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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

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 *

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 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

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package org.reactivestreams.example.unicast;

import org.reactivestreams.Publisher;

import org.reactivestreams.Subscriber;

import org.reactivestreams.Subscription;

import java.util.Iterator;

import java.util.Collections;

import java.util.concurrent.Executor;

import java.util.concurrent.atomic.AtomicBoolean;

import java.util.concurrent.ConcurrentLinkedQueue;

/**

 * AsyncIterablePublisher is an implementation of Reactive Streams `Publisher`

 * which executes asynchronously, using a provided `Executor` and produces elements

 * from a given `Iterable` in a "unicast" configuration to its `Subscribers`.

 *

 * NOTE: The code below uses a lot of try-catches to show the reader where exceptions can be expected, and where they are forbidden.

 */

public class AsyncIterablePublisher<T> implements Publisher<T> {

  private final static int DEFAULT_BATCHSIZE = 1024;

  private final Iterable<T> elements; // This is our data source / generator

  private final Executor executor; // This is our thread pool, which will make sure that our Publisher runs asynchronously to its Subscribers

  private final int batchSize; // In general, if one uses an `Executor`, one should be nice nad not hog a thread for too long, this is the cap for that, in elements

  public AsyncIterablePublisher(final Iterable<T> elements, final Executor executor) {

    this(elements, DEFAULT_BATCHSIZE, executor);

  }

  public AsyncIterablePublisher(final Iterable<T> elements, final int batchSize, final Executor executor) {

    if (elements == null) throw null;

    if (executor == null) throw null;

    if (batchSize < 1) throw new IllegalArgumentException("batchSize must be greater than zero!");

    this.elements = elements;

    this.executor = executor;

    this.batchSize = batchSize;

  }

  @Override

  public void subscribe(final Subscriber<? super T> s) {

    // As per rule 1.11, we have decided to support multiple subscribers in a unicast configuration

    // for this `Publisher` implementation.

    // As per 2.13, this method must return normally (i.e. not throw)

    new SubscriptionImpl(s).init();

  }

  // These represent the protocol of the `AsyncIterablePublishers` SubscriptionImpls

  static interface Signal {};

  enum Cancel implements Signal { Instance; };

  enum Subscribe implements Signal { Instance; };

  enum Send implements Signal { Instance; };

  static final class Request implements Signal {

    final long n;

    Request(final long n) {

      this.n = n;

    }

  };

  // This is our implementation of the Reactive Streams `Subscription`,

  // which represents the association between a `Publisher` and a `Subscriber`.

  final class SubscriptionImpl implements Subscription, Runnable {

    final Subscriber<? super T> subscriber; // We need a reference to the `Subscriber` so we can talk to it

    private boolean cancelled = false; // This flag will track whether this `Subscription` is to be considered cancelled or not

    private long demand = 0; // Here we track the current demand, i.e. what has been requested but not yet delivered

    private Iterator<T> iterator; // This is our cursor into the data stream, which we will send to the `Subscriber`

    SubscriptionImpl(final Subscriber<? super T> subscriber) {

      // As per rule 1.09, we need to throw a `java.lang.NullPointerException` if the `Subscriber` is `null`

      if (subscriber == null) throw null;

      this.subscriber = subscriber;

    }

    // This `ConcurrentLinkedQueue` will track signals that are sent to this `Subscription`, like `request` and `cancel`

    private final ConcurrentLinkedQueue<Signal> inboundSignals = new ConcurrentLinkedQueue<Signal>();

    // We are using this `AtomicBoolean` to make sure that this `Subscription` doesn't run concurrently with itself,

    // which would violate rule 1.3 among others (no concurrent notifications).

    private final AtomicBoolean on = new AtomicBoolean(false);

    // This method will register inbound demand from our `Subscriber` and validate it against rule 3.9 and rule 3.17

    private void doRequest(final long n) {

      if (n < 1)

        terminateDueTo(new IllegalArgumentException(subscriber + " violated the Reactive Streams rule 3.9 by requesting a non-positive number of elements."));

      else if (demand + n < 1) {

        // As governed by rule 3.17, when demand overflows `Long.MAX_VALUE` we treat the signalled demand as "effectively unbounded"

        demand = Long.MAX_VALUE;  // Here we protect from the overflow and treat it as "effectively unbounded"

        doSend(); // Then we proceed with sending data downstream

      } else {

        demand += n; // Here we record the downstream demand

        doSend(); // Then we can proceed with sending data downstream

      }

    }

    // This handles cancellation requests, and is idempotent, thread-safe and not synchronously performing heavy computations as specified in rule 3.5

    private void doCancel() {

      cancelled = true;

    }

    // Instead of executing `subscriber.onSubscribe` synchronously from within `Publisher.subscribe`

    // we execute it asynchronously, this is to avoid executing the user code (`Iterable.iterator`) on the calling thread.

    // It also makes it easier to follow rule 1.9

    private void doSubscribe() {

      try {

        iterator = elements.iterator();

        if (iterator == null)

          iterator = Collections.<T>emptyList().iterator(); // So we can assume that `iterator` is never null

      } catch(final Throwable t) {

        subscriber.onSubscribe(new Subscription() { // We need to make sure we signal onSubscribe before onError, obeying rule 1.9

          @Override public void cancel() {}

          @Override public void request(long n) {}

        });

        terminateDueTo(t); // Here we send onError, obeying rule 1.09

      }

      if (!cancelled) {

        // Deal with setting up the subscription with the subscriber

        try {

          subscriber.onSubscribe(this);

        } catch(final Throwable t) { // Due diligence to obey 2.13

          terminateDueTo(new IllegalStateException(subscriber + " violated the Reactive Streams rule 2.13 by throwing an exception from onSubscribe.", t));

        }

        // Deal with already complete iterators promptly

        boolean hasElements = false;

        try {

          hasElements = iterator.hasNext();

        } catch(final Throwable t) {

          terminateDueTo(t); // If hasNext throws, there's something wrong and we need to signal onError as per 1.2, 1.4,

        }

        // If we don't have anything to deliver, we're already done, so lets do the right thing and

        // not wait for demand to deliver `onComplete` as per rule 1.2 and 1.3

        if (!hasElements) {

          try {

            doCancel(); // Rule 1.6 says we need to consider the `Subscription` cancelled when `onComplete` is signalled

            subscriber.onComplete();

          } catch(final Throwable t) { // As per rule 2.13, `onComplete` is not allowed to throw exceptions, so we do what we can, and log this.

