#include <FastLED.h>

const uint8_t kMatrixWidth = 5;
const uint8_t kMatrixHeight = 30;
const bool    kMatrixSerpentineLayout = false; 

uint16_t XY( uint8_t x, uint8_t y)
{
  uint16_t i;

  if( kMatrixSerpentineLayout == false) {
    i = (y * kMatrixWidth) + x;
  }

  if( kMatrixSerpentineLayout == true) {
    if( y & 0x01) {
      // Odd rows run backwards
      uint8_t reverseX = (kMatrixWidth - 1) - x;
      i = (y * kMatrixWidth) + reverseX;
    } else {
      // Even rows run forwards
      i = (y * kMatrixWidth) + x;
    }
  }

  return i;
}

#define NUM_LEDS (kMatrixWidth * kMatrixHeight)
CRGB leds_plus_safety_pixel[ NUM_LEDS + 1];
CRGB* const leds( leds_plus_safety_pixel + 1);
uint16_t XYsafe( uint8_t x, uint8_t y)
{
  if( x >= kMatrixWidth) return -1;
  if( y >= kMatrixHeight) return -1;
  return XY(x,y);
}

int16_t XYsafe_signed( uint8_t x, uint8_t y)
{
  if( x >= kMatrixWidth) return -1;
  if( y >= kMatrixHeight) return -1;
  return XY(x,y);
}

void loop() {
  delay(1000);
}

void print_rgb(int32_t ledno) {
  Serial.print("leds_plus_safety_pixel[");
  Serial.print(ledno);
  Serial.print("] = ");
  Serial.print(leds_plus_safety_pixel[ledno].r);
  Serial.print(", ");
  Serial.print(leds_plus_safety_pixel[ledno].g);
  Serial.print(", ");
  Serial.println(leds_plus_safety_pixel[ledno].b);
}

void setup() {
  Serial.begin(9600);
  print_rgb(0);

  Serial.println("Setting leds[XYsafe(255, 255)] to white.");
  leds[XYsafe(255, 255)] = CRGB::White;
  print_rgb(0);

  Serial.println("Setting leds[XYsafe_signed(255, 255)] to white.");
  leds[XYsafe_signed(255, 255)] = CRGB::White;
  print_rgb(0);
}