipher)STRING:
 (
   CHAR null = REPR 0;
   STRING out;
   INT state := 0;
   FOR i FROM LWB cipher TO UPB cipher DO
     INT n := ABS cipher[i] - ABS "0";
     CHAR next :=
       CASE state IN
         #1:# (second OF self)[n],
         #2:# (third OF self)[n],
         #3:# cipher[i]
       OUT
         IF n = row u OF self THEN
           state := 1; null
         ELIF n = row v OF self THEN
           state := 2; null
         ELSE
           (first OF self)[n]
         FI
       ESAC;
     IF next = "/" THEN
       state := 3
     ELIF next NE null THEN
       state := 0;
       out +:= next
     FI
   OD;
   out # EXIT #
 )

);

main: (

 STRADDLINGCHECKERBOARD sc; (init OF sc class)(sc, "HOLMESRTABCDFGIJKNPQUVWXYZ./", 3, 7);
 STRING original := "One night-it was on the twentieth of March, 1888-I was returning"[@0];
 STRING en := (encode OF sc class)(sc, original);
 STRING de := (decode OF sc class)(sc, en);
 printf(($ggl$,
         "Original: ", original,
         "Encoded:  ", en,
         "Decoded:  ", de
 ))

)</lang> Output:

Original: One night-it was on the twentieth of March, 1888-I was returning
Encoded:  139539363509369743061399059745399365901344308320791798798798367430685972839363935
Decoded:  ONENIGHTITWASONTHETWENTIETHOFMARCH1888IWASRETURNING

AutoHotkey

<lang AutoHotkey>board := " ( ET AON RIS BCDFGHJKLM PQ/UVWXYZ. )" Text = One night-it was on the twentieth of March, 1888-I was returning StringUpper, Text, Text Text := RegExReplace(text, "[^A-Z0-9]") Num2 := InStr(board, A_Space) -1 Num3 := InStr(board, A_Space, true, Num1+1) -1 Loop Parse, Text { char := A_LoopField Loop Parse, board, `n { If (Pos := InStr(A_LoopField, char)) out .= Num%A_Index% . Pos-1 else if (Pos := InStr(A_LoopField, "/")) && InStr("0123456789", char) out .= Num%A_Index% . Pos-1 . char } } MsgBox % out

i := 0 While (LoopField := SubStr(out, ++i, 1)) <> "" { If (LoopField = num2) or (LoopField = num3) col := SubStr(out, ++i, 1) else col := 0 Loop Parse, board, `n { If !col && A_Index = 1 dec .= SubStr(A_LoopField, LoopField+1, 1) Else If (Num%A_Index% = LoopField) && col dec .= SubStr(A_LoopField, col+1, 1) If SubStr(dec, 0) = "/" dec := SubStr(dec, 1, StrLen(dec)-1) . SubStr(out, ++i, 1) } } MsgBox % dec</lang>

C

Library: GLib

<lang c>#include <stdio.h>

  1. include <stdlib.h>
  2. include <string.h>
  3. include <stdbool.h>
  4. include <ctype.h>
  5. include <glib.h>
  1. define ROWS 4
  2. define COLS 10
  3. define NPRX "/"

/* wikipedia table const char *table[ROWS][COLS] = {

 { "0", "1", "2",  "3", "4", "5", "6",  "7", "8", "9" },
 { "E", "T", NULL, "A", "O", "N", NULL, "R", "I", "S  },
 { "B", "C", "D",  "F", "G", "H", "J",  "K", "L", "M" },
 { "P", "Q", NPRX, "U", "V", "W", "X",  "Y", "Z", "." }

};

  • /

/* example of extending the table, COLS must be 11 const char *table[ROWS][COLS] = {

 { "0", "1", "2", "3",  "4", "5", "6", "7",  "8", "9",  ":"  },
 { "H", "O", "L", NULL, "M", "E", "S", NULL, "R", "T",  ","  },
 { "A", "B", "C", "D",  "F", "G", "I", "J",  "K", "N",  "-"  },
 { "P", "Q", "U", "V",  "W", "X", "Y", "Z",  ".", NPRX, "?"  }

};

  • /

// task table const char *table[ROWS][COLS] = {

 { "0", "1", "2", "3",  "4", "5", "6", "7",  "8", "9"  },
 { "H", "O", "L", NULL, "M", "E", "S", NULL, "R", "T"  },
 { "A", "B", "C", "D",  "F", "G", "I", "J",  "K", "N"  },
 { "P", "Q", "U", "V",  "W", "X", "Y", "Z",  ".", NPRX }

};


GHashTable *create_table_from_array(const char *table[ROWS][COLS], bool is_encoding) {

 char buf[16];
 GHashTable *r = g_hash_table_new_full(g_str_hash, g_str_equal, free, free);
 size_t i, j, k, m;
 for(i = 0, m = 0; i < COLS; i++)
 {
   if (table[1][i] == NULL) m++;
 }
 const size_t SELNUM = m;
 
 size_t selectors[SELNUM];
 size_t numprefix_row, numprefix_col;
 bool has_numprefix = false;
 // selectors keep the indexes of the symbols to select 2nd and 3rd real row;
 // nulls must be placed into the 2nd row of the table
 for(i = 0, k = 0; i < COLS && k < SELNUM; i++)
 {
   if ( table[1][i] == NULL )
   {
     selectors[k] = i;
     k++;
   }
 }
 // numprefix is the prefix to insert symbols from the 1st row of table (numbers)
 for(j = 1; j < ROWS; j++)
 {
   for(i = 0; i < COLS; i++)
   {
     if (table[j][i] == NULL) continue;
     if ( strcmp(table[j][i], NPRX) == 0 )
     {
       numprefix_col = i;
       numprefix_row = j;
       has_numprefix = true;

break;

     }
   }
 }
 // create the map for each symbol
 for(i = has_numprefix ? 0 : 1; i < ROWS; i++)
 {
   for(j = 0; j < COLS; j++)
   {
     if (table[i][j] == NULL) continue;
     if (strlen(table[i][j]) > 1)
     {

fprintf(stderr, "symbols must be 1 byte long\n"); continue; // we continue just ignoring the issue

     }
     if (has_numprefix && i == (ROWS-1) && j == numprefix_col && i == numprefix_row) continue;
     if (has_numprefix && i == 0)
     {

snprintf(buf, sizeof(buf), "%s%s%s", table[0][selectors[SELNUM-1]], table[0][numprefix_col], table[0][j]);

     }
     else if (i == 1)
     {

snprintf(buf, sizeof(buf), "%s", table[0][j]);

     }
     else
     {

snprintf(buf, sizeof(buf), "%s%s", table[0][selectors[i-2]], table[0][j]);

