common-utils/src/main/java/org/warp/commonutils/locks/LeftRightLock.java

package org.warp.commonutils.locks;
import java.util.concurrent.locks.AbstractQueuedSynchronizer;
import java.util.concurrent.locks.Lock;

/**
 * A binary mutex with the following properties:
 *
 * Exposes two different {@link Lock}s: LEFT, RIGHT.
 *
 * When LEFT is held other threads can acquire LEFT but thread trying to acquire RIGHT will be
 * blocked. When RIGHT is held other threads can acquire RIGHT but thread trying to acquire LEFT
 * will be blocked.
 */
public class LeftRightLock {

	public static final int ACQUISITION_FAILED = -1;
	public static final int ACQUISITION_SUCCEEDED = 1;

	private final LeftRightSync sync = new LeftRightSync();

	public void lockLeft() {
		sync.acquireShared(LockSide.LEFT.getV());
	}

	public void lockRight() {
		sync.acquireShared(LockSide.RIGHT.getV());
	}

	public void releaseLeft() {
		sync.releaseShared(LockSide.LEFT.getV());
	}

	public void releaseRight() {
		sync.releaseShared(LockSide.RIGHT.getV());
	}

	public boolean tryLockLeft() {
		return sync.tryAcquireShared(LockSide.LEFT) == ACQUISITION_SUCCEEDED;
	}

	public boolean tryLockRight() {
		return sync.tryAcquireShared(LockSide.RIGHT) == ACQUISITION_SUCCEEDED;
	}

	private enum LockSide {
		LEFT(-1), NONE(0), RIGHT(1);

		private final int v;

		LockSide(int v) {
			this.v = v;
		}

		public int getV() {
			return v;
		}
	}

	/**
	 * <p>
	 * Keep count the count of threads holding either the LEFT or the RIGHT lock.
	 * </p>
	 *
	 * <li>A state ({@link AbstractQueuedSynchronizer#getState()}) greater than 0 means one or more threads are holding RIGHT lock. </li>
	 * <li>A state ({@link AbstractQueuedSynchronizer#getState()}) lower than 0 means one or more threads are holding LEFT lock.</li>
	 * <li>A state ({@link AbstractQueuedSynchronizer#getState()}) equal to zero means no thread is holding any lock.</li>
	 */
	private static final class LeftRightSync extends AbstractQueuedSynchronizer {


		@Override
		protected int tryAcquireShared(int requiredSide) {
			return (tryChangeThreadCountHoldingCurrentLock(requiredSide, ChangeType.ADD) ? ACQUISITION_SUCCEEDED : ACQUISITION_FAILED);
		}

		@Override
		protected boolean tryReleaseShared(int requiredSide) {
			return tryChangeThreadCountHoldingCurrentLock(requiredSide, ChangeType.REMOVE);
		}

		public boolean tryChangeThreadCountHoldingCurrentLock(int requiredSide, ChangeType changeType) {
			if (requiredSide != 1 && requiredSide != -1)
				throw new AssertionError("You can either lock LEFT or RIGHT (-1 or +1)");

			int curState;
			int newState;
			do {
				curState = this.getState();
				if (!sameSide(curState, requiredSide)) {
					return false;
				}

				if (changeType == ChangeType.ADD) {
					newState = curState + requiredSide;
				} else {
					newState = curState - requiredSide;
				}
				//TODO: protect against int overflow (hopefully you won't have so many threads)
			} while (!this.compareAndSetState(curState, newState));
			return true;
		}

		final int tryAcquireShared(LockSide lockSide) {
			return this.tryAcquireShared(lockSide.getV());
		}

		final boolean tryReleaseShared(LockSide lockSide) {
			return this.tryReleaseShared(lockSide.getV());
		}

		private boolean sameSide(int curState, int requiredSide) {
			return curState == 0 || sameSign(curState, requiredSide);
		}

		private boolean sameSign(int a, int b) {
			return (a >= 0) ^ (b < 0);
		}

		public enum ChangeType {
			ADD, REMOVE
		}
	}
}