// Speed making histogram from BWImage: ~1 GPixels/s

// Time to get median brightness: ~1us

sclass BWImageHistogram {

  IBWImage image; // optional

  int[] data; // pixel count for each brightness from 0 to 255

  int nPixels; // sum of pixel count

  int medianBrightness = -1;

  int medianSum;

  *(BufferedImage image) {

    this(BWImage(image));

  }

  *(BWImage *image) {

    nPixels = area(image);

    data = bwImageHistogramArray(image);

  }

  toString { ret "BWImageHistogram of " + nPixels(nPixels)

    + (image == null ? "" : appendBracketed(str_px(image))); }

  int[] getArray() { ret data; }

  int medianSumTarget() { ret (nPixels+1)/2; }

  int medianBrightness() {

    if (medianBrightness < 0) {

      int threshold = medianSumTarget(), sum = 0;

      for i to 256: {

        sum += data[i];

        if (sum > threshold) {

          medianBrightness = i;

          medianSum = sum;

          break;

        }

      }

    }

    ret medianBrightness;

  }

  // get pixel count for brightness

  int get(int brightness) { ret data[brightness]; }

  void printMedianStats() {

    int threshold = medianBrightness();

    int prevSum = threshold == 0 ? 0 : medianSum-get(threshold-1);

    printVars(+threshold, +prevSum, target := medianSumTarget(), sum := medianSum);

  }

}