     }
     if (is_encoding) g_hash_table_insert(r, strdup(table[i][j]), strdup(buf));
     else g_hash_table_insert(r, strdup(buf), strdup(table[i][j]));
   }
 }
 if (is_encoding) g_hash_table_insert(r, strdup("mode"), strdup("encode"));
 else g_hash_table_insert(r, strdup("mode"), strdup("decode"));
 return r;

}

char *decode(GHashTable *et, const char *enctext) {

 char *r = NULL;
 if (et == NULL || enctext == NULL || strlen(enctext) == 0 ||
     g_hash_table_lookup(et, "mode") == NULL ||
     strcmp(g_hash_table_lookup(et, "mode"), "decode") != 0) return NULL;
 GString *res = g_string_new(NULL);
 GString *en = g_string_new(NULL);
 for( ; *enctext != '\0'; enctext++ )
 {
   if (en->len < 3)
   {
     g_string_append_c(en, *enctext);
     r = g_hash_table_lookup(et, en->str);
     if (r == NULL) continue;
     g_string_append(res, r);
     g_string_truncate(en, 0);
   }
   else
   {
     fprintf(stderr, "decoding error\n");
     break;
   }
 }
 
 r = res->str;
 g_string_free(res, FALSE);
 g_string_free(en, TRUE);
 return r;

}

char *encode(GHashTable *et, const char *plaintext, int (*trasf)(int), bool compress_spaces) {

 GString *s;
 char *r = NULL;
 char buf[2] = { 0 };
 if (plaintext == NULL ||
     et == NULL || g_hash_table_lookup(et, "mode") == NULL ||
     strcmp(g_hash_table_lookup(et, "mode"), "encode") != 0) return NULL;
 s = g_string_new(NULL);
 for(buf[0] = trasf ? trasf(*plaintext) : *plaintext; 
     buf[0] != '\0'; 
     buf[0] = trasf ? trasf(*++plaintext) : *++plaintext)
 {
   if ( (r = g_hash_table_lookup(et, buf)) != NULL )
   {
     g_string_append(s, r);
   }
   else if (isspace(buf[0])) 
   {
     if (!compress_spaces) g_string_append(s, buf);
   } 
   else
   {
     fprintf(stderr, "char '%s' is not encodable%s\n",

isprint(buf[0]) ? buf : "?", !compress_spaces ? ", replacing with a space" : "");

     if (!compress_spaces) g_string_append_c(s, ' ');
   }
 }
 r = s->str;
 g_string_free(s, FALSE);
 return r;

}


int main() {

 GHashTable *enctab = create_table_from_array(table, true);  // is encoding? true
 GHashTable *dectab = create_table_from_array(table, false); // is encoding? false (decoding)
 const char *text = "One night-it was on the twentieth of March, 1888-I was returning";
 char *encoded = encode(enctab, text, toupper, true);
 printf("%s\n", encoded);
 char *decoded = decode(dectab, encoded);
 printf("%s\n", decoded);
 free(decoded);
 free(encoded);
 g_hash_table_destroy(enctab);
 g_hash_table_destroy(dectab);
 return 0;

}</lang>

Shorter version

<lang C>#include <stdio.h>

  1. include <stdlib.h>

const char * board = "ET AON RIS"

                     "BCDFGHJKLM"
                     "PQ/UVWXYZ.";

char encode[128] = {0}; char decode[128] = {0}; int row[2] = {0};

void read_table(const char *s) {

       int i, code;
       for (i = 0; i < 30; i++) {
               if (s[i] == '\0') {
                       fprintf(stderr, "Table too short\n");
                       exit(1);
               }
               if (s[i] == ' ') {
                       row[  row[0] ? 1 : 0 ] = i;
                       continue;
               }
               code = ((i < 10) ? 0 : i < 20 ? row[0] : row[1])
                               * 10 + (i % 10);
               encode[0 + s[i]] = code; /* guess what 0 + s[i] does, sigh */
               decode[code] = s[i];
       }

}

void encipher(const char *in, char *out, int strip) {

  1. define PUTCODE(c) { if (c > 9) {*(out++) = c / 10 + '0'; c %= 10;} *(out++) = c + '0'; }
       int c, code;
       while ((c = *(in++)) != '\0') {
               if (c >= '0' && c <= '9') {
                       code = encode['.'];
                       c -= '0';
                       PUTCODE(code);
                       PUTCODE(c);
                       continue;
               }
               c &= ~0x20;
               if (c >= 'A' && c <= 'Z') code = encode[c];
               else if (strip && !c )    continue;
               else                      code = encode['/'];
               PUTCODE(code);
       }
       *(out++) = '\0';

}

void decipher(const char *in, char *out, int strip) {

       int c;
       while ((c = *(in++)) != '\0') {
               c -= '0';
               if (c == row[0] || c == row[1])
                       c = c * 10 + *(in++) - '0';
               c = decode[c];
               if (c == '.') c = *(in++);
               if (c == '/' && !strip) c = ' ';
               *(out++) = c;
       }
       *(out++) = '\0';

}

int main() {

       const char *msg = "In the winter 1965/we were hungry/just barely alive";
       char enc[100] = {0}, dec[100] = {0};
       read_table(board);
       printf("message: %s\n", msg);
       encipher(msg, enc, 0); printf("encoded: %s\n", enc);
       decipher(enc, dec, 0); printf("decoded: %s\n", dec);
       printf("\nNo spaces:\n");
       encipher(msg, enc, 1); printf("encoded: %s\n", enc);
       decipher(enc, dec, 1); printf("decoded: %s\n", dec);
       return 0;

}</lang>Output:<lang>message: In the winter 1965/we were hungry/just barely alive encoded: 85621250626585107626916996966956265062650706225635247676226639162203702867623288640 decoded: IN THE WINTER 1965 WE WERE HUNGRY JUST BARELY ALIVE

No spaces: encoded: 851250658510769169969669562650650702563524767622663912037028673288640 decoded: INTHEWINTER1965/WEWEREHUNGRY/JUSTBARELYALIVE</lang>

C++

<lang cpp>#include <iostream>

  1. include <string>
  2. include <map>
  3. include <algorithm> // for min, max

using namespace std;

class StraddlingCheckerboard {

 map<char, string> table;
 char first[10], second[10], third[10];
 int rowU, rowV;

public:

