// The pin numbers for the ultrasonic sensor's trigger and echo pins
const byte trigpin = 13;
const byte echopin = 12;

// An array of pin numbers for the LEDs
const byte ledpins[] = {11, 10, 9, 8};

// The wait time in seconds between each loop iteration
const float loop_wait_s = 0.1;

// Variables for storing measured distance and number of leds that need to be on
int distance;
byte onleds;

// Determine the wait time in milliseconds for use in delay
const unsigned long loop_wait_ms = loop_wait_s * 1000UL;
// Determine the total number of LEDs based on the array size
const byte numleds = sizeof(ledpins) / sizeof(byte);

// Setup
void setup() {
  // Begin serial communication at 9600 baud
  Serial.begin(9600);

  // Set the ultrasonic sensor pins
  pinMode(trigpin, OUTPUT);
  pinMode(echopin, INPUT);
  
  // Set each LED pin
  for (int i = 0; i < numleds; i++) {
    pinMode(ledpins[i], OUTPUT);
  }
}

// Send a trigger pulse for the ultrasonic sensor
// Code from Arduino lessons
void trigger() {
  digitalWrite(trigpin, LOW);
  delayMicroseconds(2);
  digitalWrite(trigpin, HIGH);
  delayMicroseconds(10);
  digitalWrite(trigpin, LOW);
}

// Loop
void loop() {
  // Trigger the ultrasonic sensor for distance measurement
  trigger();

  // Determine the distance based on the pulse duration received at the echo pin
  distance = floor((pulseIn(echopin, HIGH) * 0.0343) / 2);
  // Determine the number of leds to turn on based on distance
  onleds = distance / 100;

  // Update the leds based on the determined onleds value
  for (int i = 0; i < numleds; i++) {
    digitalWrite(ledpins[i], i < onleds ? HIGH : LOW);
  }

  // Delay to wait before starting the next loop
  delay(loop_wait_ms);
}