decode.c

   1 /**
   2  * Decode a Game Genie code into an M68000 address/data pair.
   3  * The Game Genie code is made of the characters
   4  * ABCDEFGHJKLMNPRSTVWXYZ0123456789 (notice the missing I, O, Q and U).
   5  * Where A = 00000, B = 00001, C = 00010, ... , on to 9 = 11111.
   6  *
   7  * These come out to a very scrambled bit pattern like this:
   8  * (SCRA-MBLE is just an example)
   9  *
  10  *   S     C     R     A  -  M     B     L     E
  11  * 01111 00010 01110 00000 01011 00001 01010 00100
  12  * ijklm nopIJ KLMNO PABCD EFGHd efgha bcQRS TUVWX
  13  *
  14  * Our goal is to rearrange that to this:
  15  *
  16  * 0000 0101 1001 1100 0100 0100 : 1011 0000 0111 1000
  17  * ABCD EFGH IJKL MNOP QRST UVWX : abcd efgh ijkl mnop
  18  *
  19  * which in Hexadecimal is 059C44:B078. Simple, huh? ;)
  20  *
  21  * So, then, we dutifully change memory location 059C44 to B078!
  22  * (of course, that's handled by a different source file :)
  23  */
  24 #include <stdio.h>
  25 #include <string.h>
  26 #include "decode.h"
  27
  28 static char genie_chars[] = "AaBbCcDdEeFfGgHhJjKkLlMmNnPpRrSsTtVvWwXxYyZz0O1I2233445566778899";
  29
  30 /**
  31  * Decode a Game Genie Code.
  32  * This function converts a Game Genie code to an address:data pair.
  33  * The code is given as an 8-character string, like "BJX0SA1C". It need not
  34  * be null terminated, since only the first 8 characters are taken. It is
  35  * assumed that the code is already made of valid characters, i.e. there are no
  36  * Q's, U's, or symbols. If such a character is
  37  * encountered, the function will return with a warning on stderr.
  38  *
  39  * The resulting address:data pair is returned in the struct patch pointed to
  40  * by result. If an error results, both the address and data will be set to -1.
  41  *
  42  * @param[in] code 8 character Game Genie code.
  43  * @param[out] result The resulting address:data pair is returned in the struct
  44  * patch pointed to by result. If an error results, both the address and data
  45  * will be set to -1.
  46  */
  47 void genie_decode(const char *code, struct patch *result)
  48 {
  49   int i = 0, n;
  50   char* x;
  51
  52   for(; i < 8; ++i)
  53   {
  54     /* If strchr returns NULL, we were given a bad character */
  55     if(!(x = strchr(genie_chars, code[i])))
  56     {
  57       result->addr = -1; result->data = -1;
  58       return;
  59     }
  60     n = (x - genie_chars) >> 1;
  61     /* Now, based on which character this is, fit it into the result */
  62     switch(i)
  63     {
  64     case 0:
  65       /* ____ ____ ____ ____ ____ ____ : ____ ____ ABCD E___ */
  66       result->data |= n << 3;
  67       break;
  68     case 1:
  69       /* ____ ____ DE__ ____ ____ ____ : ____ ____ ____ _ABC */
  70       result->data |= n >> 2;
  71       result->addr |= (n & 3) << 14;
  72       break;
  73     case 2:
  74       /* ____ ____ __AB CDE_ ____ ____ : ____ ____ ____ ____ */
  75       result->addr |= n << 9;
  76       break;
  77     case 3:
  78       /* BCDE ____ ____ ___A ____ ____ : ____ ____ ____ ____ */
  79       result->addr |= (n & 0xF) << 20 | (n >> 4) << 8;
  80       break;
  81     case 4:
  82       /* ____ ABCD ____ ____ ____ ____ : ___E ____ ____ ____ */
  83       result->data |= (n & 1) << 12;
  84       result->addr |= (n >> 1) << 16;
  85       break;
  86     case 5:
  87       /* ____ ____ ____ ____ ____ ____ : E___ ABCD ____ ____ */
  88       result->data |= (n & 1) << 15 | (n >> 1) << 8;
  89       break;
  90     case 6:
  91       /* ____ ____ ____ ____ CDE_ ____ : _AB_ ____ ____ ____ */
  92       result->data |= (n >> 3) << 13;
  93       result->addr |= (n & 7) << 5;
  94       break;
  95     case 7:
  96       /* ____ ____ ____ ____ ___A BCDE : ____ ____ ____ ____ */
  97       result->addr |= n;
  98       break;
  99     }
 100     /* Go around again */
 101   }
 102   return;
 103 }
 104
 105 /**
 106  * "Decode" an address/data pair into a structure. This is for "012345:ABCD"
 107  * type codes. You're more likely to find Genie codes circulating around, but
 108  * there's a chance you could come on to one of these. Which is nice, since
 109  * they're MUCH easier to implement ;) Once again, the input should be
 110  * depunctuated already.
