#include <stdio.h>
#include <string.h>
#include <stdlib.h>
void giftrans(FILE *fin, FILE *fout, int index)
{  char header[14];
  unsigned char color_table[1000];
  int flags;
  int has_global_colormap;
  int bits_per_pixel;
  int colormap_entries;
  int color_tbl_size;
  int bits_color_byte;
  int rgb_byte_mask;
  int i;
  unsigned char nextblock[8];
  
  /* The header looks like:
      byte 0 - 5:  "GIF89a" or "GIF87a"
      byte 6, 7:   width  (low order first)
      byte 8, 9:   height (low order first)
      byte 10:     various flags
      byte 11:     background color index
      byte 12:     aspect ratio
  */
  fread(header, 13, 1, fin);
  if (memcmp(header, "GIF", 3))
  {
    printf("Not a GIF\n");
    return;
  }
  if (!memcmp(header, "GIF87a", 6))
    memcpy(header, "GIF89a", 6);

  printf("width = %d, height = %d\n",
         header[6] + header[7] * 256,
         header[8] + header[9] * 256);  
    
  if (fout)  
    fwrite(header, 13, 1, fout);  
  /*  
	Look at flags (8 bits): hi[MCCCSPPP]low
	   M   = global colormap present?
	   CCC = bits/color/colormapentry - 1
	   S   = color map sorted by importance?
	   PPP = bits/pixel - 1
	 therefore
	   Bits/pixel = PPP+1
	   Number of possible colors (entries in colormap): 2 ** (PPP+1)
	                                                  : 1 << (PPP+1)
	   Size (bytes) of colormap: 3 * Number of possible colors
	                           : 3 * (1 << (PPP+1))
  */
  flags = header[10];
  has_global_colormap = flags & 0x80;

  /* Copy over the colormap if need be. */
  if (!has_global_colormap)
  {
    printf("Does not have color map");
    return;
  }  	  
  bits_per_pixel = 1 + (flags & 0x07);
  colormap_entries = 1 << bits_per_pixel;
  color_tbl_size = 3 * colormap_entries;
  printf("bits per pixel = %d, colormap_entries = %d, size = %d\n",
         bits_per_pixel, colormap_entries, color_tbl_size);

  fread(color_table, color_tbl_size, 1, fin);

  if (fout)
    fwrite(color_table, color_tbl_size, 1, fout);
  
  /* For each byte of each colormap's RGB triplit, we'll have
     to mask off bits that aren't used when looking at the
     color values.
  */   
  bits_color_byte = 1 + ((flags >> 4) & 0x07);
  rgb_byte_mask = (1 << bits_color_byte) - 1;

  printf("     r   g   b\n");
  for (i = 0; i < colormap_entries; i++)
  {
    printf("%3d : %3d %3d %3d\n",
           i, 
           color_table[i*3],
           color_table[i*3+1],
           color_table[i*3+2]);
           	  
  }

  /* The next 8 bytes will either be an already-there graphic-extension
     block, or something else that we'll not care about. In the latter
     case, we'll add a graphic-extension block saying "color such-and-
     such is transparent". If there's already one there, we'll just
     ensure that it says that.
  */

  fread(nextblock, 8, 1, fin);

  /* If extension is 0x21 and label is 0xf9, that's the magic tha means
     there's already a graphic extension there.
  */   
  
  if (nextblock[0] == 0x21 && nextblock[1] == 0xf9)
  { 
    nextblock[3] |= 1;
    nextblock[6] = index;
  } 
  else
  {
    char newblock[8];

    newblock[0] = 0x21;
    newblock[1] = 0xf9;
    newblock[2] =    4;
    newblock[3] =    1;
    newblock[4] =    0;
    newblock[5] =    0;
    newblock[6] = index;
    newblock[7] =    0;
    if (fout)
      fwrite(newblock, 8, 1, fout);
  } 
  if (fout) 
    fwrite(nextblock, 8, 1, fout);

  if (fout == NULL)
    return;
      
  /* copy the rest */
  {
    char ch = (char)fgetc(fin);

    while (!feof(fin))
    {
      fputc(ch, fout);
      ch = (char)fgetc(fin);
    }
  }
}      	    

void main(int argc, char *argv[])
{
  FILE *fin = NULL, *fout = NULL;
  int index;
  
	
  if (argc == 1)
  {
    printf("transgif <fn> [<index> <fout>]\n");
    return;
  }

  fin = fopen(argv[1], "rb");
  if (fin == NULL)
  {
    printf("no file %s\n", argv[1]);
    return;
  }
  if (argc == 4)
  {  
    index = atoi(argv[2]);
    fout = fopen(argv[3], "wb");
    if (fout == NULL)
      printf("Cannot open %s\n");
  }

  giftrans(fin, fout, index);

  fclose(fin);
  if (fout) fclose(fout);
}