package dev.banking.model;

import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

public class BankAccount {

    private final int accountId;
    private long balance;

    private ReentrantReadWriteLock lock = new ReentrantReadWriteLock();

    /*
     * Students may introduce additional fields
     * such as:
     * - Lock / ReentrantLock
     * - ReadWriteLock
     * - Object monitor
     * - etc.
     */

    public BankAccount(int accountId, long initialBalance) {
        this.accountId = accountId;
        this.balance = initialBalance;
    }

    public int getAccountId() {
        return accountId;
    }

    /*
     * TODO:
     * Return the current balance in a thread-safe way.
     *
     * Requirements:
     * - Must be safe under concurrent reads/writes
     * - Should not block unnecessarily if using read/write locks
     */
    public long getBalance() {
        lock.readLock().lock();
        try {
            return balance;
        } finally {
            lock.readLock().unlock();
        }
    }

    /*
     * TODO:
     * Increase balance atomically.
     *
     * Requirements:
     * - Must not lose updates under concurrency
     */
    public void deposit(long amount) {
        if(amount < 0)
            throw new IllegalArgumentException("Amount can't be negative!");

        lock.writeLock().lock();
        try {
            balance += amount;
        } finally {
            lock.writeLock().unlock();
        }
    }

    /*
     * TODO:
     * Decrease balance atomically.
     *
     * Requirements:
     * - Must not cause race conditions
     * - Negative balance handling is NOT required unless you decide
     *   to extend the system (optional)
     */
    public void withdraw(long amount) {
        if(amount < 0)
            throw new IllegalArgumentException("Amount can't be negative!");

        lock.writeLock().lock();
        try {
            balance -= amount;
        } finally {
            lock.writeLock().unlock();
        }
    }

    /*
     * TODO:
     * Transfer money between two accounts atomically.
     *
     * IMPORTANT REQUIREMENTS:
     * - Must be atomic (no partial transfer)
     * - Must be deadlock-free
     * - Must protect both source and target accounts
     *
     * HINT:
     * - Consider global lock ordering using accountId
     * - Or tryLock with retry strategy
     */
    public void transfer(BankAccount target, long amount) {
        if (amount < 0) {
            throw new IllegalArgumentException("Transfer amount cannot be negative");
        }
        if (this == target) {
            throw new IllegalArgumentException("Cannot transfer to the same account");
        }

        // we order them by their id so that we always lock the first one to prevent deadLocks taking place.
        BankAccount first = this.accountId < target.accountId ? this : target;
        BankAccount second = this.accountId < target.accountId ? target : first;

        first.lock.writeLock().lock();
        try {
            second.lock.writeLock().lock();
            try {
                this.balance -= amount;
                target.balance += amount;
            } finally {
                second.lock.writeLock().unlock();
            }
        } finally {
            first.lock.writeLock().unlock();
        }
    }
}