!7

import java.awt.geom.*;

p {
  BufferedImage img = loadBufferedImage(#1006973);
  BWImage bw = getAlphaChannel(img);
  Image2B alpha = new Image2B(bw);
  
  int w = alpha.w(), h = alpha.h();
  
  Path2D.Float path = new Path2D.Float();
  
  int x = w/2;
  int y = -1;
  while (y < h && !alpha.getPixel(x, y+1)) ++y;
  
  int x1 = x, y1 = y;
  path.moveTo(x, y);
  
  new HashSet<Pt> seen;
  while (!seen.add(new Pt(x, y))) {
    int q = getQuadrant(alpha, x, y);
    
    if (q == 0x0011) --x;
    else if (q == 0x1100) ++x;
    if (q == 0x0101) ++y;
    else if (q == 0x1010) --y;
    else if (q == 0x0111) { --x; ++y; }
    else if (q == 0x1011) { --x; --y; }
    else fail(q);
    // TODO

    path.lineTo(x, y);
  } // while (x != x1 || y != y1);

  showImage(alpha);
  showImage(renderShape(path, w, h));
}

static int getQuadrant(Image2B img, int x, int y) {
  ret (getQuadrant_getPixel(img, x, y) ? 0x1000 : 0) +
    (getQuadrant_getPixel(img, x+1, y) ? 0x0100 : 0) +
    (getQuadrant_getPixel(img, x, y+1) ? 0x0010 : 0) +
    (getQuadrant_getPixel(img, x+1, y+1) ? 0x0001 : 0);
}

static bool getQuadrant_getPixel(Image2B img, int x, int y) {
  ret x >= 0 && x < img.w && y >= 0 && < img.h && img.getPixel(x, y);
}