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

// using I2C lcd so wiring easier
LiquidCrystal_I2C lcd(0x27, 16, 2);

// pins
int buzzer = 19;
int trigPin = 33;
int echoPin = 18;   // changed from 34, more stable
int led = 15;
int pirPin = 13;
int gasPin = 34;   // MQ-2

long duration;
int distance;
int gasValue;
bool motionDetected;

void setup() {
  Serial.begin(9600); // for monitoring values

  pinMode(trigPin, OUTPUT);
  pinMode(echoPin, INPUT);
  pinMode(buzzer, OUTPUT);
  pinMode(led, OUTPUT);
  pinMode(pirPin, INPUT);
  pinMode(gasPin, INPUT);

  lcd.init();
  lcd.backlight();
}

void loop() {

  // read all sensors first
  motionDetected = digitalRead(pirPin);  // check movement
  gasValue = analogRead(gasPin);         // gas level

  // ultrasonic part to get distance
  digitalWrite(trigPin, LOW);
  delayMicroseconds(2);
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10);
  digitalWrite(trigPin, LOW);

  duration = pulseIn(echoPin, HIGH);
  distance = duration * 0.034 / 2; // convert to cm

  // print everything in one line so easier to see
  Serial.print("Distance: ");
  Serial.print(distance);
  Serial.print(" cm | Gas Value: ");
  Serial.print(gasValue);
  Serial.print(" | Motion: ");

  if (motionDetected == HIGH) {
    Serial.println("Detected");
  } else {
    Serial.println("No Motion");
  }

  // priority logic, gas first cause most dangerous
  if (gasValue > 2000) {
    lcd.clear();
    lcd.setCursor(0,0);
    lcd.print("Gas Leak!");

    digitalWrite(buzzer, HIGH);
    digitalWrite(led, HIGH);
  }

  // then motion
  else if (motionDetected == HIGH) {
    lcd.clear();
    lcd.setCursor(0,0);
    lcd.print("Motion Detected");

    // buzzer + led for few seconds
    for (int i = 0; i < 5; i++) {
      digitalWrite(buzzer, HIGH);
      digitalWrite(led, HIGH);
      delay(500);

      digitalWrite(led, LOW);
      delay(500);
    }

    digitalWrite(buzzer, LOW);
  }

  // then obstacle
  else if (distance < 200) {
    lcd.clear();
    lcd.setCursor(0,0);
    lcd.print("Obstacle:");
    lcd.setCursor(0,1);
    lcd.print(distance);
    lcd.print(" cm");

    digitalWrite(led, HIGH);
    digitalWrite(buzzer, LOW);
  }

  // nothing happening
  else {
    lcd.clear();
    lcd.setCursor(0,0);
    lcd.print("System Normal");

    digitalWrite(led, LOW);
    digitalWrite(buzzer, LOW);
  }

  delay(500); // small delay so not too fast
}