// returns an array of length 256, each cell indicating how often
// each brightness (0 to 255) was encountered
static int[] bwImageHistogramArray(BWImage img) {
  int[] histogram = new[256];
  byte[] pixels = img.pixels;
  for i over pixels:
    histogram[ubyteToInt(pixels[i])]++;
  ret histogram;
}

static int[] bwImageHistogramArray(BufferedImage img) {
  ret bwImageHistogramArray(BWImage(img));
}