Obstacle-avoiding car on an Arduino Uno

// Obstacle-avoiding car: a 2WD chassis, an L298N and an HC-SR04.
//
//   L298N    ENA pin 5, IN1 pin 6, IN2 pin 7    left motor (ENA jumper off)
//            ENB pin 10, IN3 pin 8, IN4 pin 9   right motor (ENB jumper off)
//            12V from the 7.4 V pack, GND to the Uno's GND, regulator jumper on
//   HC-SR04  TRIG pin 12, ECHO pin 11, VCC to 5V, GND
//
// The whole robot is one loop: look, then decide. It pings every 60 ms,
// because the datasheet asks for that much quiet between pings so the last
// echo has died away. Clear ahead, it drives. Something closer than STOP_MM,
// it stops, backs off, turns right on the spot and looks again.
//
// On the canvas, drag the HC-SR04's slider to put something in front of the
// car, and watch it back away and turn on the floor drawn in the chassis.
// A reading of 0 means no echo at all, which is "nothing there", not "right
// against the sensor".

#include "mokxi_hcsr04.h"

const int ENA = 5;
const int IN1 = 6;
const int IN2 = 7;
const int ENB = 10;
const int IN3 = 8;
const int IN4 = 9;

const int TRIG = 12;
const int ECHO = 11;

const uint16_t STOP_MM = 250;
const int CRUISE = 200;

Sonar sonar(TRIG, ECHO);

// One side of the car, as a signed speed from -255 to 255. Zero brakes.
void side(int en, int a, int b, int speed) {
  digitalWrite(a, speed > 0 ? HIGH : LOW);
  digitalWrite(b, speed < 0 ? HIGH : LOW);
  if (speed < 0) speed = -speed;
  analogWrite(en, speed == 0 ? 255 : (speed > 255 ? 255 : speed));
}

void drive(int left, int right) {
  side(ENA, IN1, IN2, left);
  side(ENB, IN3, IN4, right);
}

void setup() {
  const int pins[] = {ENA, IN1, IN2, ENB, IN3, IN4};
  for (int i = 0; i < 6; i++) {
    pinMode(pins[i], OUTPUT);
    digitalWrite(pins[i], LOW);
  }
  sonar.begin();
  Serial.begin(115200);
  Serial.println("obstacle-avoiding car: ready");
}

void loop() {
  uint16_t mm = sonar.read();
  if (mm != 0 && mm < STOP_MM) {
    Serial.print("obstacle at ");
    Serial.print(mm);
    Serial.println(" mm: back up and turn");
    drive(0, 0);
    delay(200);
    drive(-CRUISE, -CRUISE);
    delay(400);
    drive(CRUISE, -CRUISE);
    delay(450);
    drive(0, 0);
    delay(100);
  } else {
    drive(CRUISE, CRUISE);
  }
  delay(60);
}