Lunar New Year lantern on an Arduino Uno

// Lunar New Year lantern: a red paper lantern with a ring of LEDs inside that
// glows and flickers like a candle, and turns itself on at dusk.
//
//   light sensor  module AO -> A0, VCC -> 5 V, GND -> GND
//   LED ring      12 WS2812B LEDs, DIN <- 330 ohms <- pin 6, VDD -> 5 V, VSS -> GND
//
// Red lanterns are hung for Lunar New Year in many communities, and for the
// Lantern Festival that closes the celebration. This one is red and gold:
// every LED is a warm red, and now and then one warms toward gold, the way a
// flame wanders.
//
// Why it flickers this way: a real flame is never still, but it does not
// strobe either. Each LED drifts toward a new random brightness one step at a
// time, so the glow breathes.
//
// The light module's reading rises as the room gets darker. Two thresholds,
// one to turn on and a brighter one to turn off, keep it from blinking at
// dusk while the light hovers near one number.
//
// Things to try. Drag the light sensor's slider down to make it dusk. Change
// GOLD_CHANCE for a calmer or livelier flame.

#include <Adafruit_NeoPixel.h>

const int RING_PIN = 6;
const int LEDS = 12;
const int DARK_ABOVE = 700;    // A0 reading: on above this (about 25 lux)
const int LIGHT_BELOW = 600;   // and off below this
const int GOLD_CHANCE = 20;    // 1 in this many frames, an LED warms toward gold

Adafruit_NeoPixel ring(LEDS, RING_PIN, NEO_GRB + NEO_KHZ800);

uint8_t level[LEDS], goal[LEDS], gold[LEDS];
bool lit = false;
unsigned long nextFrame = 0;

void setup() {
  Serial.begin(9600);
  ring.begin();
  ring.setBrightness(170);
  ring.show();
  for (int i = 0; i < LEDS; i++) {
    level[i] = 150;
    goal[i] = 150;
  }
  Serial.println("Lantern ready: it lights when the room gets dark");
}

void flicker() {
  for (int i = 0; i < LEDS; i++) {
    if (level[i] == goal[i]) goal[i] = random(110, 256);
    if (level[i] < goal[i]) level[i]++;
    else level[i]--;
    if (random(0, GOLD_CHANCE) == 0) gold[i] = 120;
    if (gold[i] > 0) gold[i]--;
    uint8_t v = level[i];
    // Red, with some green mixed in for the gold moments.
    ring.setPixelColor(i, v, (uint16_t)gold[i] * v / 255, 0);
  }
}

void loop() {
  int light = analogRead(A0);
  if (!lit && light > DARK_ABOVE) {
    lit = true;
    Serial.println("Dusk: lantern on");
  } else if (lit && light < LIGHT_BELOW) {
    lit = false;
    ring.clear();
    ring.show();
    Serial.println("Daylight: lantern off");
  }
  if (!lit || millis() < nextFrame) return;
  nextFrame = millis() + 15;
  flicker();
  ring.show();
}