Dump a PPM image of a Wren map's hash buckets

dump_map.c

// Generates a PPM image showing the entries in a Wren Map.
//
// Each pixel represents an entry. It will be black if the entry is empty is
// empty. Otherwise, it will be some other color. The red and green channels
// represents how far the key in that entry is from where it would ideally be.
// In other words, how many collisions occurred before it could be inserted.
// Brighter colors are worse. In particular, brighter green colors are much
// worse.
//
// A good hashing function will lead to an image containing a random-looking
// noise of black and dark blue pixels. A bad function will show clear patterns.
// If there are many brighter and greener pixels, then the map's performance is
// degrading from collisions.
void wrenMapDumpImage(ObjMap* map)
{
  char filename[100];
  sprintf(filename, "map%d.ppm", map->count);
  FILE *fp = fopen(filename, "wb");
  int width = 1024;
  int height = map->capacity / 1024;
  fprintf(fp, "P6\n%d %d\n255\n", width, height);
  for (int y = 0; y < height; y++)
  {
    for (int x = 0; x < width; x++)
    {
      static unsigned char color[3];
      color[0] = 0;  /* red */
      color[1] = 0;  /* green */
      color[2] = 0;  /* blue */
      
      int32_t index = y * width + x;
      MapEntry* entry = &map->entries[index];
      if (!IS_UNDEFINED(entry->key))
      {
        int32_t ideal = hashValue(entry->key) % map->capacity;
        int64_t distance = (int64_t)index - (int64_t)ideal;
        
        if (distance < 0) distance += map->capacity;
        color[0] = distance % 255;
        color[1] = (distance / 255) % 255;
        color[2] = 128;
      }
      
      fwrite(color, 1, 3, fp);
    }
  }
  fclose(fp);
}