 StraddlingCheckerboard(const string &alphabet, int u, int v)
 {
   rowU = min(u, v);
   rowV = max(u, v);
   for(int i = 0, j = 0; i < 10; ++i)
   {
     if(i != u && i != v)
     {
       first[i] = alphabet[j];
       table[alphabet[j]] = '0' + i;
       ++j;
     }
     second[i] = alphabet[i+8];
     table[alphabet[i+8]] = '0' + rowU;
     table[alphabet[i+8]] += '0' + i;
     third[i] = alphabet[i+18];
     table[alphabet[i+18]] = '0' + rowV;
     table[alphabet[i+18]] += '0' + i;
   }
 }
 string encode(const string &plain)
 {
   string out;
   for(int i = 0; i < plain.size(); ++i)
   {
     char c = plain[i];
     if(c >= 'a' && c <= 'z')
       c += 'A' - 'a';
     if(c >= 'A' && c <= 'Z')
       out += table[c];
     else if(c >= '0' && c <= '9')
     {
       out += table['/'];
       out += c;
     }
   }
   return out;
 }
 string decode(const string &cipher)
 {
   string out;
   int state = 0;
   for(int i = 0; i < cipher.size(); ++i)
   {
     int n = cipher[i] - '0';
     char next = 0;
     if(state == 1)
       next = second[n];
     else if(state == 2)
       next = third[n];
     else if(state == 3)
       next = cipher[i];
     else if(n == rowU)
       state = 1;
     else if(n == rowV)
       state = 2;
     else
       next = first[n];
     if(next == '/')
       state = 3;
     else if(next != 0)
     {
       state = 0;
       out += next;
     }
   }
   return out;
 }

};</lang>

Test program: <lang cpp>int main() {

 StraddlingCheckerboard sc("HOLMESRTABCDFGIJKNPQUVWXYZ./", 3, 7);
 
 string original = "One night-it was on the twentieth of March, 1888-I was returning";
 string en = sc.encode(original);
 string de = sc.decode(en);
 cout << "Original: " << original << endl;
 cout << "Encoded:  " << en << endl;
 cout << "Decoded:  " << de << endl;
 return 0;

}</lang>

Output:

Original: One night-it was on the twentieth of March, 1888-I was returning
Encoded:  139539363509369743061399059745399365901344308320791798798798367430685972839363935
Decoded:  ONENIGHTITWASONTHETWENTIETHOFMARCH1888IWASRETURNING

D

Partially based on the PicoLisp version: <lang d>import std.stdio, std.algorithm, std.string, std.array;

immutable T = ["79|0|1|2|3|4|5|6|7|8|9", "|H|O|L||M|E|S||R|T",

              "3|A|B|C|D|F|G|I|J|K|N", "7|P|Q|U|V|W|X|Y|Z|.|/"]
             .map!(r => r.split("|")).array;

enum straddle = (in string s) pure /*nothrow @safe*/ =>

   toUpper(s)
   .split("")
   .cartesianProduct(T)
   .filter!(cL => cL[1].canFind(cL[0]))
   .map!(cL => cL[1][0] ~ T[0][cL[1].countUntil(cL[0])])
   .join;

string unStraddle(string s) pure nothrow @safe {

   string result;
   for (; !s.empty; s.popFront) {
       immutable i = [T[2][0], T[3][0]].countUntil([s[0]]);
       if (i >= 0) {
           s.popFront;
           immutable n = T[2 + i][T[0].countUntil([s[0]])];
           if (n == "/") {
               s.popFront;
               result ~= s[0];
           } else result ~= n;
       } else
           result ~= T[1][T[0].countUntil([s[0]])];
   }
   return result;

}

void main() {

   immutable O = "One night-it was on the twentieth of March, 1888-I was returning";
   writeln("Encoded: ", O.straddle);
   writeln("Decoded: ", O.straddle.unStraddle);

}</lang>

Output:
Encoded: 139539363509369743061399059745399365901344308320791798798798367430685972839363935
Decoded: ONENIGHTITWASONTHETWENTIETHOFMARCH1888IWASRETURNING

Alternative Version

Translation of: C++

Same output: <lang d>import std.stdio, std.algorithm, std.ascii;

struct StraddlingCheckerboard {

 private:
   string[char] table;
   char[10] first, second, third;
   const int rowU, rowV;
 public:
   this(in string alphabet, in int u, in int v) pure nothrow {
       rowU = min(u, v);
       rowV = max(u, v);
       int j = 0;
       foreach (immutable i; 0 .. 10) {
           if (i != u && i != v) {
               first[i] = alphabet[j];
               table[alphabet[j]] = digits[i .. i + 1];
               j++;
           }
           second[i] = alphabet[i + 8];
           table[alphabet[i + 8]] = [digits[rowU], digits[i]];
           third[i] = alphabet[i + 18];
           table[alphabet[i + 18]] = [digits[rowV], digits[i]];
       }
   }
   string encode(in string plain) const pure nothrow {
       string r;
       foreach (immutable char c; plain) {
           if      (c.isLower) r ~= table[c.toUpper];
           else if (c.isUpper) r ~= table[c];
           else if (c.isDigit) r ~= table['/'] ~ c;
       }
       return r;
   }
   string decode(in string cipher) const pure nothrow {
       string r;
       int state = 0;
       foreach (immutable char c; cipher) {
           immutable int n = c - '0';
           char next = '\0';
           if (state == 1)      next = second[n];
           else if (state == 2) next = third[n];
           else if (state == 3) next = c;
           else if (n == rowU)  state = 1;
           else if (n == rowV)  state = 2;
           else                 next = first[n];
           if (next == '/')
               state = 3;
           else if (next != 0) {
               state = 0;
               r ~= next;
           }
       }
       return r;
   }

}

void main() {

   immutable orig =
   "One night-it was on the twentieth of March, 1888-I was returning";
   writeln("Original: ", orig);
   const sc = StraddlingCheckerboard("HOLMESRTABCDFGIJKNPQUVWXYZ./",
                                     3, 7);
   const en = sc.encode(orig);
   writeln("Encoded:  ", en);
   writeln("Decoded:  ", sc.decode(en));

}</lang>

Dictionary-Based Version

Translation of: Java

<lang d>import std.stdio, std.string, std.algorithm, std.regex, std.array, std.range, std.typecons;

immutable string[const string] val2key, key2val;

static this() pure /*nothrow @safe*/ {

   immutable aa = ["A":"30", "B":"31", "C":"32", "D":"33", "E":"5", "F":"34", "G":"35",
       "H":"0", "I":"36", "J":"37", "K":"38", "L":"2", "M":"4", ".":"78", "N":"39",
       "/":"79", "O":"1", "0":"790", "P":"70", "1":"791", "Q":"71", "2":"792",
       "R":"8", "3":"793", "S":"6", "4":"794", "T":"9", "5":"795", "U":"72",
       "6":"796", "V":"73", "7":"797", "W":"74", "8":"798", "X":"75", "9":"799",
       "Y":"76", "Z":"77"];
   val2key = aa;
   key2val = aa.byKeyValue.map!(t => tuple(t.value, t.key)).assocArray;

}

string encode(in string s) pure /*nothrow*/ @safe {

   return s.toUpper.split("").map!(c => val2key.get(c, "")).join;

