// 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.
sclass G22RegionThinner_LookupTable {
  byte[] table;
  
  *() {
    table = new byte[256/8];
    
    for imgPattern to 256:
      setBit(table, imgPattern, calculatePattern(imgPattern));
  }

  // imgPattern bit set = black  
  bool calculatePattern(int imgPattern) {
    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(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
    var 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
          
          int sides = 4-(int)
            (bwImage.getFloatPixel(1, 0)
            + bwImage.getFloatPixel(0, 1)
            + bwImage.getFloatPixel(2, 1)
            + bwImage.getFloatPixel(1, 2));
          delete = sides < 3;
        }
      }
    } else
      // we have more than one region - pixel was
      // structurally important, keep it
      {}

    ret delete;
  }
}