Spinning zoetrope on an Arduino Uno

// Spinning zoetrope: an L293D H-bridge runs a DC motor either way, at a speed
// set by a knob, with one button for on and off and one for direction.
//
//   L293D   EN12 -> pin 9 (speed, PWM), IN1 -> pin 3, IN2 -> pin 2
//           OUT1 and OUT2 -> the motor, VS -> the 9 V battery's +
//           VSS -> 5V, all four GND pins -> GND
//   pin 5   the on/off button, with a 10k pull-down
//   pin 4   the direction button, with a 10k pull-down
//   A0      the potentiometer's wiper (ends to 5V and GND)
//
// An H-bridge is four switches around the motor. IN1 high and IN2 low lets
// current through one way; swap them and it flows the other way, and the motor
// turns backward. EN12 switches the whole half-bridge on, so PWM on it sets
// the speed.
//
// Each button changes something on the press, not while it is held, so the
// sketch remembers the last reading and acts on the change from LOW to HIGH.

const int ENABLE = 9;
const int IN1 = 3;
const int IN2 = 2;
const int ON_OFF = 5;
const int DIRECTION = 4;

bool running = false;
bool forward = true;
int lastOnOff = LOW;
int lastDirection = LOW;

void report() {
  Serial.print(running ? "running" : "stopped");
  Serial.println(forward ? ", forward" : ", backward");
}

void setup() {
  pinMode(ENABLE, OUTPUT);
  pinMode(IN1, OUTPUT);
  pinMode(IN2, OUTPUT);
  pinMode(ON_OFF, INPUT);
  pinMode(DIRECTION, INPUT);
  Serial.begin(9600);
  Serial.println("Mokxi Uno: spinning zoetrope");
  report();
}

void loop() {
  int onOff = digitalRead(ON_OFF);
  if (onOff == HIGH && lastOnOff == LOW) {
    running = !running;
    report();
  }
  lastOnOff = onOff;

  int direction = digitalRead(DIRECTION);
  if (direction == HIGH && lastDirection == LOW) {
    forward = !forward;
    report();
  }
  lastDirection = direction;

  int speed = analogRead(A0) / 4;
  digitalWrite(IN1, forward ? HIGH : LOW);
  digitalWrite(IN2, forward ? LOW : HIGH);
  analogWrite(ENABLE, running ? speed : 0);
  delay(10);
}