}

string decode(in string s) /*pure nothrow*/ @safe {

   return s.matchAll("79.|3.|7.|.").map!(g => key2val.get(g[0], "")).join;

}

void main() @safe {

   immutable s = "One night-it was on the twentieth of March, 1888-I was returning";
   s.encode.writeln;
   s.encode.decode.writeln;

}</lang>

Output:
139539363509369743061399059745399365901344308320791798798798367430685972839363935
ONENIGHTITWASONTHETWENTIETHOFMARCH1888IWASRETURNING

Go

<lang go>package main

import (

   "fmt"
   "strings"

)

func main() {

   key := `
8752390146
ET AON RIS

5BC/FGHJKLM 0PQD.VWXYZU`

   p := "you have put on 7.5 pounds since I saw you."
   fmt.Println(p)
   c := enc(key, p)
   fmt.Println(c)
   fmt.Println(dec(key, c))

}

func enc(bd, pt string) (ct string) {

   enc := make(map[byte]string)
   row := strings.Split(bd, "\n")[1:]
   r2d := row[2][:1]
   r3d := row[3][:1]
   for col := 1; col <= 10; col++ {
       d := string(row[0][col])
       enc[row[1][col]] = d
       enc[row[2][col]] = r2d+d
       enc[row[3][col]] = r3d+d
   }
   num := enc['/']
   delete(enc, '/')
   delete(enc, ' ')
   for i := 0; i < len(pt); i++ {
       if c := pt[i]; c <= '9' && c >= '0' {
           ct += num + string(c)
       } else {
           if c <= 'z' && c >= 'a' {
               c -= 'a'-'A'
           }
           ct += enc[c]
       }
   }
   return

}

func dec(bd, ct string) (pt string) {

   row := strings.Split(bd, "\n")[1:]
   var cx [10]int
   for i := 1; i <= 10; i++ {
       cx[row[0][i]-'0'] = i
   }
   r2d := row[2][0]-'0'
   r3d := row[3][0]-'0'
   for i := 0; i < len(ct); i++ {
       var r int
       switch d := ct[i]-'0'; d {
       case r2d:
           r = 2
       case r3d:
           r = 3
       default:
           pt += string(row[1][cx[d]])
           continue
       }
       i++
       if b := row[r][cx[ct[i]-'0']]; b == '/' {
           i++
           pt += string(ct[i])
       } else {
           pt += string(b)
       }
   }
   return

}</lang>

Output:
you have put on 7.5 pounds since I saw you.
01306592038080673955702555083069056649578462090130602
YOUHAVEPUTON7.5POUNDSSINCEISAWYOU.

Icon and Unicon

<lang Icon>procedure main() StraddlingCheckerBoard("setup","HOLMESRTABCDFGIJKNPQUVWXYZ./", 3,7)

text := "One night. it was on the twentieth of March, 1888. I was returning" write("text = ",image(text)) write("encode = ",image(en := StraddlingCheckerBoard("encode",text))) write("decode = ",image(StraddlingCheckerBoard("decode",en))) end

procedure StraddlingCheckerBoard(act,text,b1,b2) static SCE,SCD case act of {

  "setup" : {  
     if (b1 < b2 < 10) & (*text = *cset(text) = 28) then {
        SCE := table("")   
        SCD := table()         
        esc := text[-1]                               # escape
        every text[(b1|b2)+1+:0] := " "               # blanks
        uix := ["",b1,b2]                             # 1st position
        every c := text[1 + (i := 0 to *text-1)] do   # build translation
           if c ~== " " then                          # skip blanks
              SCD[SCE[c] := SCE[map(c)] := uix[i/10+1]||(i%10) ] := c         
        every c := !&digits do 
           SCD[SCE[c] := SCE[esc] || c] := c
        delete(SCD,SCE[esc])
        delete(SCE,esc)          
        }
        else stop("Improper setup: ",image(text),", ",b1,", ",b2)
     }
  "encode" : {
     every (s := "") ||:= x := SCE[c := !text]
     return s
     }
  "decode" : {  
     s := ""
     text ? until pos(0) do 
        s ||:= \SCD[k := move(1 to 3)]
     return s       
     }
  }

end</lang>

Output:

text   = "One night. it was on the twentieth of March, 1888. I was returning"
encode = "1395393635097836974306139905974539936590134430832079179879879878367430685972839363935"
decode = "ONENIGHT.ITWASONTHETWENTIETHOFMARCH1888.IWASRETURNING"

J

Solution: <lang j>'Esc Stop'=: '/.' 'Nums Alpha'=: '0123456789';'ABCDEFGHIJKLMNOPQRSTUVWXYZ' Charset=: Nums,Alpha,Stop

escapenum=: (,@:((; Esc&,)&>) Nums) rplc~ ] NB. escape numbers unescapenum=: ((, ; ' '&,@])"1 0 Nums"_) rplc~ ] NB. unescape coded numbers (x is escape code, y is cipher) expandKeyatUV=: 0:`[`(1 #~ 2 + #@])} #inv ] makeChkBrd=: Nums , expandKeyatUV

chkbrd=: conjunction define

 'uv key'=. n
 board=. uv makeChkBrd key
 select. m
 case. 0 do.                                                      NB. encode
   digits=. board 10&#.inv@i. escapenum y
   ' ' -.~ ,(":@{:"1 digits) ,.~ (1 1 0 2{":uv) {~ {."1 digits
 case. 1 do.                                                      NB. decode
   esc=. 0 chkbrd (uv;key) Esc                                    NB. find code for Esc char
   tmp=. esc unescapenum esc,'0',y                                
   tmp=. ((":uv) ((-.@e.~ _1&|.) *. e.~) tmp) <;.1 tmp            NB. box on chars from rows 0 2 3
   idx=. (}. ,~ (1 1 0 2{":uv) ":@i. {.) each tmp                 NB. recreate indices for rows 0 2 3
   idx=. ;(2&{. , [: ((0 1 $~ +:@#) #inv!.'1' ]) 2&}.) each idx   NB. recreate indices for row 1
   }.board {~ _2 (_&".)\ idx    
 end.

)</lang> Example usage: <lang j> preprocess=: (#~ Charset e.~ ])@toupper NB. verb to compress out non-alphanumerics

  chkbrdRC=: chkbrd (3 7;'HOLMESRTABCDFGIJKNPQUVWXYZ./')  NB. define adverb by applying Rosetta Code key to chkbrd conjunction
  0 chkbrdRC preprocess 'One night-it was on the twentieth of March, 1888-I was returning'

139539363509369743061399059745399365901344308320791798798798367430685972839363935

  1 chkbrdRC 0 chkbrdRC preprocess 'One night-it was on the twentieth of March, 1888-I was returning'

ONENIGHTITWASONTHETWENTIETHOFMARCH1888IWASRETURNING</lang> Or using the rules proposed by Util on the discussion page: <lang j> preprocess=: Stop&([`([: I. Charset -.@e.~ ])`]} toupper) NB. replace all non-alphanumerics with Stop

  1 chkbrdRC 0 chkbrdRC preprocess 'One night-it was on the twentieth of March, 1888-I was returning'

ONE.NIGHT.IT.WAS.ON.THE.TWENTIETH.OF.MARCH..1888.I.WAS.RETURNING</lang>


Another method is to create a set of keys which correspond to the encoded letters in the alphabet. This allows for the relatively simple design of the encoding and decoding functions as they are essentially hash table lookups.

