Rainbow to the pot of gold on an Arduino Uno

// Rainbow to the pot of gold: an arch of LEDs runs the rainbow toward a pot
// of gold that shimmers at the end, for St. Patrick's Day.
//
//   rainbow   20 WS2812B LEDs, DIN <- 330 ohms <- pin 6
//   gold      4 WS2812B LEDs in the pot, DIN <- 330 ohms <- pin 5
//   knob      potentiometer wiper -> A0, ends to 5 V and GND: the speed
//
// The arch is a strip bent over a doorway or along a shelf, and the pot is a
// little black cup with four LEDs in it under some gold paper coins. The
// legend says a pot of gold waits at the end of every rainbow; here it is
// always there.
//
// The rainbow moves by shifting where the color wheel starts. ColorHSV takes
// a hue from 0 to 65535 all the way round, so spreading one turn across the
// 20 LEDs and adding a little to the start every frame makes the colors flow
// along the arch. The knob sets how much is added, from a slow drift to a
// quick run, and the gold LEDs twinkle on their own.
//
// Things to try. Turn the knob. Make the arch flow the other way. Give the
// pot a green shimmer instead.

#include <Adafruit_NeoPixel.h>

const int ARCH = 20, POT = 4;
Adafruit_NeoPixel arch(ARCH, 6, NEO_GRB + NEO_KHZ800);
Adafruit_NeoPixel gold(POT, 5, NEO_GRB + NEO_KHZ800);

uint16_t start = 0;
unsigned long frames = 0;

void setup() {
  Serial.begin(9600);
  arch.begin();
  gold.begin();
  arch.setBrightness(140);
  gold.setBrightness(200);
  Serial.println("Happy St. Patrick's Day! Follow the rainbow");
}

void loop() {
  int speed = map(analogRead(A0), 0, 1023, 32, 1024);
  start += speed;
  for (int i = 0; i < ARCH; i++) {
    uint16_t hue = start + (uint32_t)i * 65536 / ARCH;
    arch.setPixelColor(i, arch.gamma32(arch.ColorHSV(hue)));
  }
  for (int i = 0; i < POT; i++) {
    int glint = random(0, 10) == 0 ? 255 : random(140, 200);
    gold.setPixelColor(i, glint, glint * 3 / 5, 0);
  }
  arch.show();
  gold.show();
  frames++;
  if (frames % 200 == 1) {
    Serial.print("Rainbow speed ");
    Serial.println(speed);
  }
  delay(20);
}