/**
 * 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;
	private int writers; // introduced to reason about the code

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

	/**
	 * Reads this database.
	 */
	public void read() {
		this.beginRead();
		// read
		assert !this.writing;
		this.endRead();
	}

	/**
	 * Waits if a writer is writing.
	 */
	private synchronized void beginRead() {
		while (this.writing) {
			try {
				this.wait();
			} catch (InterruptedException e) {}
		}
		this.readers++;
	}

	/**
	 * Notifies waiting writers.
	 */
	private synchronized void endRead() {
		this.readers--;
		if (this.readers == 0) {
			this.notifyAll();
		}
	}


	/**
	 * Writes to this database.
	 */
	public void write() {
		this.beginWrite();
		// write
		assert this.readers == 0 && this.writer == 1;
		this.endWrite();
	}

	/**
	 * Waits if readers are reading.
	 */
	private synchronized void beginWrite() {
		while (this.writing || this.readers > 0) {
			try {
				this.wait();
			} catch (InterruptedException e) {}
		}
		this.writing = true;
		this.writers++;
	}
	
	/**
	 * Notifies waiting readers.
	 */
	private synchronized void endWrite() {
		this.writing = false;
		this.writers--;
		this.notifyAll();
	}
}