Reaction duel on an Arduino Uno

// Reaction duel: two players, one lamp, and whoever presses first wins.
//
//   pin 9   the go lamp, through 220 ohm to GND
//   pin 10  player one's lamp        pin 2  player one's button, to GND
//   pin 11  player two's lamp        pin 3  player two's button, to GND
//   pin 7   a passive piezo straight to GND
//
// Both buttons use the pad's internal pull-up, so a button is one wire to the
// pin and one to ground and nothing else. A round waits a random one and a
// half to four seconds, lights the go lamp, and then watches both pins until
// one of them falls.
//
// Pressing before the lamp is a false start and hands the round to the other
// player, which is the rule that makes the game worth playing: you cannot win
// by holding the button down.
//
// The margin is the gap between the two presses. Anything under twenty
// milliseconds is a tie as far as a person is concerned, and the sketch says
// so rather than printing a number nobody could have felt.

#include "mokxi_tone.h"

const int GO_LAMP = 9;
const int LAMP_A = 10;
const int LAMP_B = 11;
const int BUTTON_A = 2;
const int BUTTON_B = 3;
const int PIEZO = 7;

/** How long the round waits for the second player before calling it. */
const long LATE_MS = 400;

static bool downA() { return digitalRead(BUTTON_A) == LOW; }
static bool downB() { return digitalRead(BUTTON_B) == LOW; }

static void win(int lamp, const char *who, long ms, long margin) {
  digitalWrite(GO_LAMP, LOW);
  digitalWrite(lamp, HIGH);
  Serial.print(who);
  Serial.print(" wins in ");
  Serial.print(ms);
  Serial.print(" ms, by ");
  if (margin < 20) {
    Serial.println("a whisker");
  } else {
    Serial.print(margin);
    Serial.println(" ms");
  }
  mokxiTone(PIEZO, NOTE_C5, 90);
  mokxiTone(PIEZO, NOTE_G5, 140);
  delay(900);
  digitalWrite(lamp, LOW);
}

static void falseStart(const char *who, int otherLamp) {
  Serial.print(who);
  Serial.println(" jumped the lamp: the round goes the other way");
  mokxiTone(PIEZO, 160, 250);
  digitalWrite(otherLamp, HIGH);
  delay(700);
  digitalWrite(otherLamp, LOW);
}

void setup() {
  pinMode(GO_LAMP, OUTPUT);
  pinMode(LAMP_A, OUTPUT);
  pinMode(LAMP_B, OUTPUT);
  pinMode(PIEZO, OUTPUT);
  pinMode(BUTTON_A, INPUT_PULLUP);
  pinMode(BUTTON_B, INPUT_PULLUP);
  Serial.begin(115200);
  Serial.println("Mokxi Arduino Uno: reaction duel");
  Serial.println("wait for the middle lamp, then press first");
}

void loop() {
  digitalWrite(GO_LAMP, LOW);
  digitalWrite(LAMP_A, LOW);
  digitalWrite(LAMP_B, LOW);

  // a round starts with both buttons up
  while (downA() || downB()) {
    delay(10);
  }
  delay(400);

  unsigned long wait = 1500UL + (unsigned long)random(2500);
  unsigned long start = millis();
  while (millis() - start < wait) {
    if (downA()) {
      falseStart("player one", LAMP_B);
      return;
    }
    if (downB()) {
      falseStart("player two", LAMP_A);
      return;
    }
    delay(2);
  }

  digitalWrite(GO_LAMP, HIGH);
  unsigned long lit = millis();
  int winner = 0;
  long winnerMs = 0;
  long loserMs = -1;

  while (millis() - lit < 5000UL) {
    long now = (long)(millis() - lit);
    if (winner == 0) {
      if (downA()) {
        winner = 1;
        winnerMs = now;
      } else if (downB()) {
        winner = 2;
        winnerMs = now;
      }
    } else if (winner == 1 && downB()) {
      loserMs = now;
      break;
    } else if (winner == 2 && downA()) {
      loserMs = now;
      break;
    } else if (now - winnerMs > LATE_MS) {
      break;
    }
    delay(1);
  }

  if (winner == 0) {
    Serial.println("nobody pressed: round abandoned");
    mokxiTone(PIEZO, 220, 200);
    return;
  }

  long margin = loserMs < 0 ? LATE_MS : loserMs - winnerMs;
  if (winner == 1) {
    win(LAMP_A, "player one", winnerMs, margin);
  } else {
    win(LAMP_B, "player two", winnerMs, margin);
  }
}