/*
    chksum64 V1.2, a program to calculate the ROM checksum of Nintendo64 ROMs.
    Copyright (C) 1997  Andreas Sterbenz (stan@sbox.tu-graz.ac.at)

    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 2 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program; if not, write to the Free Software
    Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
*/

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define max2(a, b) ( (a)>(b) ? (a) : (b) )
#define min2(a, b) ( (a)<(b) ? (a) : (b) )

#define BUFSIZE 32768

#define CHECKSUM_START 0x1000
#define CHECKSUM_LENGTH 0x100000L
#define CHECKSUM_HEADERPOS 0x10
#define CHECKSUM_END (CHECKSUM_START + CHECKSUM_LENGTH)

#define CHECKSUM_STARTVALUE 0xf8ca4ddc

#define ROL(i, b) (((i)<<(b)) | ((i)>>(32-(b))))

#define BYTES2LONG(b, s) ( (((b)[0^(s)] & 0xffL) << 24) | \
                           (((b)[1^(s)] & 0xffL) << 16) | \
                           (((b)[2^(s)] & 0xffL) <<  8) | \
                           (((b)[3^(s)] & 0xffL)) )

#define LONG2BYTES(l, b, s)  (b)[0^(s)] = ((l)>>24)&0xff; \
                             (b)[1^(s)] = ((l)>>16)&0xff; \
                             (b)[2^(s)] = ((l)>> 8)&0xff; \
                             (b)[3^(s)] = ((l)    )&0xff;

#define HEADER_MAGIC 0x80371240

void usage(char *progname)
{
  fprintf(stderr, "Usage: %s [-r] [-o|-s] <File>\n\n", progname);
  fprintf(stderr, "This program calculates the ROM checksum for Nintendo64 ROM images.\n");
  fprintf(stderr, "Checksum code reverse engineered from Nagra's program.\n");
  exit(2);
}

int main(int argc, char *argv[])
{
  FILE *file1;
  char *fname1=NULL, *progname=argv[0];
  unsigned char buffer1[BUFSIZE];
  unsigned int flen1;
  unsigned int sum1, sum2;
  int swapped=-1;
  int readonly = 0;

  printf("CHKSUM64 V1.2   Copyright (C) 1997 Andreas Sterbenz (stan@sbox.tu-graz.ac.at)\n");
  printf("This program is released under the terms of the GNU public license. NO WARRANTY\n\n");

  {
    int i;

    for( i=1; i<argc; i++ ) {
      if( argv[i][0] == '-' ) {
        char *c = argv[i]+1;
        if( *(c+1) != '\0' ) usage(progname);
        switch( *c ) {
        case 'r':
          readonly = 1;
          break;
        case 'o':
          swapped = 0;
          break;
        case 's':
          swapped = 1;
          break;
        default:
          usage(progname);
        }
      } else {
        if( fname1 == NULL ) {
          fname1 = argv[i];
        } else {
          usage(progname);
        }
      }
    }
  }
  if( fname1 == NULL ) usage(progname);

  if( (readonly == 1) || (file1 = fopen(fname1, "r+b")) == NULL ) {
    if( (file1 = fopen(fname1, "rb")) == NULL ) {
      fprintf(stderr, "%s: Could not open file '%s' for reading.\n", progname, fname1);
      exit(1);
    } else {
      readonly = 1;
    }
  }
  if( fread(buffer1, 1, 12, file1) != 12 ) {
    fprintf(stderr, "%s: File is too short to be a N64 ROM Image, cannot checksum it.\n", progname);
    exit(1);
  }
  if( swapped == -1 ) {
    if( BYTES2LONG(buffer1, 0) == HEADER_MAGIC ) {
      swapped = 0;
    } else if( BYTES2LONG(buffer1, 1) == HEADER_MAGIC ) {
      swapped = 1;
    }
  }
  if( swapped != -1 ) {
    printf("The image '%s' is in %s format.\n", fname1, (swapped==0) ? "original (not swapped)" : "V64 (byte-swapped)");
  } else {
    if( (buffer1[8] == 0x80) && (buffer1[9] != 0x80) ) {
      swapped = 0;
    } else {
      if( (buffer1[8] != 0x80) && (buffer1[9] == 0x80) ) {
        swapped = 1;
      }
    }
    if( swapped != -1 ) {
      printf("WARNING: I am not 100%% certain, but the image '%s' appears to be in %s format.\n", fname1, (swapped==0) ? "original (not swapped)" : "V64 (byte-swapped)");
    } else {
      fprintf(stderr, "%s: Could not detect type of image '%s', use commandline to override.\n", progname, fname1);
      exit(1);
    }
  }
  fseek(file1, 0L, SEEK_END);
  flen1 = ftell(file1);
  if( flen1 < CHECKSUM_END ) {
    if( flen1 < CHECKSUM_START ) {
      fprintf(stderr, "%s: File is too short to be a N64 ROM Image, cannot checksum it.\n", progname);
      exit(1);
    }
    if( (flen1 & 3) != 0 ) {
      fprintf(stderr, "%s: File length is not a multiple of four, cannot calculate checksum.\n", progname);
      exit(1);
    }
    printf("File is only %d bytes long, remaining %ld to be checksummed will be assumed 0.\n", flen1, CHECKSUM_END - flen1);
  }
  fseek(file1, CHECKSUM_START, SEEK_SET);
  {
    unsigned int i;
    unsigned int c1, k1, k2;
    unsigned int t1, t2, t3, t4;
    unsigned int t5, t6;
    unsigned int n;
    unsigned clen = CHECKSUM_LENGTH;
    unsigned rlen = flen1 - CHECKSUM_START;

