#include <Adafruit_NeoPixel.h>

#define PIN         6
#define NUMPIXELS  64

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

struct PixelState {
  uint8_t r, g, b;       // base color
  float brightness;      // 0.0 to 1.0
  int direction;         // 1 = fade in, -1 = fade out
  unsigned long lastUpdate; // last fade update
  int fadeSpeed;         // ms between brightness steps
};

PixelState pixelStates[NUMPIXELS];

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

  for (int i = 0; i < NUMPIXELS; i++) {
    pixelStates[i].r = random(0, 255);
    pixelStates[i].g = random(0, 255);
    pixelStates[i].b = random(0, 255);
    pixelStates[i].brightness = 0.0;
    pixelStates[i].direction = 1; // start fading in
    pixelStates[i].lastUpdate = millis();
    pixelStates[i].fadeSpeed = random(10, 30); // speed of fade steps
  }
}

void loop() {
  unsigned long currentMillis = millis();

  for (int i = 0; i < NUMPIXELS; i++) {
    if (currentMillis - pixelStates[i].lastUpdate >= pixelStates[i].fadeSpeed) {
      pixelStates[i].lastUpdate = currentMillis;

      // adjust brightness
      pixelStates[i].brightness += 0.02 * pixelStates[i].direction;

      if (pixelStates[i].brightness >= 1.0) {
        pixelStates[i].brightness = 1.0;
        pixelStates[i].direction = -1; // start fading out
      } else if (pixelStates[i].brightness <= 0.0) {
        pixelStates[i].brightness = 0.0;
        pixelStates[i].direction = 1; // start fading in again
        // pick a new random color
        pixelStates[i].r = random(0, 255);
        pixelStates[i].g = random(0, 255);
        pixelStates[i].b = random(0, 255);
        pixelStates[i].fadeSpeed = random(10, 30);
      }

      // apply brightness scaling
      uint8_t rVal = pixelStates[i].r * pixelStates[i].brightness;
      uint8_t gVal = pixelStates[i].g * pixelStates[i].brightness;
      uint8_t bVal = pixelStates[i].b * pixelStates[i].brightness;

      pixels.setPixelColor(i, pixels.Color(rVal, gVal, bVal));
    }
  }
  pixels.show();
}