Solution: <lang j>NB. stops setcode alphabet NB. creates verbs encode and decode which change between unencoded text and lists of encoded numbers

  setcode=: 3 :0

2 6 setcode y

alphabet=: y, ,":"0 i.10 stops=. x alphkeys=. (a: , ,&.> x) ([ -.~ [: , ,&.>/) i.10 esckey=. >(alphabet i. '/'){alphkeys numkeys=. esckey&,&.> i.10 keys=. alphkeys,numkeys

encode=: ([: ; keys {~ alphabet&i.) :. decode break=. </.~ i.@# - _1|. ([: >/\.&.|. e.&stops) + _1|.2*esckey&E. decode=: (alphabet {~ keys i. break f.) :. encode i.0 0 )</lang>

Example usage: <lang j> 3 7 setcode 'HOLMESRTABCDFGIJKNPQUVWXYZ./'

  preprocess=: (#~ alphabet e.~ ])@toupper
  ,":"0 encode message=: preprocess 'One night-it was on the twentieth of March, 1888-I was returning'

139539363509369743061399059745399365901344308320791798798798367430685972839363935

  decode encode message

ONENIGHTITWASONTHETWENTIETHOFMARCH1888IWASRETURNING

  ]s=. ((10|+) 0 4 5 2$~$)&.encode message NB. scramble by taking a modular sum with 0 4 5 2 while encoded

OWVKRNEOAMTMXROWOHTMTMTROTQ4SEMRRLRZLVSTTLLOROMHALSFOHECMRWESWEE

  ((10|-) 0 4 5 2$~$)&.encode s

ONENIGHTITWASONTHETWENTIETHOFMARCH1888IWASRETURNING</lang>

Java

Works with: Java version 7

<lang java>import java.util.Map; import java.util.regex.*;

public class StraddlingCheckerboard {

   final static String[] keyvals = {"H:0", "O:1", "L:2", "M:4", "E:5", "S:6",
       "R:8", "T:9", "A:30", "B:31", "C:32", "D:33", "F:34", "G:35", "I:36",
       "J:37", "K:38", "N:39", "P:70", "Q:71", "U:72", "V:73", "W:74", "X:75",
       "Y:76", "Z:77", ".:78", "/:79", "0:790", "1:791", "2:792", "3:793",
       "4:794", "5:795", "6:796", "7:797", "8:798", "9:799"};
   final static Map<String, String> val2key = new HashMap<>();
   final static Map<String, String> key2val = new HashMap<>();
   public static void main(String[] args) {
       for (String keyval : keyvals) {
           String[] kv = keyval.split(":");
           val2key.put(kv[0], kv[1]);
           key2val.put(kv[1], kv[0]);
       }
       String enc = encode("One night-it was on the twentieth of March, "
               + "1888-I was returning");
       System.out.println(enc);
       System.out.println(decode(enc));
   }
   static String encode(String s) {
       StringBuilder sb = new StringBuilder();
       for (String c : s.toUpperCase().split("")) {
           c = val2key.get(c);
           if (c != null)
               sb.append(c);
       }
       return sb.toString();
   }
   static String decode(String s) {
       Matcher m = Pattern.compile("(79.|3.|7.|.)").matcher(s);
       StringBuilder sb = new StringBuilder();
       while (m.find()) {
           String v = key2val.get(m.group(1));
           if (v != null)
               sb.append(v);
       }
       return sb.toString();
   }

}</lang>

139539363509369743061399059745399365901344308320791798798798367430685972839363935
ONENIGHTITWASONTHETWENTIETHOFMARCH1888IWASRETURNING

JavaScript

<lang javascript><script> var alphabet=new Array("ESTONIA R","BCDFGHJKLM","PQUVWXYZ./") // scramble alphabet as you wish var prefixes=new Array("",alphabet[0].indexOf(" "),alphabet[0].lastIndexOf(" "))

function straddle(message){

 var out=""
 message=message.toUpperCase()
 message=message.replace(/([0-9])/g,"/$1") // dumb way to escape numbers
 for(var i=0;i<message.length;i++){
   var chr=message[i]

if(chr==" ")continue for(var j=0;j<3;j++){ var k=alphabet[j].indexOf(chr) if(k<0)continue out+=prefixes[j].toString()+k } if(chr=="/")out+=message[++i]

 }
 return out

}

function unstraddle(message){

 var out=""
 var n,o
 for(var i=0;i<message.length;i++){

n=message[i]*1

   switch(n){

case prefixes[1]: o=alphabet[1][message[++i]];break case prefixes[2]: o=alphabet[2][message[++i]];break default: o=alphabet[0][n] } o=="/"?out+=message[++i]:out+=o

 }
 return out

}

str="One night-it was on the twentieth of March, 1888-I was returning." document.writeln(str) document.writeln(straddle(str)) document.writeln(unstraddle(straddle(str))) </script></lang>

Output:

One night-it was on the twentieth of March, 1888-I was returning.
34045747525284613427502840425027537379697175891898898898584619028294547488
ONENIGHTITWASONTHETWENTIETHOFMARCH1888IWASRETURNING.

Perl 6

The .trans method in Perl 6 improves on Perl 5's tr/// by allowing multi-character translation tables.

We build the full table during .new, which simplifies .encode and .decode. <lang perl6>class Straddling_Checkerboard {

   has @!flat_board; # 10x3 stored as 30x1
   has $!plain2code; # Full translation table; invertable
   has @!table;      # Printable layout, like Wikipedia entry

   my $numeric_escape = '/';
   my $exclude = /<-[A..Z0..9.]>/; # Omit the escape character
   method display_table { say ~ .list for @!table };
   method decode ( Str $s --> Str ) {
       $s.trans($!plain2code.invert);
   }
   method encode ( Str $s, :$collapse? --> Str ) {
       my $replace = $collapse ??  !! '.';
       $s.uc.subst( $exclude, $replace, :g ).trans($!plain2code);
   }
   submethod BUILD ( :$alphabet, :$u where 0..9, :$v where 0..9 ) {
       die if $u == $v;
       die if $alphabet.comb.sort.join ne [~] './', 'A'..'Z';
       @!flat_board = $alphabet.uc.comb;
       @!flat_board.splice( $u min $v, 0, Any );
       @!flat_board.splice( $u max $v, 0, Any );
       @!table = [ ' ',             [ 0 ..  9]               ],
                 [ ' ', @!flat_board[ 0 ..  9].map: * // ' ' ],
                 [ $u,  @!flat_board[10 .. 19]               ],
                 [ $v,  @!flat_board[20 .. 29]               ];
       my @order = 0..9; # This may be passed as a param in the future
       my @nums = @order,
                  @order.map({ +"$u$_" }),
                  @order.map({ +"$v$_" });
       my %p2c = @!flat_board Z=> @nums;
       %p2c.delete(Any);
       %p2c{$_} = %p2c{$numeric_escape} ~ $_ for 0..9;
       $!plain2code = [%p2c.keys] => [%p2c.values];
   }

