/*
  HC‑SR04 Ultrasound Distance Measurement with Arduino – Parking Alert (LCD & Buzzer)
  Displays distance on I2C LCD. Buzzer beeps faster as object gets closer.
  Non‑blocking design using millis().
*/

#include <Wire.h>
#include <LiquidCrystal_I2C.h>

// ---------- LCD Configuration ----------
// Common I2C addresses: 0x27 or 0x3F. Change if your LCD does not work.
LiquidCrystal_I2C lcd(0x27, 16, 2);

// ---------- HC-SR04 Pins ----------
const int trigPin = 9;
const int echoPin = 8;

// ---------- Buzzer Pin ----------
const int buzzerPin = 11;

// ---------- Constants ----------
const float SOUND_SPEED_CM_US = 0.034;   // speed of sound in cm/µs
const long MAX_DISTANCE_CM = 400;        // maximum reliable distance (cm)
const int MIN_BEEP_DELAY_MS = 50;        // fastest beep (very close – 50ms apart)
const int MAX_BEEP_DELAY_MS = 1000;      // slowest beep (far – 1 second apart)

// ---------- Variables ----------
long duration;          // echo pulse duration in microseconds
float distanceCm;       // calculated distance in centimeters
int beepDelayMs;        // current delay between beeps (milliseconds)

// Timing for non‑blocking updates
unsigned long lastBeepTime = 0;          // last time we turned the buzzer on
unsigned long lastDisplayUpdate = 0;     // last time we refreshed the LCD
const unsigned long displayInterval = 200;  // refresh LCD every 200 ms

void setup() {
  Serial.begin(9600);
  
  // --- LCD Initialization ---
  lcd.init();
  lcd.backlight();
  lcd.setCursor(0, 0);
  lcd.print("Parking Alert");
  lcd.setCursor(0, 1);
  lcd.print("Initializing...");
  delay(2000);
  lcd.clear();

  // --- Sensor pin modes ---
  pinMode(trigPin, OUTPUT);
  pinMode(echoPin, INPUT);

  // --- Buzzer pin ---
  pinMode(buzzerPin, OUTPUT);
  digitalWrite(buzzerPin, LOW);

  Serial.println("HC‑SR04 Parking Alert Ready");
}

void loop() {
  // --- Step 1: Measure Distance (runs every loop) ---
  // Send a 10 µs HIGH pulse to trigger the sensor
  digitalWrite(trigPin, LOW);
  delayMicroseconds(2);
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10);
  digitalWrite(trigPin, LOW);

  // Measure the echo pulse duration (µs). Timeout after 30ms (≈5 meters).
  duration = pulseIn(echoPin, HIGH, 30000);

  if (duration == 0) {
    // No echo received – object is out of range
    distanceCm = MAX_DISTANCE_CM;
  } else {
    // Convert duration to centimeters: (time × speed) / 2
    distanceCm = duration * SOUND_SPEED_CM_US / 2.0;
  }

  // Clamp distance to a sensible range
  if (distanceCm > MAX_DISTANCE_CM) distanceCm = MAX_DISTANCE_CM;
  if (distanceCm < 0) distanceCm = 0;

  // --- Step 2: Map distance to buzzer beep delay ---
  // Closer distance → smaller delay → faster beeps
  beepDelayMs = map(distanceCm, 0, MAX_DISTANCE_CM, MIN_BEEP_DELAY_MS, MAX_BEEP_DELAY_MS);
  beepDelayMs = constrain(beepDelayMs, MIN_BEEP_DELAY_MS, MAX_BEEP_DELAY_MS);

  // --- Step 3: Update LCD (every 200 ms) ---
  if (millis() - lastDisplayUpdate >= displayInterval) {
    lastDisplayUpdate = millis();

    // First line: distance
    lcd.setCursor(0, 0);
    lcd.print("Distance:      ");   // clear old text
    lcd.setCursor(0, 0);
    lcd.print("Distance:");
    lcd.setCursor(9, 0);
    lcd.print(distanceCm, 1);
    lcd.print(" cm ");

    // Second line: warning or safe message
    lcd.setCursor(0, 1);
    if (distanceCm < 20) {
      lcd.print(">> TOO CLOSE! <<");
    } else {
      lcd.print("                "); // clear line
      lcd.setCursor(0, 1);
      lcd.print("Safe distance   ");
    }
  }

  // --- Step 4: Buzzer control (non‑blocking) ---
  if (distanceCm < MAX_DISTANCE_CM) {
    // Only beep if an object is detected within range
    if (millis() - lastBeepTime >= beepDelayMs) {
      lastBeepTime = millis();
      digitalWrite(buzzerPin, HIGH);
      delay(50);                // short beep (50 ms)
      digitalWrite(buzzerPin, LOW);
    }
  } else {
    // No object – ensure buzzer is off
    digitalWrite(buzzerPin, LOW);
  }

  // A tiny delay to prevent the loop from running too aggressively (optional)
  delay(10);
}