// 1. LIBRARIES 

#include <Wire.h>            //for I2C communication (SDA/SCL)
#include <LiquidCrystal_I2C.h> //for the 4-pin I2C LCD
#include <Servo.h>           // for servo motor control
#include <Keypad.h>          // Library for handling keypad input
#include <math.h>            // for the ceil() function in fee calculation


// 2. CONFIGURATION & CONSTANTS

const int TOTAL_SLOTS = 5; // Define the total capacity of the parking lot, this can be changed

// Define time and fee structure
const int FIRST_HOUR_RATE_CENTS = 100; // $1.00 for the first hour (in cents)
const int HOURLY_RATE_CENTS = 50;      // $0.50 for every subsequent hour
const long ONE_HOUR_MS = 3600000;      // 1 hour in milliseconds (3600 * 1000)

// Ultrasonic Sensor Threshold (Distance in cm)
const int DISTANCE_THRESHOLD = 20; 


// 3. PIN MAPPING (Arduino Mega 2560)

// Keypad setup (4x4)
const byte ROWS = 4;
const byte COLS = 4;
char keys[ROWS][COLS] = { //defining keypad characters
  {'1', '2', '3', 'A'},
  {'4', '5', '6', 'B'},
  {'7', '8', '9', 'C'},
  {'*', '0', '#', 'D'}
};
//assigning pins on the mcu to rows and columns of the keypad
byte rowPins[ROWS] = {22, 23, 24, 25}; 
byte colPins[COLS] = {26, 27, 28, 29};
//bringing all together
Keypad keypad = Keypad(makeKeymap(keys), rowPins, colPins, ROWS, COLS);

// LCD setup: I2C LCD (4 pins VCC, GND, SDA, SCL)
// 0x27 represents the I2C address of the I2C adapter connected to the LCD display.
LiquidCrystal_I2C lcd(0x27, 20, 4); 

// Gate Pins i.e. of the servo motors and ultrasonic sensors at entry and exit
const int ENTRY_TRIG_PIN = 30; 
const int ENTRY_ECHO_PIN = 31; 
const int ENTRY_SERVO_PIN = 8; 
const int EXIT_TRIG_PIN = 32;  
const int EXIT_ECHO_PIN = 33;  
const int EXIT_SERVO_PIN = 9;  

// Parking Slot Pins (Mega Digital Pins 34 to 53)- for leds and ultrasonic sensors
int trigPins[TOTAL_SLOTS]  = {34, 38, 42, 46, 50}; 
int echoPins[TOTAL_SLOTS]  = {35, 39, 43, 47, 51}; 
int greenLEDs[TOTAL_SLOTS] = {36, 40, 44, 48, 52}; 
int redLEDs[TOTAL_SLOTS]   = {37, 41, 45, 49, 53}; 

// 4. GLOBAL VARIABLES

Servo entryServo;
Servo exitServo;
int availableSlots = TOTAL_SLOTS;
long entryTime[TOTAL_SLOTS] = {0, 0, 0, 0, 0}; 
bool slotOccupied[TOTAL_SLOTS] = {false};  //initialization


// 5. FUNCTIONS

// Reads distance from an Ultrasonic sensor 
long readDistance(int trigPin, int echoPin) {
  digitalWrite(trigPin, LOW);
  delayMicroseconds(2);
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10);
  digitalWrite(trigPin, LOW);
  long duration = pulseIn(echoPin, HIGH);
  return duration * 0.034 / 2; // Returns distance in cm
}

// Controls the servo motor angle (0=Closed, 90=Open)
void setGate(Servo& servo, int angle) {
  servo.write(angle);
}

long calculateFee(long duration_ms) {
  if (duration_ms <= 0) return 0; 

  float duration_hours = (float)duration_ms / ONE_HOUR_MS;
  long totalFee = 0;
  
  if (duration_hours <= 1.0) {
    totalFee = FIRST_HOUR_RATE_CENTS;
  } else {
    totalFee = FIRST_HOUR_RATE_CENTS;
    
    float additionalTime = duration_hours - 1.0;
    int additionalHours = (int)ceil(additionalTime); 
    
    totalFee += additionalHours * HOURLY_RATE_CENTS;
  }
  return totalFee;
}


// 6. SETUP 


void setup() {
  Serial.begin(9600); 
  
  // LCD Initialization
  lcd.init();        // Initialize the I2C LCD
  lcd.backlight();   // Turn on the backlight 
  
  lcd.print("Smart Parking System");
  lcd.setCursor(0, 1);
  lcd.print(" Total Slots:      ");;
  
