/* -*- mode: Java; c-basic-offset: 2; indent-tabs-mode: nil; coding: utf-8-unix -*-
 *
 * Copyright © 2017-2018 microBean.
 *
 * 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.microbean.kubernetes.controller;

import java.io.IOException;

import java.time.Duration;
import java.time.Instant;

import java.util.ArrayList;
import java.util.Collection;
import java.util.Iterator;
import java.util.Map;
import java.util.Objects;

import java.util.concurrent.BlockingQueue;
import java.util.concurrent.Executor;
import java.util.concurrent.Executors;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Future;
import java.util.concurrent.CopyOnWriteArrayList;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.ScheduledThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

import java.util.function.Consumer;

import io.fabric8.kubernetes.api.model.HasMetadata;

import net.jcip.annotations.Immutable;
import net.jcip.annotations.GuardedBy;
import net.jcip.annotations.ThreadSafe;

/**
 * A {@link ResourceTrackingEventQueueConsumer} that {@linkplain
 * ResourceTrackingEventQueueConsumer#accept(EventQueue) consumes
 * <tt>EventQueue</tt> instances} by feeding each {@link
 * AbstractEvent} in the {@link EventQueue} being consumed to {@link
 * Consumer}s of {@link AbstractEvent}s that have been {@linkplain
 * #addConsumer(Consumer) registered}.
 *
 * <p>{@link EventDistributor} instances must be {@linkplain #close()
 * closed} and discarded after use.</p>
 *
 * @param <T> a type of Kubernetes resource
 *
 * @author <a href="https://about.me/lairdnelson"
 * target="_parent">Laird Nelson</a>
 *
 * @see #addConsumer(Consumer)
 *
 * @see #removeConsumer(Consumer)
 */
@Immutable
@ThreadSafe
public final class EventDistributor<T extends HasMetadata> extends ResourceTrackingEventQueueConsumer<T> implements AutoCloseable {


  /*
   * Instance fields.
   */
  

  @GuardedBy("readLock && writeLock")
  private final Collection<Pump<T>> distributors;

  @GuardedBy("readLock && writeLock")
  private final Collection<Pump<T>> synchronizingDistributors;

  private final Duration synchronizationInterval;

  private final Lock readLock;

  private final Lock writeLock;


  /*
   * Constructors.
   */


  /**
   * Creates a new {@link EventDistributor}.
   *
   * @param knownObjects a mutable {@link Map} of Kubernetes resources
   * that contains or will contain Kubernetes resources known to this
   * {@link EventDistributor} and whatever mechanism (such as a {@link
   * Controller}) is feeding it; may be {@code null}
   *
   * @see
   * ResourceTrackingEventQueueConsumer#ResourceTrackingEventQueueConsumer(Map)
   */
  public EventDistributor(final Map<Object, T> knownObjects) {
    super(knownObjects);
    final ReadWriteLock lock = new ReentrantReadWriteLock();
    this.readLock = lock.readLock();
    this.writeLock = lock.writeLock();
    this.distributors = new ArrayList<>();    
    this.synchronizingDistributors = new ArrayList<>();
    this.synchronizationInterval = null; // TODO: implement/fix
  }


  /*
   * Instance methods.
   */
  

  /**
   * Adds the supplied {@link Consumer} to this {@link
   * EventDistributor} as a listener that will be notified of each
   * {@link AbstractEvent} this {@link EventDistributor} receives.
   *
   * <p>The supplied {@link Consumer}'s {@link
   * Consumer#accept(Object)} method may be called later on a separate
   * thread of execution.</p>
   *
   * @param consumer a {@link Consumer} of {@link AbstractEvent}s; may
   * be {@code null} in which case no action will be taken
   *
   * @see #removeConsumer(Consumer)
   */
  public final void addConsumer(final Consumer<? super AbstractEvent<? extends T>> consumer) {
    if (consumer != null) {
      this.writeLock.lock();
      try {
        final Pump<T> distributor = new Pump<>(this.synchronizationInterval, consumer);
        this.distributors.add(distributor);
        this.synchronizingDistributors.add(distributor);
      } finally {
        this.writeLock.unlock();
      }
    }
  }

