// Lookup table to speed up the last phase of G22RegionThinner_v2.

// There are 256 possible neighborhood images for a 3x3 neighborhood

// excluding the central pixel.

// For each of these, we store whether the central pixel should be

// deleted (bit is 1 = deletable)

sclass G22RegionThinner_LookupTable {

  byte[] table;

  settable int lastPhaseThreshold1 = 2;

  settable int lastPhaseThreshold = 5;

  static byte[] defaultTable = null/*bytesFromHex(

    "80c8c84c00ce0004004e5c17000715010000dc04000000000007150101031100"

  )*/;

  // for debugging

  L<IImageRegion<BWImage>> regions;

  int sides;

  byte[] get aka table aka getTable() {

    if (table == null) {

      if (defaultTable != null && lastPhaseThreshold1 == 2 && lastPhaseThreshold == 5)

        table = defaultTable;

      else {

        table = new byte[256/8];

        for imgPattern to 256:

          setBit(table, imgPattern, calculatePattern(imgPattern));

      }

    }

    ret table;

  }

  S toHex() { ret bytesToHex(table()); }

  // imgPattern bit set = black  

  bool calculatePattern(int imgPattern) {

    // special case "cross" (don't delete center point)

    if (imgPattern == 0b01011010) false;

    Image2B img = new(3, 3);

    imgPattern = ~imgPattern;

    img.setPixel(0, 0, testBit(imgPattern, 0));

    img.setPixel(1, 0, testBit(imgPattern, 1));

    img.setPixel(2, 0, testBit(imgPattern, 2));

    img.setPixel(0, 1, testBit(imgPattern, 3));

    img.setPixel(1, 1, true);

    img.setPixel(2, 1, testBit(imgPattern, 4));

    img.setPixel(0, 2, testBit(imgPattern, 5));

    img.setPixel(1, 2, testBit(imgPattern, 6));

    img.setPixel(2, 2, testBit(imgPattern, 7));

    var bwImage = img.toBW();

    FastRegions_BWImage regionMaker = new(bwImage);

    regionMaker.withDiagonals(true);

    regionMaker.run();

    // get all the black regions out of the 3x3 image

    regions = regionMaker.regions();

    regions = filter(regions, r -> r.brightness() == 0);

    bool delete = false;

    int pixels = 0;

    // no regions? it's a lonely pixel - keep it

    if (empty(regions)) {}

    else if (l(regions) == 1) {

      // one region

      pixels = first(regions).numberOfPixels();

      // if it's only one or two pixels, we are at the end of a line - keep

      if (pixels <= lastPhaseThreshold1)

        delete = false;

      else {

        // delete pixel if the region is small, but not too small

        // (lastPhaseThreshold is 5 by default)

        if (pixels < lastPhaseThreshold)

          delete = true;

        else if (pixels == lastPhaseThreshold) {

          // if it's exactly 5 pixels, check out how far

          // the region "surrounds" the pixel (does it touch 2 or 3 sides?)

          // delete pixel if it is not surrounded

          sides = 4-(int)

            (bwImage.getFloatPixel(1, 0)

            + bwImage.getFloatPixel(0, 1)

            + bwImage.getFloatPixel(2, 1)

            + bwImage.getFloatPixel(1, 2));

          delete = sides <= 3; // XXX - formerly: < 3

        }

      }

    } else

      // we have more than one region - pixel was

      // structurally important, keep it

      {}

    ret delete;

  }

}