Defuse the doomsday device on an Arduino Uno

// Defuse the doomsday device: a TM1637 timer, three "wires", a clue LED and a
// piezo, on an Uno.
//
//   A0, A1        the TM1637's CLK and DIO (analog pins work as digital ones)
//   pin 2         clue LED (yellow)
//   pin 3         SAFE LED (green)
//   pin 4         BOOM LED (red)
//   pins 6, 7, 8  wire 1, wire 2, wire 3, each a button to GND
//   pin 10        piezo
//
// The clock starts at 60 seconds. The clue LED blinks one, two or three times,
// pauses, and blinks again. Cut the wire with that number: press its button.
// The right wire stops the clock and the green LED lights. A wrong wire costs
// ten seconds and a strike, and the third strike, or zero on the clock, sets
// off the red LED. The piezo ticks once a second, and twice as often in the
// last ten. Power-cycle (Stop, then Run) to get a new bomb.
//
// # A game is a state machine
//
// There are only three states: ARMED, SAFE and BOOM. loop() checks which one it
// is in and does that state's work, and the only way out of ARMED is through
// the checks in cutWire() and tick(). Writing the states down first makes it
// hard to get a bomb that is both defused and exploded.
//
// # No delay() anywhere
//
// A delay() would freeze the buttons while the clue LED blinked. Instead each
// job keeps the time it last ran, and loop() asks "is it my turn yet?" with
// millis(). The clock, the clue pattern and the tick all share one loop.

#include <TM1637Display.h>

const uint8_t WIRE[3] = {6, 7, 8};
const uint8_t CLUE = 2;
const uint8_t SAFE_LED = 3;
const uint8_t BOOM_LED = 4;
const uint8_t PIEZO = 10;

const int START_SECONDS = 60;
const int PENALTY = 10;
const int MAX_STRIKES = 3;

enum State { ARMED, SAFE, BOOM };

TM1637Display display(A0, A1);

State state = ARMED;
int secondsLeft = START_SECONDS;
int strikes = 0;
int answer;                      // 1 to 3: the wire that defuses it
bool cut[3];                     // wires already cut
bool wasDown[3];                 // for catching the press, not the hold
unsigned long lastSecond = 0;
unsigned long clueStart = 0;
bool halfTicked = false;      // the faster tick in the last ten seconds

void show() {
  // minutes and seconds with the colon lit
  display.showNumberDecEx((secondsLeft / 60) * 100 + secondsLeft % 60, 0x40, true);
}

void explode() {
  state = BOOM;
  digitalWrite(BOOM_LED, HIGH);
  digitalWrite(CLUE, LOW);
  tone(PIEZO, 120, 1500);
  Serial.println("BOOM");
}

void defuse() {
  state = SAFE;
  digitalWrite(SAFE_LED, HIGH);
  digitalWrite(CLUE, LOW);
  tone(PIEZO, 1800, 400);
  Serial.print("defused with ");
  Serial.print(secondsLeft);
  Serial.println(" seconds left");
}

void cutWire(int n) {  // n is 1 to 3
  if (cut[n - 1]) return;
  cut[n - 1] = true;
  Serial.print("cut wire ");
  Serial.println(n);
  if (n == answer) {
    defuse();
    return;
  }
  strikes++;
  secondsLeft = max(0, secondsLeft - PENALTY);
  Serial.print("strike ");
  Serial.println(strikes);
  tone(PIEZO, 300, 300);
  show();
  if (strikes >= MAX_STRIKES || secondsLeft == 0) explode();
}

// The clue: `answer` blinks of 150 ms, then a 1.5 s pause, repeated.
void drawClue(unsigned long now) {
  unsigned long t = (now - clueStart) % (answer * 300UL + 1500UL);
  bool on = t < answer * 300UL && (t % 300UL) < 150UL;
  digitalWrite(CLUE, on ? HIGH : LOW);
}

void setup() {
  Serial.begin(115200);
  for (int i = 0; i < 3; i++) pinMode(WIRE[i], INPUT_PULLUP);
  pinMode(CLUE, OUTPUT);
  pinMode(SAFE_LED, OUTPUT);
  pinMode(BOOM_LED, OUTPUT);
  pinMode(PIEZO, OUTPUT);
  display.setBrightness(0x0f);
  // Nothing is wired to A2, so it reads noise: a different bomb each Run.
  randomSeed(analogRead(A2));
  answer = random(1, 4);
  show();
  Serial.println("doomsday device armed: count the blinks, cut that wire");
  clueStart = lastSecond = millis();
}

void loop() {
  unsigned long now = millis();

  if (state == ARMED) {
    // a press is a pin that is low now and was not low last time round
    for (int i = 0; i < 3; i++) {
      bool down = digitalRead(WIRE[i]) == LOW;
      if (down && !wasDown[i]) cutWire(i + 1);
      wasDown[i] = down;
    }
  }

  if (state == ARMED) {
    drawClue(now);
    if (now - lastSecond >= 1000) {
      lastSecond += 1000;
      halfTicked = false;
      secondsLeft--;
      show();
      tone(PIEZO, 2400, 20);
      if (secondsLeft == 0) explode();
    } else if (secondsLeft <= 10 && !halfTicked && now - lastSecond >= 500) {
      halfTicked = true;
      tone(PIEZO, 2400, 20);
    }
  }
}