  /**
   * Removes any {@link Consumer} {@linkplain Object#equals(Object)
   * equal to} a {@link Consumer} previously {@linkplain
   * #addConsumer(Consumer) added} to this {@link EventDistributor}.
   *
   * @param consumer the {@link Consumer} to remove; may be {@code
   * null} in which case no action will be taken
   *
   * @see #addConsumer(Consumer)
   */
  public final void removeConsumer(final Consumer<? super AbstractEvent<? extends T>> consumer) {
    if (consumer != null) {
      this.writeLock.lock();
      try {
        final Iterator<? extends Pump<?>> iterator = this.distributors.iterator();
        assert iterator != null;
        while (iterator.hasNext()) {
          final Pump<?> distributor = iterator.next();
          if (distributor != null && consumer.equals(distributor.getEventConsumer())) {
            iterator.remove();
            break;
          }
        }
      } finally {
        this.writeLock.unlock();
      }
    }
  }

  /**
   * Releases resources held by this {@link EventDistributor} during
   * its execution.
   */
  @Override
  public final void close() {
    this.writeLock.lock();
    try {
      this.distributors.parallelStream()
        .forEach(distributor -> {
            distributor.close();
          });
      this.synchronizingDistributors.clear();
      this.distributors.clear();
    } finally {
      this.writeLock.unlock();
    }
  }

  /**
   * Returns {@code true} if this {@link EventDistributor} should
   * <em>synchronize</em> with its upstream source.
   *
   * <h2>Design Notes</h2>
   *
   * <p>The Kubernetes {@code tools/cache} package spreads
   * synchronization out among the reflector, controller, event cache
   * and event processor constructs for no seemingly good reason.
   * They should probably be consolidated, particularly in an
   * object-oriented environment such as Java.</p>
   *
   * @return {@code true} if synchronization should occur; {@code
   * false} otherwise
   *
   * @see EventCache#synchronize()
   */
  public final boolean shouldSynchronize() {
    boolean returnValue = false;
    this.writeLock.lock();
    try {
      this.synchronizingDistributors.clear();
      final Instant now = Instant.now();
      this.distributors.parallelStream()
        .filter(distributor -> distributor.shouldSynchronize(now))
        .forEach(distributor -> {
            this.synchronizingDistributors.add(distributor);
            distributor.determineNextSynchronizationInterval(now);
          });
      returnValue = !this.synchronizingDistributors.isEmpty();
    } finally {
      this.writeLock.unlock();
    }
    return returnValue;
  }

  /**
   * Consumes the supplied {@link AbstractEvent} by forwarding it to
   * the {@link Consumer#accept(Object)} method of each {@link
   * Consumer} {@linkplain #addConsumer(Consumer) registered} with
   * this {@link EventDistributor}.
   *
   * @param event the {@link AbstractEvent} to forward; may be {@code
   * null} in which case no action is taken
   *
   * @see #addConsumer(Consumer)
   */
  @Override
  protected final void accept(final AbstractEvent<? extends T> event) {
    if (event != null) {
      if (event instanceof SynchronizationEvent) {
        this.accept((SynchronizationEvent<? extends T>)event);
      } else if (event instanceof Event) {
        this.accept((Event<? extends T>)event);
      } else {
        assert false : "Unexpected event type: " + event.getClass();
      }
    }
  }

  private final void accept(final SynchronizationEvent<? extends T> event) {
    this.readLock.lock();
    try {
      if (!this.synchronizingDistributors.isEmpty()) {
        this.synchronizingDistributors.parallelStream()
          .forEach(distributor -> distributor.accept(event));
      }
    } finally {
      this.readLock.unlock();
    }
  }

  private final void accept(final Event<? extends T> event) {
    this.readLock.lock();
    try {
      if (!this.distributors.isEmpty()) {
        this.distributors.parallelStream()
          .forEach(distributor -> distributor.accept(event));
      }
    } finally {
      this.readLock.unlock();
    }
  }


