P-Controller

const int trigPin = 9;
const int echoPin = 10;

const int ENA = 5;
const int IN1 = 2;
const int IN2 = 3;

const int ENB = 6;
const int IN3 = 4;
const int IN4 = 7;

float set_point = 15.0;  // Stop 15 cm from wall
float gain = 10.0;      // Proportional gain

void setup() {
  pinMode(trigPin, OUTPUT);
  pinMode(echoPin, INPUT);

  pinMode(ENA, OUTPUT);
  pinMode(ENB, OUTPUT);
  pinMode(IN1, OUTPUT);
  pinMode(IN2, OUTPUT);
  pinMode(IN3, OUTPUT);
  pinMode(IN4, OUTPUT);
}

void loop() {
  float distance = getDistance();

  // P controller
  float error = distance - set_point;
  int speed = gain * error;

  drive(speed);

  delay(50);
}

void stopMotors() {
  analogWrite(ENA, 0);
  analogWrite(ENB, 0);
}

float getDistance() {
  // Measure distance
  digitalWrite(trigPin, LOW);
  delayMicroseconds(2);
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10);
  digitalWrite(trigPin, LOW);

  long duration = pulseIn(echoPin, HIGH, 30000);

  float distance = duration * 0.0343 / 2;
  return distance;
}

float drive(int speed) {
  // consider constraining speed to 0-255
  if (speed >= 0) {
    // Drive forward
    digitalWrite(IN1, HIGH);
    digitalWrite(IN2, LOW);
    digitalWrite(IN3, HIGH);
    digitalWrite(IN4, LOW);

    analogWrite(ENA, speed);
    analogWrite(ENB, speed);
  } else if (speed <= 0) {
    // Drive backwards
    digitalWrite(IN1, LOW);
    digitalWrite(IN2, HIGH);
    digitalWrite(IN3, LOW);
    digitalWrite(IN4, HIGH);

    analogWrite(ENA, speed);
    analogWrite(ENB, speed);

  }

}