  // Servo Initialization
  entryServo.attach(ENTRY_SERVO_PIN);
  exitServo.attach(EXIT_SERVO_PIN);
  // Close both gates
  setGate(entryServo, 0); 
  setGate(exitServo, 0);

  // Pin Mode Setup for all slot sensors and LEDs
  for (int i = 0; i < TOTAL_SLOTS; i++) {
    pinMode(trigPins[i], OUTPUT);
    pinMode(echoPins[i], INPUT);
    pinMode(greenLEDs[i], OUTPUT);
    pinMode(redLEDs[i], OUTPUT);
    digitalWrite(greenLEDs[i], HIGH); 
    digitalWrite(redLEDs[i], LOW);
  }
  
  // Setup for gate sensors
  pinMode(ENTRY_TRIG_PIN, OUTPUT);
  pinMode(ENTRY_ECHO_PIN, INPUT);
  pinMode(EXIT_TRIG_PIN, OUTPUT);
  pinMode(EXIT_ECHO_PIN, INPUT);
}


// 7. MAIN LOOP


void loop() {
  // A. UPDATE PARKING SLOT STATUS (LEDs)
  for (int i = 0; i < TOTAL_SLOTS; i++) {
    long slotDist = readDistance(trigPins[i], echoPins[i]);
    if (slotDist < DISTANCE_THRESHOLD) {
      slotOccupied[i] = true;
      digitalWrite(redLEDs[i], HIGH);
      digitalWrite(greenLEDs[i], LOW);
    } else {
      if (entryTime[i] == 0) {
        slotOccupied[i] = false;
        digitalWrite(redLEDs[i], LOW);
        digitalWrite(greenLEDs[i], HIGH);
      }
    }
  }

  // B. HANDLE ENTRY GATE LOGIC
  long entryDist = readDistance(ENTRY_TRIG_PIN, ENTRY_ECHO_PIN);
  if (entryDist < DISTANCE_THRESHOLD) {
    if (availableSlots > 0) {
      // Logic: Allow Entry
      lcd.setCursor(0, 2);
      lcd.print("Gate Opening...      ");
      setGate(entryServo, 90); 
      
      for (int i = 0; i < TOTAL_SLOTS; i++) {
        if (entryTime[i] == 0) {
          entryTime[i] = millis(); 
          availableSlots--;
          lcd.setCursor(0, 3);
          lcd.print("Park at Slot: ");
          lcd.print(i + 1);
          delay(4000); 
          setGate(entryServo, 0); // Close gate
          break;
        }
      }
    } else {
      // Logic: Parking is Full
      lcd.setCursor(0, 2);
      lcd.print("PARKING FULL!        ");
      lcd.setCursor(0, 3);
      lcd.print("Gate Closed.         ");
      setGate(entryServo, 0); 
    }
  }
  
  // C. HANDLE EXIT GATE & BILLING LOGIC
  long exitDist = readDistance(EXIT_TRIG_PIN, EXIT_ECHO_PIN);
  char key = keypad.getKey(); 
  
  if (exitDist < DISTANCE_THRESHOLD) {
    lcd.setCursor(0, 2);
    lcd.print("Enter Slot ID (1-5): ");

    if (key >= '1' && key <= ('0' + TOTAL_SLOTS)) {
      int slotID = key - '0';
      int index = slotID - 1;

      if (entryTime[index] != 0) { 
        long duration_ms = millis() - entryTime[index];
        long fee_cents = calculateFee(duration_ms);
        float fee_dollars = (float)fee_cents / 100.0;
        
        lcd.setCursor(0, 3);
        lcd.print("Fee: $");
        lcd.print(fee_dollars, 2); 
        lcd.print(" Press # to Pay");

        while (key != '#') {
            key = keypad.getKey();
            if (key == '*') break; // Allows escape
            delay(10); 
        }

        if (key == '#') {
          // Logic: Payment confirmed, allow exit
          lcd.setCursor(0, 2);
          lcd.print("Payment OK. EXITING! ");
          lcd.setCursor(0, 3);
          lcd.print("Thank You.           ");
          
          setGate(exitServo, 90); 
          
          entryTime[index] = 0;
          availableSlots++;
          
          delay(4000); 
          setGate(exitServo, 0); // Close gate
        }
      } else {
        lcd.setCursor(0, 3);
        lcd.print("Vacant Slot or Invalid ID.");
      }
    }
  }

  // D. UPDATE LCD SLOT COUNT 
  lcd.setCursor(14, 1);
  lcd.print(availableSlots);
  lcd.print("  "); 
}