/**
 * A database that can be read by multiple readers
 * at the same time, but can only be written by a
 * single writer.
 * 
 * @author Franck van Breugel
 */
public class Database {
	private boolean writing;
	private int readers;	

	/**
	 * Initializes this database.
	 */
	public Database() {
		this.writing = false;
		this.readers = 0;
	}

	/**
	 * Reads this database.
	 * 
	 * @throws InterruptedException if thread is interrupted while 
	 * waiting to read.
	 */
	public void read() throws InterruptedException {
		synchronized (this) {
			while (this.writing) {
				this.wait();
			}
			this.readers++;
		}
		// read
		synchronized (this) {
			this.readers--;
			if (this.readers == 0) {
				this.notify();
			}
		}
	}

	/**
	 * Writes to this database.
	 * 
	 * @throws InterruptedException if thread is interrupted while 
	 * waiting to write.
	 */
	public void write() throws InterruptedException {
		synchronized (this) {
			while (this.writing || this.readers > 0) {
				this.wait();
			}
			this.writing = true;
		}
		// write
		synchronized (this) {
			this.writing = false;
			this.notifyAll();
		}
	}
}