# https://sider.ai/share/e04a80d5d85ffb9350ffc4f3ac68a2d4  for explaination





#include <Adafruit_NeoPixel.h>

#define PIN       3
#define NUMPIXELS 256 // 16x16 matrix

Adafruit_NeoPixel pixels = Adafruit_NeoPixel(NUMPIXELS, PIN, NEO_GRB + NEO_KHZ800);

void setup() {
  pixels.begin();
  pixels.clear();
  pixels.show();
}

void loop() {
  rainbow(10000);
}

void rainbow(unsigned long duration) {
  uint16_t i, j;

  for(j=0; j<256; j++) {
    for(i=0; i<NUMPIXELS; i++) {
      pixels.setPixelColor(i, Wheel((i+j) & 255));
    }
    pixels.show();
    delay(duration / 256);
  }
}

// Input a value 0 to 255 to get a color value.
// The colours are a transition r - g - b - back to r.
uint32_t Wheel(byte WheelPos) {
  WheelPos = 255 - WheelPos;
  if(WheelPos < 85) {
    return pixels.Color(255 - WheelPos * 2, 0, WheelPos * 3);
  }
  if(WheelPos < 170) {
    WheelPos -= 85;
    return pixels.Color(0, WheelPos * 2, 255 - WheelPos * 3);
  }
  WheelPos -= 170;
  return pixels.Color(WheelPos * 2, 255 - WheelPos * 3, 0);
}