 111  *
 112  * @param[in] code 8 character Game Genie code.
 113  * @param[out] result The resulting address:data pair is returned in the struct
 114  * patch pointed to by result. If an error results, both the address and data
 115  * will be set to -1.
 116  */
 117 void hex_decode(const char *code, struct patch *result)
 118 {
 119   static char hex_chars[] = "00112233445566778899AaBbCcDdEeFf";
 120   char *x;
 121   int i;
 122   /* 6 digits for address */
 123   for(i = 0; i < 6; ++i)
 124     {
 125       if(!(x = strchr(hex_chars, code[i])))
 126       {
 127         result->addr = result->data = -1;
 128         return;
 129       }
 130       result->addr = (result->addr << 4) | ((x - hex_chars) >> 1);
 131     }
 132   /* 4 digits for data */
 133   for(i = 6; i < 10; ++i)
 134     {
 135       if(!(x = strchr(hex_chars, code[i])))
 136       {
 137         result->addr = result->data = -1;
 138         return;
 139       }
 140       result->data = (result->data << 4) | ((x - hex_chars) >> 1);
 141     }
 142 }
 143
 144 /**
 145  * THIS is the function you call from the MegaDrive or whatever. This figures
 146  * out whether it's a genie or hex code, depunctuates it, and calls the proper
 147  * decoder.
 148  *
 149  * @param[in] code Game Genie or hex code.
 150  * @param[out] result The resulting address:data pair is returned in the struct
 151  * patch pointed to by result. If an error results, both the address and data
 152  * will be set to -1.
 153  */
 154 void decode(const char *code, struct patch *result)
 155 {
 156   int len = strlen(code), i, j;
 157   char code_to_pass[16], *x;
 158   const char *ad, *da;
 159   int adl, dal;
 160
 161   /* Initialize the result */
 162   result->addr = result->data = 0;
 163
 164   /* If it's 9 chars long and the 5th is a hyphen, we have a Game Genie
 165    * code. */
 166   if(len == 9 && code[4] == '-')
 167     {
 168       /* Remove the hyphen and pass to genie_decode */
 169       code_to_pass[0] = code[0];
 170       code_to_pass[1] = code[1];
 171       code_to_pass[2] = code[2];
 172       code_to_pass[3] = code[3];
 173       code_to_pass[4] = code[5];
 174       code_to_pass[5] = code[6];
 175       code_to_pass[6] = code[7];
 176       code_to_pass[7] = code[8];
 177       code_to_pass[8] = '\0';
 178       genie_decode(code_to_pass, result);
 179       return;
 180     }
 181   /* Otherwise, we assume it's a hex code.
 182    * Find the colon so we know where address ends and data starts. If there's
 183    * no colon, then we haven't a code at all! */
 184   if(!(x = strchr(code, ':'))) goto bad_code;
 185   ad = code; da = x + 1; adl = x - code; dal = len - adl - 1;
 186   /* If a section is empty or too long, toss it */
 187   if(adl == 0 || adl > 6 || dal == 0 || dal > 4) goto bad_code;
 188   /* Pad the address with zeros, then fill it with the value */
 189   for(i = 0; i < (6 - adl); ++i) code_to_pass[i] = '0';
 190   for(j = 0; i < 6; ++i, ++j) code_to_pass[i] = ad[j];
 191   /* Do the same for data */
 192   for(i = 6; i < (10 - dal); ++i) code_to_pass[i] = '0';
 193   for(j = 0; i < 10; ++i, ++j) code_to_pass[i] = da[j];
 194   code_to_pass[10] = '\0';
 195   /* Decode and goodbye */
 196   hex_decode(code_to_pass, result);
 197   return;
 198
 199 bad_code:
 200   /* AGH! Invalid code! */
 201   result->data = result->addr = -1;
 202   return;
 203 }