    /* Below is the actual checksum calculation algorithm, which was
       reverse engineered out of Nagra's program.

       As you can see, the algorithm is total crap. Obviously it was
       designed to be difficult to guess and reverse engineer, and not
       to give a good checksum. A simple XOR and ROL 13 would give a
       just as good checksum. The ifs and the data dependent ROL are really
       extreme nonsense.
    */

    t1 = CHECKSUM_STARTVALUE;
    t2 = CHECKSUM_STARTVALUE;
    t3 = CHECKSUM_STARTVALUE;
    t4 = CHECKSUM_STARTVALUE;
    t5 = CHECKSUM_STARTVALUE;
    t6 = CHECKSUM_STARTVALUE;

    for( ;; ) {
      if( rlen > 0 ) {
        n = fread(buffer1, 1, min2(BUFSIZE, clen), file1);
        if( (n & 0x03) != 0 ) {
          n += fread(buffer1+n, 1, 4-(n&3), file1);
        }
      } else {
        n = min2(BUFSIZE, clen);
      }
      if( (n == 0) || ((n&3) != 0) ) {
        if( (clen != 0) || (n != 0) ) {
          fprintf(stderr, "WARNING: Short read, checksum may be incorrect.\n");
        }
        break;
      }
      for( i=0; i<n; i+=4 ) {
        c1 = BYTES2LONG(&buffer1[i], swapped);
        k1 = t6 + c1;
        if( k1 < t6 ) t4++;
        t6 = k1;
        t3 ^= c1;
        k2 = c1 & 0x1f;
        k1 = ROL(c1, k2);
        t5 += k1;
        if( c1 < t2 ) {
          t2 ^= k1;
        } else {
          t2 ^= t6 ^ c1;
        }
        t1 += c1 ^ t5;
      }
      if( rlen > 0 ) {
        rlen -= n;
        if( rlen <= 0 ) memset(buffer1, 0, BUFSIZE);
      }
      clen -= n;
    }
    sum1 = t6 ^ t4 ^ t3;
    sum2 = t5 ^ t2 ^ t1;
  }
  LONG2BYTES(sum1, &buffer1[0], 0);
  LONG2BYTES(sum2, &buffer1[4], 0);
  fseek(file1, CHECKSUM_HEADERPOS, SEEK_SET);
  fread(buffer1+8, 1, 8, file1);
  printf("Old Checksum:        ");
  printf("%02X %02X %02X %02X  ", buffer1[ 8^swapped], buffer1[ 9^swapped], buffer1[10^swapped], buffer1[11^swapped]);
  printf("%02X %02X %02X %02X\n", buffer1[12^swapped], buffer1[13^swapped], buffer1[14^swapped], buffer1[15^swapped]);
  printf("Calculated Checksum: ");
  printf("%02X %02X %02X %02X  ", buffer1[0], buffer1[1], buffer1[2], buffer1[3]);
  printf("%02X %02X %02X %02X\n", buffer1[4], buffer1[5], buffer1[6], buffer1[7]);
  if( readonly == 0 ) {
    fseek(file1, CHECKSUM_HEADERPOS, SEEK_SET);
    LONG2BYTES(sum1, &buffer1[16], swapped);
    LONG2BYTES(sum2, &buffer1[20], swapped);
    if( fwrite(buffer1+16, 1, 8, file1) != 8 ) {
      fprintf(stderr, "%s: Could not write new checksum to file '%s'!\n", progname, fname1);
      exit(1);
    } else {
      printf("New checksum successfully written.\n");
    }
  } else {
    printf("File opened in read-only mode, new checksum not written.\n");
  }
  fclose(file1);
  return 0;
}