sclass GSI is HasBounds {
  int y1; // starting y coordinate
  int[] rowStarts; // where in data each row starts
  int[] data; // all x coordinates in a long list (x1, x2, x1, x2, ...)
  
  *(int *y1, int[] *rowStarts, int[] *data) {}
  
  public int getHeight() { ret l(data)/2; }
  int y2() { ret y1+rowStarts.length; }
  
  // bounds are calculated & cached transiently
  public simplyCached Rect bounds() {
    int x1 = Int.MAX_VALUE, x2 = 0;
    for (int i = 0; i < l(data); i += 2) {
      x1 = min(x1, data[i]);
      x2 = max(x2, data[i+1]);
    }
    ret rectFromPoints(x1, y1, x2, y2());
  }
  
  public bool contains(Pt p) { ret contains(p.x, p.y); }
  public bool contains(int x, int y) {
    if (y < y1) false;
    int i = (y-y1)*2;
    if (i >= l(data)) false;
    ret x >= data[i] && x < data[i+1];
  }
  
  int rowStart(int y) { ret rowStarts[y-y1]; }
  int rowEnd(int y) {
    ret y == y2()-1 ? data.length : data[y-y1+1];
  }
}