decode.c

/* Decode a Game Genie code into an M68000 address/data pair.
 * The Game Genie code is made of the characters
 * ABCDEFGHJKLMNPRSTVWXYZ0123456789 (notice the missing I, O, Q and U).
 * Where A = 00000, B = 00001, C = 00010, ... , on to 9 = 11111.
 * 
 * These come out to a very scrambled bit pattern like this:
 * (SCRA-MBLE is just an example)
 *
 *   S     C     R     A  -  M     B     L     E
 * 01111 00010 01110 00000 01011 00001 01010 00100
 * ijklm nopIJ KLMNO PABCD EFGHd efgha bcQRS TUVWX
 *
 * Our goal is to rearrange that to this:
 *
 * 0000 0101 1001 1100 0100 0100 : 1011 0000 0111 1000
 * ABCD EFGH IJKL MNOP QRST UVWX : abcd efgh ijkl mnop
 *
 * which in Hexadecimal is 059C44:B078. Simple, huh? ;)
 *
 * So, then, we dutifully change memory location 059C44 to B078!
 * (of course, that's handled by a different source file :)
 */
#include <stdio.h>
#include <string.h>
#include "decode.h"

static char genie_chars[] = "AaBbCcDdEeFfGgHhJjKkLlMmNnPpRrSsTtVvWwXxYyZz0O1I2233445566778899";

/* genie_decode
 * This function converts a Game Genie code to an address:data pair.
 * The code is given as an 8-character string, like "BJX0SA1C". It need not
 * be null terminated, since only the first 8 characters are taken. It is
 * assumed that the code is already made of valid characters, i.e. there are no
 * Q's, U's, or symbols. If such a character is
 * encountered, the function will return with a warning on stderr.
 *
 * The resulting address:data pair is returned in the struct patch pointed to
 * by result. If an error results, both the address and data will be set to -1.
 */

static void genie_decode(const char* code, struct patch* result)
{
  int i = 0, n;
  char* x;

  for(; i < 8; ++i)
  {
    /* If strchr returns NULL, we were given a bad character */
    if(!(x = strchr(genie_chars, code[i])))
    {
      result->addr = -1; result->data = -1;
      return;
    }
    n = (x - genie_chars) >> 1;
    /* Now, based on which character this is, fit it into the result */
    switch(i)
    {
    case 0:
      /* ____ ____ ____ ____ ____ ____ : ____ ____ ABCD E___ */
      result->data |= n << 3;
      break;
    case 1:
      /* ____ ____ DE__ ____ ____ ____ : ____ ____ ____ _ABC */
      result->data |= n >> 2;
      result->addr |= (n & 3) << 14;
      break;
    case 2:
      /* ____ ____ __AB CDE_ ____ ____ : ____ ____ ____ ____ */
      result->addr |= n << 9;
      break;
    case 3:
      /* BCDE ____ ____ ___A ____ ____ : ____ ____ ____ ____ */
      result->addr |= (n & 0xF) << 20 | (n >> 4) << 8;
      break;
    case 4:
      /* ____ ABCD ____ ____ ____ ____ : ___E ____ ____ ____ */
      result->data |= (n & 1) << 12;
      result->addr |= (n >> 1) << 16;
      break;
    case 5:
      /* ____ ____ ____ ____ ____ ____ : E___ ABCD ____ ____ */
      result->data |= (n & 1) << 15 | (n >> 1) << 8;
      break;
    case 6:
      /* ____ ____ ____ ____ CDE_ ____ : _AB_ ____ ____ ____ */
      result->data |= (n >> 3) << 13;
      result->addr |= (n & 7) << 5;
      break;
    case 7:
      /* ____ ____ ____ ____ ___A BCDE : ____ ____ ____ ____ */
      result->addr |= n;
      break;
    }
    /* Go around again */
  }
  return;
}

/* "Decode" an address/data pair into a structure. This is for "012345:ABCD"
 * type codes. You're more likely to find Genie codes circulating around, but
 * there's a chance you could come on to one of these. Which is nice, since
 * they're MUCH easier to implement ;) Once again, the input should be depunc-
 * tuated already. */

static char hex_chars[] = "00112233445566778899AaBbCcDdEeFf";

static void hex_decode(const char *code, struct patch *result)
{
  char *x;
  int i;
  /* 6 digits for address */
  for(i = 0; i < 6; ++i)
    {
      if(!(x = strchr(hex_chars, code[i])))
      {
	result->addr = result->data = -1;
	return;
      }
      result->addr = (result->addr << 4) | ((x - hex_chars) >> 1);
    }
  /* 4 digits for data */
  for(i = 6; i < 10; ++i)
    {
      if(!(x = strchr(hex_chars, code[i])))
      {
	result->addr = result->data = -1;
	return;
      }
      result->data = (result->data << 4) | ((x - hex_chars) >> 1);
    }
}

/* THIS is the function you call from the MegaDrive or whatever. This figures
 * out whether it's a genie or hex code, depunctuates it, and calls the proper
 * decoder. */
void decode(const char* code, struct patch* result)
{
  int len = strlen(code), i, j;
  char code_to_pass[16], *x;
  const char *ad, *da;
  int adl, dal;

  /* Initialize the result */
  result->addr = result->data = 0;
  
  /* If it's 9 chars long and the 5th is a hyphen, we have a Game Genie
   * code. */
  if(len == 9 && code[4] == '-')
    {
      /* Remove the hyphen and pass to genie_decode */
      code_to_pass[0] = code[0];
      code_to_pass[1] = code[1];
      code_to_pass[2] = code[2];
      code_to_pass[3] = code[3];
      code_to_pass[4] = code[5];
      code_to_pass[5] = code[6];
      code_to_pass[6] = code[7];
      code_to_pass[7] = code[8];
      code_to_pass[8] = '\0';
      genie_decode(code_to_pass, result);
      return;
    }
  /* Otherwise, we assume it's a hex code.
   * Find the colon so we know where address ends and data starts. If there's
   * no colon, then we haven't a code at all! */
  if(!(x = strchr(code, ':'))) goto bad_code;
  ad = code; da = x + 1; adl = x - code; dal = len - adl - 1;
  /* If a section is empty or too long, toss it */
  if(adl == 0 || adl > 6 || dal == 0 || dal > 4) goto bad_code;
  /* Pad the address with zeros, then fill it with the value */
  for(i = 0; i < (6 - adl); ++i) code_to_pass[i] = '0';
  for(j = 0; i < 6; ++i, ++j) code_to_pass[i] = ad[j];
  /* Do the same for data */
  for(i = 6; i < (10 - dal); ++i) code_to_pass[i] = '0';
  for(j = 0; i < 10; ++i, ++j) code_to_pass[i] = da[j];
  code_to_pass[10] = '\0';
  /* Decode and goodbye */
  hex_decode(code_to_pass, result);
  return;

bad_code:
  /* AGH! Invalid code! */
  result->data = result->addr = -1;
  return;
}