}

sub MAIN ( :$u = 3, :$v = 7, :$alphabet = 'HOLMESRTABCDFGIJKNPQUVWXYZ./' ) {

   my Straddling_Checkerboard $sc .= new: :$u, :$v, :$alphabet;
   $sc.display_table;
   for 0..1 -> $collapse {
       my $original = 'One night-it was on the twentieth of March, 1888-I was returning';
       my $en = $sc.encode($original, :$collapse);
       my $de = $sc.decode($en);
       say;
       say "Original: $original";
       say "Encoded:  $en";
       say "Decoded:  $de";
   }

}</lang>

Output:

  0 1 2 3 4 5 6 7 8 9
  H O L   M E S   R T
3 A B C D F G I J K N
7 P Q U V W X Y Z . /

Original: One night-it was on the twentieth of March, 1888-I was returning
Encoded:  13957839363509783697874306781397890578974539936590781347843083207878791798798798783678743067885972839363935
Decoded:  ONE.NIGHT.IT.WAS.ON.THE.TWENTIETH.OF.MARCH..1888.I.WAS.RETURNING

Original: One night-it was on the twentieth of March, 1888-I was returning
Encoded:  139539363509369743061399059745399365901344308320791798798798367430685972839363935
Decoded:  ONENIGHTITWASONTHETWENTIETHOFMARCH1888IWASRETURNING

PicoLisp

<lang PicoLisp>(de *Straddling

  (NIL  "H"  "O"  "L"  NIL  "M"  "E"  "S"  NIL  "R"  "T")
  ("3"  "A"  "B"  "C"  "D"  "F"  "G"  "I"  "J"  "K"  "N")
  ("7"  "P"  "Q"  "U"  "V"  "W"  "X"  "Y"  "Z"  "."  "/")
  ("79" "0"  "1"  "2"  "3"  "4"  "5"  "6"  "7"  "8"  "9") )

(de straddle (Str)

  (pack
     (mapcar
        '((C)
           (pick
              '((L)
                 (and
                    (index C (cdr L))
                    (cons (car L) (dec @)) ) )
              *Straddling ) )
        (chop (uppc Str)) ) ) )

(de unStraddle (Str)

  (pack
     (make
        (for (L (chop Str)  L)
           (let C (pop 'L)
              (setq C
                 (if (assoc C *Straddling)
                    (get (cdr @) (inc (format (pop 'L))))
                    (get (cdar *Straddling) (inc (format C))) ) )
              (link (if (= "/" C) (pop 'L) C)) ) ) ) ) )</lang>

Output:

: (straddle "One night-it was on the twentieth of March, 1888-I was returning")
-> "139539363509369743061399059745399365901344308320791798798798367430685972839363935"

: (unStraddle @)
-> "ONENIGHTITWASONTHETWENTIETHOFMARCH1888IWASRETURNING"

PureBasic

<lang purebasic>Procedure.s encrypt_SC(originalText.s, alphabet.s, blank_1, blank_2)

 Static notEscape.s = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ."
 Protected preDigit_1.s = Str(blank_1), preDigit_2.s = Str(blank_2)
 Protected i, index, curChar.s, escapeChar.s, outputText.s
 Protected NewMap cipher.s()
 
 ;build cipher reference
 alphabet = UCase(alphabet)
 For i = 1 To 28
   curChar = Mid(alphabet, i, 1)
   If Not FindString(notEscape, curChar)
     escapeChar = curChar
   EndIf 
   
   Select i
     Case 1 To 8
       If index = blank_1 Or index = blank_2: index + 1: EndIf ;adjust index for blank
       
       cipher(curChar) = Str(index)
       index + 1
     Case 9 To 18
       cipher(curChar) = preDigit_1 + Str(i - 9)
     Case 19 To 28
       cipher(curChar) = preDigit_2 + Str(i - 19)
   EndSelect
 Next
 
 For i = 0 To 9: cipher(Str(i)) = cipher(escapeChar) + Str(i): Next 
 
 ;encrypt each character
 originalText = UCase(originalText)
 Protected length = Len(originalText)
 For i = 1 To length
   outputText + cipher(Mid(originalText, i, 1))
 Next 
 
 ProcedureReturn outputText

EndProcedure

Procedure.s decrypt_SC(cipherText.s, alphabet.s, blank_1, blank_2)

 Static notEscape.s = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ."
 Protected preDigit_1.s = Str(blank_1), preDigit_2.s = Str(blank_2)
 Protected i, index, curChar.s, escapeCipher.s, outputText.s
 Protected NewMap cipher.s()
 
 ;build decipher reference
 alphabet = UCase(alphabet)
 For i = 1 To 28
   curChar = Mid(alphabet, i, 1)
   
   Select i
     Case 1 To 8
       If index = blank_1 Or index = blank_2: index + 1: EndIf ;adjust index for blank
       cipher(Str(index)) = curChar
       index + 1
     Case 9 To 18
       cipher(preDigit_1 + Str(i - 9)) = curChar
     Case 19 To 28
       cipher(preDigit_2 + Str(i - 19)) = curChar
   EndSelect
   If Not FindString(notEscape, curChar)
     escapeCipher = MapKey(cipher())
   EndIf
 Next
 
 For i = 0 To 9: cipher(escapeCipher + Str(i)) = Str(i): Next 
 
 ;decrypt each character
 Protected length = Len(cipherText)
 index = 1
 While index <=length
   curChar = Mid(cipherText, index, 1)
   If curChar = preDigit_1 Or curChar = preDigit_2
     curChar = Mid(cipherText, index, 2)
     If curChar = escapeCipher: curChar = Mid(cipherText, index, 3): EndIf
   EndIf 
   outputText + cipher(curChar)
   index + Len(curChar)
 Wend
 
 ProcedureReturn outputText

EndProcedure

If OpenConsole()

 Define message.s = "One night-it was on the twentieth of March, 1888-I was returning"
 Define cipher.s = "HOLMESRTABCDFGIJKNPQUVWXYZ./", encoded.s
 