  /*
   * Inner and nested classes.
   */
  

  /**
   * A {@link Consumer} of {@link AbstractEvent} instances that puts
   * them on an internal queue and, in a separate thread, removes them
   * from the queue and forwards them to the "real" {@link Consumer}
   * supplied at construction time.
   *
   * @author <a href="https://about.me/lairdnelson"
   * target="_parent">Laird Nelson</a>
   */
  private static final class Pump<T extends HasMetadata> implements Consumer<AbstractEvent<? extends T>>, AutoCloseable {

    private volatile boolean closing;
    
    private volatile Instant nextSynchronizationInstant;
    
    private volatile Duration synchronizationInterval;
    
    final BlockingQueue<AbstractEvent<? extends T>> queue;
    
    private final ScheduledExecutorService executor;

    private final Future<?> task;
    
    private final Consumer<? super AbstractEvent<? extends T>> eventConsumer;
    
    private Pump(final Duration synchronizationInterval, final Consumer<? super AbstractEvent<? extends T>> eventConsumer) {
      super();
      Objects.requireNonNull(eventConsumer);
      this.eventConsumer = eventConsumer;
      this.executor = this.createScheduledThreadPoolExecutor();
      if (this.executor == null) {
        throw new IllegalStateException("createScheduledThreadPoolExecutor() == null");
      }
      this.queue = new LinkedBlockingQueue<>();

      // Schedule a hopefully never-ending task to pump events from
      // our queue to the supplied eventConsumer.  We schedule this
      // instead of simply executing it so that if for any reason it
      // exits it will get restarted.  Cancelling a scheduled task
      // will also cancel all resubmissions of it, so this is the most
      // robust thing to do.  The delay of one second is arbitrary.
      this.task = this.executor.scheduleWithFixedDelay(() -> {
          try {
            while (!Thread.currentThread().isInterrupted()) {
              this.eventConsumer.accept(this.queue.take());
            }
          } catch (final InterruptedException interruptedException) {
            Thread.currentThread().interrupt();
          }
        }, 0L, 1L, TimeUnit.SECONDS);
      
      this.setSynchronizationInterval(synchronizationInterval);
    }

    private final ScheduledExecutorService createScheduledThreadPoolExecutor() {
      final ScheduledThreadPoolExecutor executor = new ScheduledThreadPoolExecutor(1);
      executor.setRemoveOnCancelPolicy(true);
      return executor;
    }
    
    private final Consumer<? super AbstractEvent<? extends T>> getEventConsumer() {
      return this.eventConsumer;
    }
    
    /**
     * Adds the supplied {@link AbstractEvent} to an internal {@link
     * BlockingQueue} and schedules a task to consume it.
     *
     * @param event the {@link AbstractEvent} to add; may be {@code
     * null} in which case no action is taken
     */
    @Override
    public final void accept(final AbstractEvent<? extends T> event) {
      if (this.closing) {
        throw new IllegalStateException();
      }
      if (event != null) {
        final boolean added = this.queue.add(event);
        assert added;
      }
    }
    
    @Override
    public final void close() {
      this.closing = true;
      this.executor.shutdown();
      this.task.cancel(true);
      try {
        if (!this.executor.awaitTermination(60, TimeUnit.SECONDS)) {
          this.executor.shutdownNow();
          if (!this.executor.awaitTermination(60, TimeUnit.SECONDS)) {
            // TODO: log
          }
        }
      } catch (final InterruptedException interruptedException) {
        this.executor.shutdownNow();
        Thread.currentThread().interrupt();
      }
    }
    
    
    /*
     * Synchronization-related methods.  It seems odd that one of these
     * listeners would need to report details about synchronization, but
     * that's what the Go code does.  Maybe this functionality could be
     * relocated "higher up".
     */
    
    
    private final boolean shouldSynchronize(Instant now) {
      if (now == null) {
        now = Instant.now();
      }
      final Duration interval = this.getSynchronizationInterval();
      final boolean returnValue = interval != null && !interval.isZero() && now.compareTo(this.nextSynchronizationInstant) >= 0;
      return returnValue;
    }
    
    private final void determineNextSynchronizationInterval(Instant now) {
      if (now == null) {
        now = Instant.now();
      }
      this.nextSynchronizationInstant = now.plus(this.synchronizationInterval);
    }
    
    public final void setSynchronizationInterval(final Duration synchronizationInterval) {
      this.synchronizationInterval = synchronizationInterval;
    }
    
    public final Duration getSynchronizationInterval() {
      return this.synchronizationInterval;
    }
  }
  
}