Marigold path on an Arduino Uno

// Marigold path: a strip of LEDs laid along the floor like a path of marigold
// petals, with a glow of orange and gold that travels from the door to the
// ofrenda, where a cluster of marigold lights glows warm.
//
//   path      16 WS2812B LEDs, DIN <- 330 ohms <- pin 6 (LED 0 at the door)
//   altar     4 WS2812B LEDs, DIN <- 330 ohms <- pin 5
//   knob      potentiometer wiper -> A0, ends to 5 V and GND: the pace
//
// For Día de los Muertos, families scatter petals of cempasúchil, the Mexican
// marigold, in a path from the front door to the ofrenda. Their bright color
// and scent are said to guide the spirits home.
//
// The path glows a soft marigold all along, and a brighter wave of gold
// travels slowly from the door toward the altar, again and again. The altar's
// four LEDs breathe in deep orange. Every LED's color is a mix of red and a
// little less green: that is the whole range from orange to gold.
//
// Things to try. Turn the knob to slow the wave. Make the wave longer with
// WAVE. Lay the strip in a curve around the room.

#include <Adafruit_NeoPixel.h>

const int PATH = 16, ALTAR = 4;
const int WAVE = 4;   // how many LEDs the bright wave covers

Adafruit_NeoPixel path(PATH, 6, NEO_GRB + NEO_KHZ800);
Adafruit_NeoPixel altar(ALTAR, 5, NEO_GRB + NEO_KHZ800);

int head = 0;   // in 1/8ths of an LED
unsigned long frames = 0;

void setup() {
  Serial.begin(9600);
  path.begin();
  altar.begin();
  path.setBrightness(160);
  altar.setBrightness(200);
  Serial.println("The marigold path is glowing");
}

void loop() {
  int pace = map(analogRead(A0), 0, 1023, 1, 6);
  head += pace;
  if (head >= (PATH + WAVE) * 8) {
    head = 0;
    Serial.println("The glow reached the ofrenda");
  }

  for (int i = 0; i < PATH; i++) {
    int r = 120, g = 45;   // the resting marigold
    int d = head / 8 - i;  // how far behind the wave's front
    if (d >= 0 && d < WAVE) {
      r = 255;
      g = 150 - d * 25;    // gold at the front, orange behind
    }
    path.setPixelColor(i, r, g, 0);
  }

  // The altar breathes slowly between deep orange and gold.
  int t = (frames * 3) % 512;
  int glow = t < 256 ? t : 511 - t;
  for (int i = 0; i < ALTAR; i++) altar.setPixelColor(i, 200 + glow / 5, 60 + glow / 4, 0);

  path.show();
  altar.show();
  frames++;
  delay(25);
}