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

// Initialize the LCD with I2C address 0x27, 16 columns, and 2 rows
LiquidCrystal_I2C lcd(0x27, 16, 2);

// Pin Definitions
const int IR_SENSOR_1 = 2; // Entry Sensor
const int IR_SENSOR_2 = 3; // Exit Sensor
const int BUZZER = 13;

// Physics Configuration
const float DISTANCE_BETWEEN_SENSORS = 0.10; // Distance in meters (e.g., 10cm = 0.10m)
const unsigned long TIMEOUT_MS = 5000;       // Reset if car stops halfway (5 seconds)
const float SPEED_LIMIT_KMH = 40.0;          // Threshold to trigger buzzer warning

// State Variables
unsigned long startTime = 0;
unsigned long endTime = 0;
bool carDetected = false;
bool timingActive = false;

void setup() {
  // Initialize serial communication for debugging
  Serial.begin(9600);
  
  // Configure Pin Modes
  pinMode(IR_SENSOR_1, INPUT);
  pinMode(IR_SENSOR_2, INPUT);
  pinMode(BUZZER, OUTPUT);
  
  // Ensure buzzer is off at startup
  digitalWrite(BUZZER, LOW);
  
  // Initialize and turn on the LCD backlight
  lcd.init();
  lcd.backlight();
  
  // Display Startup Splash Screen
  lcd.setCursor(0, 0);
  lcd.print("  SPEED DETECTOR ");
  lcd.setCursor(0, 1);
  lcd.print("   INITIALIZING  ");
  delay(2000);
  
  showDefaultMessage();
}

void loop() {
  // Read sensor inputs (Active LOW or HIGH depending on your module. Usually LOW when obstructed)
  // Adjust to LOW if your specific IR modules pull the pin LOW upon vehicle detection.
  bool sensor1_triggered = (digitalRead(IR_SENSOR_1) == HIGH); 
  bool sensor2_triggered = (digitalRead(IR_SENSOR_2) == HIGH);

  // 1. Car enters the trap zone (triggers Sensor 1)
  if (sensor1_triggered && !timingActive && !carDetected) {
    startTime = millis();
    timingActive = true;
    carDetected = true;
    
    lcd.clear();
    lcd.setCursor(0, 0);
    lcd.print("CAR DETECTED... ");
    lcd.setCursor(0, 1);
    lcd.print("TIMING RUN...   ");
  }

  // 2. Car leaves the trap zone (triggers Sensor 2)
  if (sensor2_triggered && timingActive) {
    endTime = millis();
    timingActive = false;
    
    // Calculate total elapsed duration in seconds
    float elapsedSeconds = (endTime - startTime) / 1000.0;
    
    // Safety check to prevent division by zero
    if (elapsedSeconds > 0) {
      // Speed = Distance / Time (m/s)
      float speedMPS = DISTANCE_BETWEEN_SENSORS / elapsedSeconds;
      // Convert m/s to km/h
      float speedKMH = speedMPS * 3.6;
      
      // Update Display Panel
      lcd.clear();
      lcd.setCursor(0, 0);
      lcd.print("SPEED: ");
      lcd.print(speedKMH, 1);
      lcd.print(" km/h");
      
      // Speed check for buzzer warning activation
      if (speedKMH > SPEED_LIMIT_KMH) {
        lcd.setCursor(0, 1);
        lcd.print("! SPEED LIMIT !");
        
        // Alert sound cadence
        for (int i = 0; i < 3; i++) {
          digitalWrite(BUZZER, HIGH);
          delay(150);
          digitalWrite(BUZZER, LOW);
          delay(100);
        }
      } else {
        lcd.setCursor(0, 1);
        lcd.print("   SPEED OK     ");
      }
      
      // Hold speed metrics on screen before reset window opens
      delay(4000); 
    }
    
    // Reset control variables for the next measurement loop
    carDetected = false;
    showDefaultMessage();
  }

  // 3. Timeout fallback safeguard
  if (timingActive && (millis() - startTime > TIMEOUT_MS)) {
    timingActive = false;
    carDetected = false;
    
    lcd.clear();
    lcd.setCursor(0, 0);
    lcd.print(" TIMEOUT ERROR  ");
    lcd.setCursor(0, 1);
    lcd.print("  RESETTING...  ");
    delay(2000);
    
    showDefaultMessage();
  }
}

// Helper function to restore default static view
void showDefaultMessage() {
  lcd.clear();
  lcd.setCursor(0, 0);
  lcd.print("  RADAR ACTIVE  ");
  lcd.setCursor(0, 2); // Center aligns 'NO CAR FOUND' on row 2
  lcd.setCursor(2, 1);
  lcd.print("NO CAR FOUND");
}