Tug of war game on an Arduino Nano

// Tug of war: two players, two buttons and a strip of 15 NeoPixels on an
// Arduino Nano. Whoever presses faster pulls the light to their end.
//
//   red button    D2 and GND (INPUT_PULLUP: a press reads LOW)
//   blue button   D12 and GND
//   LED strip     DIN -> D11, 5V -> 5V, GND -> GND (15 WS2812B LEDs)
//   piezo         D13 and GND
//
// The white light is the knot in the rope. It starts in the middle. Every
// press by red pulls it one LED toward red's end of the strip (the first LED,
// at DIN), every press by blue one LED toward the last. The LEDs on each side
// of the knot glow in the color of the player who owns that end. Pull the
// knot right off your end to win the round.
//
// A round starts with a countdown: three short beeps and a high one for GO.
// Presses before GO do not count. After a win the strip flashes the winner's
// color, the score goes to the Serial Monitor, and a new round starts.
//
// # Counting presses fairly
//
// A press is the moment a pin goes from HIGH to LOW. The sketch remembers
// each pin's last reading and only counts that edge, so holding a button down
// pulls once, not over and over. Contacts bounce for a few milliseconds, so a
// second edge within 30 ms of the first is ignored.

#include <Adafruit_NeoPixel.h>

const uint8_t RED_BUTTON = 2;
const uint8_t BLUE_BUTTON = 12;
const uint8_t STRIP_PIN = 11;
const uint8_t PIEZO = 13;
const uint8_t LEDS = 15;
const int8_t END = LEDS / 2;  // 7: the knot wins at -8 or +8

Adafruit_NeoPixel strip(LEDS, STRIP_PIN, NEO_GRB + NEO_KHZ800);

// Where the knot is: 0 is the middle, negative is toward red.
int8_t knot = 0;
uint8_t redWins = 0;
uint8_t blueWins = 0;

struct Player {
  uint8_t pin;
  bool last;
  unsigned long at;
};
Player red = {RED_BUTTON, HIGH, 0};
Player blue = {BLUE_BUTTON, HIGH, 0};

// True once for each new press.
bool pressed(Player &p) {
  bool now = digitalRead(p.pin);
  bool edge = now == LOW && p.last == HIGH && millis() - p.at > 30;
  if (now != p.last) p.at = millis();
  p.last = now;
  return edge;
}

void draw() {
  int8_t at = END + knot;  // the knot's LED, 0 to 14
  for (int8_t i = 0; i < LEDS; i++) {
    if (i == at) {
      strip.setPixelColor(i, strip.Color(255, 255, 255));
    } else if (i < at) {
      strip.setPixelColor(i, strip.Color(60, 0, 0));
    } else {
      strip.setPixelColor(i, strip.Color(0, 0, 60));
    }
  }
  strip.show();
}

void countdown() {
  strip.clear();
  strip.show();
  Serial.println("ready...");
  for (uint8_t i = 0; i < 3; i++) {
    tone(PIEZO, 440, 150);
    delay(600);
  }
  tone(PIEZO, 880, 300);
  Serial.println("GO!");
  knot = 0;
  // Forget anything pressed during the countdown.
  pressed(red);
  pressed(blue);
  draw();
}

void win(bool redWon) {
  if (redWon) {
    redWins++;
  } else {
    blueWins++;
  }
  Serial.print(redWon ? "red wins! " : "blue wins! ");
  Serial.print("score: red ");
  Serial.print(redWins);
  Serial.print(", blue ");
  Serial.println(blueWins);
  uint32_t color = redWon ? strip.Color(255, 0, 0) : strip.Color(0, 0, 255);
  // a little fanfare, the strip flashing in time
  const unsigned int tune[4] = {523, 659, 784, 1047};
  for (uint8_t i = 0; i < 4; i++) {
    strip.fill(color);
    strip.show();
    tone(PIEZO, tune[i], 150);
    delay(180);
    strip.clear();
    strip.show();
    delay(70);
  }
  strip.fill(color);
  strip.show();
  delay(1500);
}

void setup() {
  Serial.begin(115200);
  pinMode(RED_BUTTON, INPUT_PULLUP);
  pinMode(BLUE_BUTTON, INPUT_PULLUP);
  pinMode(PIEZO, OUTPUT);
  strip.begin();
  strip.setBrightness(80);
  Serial.println("Tug of war: red on D2, blue on D12");
  countdown();
}

void loop() {
  bool moved = false;
  if (pressed(red)) {
    knot--;
    moved = true;
  }
  if (pressed(blue)) {
    knot++;
    moved = true;
  }
  if (!moved) return;
  tone(PIEZO, 300 + 40 * (knot + END), 15);
  if (knot < -END) {
    win(true);
    countdown();
  } else if (knot > END) {
    win(false);
    countdown();
  } else {
    draw();
    Serial.print("knot at ");
    Serial.println(knot);
  }
}