import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

/**
 * Object pool manager: Responsible for DatabaseConnection creation, reuse, and recycling
 * @author chuyang
 * @version 1.0
 */
public class ConnectionPoolManager {

	/**
	 * Connection pool storage
	 */
	private final List<DatabaseConnection> connectionPool;
	/**
	 * Database URL
	 */
	private final String databaseUrl;
	/**
	 * Initial pool size
	 */
	private final int initialSize;
	/**
	 * Maximum pool size
	 */
	private final int maxSize;
	/**
	 * For thread safety
	 */
	private final Lock poolLock = new ReentrantLock();
	public DatabaseConnection m_DatabaseConnection;

	public ConnectionPoolManager(){
		this.connectionPool = new ArrayList<>();
		this.databaseUrl = "";
		this.initialSize = 0;
		this.maxSize = 0;
	}

	@Override
	public void finalize() throws Throwable {
		// Close all connections
		poolLock.lock();
		try {
			for (DatabaseConnection conn : connectionPool) {
				System.out.println("Closing connection " + conn.getConnectionId());
			}
			connectionPool.clear();
		} finally {
			poolLock.unlock();
		}
	}
	/**
	 * Constructor: Initialize object pool
	 * 
	 * @param initialSize Initial number of connections
	 * @param maxSize Maximum number of connections
	 * @param databaseUrl Database URL
	 */
	public ConnectionPoolManager(int initialSize, int maxSize, String databaseUrl){
		if (initialSize < 0 || maxSize < initialSize) {
			throw new IllegalArgumentException("Invalid pool size parameters");
		}
		this.connectionPool = new ArrayList<>(maxSize);
		this.databaseUrl = databaseUrl;
		this.initialSize = initialSize;
		this.maxSize = maxSize;
		
		// Initialize connection pool
		initializePool();
	}

	/**
	 * Initialize the connection pool, create initial connections
	 */
	private void initializePool() {
		System.out.println("Initializing connection pool with " + initialSize + " connections");
		for (int i = 0; i < initialSize; i++) {
			String connId = "conn-" + i;
			DatabaseConnection conn = new DatabaseConnection(connId, databaseUrl);
			connectionPool.add(conn);
		}
		System.out.println("Connection pool initialized with " + connectionPool.size() + " connections");
	}

	/**
	 * Borrow a connection from the pool
	 * @return Available connection, or null (pool is full)
	 */
	public DatabaseConnection borrowConnection(){
		poolLock.lock();
		try {
			// 1. Look for idle connection
			for (DatabaseConnection conn : connectionPool) {
				if (!conn.isInUse()) {
					conn.setInUse(true);
					System.out.println("Borrowed existing connection: " + conn.getConnectionId());
					return conn;
				}
			}
			
			// 2. If no idle connections and max size not reached, create new connection
			if (connectionPool.size() < maxSize) {
				String connId = "conn-" + connectionPool.size();
				DatabaseConnection newConn = new DatabaseConnection(connId, databaseUrl);
				newConn.setInUse(true);
				connectionPool.add(newConn);
				System.out.println("Created and borrowed new connection: " + newConn.getConnectionId());
				return newConn;
			}
			
			// 3. Pool is full, return null
			System.out.println("Connection pool is full, no available connections");
			return null;
		} finally {
			poolLock.unlock();
		}
	}

	/**
	 * Return connection to the pool
	 * 
	 * @param conn Connection to return
	 */
	public void returnConnection(DatabaseConnection conn){
		if (conn == null) {
			return;
		}
		
		poolLock.lock();
		try {
			// Check if connection belongs to this pool
			if (connectionPool.contains(conn)) {
				// Reset connection state
				conn.reset();
				conn.setInUse(false);
				System.out.println("Connection returned to pool: " + conn.getConnectionId());
			} else {
				System.out.println("Warning: Attempting to return a connection not managed by this pool");
			}
		} finally {
			poolLock.unlock();
		}
	}

	/**
	 * Shrink pool: Destroy idle connections
	 */
	public void shrinkPool(){
		poolLock.lock();
		try {
			// Only keep initial size of connections
			List<DatabaseConnection> connectionsToRemove = new ArrayList<>();
			int activeConnections = 0;
			
			for (DatabaseConnection conn : connectionPool) {
				if (conn.isInUse()) {
					activeConnections++;
				} else if (connectionPool.size() - connectionsToRemove.size() > initialSize) {
					connectionsToRemove.add(conn);
					System.out.println("Removing idle connection: " + conn.getConnectionId());
				}
			}
			
			connectionPool.removeAll(connectionsToRemove);
			System.out.println("Pool shrunk. Active connections: " + activeConnections + ", Idle connections: " + connectionPool.size() + ", Removed: " + connectionsToRemove.size());
		} finally {
			poolLock.unlock();
		}
	}
	
	/**
	 * Get pool status information
	 */
	public String getPoolStatus() {
		poolLock.lock();
		try {
			int activeConnections = 0;
			for (DatabaseConnection conn : connectionPool) {
				if (conn.isInUse()) {
					activeConnections++;
				}
			}
			return "Pool status: " + connectionPool.size() + " total connections, " + 
				   activeConnections + " active, " + 
				   (connectionPool.size() - activeConnections) + " idle, " +
				   "Max capacity: " + maxSize;
		} finally {
			poolLock.unlock();
		}
	}
}