#include "FastLED.h"

#define NUM_ROWS 16 
#define NUM_COLS 16 
#define NUM_LEDS 256
#define DATA_PIN 3

CRGB leds[NUM_LEDS];  
 
void setup() {

  FastLED.addLeds<NEOPIXEL, DATA_PIN>(leds, NUM_LEDS);
  FastLED.setBrightness(126);  
  
} 

void loop() { 

    for (byte j = 0; j < NUM_COLS; j++) {

        for (byte i = 0; i < NUM_ROWS; i++) {

            leds[XY(i, j)] = CRGB(126, 0, 0);
            FastLED.show();
            delay(500);
            leds[XY(i, j)] = CRGB(0,   0, 0);     

        }

    }

}

byte XY(byte x, byte y) { 

    if (y % 2 == 0) {

        return y * NUM_COLS + x;

    } else {

        return ((y + 1) * NUM_COLS - 1) - x;

    }

}