persistable sclass ScanlineBitMatrix extends AbstractMatrix<Bool> is BitMatrix {
  int bx1 = -1, by1, bx2, by2; // bounding box of positive bits
  
  // format for each line: start x, end x (excl.), start x, end x, ...
  new IntBuffer scanlineData;
  
  // pointer into scanlineData for each line
  new IntBuffer scanlineStarts;
  
  *(BitMatrix matrix) {
    super(matrix.getWidth(), matrix.getHeight());
    
    int ptr = 0;
    for y to h: {
      scanlineStarts.add(ptr));
    }
  }
  
  public Bool get(int x, int y) {
    if (x < bx1 || y < by1 || x >= bx2 || x >= by2) false;
    int ptr = scanlineStarts.get(y-by1);
    
    int end = y == by2-1 ? scanlineData.size() : scanlineStarts.get(y-by1+1);
    x -= bx1;
    
    // linear search for now
    while (ptr < end) {
      int x1 = scanlineData.get(ptr++);
      if (x < x1) false;
      int x2 = scanlineData.get(ptr++);
      if (x < x2) true;
    }
    
    false;
  }
  
  public void set(int x, int y, Bool a) { unimplemented(); }
  
  Rect boundingBoxOfTrueBits() { ret bx1 < 0 ? null : rect(bx1, by1, bx2, by2); }
}