gameboy tile viewer

#include <SDL/SDL.h>

// gcc `sdl-config --cflags --libs` gbtiles.c

// VIEW TILES FROM A GAMEBOY ROM
// ARROW KEYS TO MOVE

#define  cols 64
#define  rows 64

void print_tile(unsigned char *bytes, int x, int y, int l, char *data) {
  int i, j;
  int b1, b2;
  int b1_bit, b2_bit;
  for(i = 0; i < 16; i += 2) {
    b1 = bytes[i];
    b2 = bytes[i+1];
    for(j = 0; j < 8; j++) {
      b1_bit = !(!(b1 & (0x80>>j))); 
      b2_bit = !(!(b2 & (0x80>>j))); 
      data[(x+j)+(y+i/2)*l] = 0xff-((2*b2_bit+b1_bit))*(0xff/4);
    }
    //puts("");
  }
}

SDL_Surface *read_rom(char *filename, int doff, int *dout) {
  int w, h, i;
  
  unsigned char bytes[] = {0x7C,0x7C,0x00,0xC6,0xC6,0x00,0x00,0xFE,0xC6,0xC6,0x00,0xC6,0xC6,0x00,0x00,0x00};
  
  w = 8*cols;
  h = 8*rows;
  
  char *pixels = malloc(w*h*8);
  for(i = 0; i < w*h*8; i++) {
    pixels[i] = 0; //(i*0xff)/(w*h*8);
  }
  
  FILE *fptr;
  int er;
  if(!(fptr = fopen(filename, "rb"))) { puts("fopen fail"); return NULL; }
  int size;// = 65536;
  
  fseek(fptr, 0, SEEK_END);
  size = ftell(fptr);
  fseek(fptr, 0, SEEK_SET);
  
  unsigned char *data = malloc(size);
  if((er = fread(data, size, 1, fptr)) != 1) { printf("fread fail %d\n", er); return NULL; }
  
  int x = 0, y = 0, d = 0;
  for(d = doff; d < size; d += 16) {
    print_tile(data+d, x*8, y*8, w, pixels);
    x++;
    if(x>= cols) {
      x=0;
      y++;
    }
    if(y >= rows) break;
  }
  d+=16;
  *dout = d;
  //if(d < size)
  //  printf("didn't show everything! %d/%d\n", d, size);
  
  return SDL_CreateRGBSurfaceFrom(pixels, w, h,
				  8, w,
				  0xff, 0xff, 0xff, 0);
}

int main(int argc, char* args[]) {
  SDL_Surface* tiles = NULL;
  SDL_Surface* screen = NULL;
  
  int oldd, d;
  
  if(argc != 2) { puts("give a gb rom as input"); return 0; }
  
  SDL_Init(SDL_INIT_EVERYTHING);
  screen = SDL_SetVideoMode( cols*8, rows*8, 32, SDL_SWSURFACE );
  
  oldd = 0;
  tiles = read_rom(args[1], 0, &d);
  if(!tiles) { puts("error"); return; }
  
  SDL_BlitSurface(tiles, NULL, screen, NULL);
  SDL_Flip(screen);
  
  SDL_Event e;
  while(SDL_WaitEvent(&e)) {
    if(e.type == SDL_QUIT)
      break;
    if(e.type == SDL_KEYDOWN) {
      if(e.key.keysym.sym == SDLK_LEFT) {
	if(oldd>0) oldd--;
      }
      else if(e.key.keysym.sym == SDLK_RIGHT) {
	oldd++;
      }
      else if(e.key.keysym.sym == SDLK_UP) {
	oldd = 0;
      }
      else {
	oldd = d;
      }
      
      tiles = read_rom(args[1], oldd, &d);
      if(!tiles) { puts("error"); return; }
      SDL_BlitSurface(tiles, NULL, screen, NULL);
      SDL_Flip(screen);
    }
  }
  
  SDL_Quit();
  
  return 0;
}