Diwali diya row on an Arduino Uno

// Diya row: a row of eight LED diyas for Diwali that light one after another,
// left to right, and then flicker together like small oil lamps along a
// windowsill or a doorstep.
//
//   diyas     8 WS2812B LEDs, DIN <- 330 ohms <- pin 6, one in each lamp
//   button    pin 2 -> button -> GND        (INPUT_PULLUP): light them again
//
// Diwali, the festival of lights, is celebrated by Hindus, Sikhs, Jains and
// many Buddhists. The name comes from the Sanskrit for "row of lights", and
// families line their homes with diyas, small clay lamps with a cotton wick
// in oil or ghee. Put one LED inside each clay diya, or behind a paper cutout
// of one.
//
// Each flame has its own state: a brightness that drifts toward a random goal
// and a small chance each frame of a quick dip, so the row never pulses in
// step. The color is a deep yellow with a touch of orange, the color of a
// small oil flame.
//
// The diyas light by themselves at power-up. Press the button and they go out
// and light again, one by one.
//
// Things to try. Press the button. Make it a longer row with DIYAS. Light
// them from both ends toward the middle.

#include <Adafruit_NeoPixel.h>

const int DIYAS = 8;
const int BUTTON_PIN = 2;

Adafruit_NeoPixel row(DIYAS, 6, NEO_GRB + NEO_KHZ800);

uint8_t level[DIYAS], goal[DIYAS];
int lit = 0;
unsigned long nextLight = 800;

void setup() {
  Serial.begin(9600);
  pinMode(BUTTON_PIN, INPUT_PULLUP);
  row.begin();
  row.setBrightness(180);
  row.show();
  for (int i = 0; i < DIYAS; i++) level[i] = goal[i] = 180;
  randomSeed(analogRead(A0));
  Serial.println("Happy Diwali! Lighting the diyas");
}

void loop() {
  unsigned long now = millis();
  if (digitalRead(BUTTON_PIN) == LOW) {
    lit = 0;
    nextLight = now + 600;
    row.clear();
    row.show();
    Serial.println("Lighting them again");
    while (digitalRead(BUTTON_PIN) == LOW) delay(10);
  }
  if (lit < DIYAS && now >= nextLight) {
    lit++;
    nextLight = now + 700;
    Serial.print("Diya ");
    Serial.print(lit);
    Serial.println(" lit");
    if (lit == DIYAS) Serial.println("The whole row is glowing");
  }

  for (int i = 0; i < DIYAS; i++) {
    if (i >= lit) {
      row.setPixelColor(i, 0);
      continue;
    }
    if (abs(level[i] - goal[i]) < 3) goal[i] = random(150, 253);
    level[i] += level[i] < goal[i] ? 3 : -3;
    int v = level[i];
    if (random(0, 40) == 0) v = v * 2 / 3;   // a quick dip in a draft
    row.setPixelColor(i, v, v * 11 / 20, v / 16);
  }
  row.show();
  delay(20);
}