 PrintN("Original: " + message)
 encoded = encrypt_SC(message, cipher, 3, 7)
 PrintN("encoded: " + encoded)
 PrintN("decoded: " + decrypt_SC(encoded, cipher, 3, 7))
 
 Print(#CRLF$ + #CRLF$ + "Press ENTER to exit"): Input()
 CloseConsole()

EndIf</lang> Sample output:

Original: One night-it was on the twentieth of March, 1888-I was returning
encoded: 13953936350936974306139905974539936590134430832079179879879836743068597
2839363935
decoded: ONENIGHTITWASONTHETWENTIETHOFMARCH1888IWASRETURNING

Python

Partially based on the PicoLisp version: <lang python>T = [["79", "0", "1", "2", "3", "4", "5", "6", "7", "8", "9"],

    ["",   "H", "O", "L", "",  "M", "E", "S", "",  "R", "T"],
    ["3",  "A", "B", "C", "D", "F", "G", "I", "J", "K", "N"],
    ["7",  "P", "Q", "U", "V", "W", "X", "Y", "Z", ".", "/"]]

def straddle(s):

   return "".join(L[0]+T[0][L.index(c)] for c in s.upper() for L in T if c in L)

def unstraddle(s):

   s = iter(s)
   for c in s:
       if c in [T[2][0], T[3][0]]:
           i = [T[2][0], T[3][0]].index(c)
           n = T[2 + i][T[0].index(s.next())]
           yield s.next() if n == "/" else n
       else:
           yield T[1][T[0].index(c)]

O = "One night-it was on the twentieth of March, 1888-I was returning" print "Encoded:", straddle(O) print "Decoded:", "".join(unstraddle(straddle(O)))</lang> Output:

Encoded: 139539363509369743061399059745399365901344308320791798798798367430685972839363935
Decoded: ONENIGHTITWASONTHETWENTIETHOFMARCH1888IWASRETURNING

Racket

Based in the PicoLisp version.

We store the straddling checkerboard in a structure, so it can be reused or changed for the different examples. The “constructor” transforms the lines of the checkboard into an internal representation. <lang Racket>#lang racket

(struct *straddling (header main original)

 #:reflection-name 'straddling
 #:methods gen:custom-write 
 [(define (write-proc board port mode)
    (write-string "#<straddling " port)
    (write (*straddling-original board) port)
    (write-string ">" port))]) 

(define string->vector (compose list->vector string->list))

(define (straddling . lines)

 (define header-tail (reverse 
                      (for/fold ([rev-ret '()]) 
                                ([char (in-list (string->list (car lines)))]
                                 [i (in-naturals)])
                        (if (equal? char #\space)
                            (cons (number->string i) rev-ret)
                            rev-ret))))
 (define main (list->vector
               (map string->vector
                    (cons "0123456789" 
                          (map string-upcase 
                               lines)))))
 (define temporal-board 
   (*straddling (list->vector (list* "?" "" header-tail)) main lines))
 (define escape (straddling-encode-char #\/ temporal-board))
 (*straddling (list->vector (list* escape "" header-tail)) main lines))</lang>

Now we define the functions to straddle and unstraddle the message. <lang Racket>(define (straddling-encode-char char board)

 (or (for/or ([head (in-vector (*straddling-header board))]
               [line (in-vector (*straddling-main board))]) 
       (let ([pos (vector-member char line)])
         (if pos
             (string-append head (number->string pos))
             #f)))
     ""))

(define (straddle message board)

 (apply string-append 
        (map (lambda (char)
               (if (or (equal? char #\space) (equal? char #\/))
                   ""
                   (straddling-encode-char char board)))
             (string->list (string-upcase message)))))


(define (unstraddle message board)

 (define char->string string)
 (define (straddling-decode-char str row)
   (vector-ref (vector-ref (*straddling-main board) row) (string->number str)))
 (list->string
  (reverse
   (let-values ([(row rev-ret)
                 (for/fold ([row #f] ;row to read in multichar codes 
                                     ;#f means start of new code
                            [rev-ret '()]) ;result
                           ([str (map char->string (string->list (string-upcase message)))])
                   (if (not row)
                       (let ([pos (vector-member str (*straddling-header board))])
                         (if pos 
                             (values pos rev-ret)
                             (values #f (cons (straddling-decode-char str 1) rev-ret))))
                       (let ([decoded (straddling-decode-char str row)])
                         (if (equal? decoded #\/)
                             (values 0 rev-ret)
                             (values #f (cons decoded rev-ret))))))])
     (unless row ;check that last number was not missing
       rev-ret)))))</lang>

Two examples: <lang Racket>(define (test-straddling message board)

 (let* ([encoded (straddle message board)]
        [decoded (unstraddle encoded board)])
   (displayln board)
   (displayln message)
   (displayln encoded)
   (displayln decoded)))
 

(test-straddling "One night-it was on the twentieth of March, 1888-I was returning"

                (straddling "HOL MES RT"
                            "ABCDFGIJKN"
                            "PQUVWXYZ./"))

(newline) (test-straddling "One night-it was on the twentieth of March, 1888-I was returning"

                (straddling "ET AON RIS"
                            "BCDFGHJKLM"
                            "PQ/UVWXYZ."))</lang>
Output:
#<straddling ("HOL MES RT" "ABCDFGIJKN" "PQUVWXYZ./")>
One night-it was on the twentieth of March, 1888-I was returning
139539363509369743061399059745399365901344308320791798798798367430685972839363935
ONENIGHTITWASONTHETWENTIETHOFMARCH1888IWASRETURNING

#<straddling ("ET AON RIS" "BCDFGHJKLM" "PQ/UVWXYZ.")>
One night-it was on the twentieth of March, 1888-I was returning
450582425181653945125016505180125423293721256216286286288653970163758524
ONENIGHTITWASONTHETWENTIETHOFMARCH1888IWASRETURNING

Ruby

Works with: Ruby version 2.0

<lang ruby>class StraddlingCheckerboard

 EncodableChars = "A-Z0-9."
 SortedChars = "  ./" + [*"A".."Z"].join
 
 def initialize(board = nil)
   if board.nil?
     # create a new random board
     rest = "BCDFGHJKLMPQUVWXYZ/.".chars.shuffle
     @board = ["  ESTONIAR".chars.shuffle, rest[0..9], rest[10..19]]
   elsif board.chars.sort.join == SortedChars
     @board = board.chars.each_slice(10).to_a
   else
     raise ArgumentError, "invalid #{self.class}: #{board}"
   end
   # find the indices of the first row blanks
   @row_labels = @board[0].each_with_index.select {|v, i| v == " "}.map {|v,i| i}
   
