// translates from x, y into an index into the LED array and
// finds the right index for a S shaped matrix
int XY(int x, int y) {
  if(y > HEIGHT) {
    y = HEIGHT;
  }
  if(y < 0) {
    y = 0;
  }
  if(x > WIDTH) {
    x = WIDTH;
  }
  if(x < 0) {
    x = 0;
  }
  // for a serpentine layout reverse every 2nd row:
  if(x % 2 == 1) {
    return (x * (WIDTH) + (HEIGHT - y -1));
  }
  else {
    // use that line only, if you have all rows beginning at the same side
    return (x * (WIDTH) + y);
  }
}