uint8_t XY (uint8_t x, uint8_t y) {
  // any out of bounds address maps to the first hidden pixel
  if ( (x >= LED_X) || (y >= LED_Y) ) {
    return (LAST_VISIBLE_LED + 1);
  }

  const uint8_t XYTable[] = {
     7,   8,  23,  24,
     6,   9,  22,  25,
     5,  10,  21,  26,
     4,  11,  20,  27,
     3,  12,  19,  28,
     2,  13,  18,  29,
     1,  14,  17,  30,
     0,  15,  16,  31
  };

  uint8_t i = (y * LED_X) + x;
  uint8_t j = XYTable[i];
  return j;
}