/*
 * Copyright 2017 the original author or authors.
 *
 * 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 org.gradle.internal.filewatch;

import org.gradle.api.Action;
import org.gradle.api.internal.file.FileSystemSubset;
import org.gradle.initialization.BuildCancellationToken;
import org.gradle.internal.UncheckedException;

import java.io.IOException;
import java.util.ArrayList;
import java.util.Collection;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicLong;
import java.util.concurrent.atomic.AtomicReference;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

class DefaultFileSystemChangeWaiter implements FileSystemChangeWaiter {
    private final long quietPeriodMillis;
    private final BuildCancellationToken cancellationToken;
    private final AtomicReference<Throwable> error = new AtomicReference<Throwable>();
    private final Lock lock = new ReentrantLock();
    private final Condition condition = lock.newCondition();
    private final AtomicLong lastChangeAt = new AtomicLong(0);
    private final FileWatcher watcher;
    private final Action<Throwable> onError;
    private boolean watching;
    private FileWatcherEventListener eventListener;
    private final Collection<FileWatcherEvent> eventsBeforeListening = new ArrayList<FileWatcherEvent>();
    private final Lock eventDeliveryLock = new ReentrantLock();

    private static long monotonicClockMillis() {
        return System.nanoTime() / 1000000L;
    }

    private static void signal(Lock lock, Condition condition, Runnable runnable) {
        boolean interrupted = Thread.interrupted();
        lock.lock();
        try {
            runnable.run();
            condition.signal();
        } finally {
            lock.unlock();
            if (interrupted) {
                Thread.currentThread().interrupt();
            }
        }
    }

    private static void signal(Lock lock, Condition condition) {
        signal(lock, condition, new Runnable() {
            @Override
            public void run() {

            }
        });
    }

    DefaultFileSystemChangeWaiter(FileWatcherFactory fileWatcherFactory, final PendingChangesListener pendingChangesListener, long quietPeriodMillis, BuildCancellationToken cancellationToken) {
        this.quietPeriodMillis = quietPeriodMillis;
        this.cancellationToken = cancellationToken;
        this.onError = new Action<Throwable>() {
            @Override
            public void execute(Throwable throwable) {
                error.set(throwable);
                signal(lock, condition);
            }
        };
        watcher = fileWatcherFactory.watch(
            onError,
            new FileWatcherListener() {
                @Override
                public void onChange(final FileWatcher watcher, FileWatcherEvent event) {
                    if (!(event.getType() == FileWatcherEvent.Type.MODIFY && event.getFile().isDirectory())) {
                        deliverEvent(event);
                        signal(lock, condition, new Runnable() {
                            @Override
                            public void run() {
                                lastChangeAt.set(monotonicClockMillis());
                                pendingChangesListener.onPendingChanges();
                            }
                        });
                    }
                }
            }
        );
    }

    @Override
    public void watch(FileSystemSubset fileSystemSubset) {
        try {
            if (!fileSystemSubset.isEmpty()) {
                watching = true;
                watcher.watch(fileSystemSubset);
            }
        } catch (IOException e) {
            onError.execute(e);
        }
    }

    public void wait(Runnable notifier, FileWatcherEventListener eventListener) {
        Runnable cancellationHandler = new Runnable() {
            @Override
            public void run() {
                signal(lock, condition);
            }
        };
        try {
            attachEventListener(eventListener);
            if (cancellationToken.isCancellationRequested()) {
                return;
            }
            cancellationToken.addCallback(cancellationHandler);
            notifier.run();
            lock.lock();
            try {
                long lastChangeAtValue = lastChangeAt.get();
                while (!cancellationToken.isCancellationRequested() && waitingForChanges(lastChangeAtValue)) {
                    condition.await(quietPeriodMillis, TimeUnit.MILLISECONDS);
                    lastChangeAtValue = lastChangeAt.get();
                }
            } finally {
                lock.unlock();
            }
            Throwable throwable = error.get();
            if (throwable != null) {
                throw throwable;
            }
        } catch (Throwable e) {
            throw UncheckedException.throwAsUncheckedException(e);
        } finally {
            detachEventListener();
            cancellationToken.removeCallback(cancellationHandler);
            watcher.stop();
        }
    }

    private boolean waitingForChanges(long lastChangeAtValue) {
        return error.get() == null && shouldKeepWaitingForQuietPeriod(lastChangeAtValue);
    }

    private void deliverEvent(FileWatcherEvent event) {
        eventDeliveryLock.lock();
        try {
            if (eventListener != null) {
                eventListener.onChange(event);
            } else {
                eventsBeforeListening.add(event);
            }
        } finally {
            eventDeliveryLock.unlock();
        }
    }

    private void attachEventListener(FileWatcherEventListener eventListener) {
        eventDeliveryLock.lock();
        try {
            this.eventListener = eventListener;
            for (FileWatcherEvent event : eventsBeforeListening) {
                eventListener.onChange(event);
            }
            eventsBeforeListening.clear();
        } finally {
            eventDeliveryLock.unlock();
        }
    }

    private void detachEventListener() {
        eventDeliveryLock.lock();
        try {
            this.eventListener = null;
        } finally {
            eventDeliveryLock.unlock();
        }
    }

    private boolean shouldKeepWaitingForQuietPeriod(long lastChangeAtValue) {
        long now = monotonicClockMillis();
        return lastChangeAtValue == 0   // no changes yet
            || now < lastChangeAtValue  // handle case where monotonic clock isn't monotonic
            || now - lastChangeAtValue < quietPeriodMillis;
    }

    @Override
    public boolean isWatching() {
        return watching;
    }

    @Override
    public void stop() {
        watcher.stop();
    }
}
