/*
 * Copyright (C) 2013 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.android.inputmethod.latin.utils;

import java.util.Queue;
import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.ConcurrentLinkedQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

/**
 * An object that executes submitted tasks using a thread.
 */
public class PrioritizedSerialExecutor {
    public static final String TAG = PrioritizedSerialExecutor.class.getSimpleName();

    private final Object mLock = new Object();

    private final Queue<Runnable> mTasks;
    private final Queue<Runnable> mPrioritizedTasks;
    private boolean mIsShutdown;
    private final ThreadPoolExecutor mThreadPoolExecutor;

    // The task which is running now.
    private Runnable mActive;

    public PrioritizedSerialExecutor() {
        mTasks = new ConcurrentLinkedQueue<Runnable>();
        mPrioritizedTasks = new ConcurrentLinkedQueue<Runnable>();
        mIsShutdown = false;
        mThreadPoolExecutor = new ThreadPoolExecutor(1 /* corePoolSize */, 1 /* maximumPoolSize */,
                0 /* keepAliveTime */, TimeUnit.MILLISECONDS, new ArrayBlockingQueue<Runnable>(1));
    }

    /**
     * Clears all queued tasks.
     */
    public void clearAllTasks() {
        synchronized(mLock) {
            mTasks.clear();
            mPrioritizedTasks.clear();
        }
    }

    /**
     * Enqueues the given task into the task queue.
     * @param r the enqueued task
     */
    public void execute(final Runnable r) {
        synchronized(mLock) {
            if (!mIsShutdown) {
                mTasks.offer(new Runnable() {
                    @Override
                    public void run() {
                        try {
                            r.run();
                        } finally {
                            scheduleNext();
                        }
                    }
                });
                if (mActive == null) {
                    scheduleNext();
                }
            }
        }
    }

    /**
     * Enqueues the given task into the prioritized task queue.
     * @param r the enqueued task
     */
    public void executePrioritized(final Runnable r) {
        synchronized(mLock) {
            if (!mIsShutdown) {
                mPrioritizedTasks.offer(new Runnable() {
                    @Override
                    public void run() {
                        try {
                            r.run();
                        } finally {
                            scheduleNext();
                        }
                    }
                });
                if (mActive == null) {
                    scheduleNext();
                }
            }
        }
    }

    private boolean fetchNextTasksLocked() {
        mActive = mPrioritizedTasks.poll();
        if (mActive == null) {
            mActive = mTasks.poll();
        }
        return mActive != null;
    }

    private void scheduleNext() {
        synchronized(mLock) {
            if (fetchNextTasksLocked()) {
                mThreadPoolExecutor.execute(mActive);
            }
        }
    }

    public void remove(final Runnable r) {
        synchronized(mLock) {
            mTasks.remove(r);
            mPrioritizedTasks.remove(r);
        }
    }

    public void replaceAndExecute(final Runnable oldTask, final Runnable newTask) {
        synchronized(mLock) {
            if (oldTask != null) remove(oldTask);
            execute(newTask);
        }
    }

    public void shutdown() {
        synchronized(mLock) {
            mIsShutdown = true;
        }
    }

    public boolean isTerminated() {
        synchronized(mLock) {
            if (!mIsShutdown) {
                return false;
            }
            return mPrioritizedTasks.isEmpty() && mTasks.isEmpty() && mActive == null;
        }
    }
}