   @mapping = {}
   @board[0].each_with_index {|char, idx| @mapping[char] = idx.to_s unless char == " "}
   @board[1..2].each_with_index do |row, row_idx|
     row.each_with_index do |char, idx|
       @mapping[char] = "%d%d" % [@row_labels[row_idx], idx]
     end
   end
 end
 
 def encode(message)
   msg = message.upcase.delete("^#{EncodableChars}")
   msg.chars.inject("") do |crypt, char|
     crypt << (char =~ /[0-9]/ ? @mapping["/"] + char : @mapping[char])
   end
 end
 
 def decode(code)
   msg = ""
   tokens = code.chars
   until tokens.empty?
     token = tokens.shift
     itoken = token.to_i
     unless @row_labels.include?(itoken)
       msg << @board[0][itoken]
     else
       token2 = tokens.shift
       if @mapping["/"] == token + token2
         msg << tokens.shift
       else
         msg << @board[1+@row_labels.index(itoken)][token2.to_i]
       end
     end
   end
   msg
 end
 
 def to_s
   @board.inject("") {|res, row| res << row.join}
 end
 
 def inspect
   "#<%s board=%p, row_labels=%p, mapping=%p>" % [self.class, to_s, @row_labels, @mapping]
 end

end</lang>

The test suite <lang ruby>require 'test/unit' class StraddlingCheckerboardTest < Test::Unit::TestCase

 def setup
   @msg = "One night-it was on the twentieth of March, 1888-I was returning"
   @expected = "ONENIGHTITWASONTHETWENTIETHOFMARCH1888IWASRETURNING"
 end
 
 def test1
   sc = StraddlingCheckerboard.new "ET AON RISBCDFGHJKLMPQ/UVWXYZ."
   code = sc.encode(@msg)
   plaintext = sc.decode(code)
   
   puts "using checkerboard: #{sc}"
   puts "original: #{@msg}"
   puts "encoded: #{code}"
   puts "decoded: #{plaintext}"
   assert_equal("450582425181653945125016505180125423293721256216286286288653970163758524", code)
   assert_equal(@expected, plaintext)
 end
 
 def test_board_with_space_in_first_char
   sc = StraddlingCheckerboard.new " RIOESN ATG./LXYBDKQMJUHWPVCFZ"
   code = sc.encode(@msg)
   plaintext = sc.decode(code)
   #p sc
   #puts "encoded: #{code}"
   #puts "decoded: #{plaintext}"
   assert_equal(@expected, plaintext)
 end
 
 def test_random_board
   sc = StraddlingCheckerboard.new
   plaintext = sc.decode(sc.encode(@msg))
   assert_equal(@expected, plaintext)
 end
 
 def test_invalid_input
   assert_raise(ArgumentError) {StraddlingCheckerboard.new "ET ON RISBCDFGHJKLMPQ/UVWXYZ.!"}
 end

end</lang>

output

Run options: 

# Running tests:

using checkerboard: ET AON RISBCDFGHJKLMPQ/UVWXYZ.
original: One night-it was on the twentieth of March, 1888-I was returning
encoded: 450582425181653945125016505180125423293721256216286286288653970163758524
decoded: ONENIGHTITWASONTHETWENTIETHOFMARCH1888IWASRETURNING
....

Finished tests in 0.007873s, 508.0378 tests/s, 635.0472 assertions/s.

4 tests, 5 assertions, 0 failures, 0 errors, 0 skips

Tcl

Structured as a class, the instances of which contain encoding and decoding mappings suitable for use with Tcl's built in string remapping engine. This uses the fact that no sequence of digits in the encoded form that maps to one character is a prefix of the sequence for any other character. (Inspired by the description of the Perl 6 solution.) <lang tcl>package require Tcl 8.6

oo::class create StraddlingCheckerboardCypher {

   variable encmap decmap
   constructor table {

# Sanity check the table foreach ch [lindex $table 0] i {"" 0 1 2 3 4 5 6 7 8 9} { if {$ch eq "" && $i ne "" && [lsearch -index 0 $table $i] < 1} { error "bad checkerboard table" } } # Synthesize the escaped number row foreach row $table { if {"/" ni $row} continue set pfx [lindex $row 0][expr {[lsearch -exact $row "/"]-1}] lappend table [list $pfx 0 1 2 3 4 5 6 7 8 9] break } # Build the actual per-character mapping foreach row $table { foreach ch [lrange $row 1 end] n {0 1 2 3 4 5 6 7 8 9} { if {$ch in {"" "/"}} continue; # Skip escape cases lappend encmap $ch [lindex $row 0]$n lappend decmap [lindex $row 0]$n $ch } }

   }
   # These methods just sanitize their input and apply the map
   method encode msg {

string map $encmap [regsub -all {[^A-Z0-9.]} [string toupper $msg] ""]

   }
   method decode msg {

string map $decmap [regsub -all {[^0-9]} $msg ""]

   }

}</lang> Demonstration code: <lang tcl>StraddlingCheckerboardCypher create demo {

   {{} E T {} A O N {} R I S}
   {2  B C D  F G H J  K L M}
   {6  P Q /  U V W X  Y Z .}

} set input "One night-it was on the twentieth of March, 1888-I was returning" set encrypted [demo encode $input] set output [demo decode $encrypted] puts "Input: $input" puts "Encoded: $encrypted" puts "Decoded: $output"</lang> Output:

Input:   One night-it was on the twentieth of March, 1888-I was returning
Encoded: 450582425181653945125016505180125423293721256216286286288653970163758524
Decoded: ONENIGHTITWASONTHETWENTIETHOFMARCH1888IWASRETURNING

zkl

Translation of: D

<lang zkl>var [const]

  val2key=Dictionary(
   "A","30",  "B","31",  "C","32",  "D","33",  "E","5",   "F","34",  "G","35",
   "H","0",   "I","36",  "J","37",  "K","38",  "L","2",   "M","4",   ".","78", 
   "N","39",  "/","79",  "O","1",   "0","790", "P","70",  "1","791", "Q","71", 
   "2","792", "R","8",   "3","793", "S","6",   "4","794" ,"T","9",   "5","795",
   "U","72",  "6","796", "V","73",  "7","797", "W","74",  "8","798", "X","75",
   "9","799", "Y","76",  "Z","77"),
  key2val=val2key.pump(Dictionary(),"reverse");

fcn encode(txt){ txt.toUpper().pump(String,val2key.find.fp1("")) } fcn decode(str){ RegExp("79.|3.|7.|.").munch(str).pump(String,key2val.get) }</lang> <lang zkl>txt:="One night-it was on the twentieth of March, 1888-I was returning"; str:=encode(txt); println("Encoded: ", str); println("Decoded: ", decode(str));</lang>

Output:
ncoded: 139539363509369743061399059745399365901344308320791798798798367430685972839363935
Decoded: ONENIGHTITWASONTHETWENTIETHOFMARCH1888IWASRETURNING