/*
    * Copyright (C) 2010, Google Inc. and others
    *
    * This program and the accompanying materials are made available under the
    * terms of the Eclipse Distribution License v. 1.0 which is available at
    * https://www.eclipse.org/org/documents/edl-v10.php.
    *
    * SPDX-License-Identifier: BSD-3-Clause
    */
  
  package org.eclipse.jgit.lib;
  
  import java.util.concurrent.Semaphore;
  import java.util.concurrent.atomic.AtomicInteger;
  import java.util.concurrent.locks.ReentrantLock;
  
  /**
   * Wrapper around the general {@link org.eclipse.jgit.lib.ProgressMonitor} to
   * make it thread safe.
   *
   * Updates to the underlying ProgressMonitor are made only from the thread that
   * allocated this wrapper. Callers are responsible for ensuring the allocating
   * thread uses {@link #pollForUpdates()} or {@link #waitForCompletion()} to
   * update the underlying ProgressMonitor.
   *
   * Only {@link #update(int)}, {@link #isCancelled()}, and {@link #endWorker()}
   * may be invoked from a worker thread. All other methods of the ProgressMonitor
   * interface can only be called from the thread that allocates this wrapper.
   */
  public class ThreadSafeProgressMonitor implements ProgressMonitor {
  	private final ProgressMonitor pm;
  
  	private final ReentrantLock lock;
  
  	private final Thread mainThread;
  
  	private final AtomicInteger workers;
  
  	private final AtomicInteger pendingUpdates;
  
  	private final Semaphore process;
  
  	/**
  	 * Wrap a ProgressMonitor to be thread safe.
  	 *
  	 * @param pm
  	 *            the underlying monitor to receive events.
  	 */
  	public ThreadSafeProgressMonitor(ProgressMonitor pm) {
  		this.pm = pm;
  		this.lock = new ReentrantLock();
  		this.mainThread = Thread.currentThread();
  		this.workers = new AtomicInteger(0);
  		this.pendingUpdates = new AtomicInteger(0);
  		this.process = new Semaphore(0);
  	}
  
  	/** {@inheritDoc} */
  	@Override
  	public void start(int totalTasks) {
  		if (!isMainThread())
  			throw new IllegalStateException();
  		pm.start(totalTasks);
  	}
  
  	/** {@inheritDoc} */
  	@Override
  	public void beginTask(String title, int totalWork) {
  		if (!isMainThread())
  			throw new IllegalStateException();
  		pm.beginTask(title, totalWork);
  	}
  
  	/**
  	 * Notify the monitor a worker is starting.
  	 */
  	public void startWorker() {
  		startWorkers(1);
  	}
  
  	/**
  	 * Notify the monitor of workers starting.
  	 *
  	 * @param count
  	 *            the number of worker threads that are starting.
  	 */
  	public void startWorkers(int count) {
  		workers.addAndGet(count);
  	}
  
  	/**
  	 * Notify the monitor a worker is finished.
  	 */
  	public void endWorker() {
  		if (workers.decrementAndGet() == 0)
  			process.release();
  	}
  
  	/**
 	 * Non-blocking poll for pending updates.
 	 *
 	 * This method can only be invoked by the same thread that allocated this
 	 * ThreadSafeProgressMonior.
 	 */
 	public void pollForUpdates() {
 		assert isMainThread();
 		doUpdates();
 	}
 
 	/**
 	 * Process pending updates and wait for workers to finish.
 	 *
 	 * This method can only be invoked by the same thread that allocated this
 	 * ThreadSafeProgressMonior.
 	 *
 	 * @throws java.lang.InterruptedException
 	 *             if the main thread is interrupted while waiting for
 	 *             completion of workers.
 	 */
 	public void waitForCompletion() throws InterruptedException {
 		assert isMainThread();
 		while (0 < workers.get()) {
 			doUpdates();
 			process.acquire();
 		}
 		doUpdates();
 	}
 
 	private void doUpdates() {
 		int cnt = pendingUpdates.getAndSet(0);
 		if (0 < cnt)
 			pm.update(cnt);
 	}
 
 	/** {@inheritDoc} */
 	@Override
 	public void update(int completed) {
 		if (0 == pendingUpdates.getAndAdd(completed))
 			process.release();
 	}
 
 	/** {@inheritDoc} */
 	@Override
 	public boolean isCancelled() {
 		lock.lock();
 		try {
 			return pm.isCancelled();
 		} finally {
 			lock.unlock();
 		}
 	}
 
 	/** {@inheritDoc} */
 	@Override
 	public void endTask() {
 		if (!isMainThread())
 			throw new IllegalStateException();
 		pm.endTask();
 	}
 
 	private boolean isMainThread() {
 		return Thread.currentThread() == mainThread;
 	}
 }