            (new IllegalStateException(subscriber + " violated the Reactive Streams rule 2.13 by throwing an exception from onComplete.", t)).printStackTrace(System.err);

          }

        }

      }

    }

    // This is our behavior for producing elements downstream

    private void doSend() {

      try {

        // In order to play nice with the `Executor` we will only send at-most `batchSize` before

        // rescheduing ourselves and relinquishing the current thread.

        int leftInBatch = batchSize;

        do {

          T next;

          boolean hasNext;

          try {

            next = iterator.next(); // We have already checked `hasNext` when subscribing, so we can fall back to testing -after- `next` is called.

            hasNext = iterator.hasNext(); // Need to keep track of End-of-Stream

          } catch (final Throwable t) {

            terminateDueTo(t); // If `next` or `hasNext` throws (they can, since it is user-provided), we need to treat the stream as errored as per rule 1.4

            return;

          }

          subscriber.onNext(next); // Then we signal the next element downstream to the `Subscriber`

          if (!hasNext) { // If we are at End-of-Stream

            doCancel(); // We need to consider this `Subscription` as cancelled as per rule 1.6

            subscriber.onComplete(); // Then we signal `onComplete` as per rule 1.2 and 1.5

          }

        } while (!cancelled           // This makes sure that rule 1.8 is upheld, i.e. we need to stop signalling "eventually"

                 && --leftInBatch > 0 // This makes sure that we only send `batchSize` number of elements in one go (so we can yield to other Runnables)

                 && --demand > 0);    // This makes sure that rule 1.1 is upheld (sending more than was demanded)

        if (!cancelled && demand > 0) // If the `Subscription` is still alive and well, and we have demand to satisfy, we signal ourselves to send more data

          signal(Send.Instance);

      } catch(final Throwable t) {

        // We can only get here if `onNext` or `onComplete` threw, and they are not allowed to according to 2.13, so we can only cancel and log here.

        doCancel(); // Make sure that we are cancelled, since we cannot do anything else since the `Subscriber` is faulty.

        (new IllegalStateException(subscriber + " violated the Reactive Streams rule 2.13 by throwing an exception from onNext or onComplete.", t)).printStackTrace(System.err);

      }

    }

    // This is a helper method to ensure that we always `cancel` when we signal `onError` as per rule 1.6

    private void terminateDueTo(final Throwable t) {

      cancelled = true; // When we signal onError, the subscription must be considered as cancelled, as per rule 1.6

      try {

        subscriber.onError(t); // Then we signal the error downstream, to the `Subscriber`

      } catch(final Throwable t2) { // If `onError` throws an exception, this is a spec violation according to rule 1.9, and all we can do is to log it.

        (new IllegalStateException(subscriber + " violated the Reactive Streams rule 2.13 by throwing an exception from onError.", t2)).printStackTrace(System.err);

      }

    }

    // What `signal` does is that it sends signals to the `Subscription` asynchronously

    private void signal(final Signal signal) {

      if (inboundSignals.offer(signal)) // No need to null-check here as ConcurrentLinkedQueue does this for us

        tryScheduleToExecute(); // Then we try to schedule it for execution, if it isn't already

    }

    // This is the main "event loop" if you so will

    @Override public final void run() {

      if(on.get()) { // establishes a happens-before relationship with the end of the previous run

        try {

          final Signal s = inboundSignals.poll(); // We take a signal off the queue

          if (!cancelled) { // to make sure that we follow rule 1.8, 3.6 and 3.7

            // Below we simply unpack the `Signal`s and invoke the corresponding methods

            if (s instanceof Request)

              doRequest(((Request)s).n);

            else if (s == Send.Instance)

              doSend();

            else if (s == Cancel.Instance)

              doCancel();

            else if (s == Subscribe.Instance)

              doSubscribe();

          }

        } finally {

          on.set(false); // establishes a happens-before relationship with the beginning of the next run

          if(!inboundSignals.isEmpty()) // If we still have signals to process

            tryScheduleToExecute(); // Then we try to schedule ourselves to execute again

        }

      }

    }

    // This method makes sure that this `Subscription` is only running on one Thread at a time,

    // this is important to make sure that we follow rule 1.3

    private final void tryScheduleToExecute() {

      if(on.compareAndSet(false, true)) {

        try {

          executor.execute(this);

        } catch(Throwable t) { // If we can't run on the `Executor`, we need to fail gracefully

          if (!cancelled) {

            doCancel(); // First of all, this failure is not recoverable, so we need to follow rule 1.4 and 1.6

            try {

              terminateDueTo(new IllegalStateException("Publisher terminated due to unavailable Executor.", t));

            } finally {

              inboundSignals.clear(); // We're not going to need these anymore

              // This subscription is cancelled by now, but letting it become schedulable again means

              // that we can drain the inboundSignals queue if anything arrives after clearing

              on.set(false);

            }

          }

        }

      }

    }

    // Our implementation of `Subscription.request` sends a signal to the Subscription that more elements are in demand

    @Override public void request(final long n) {

      signal(new Request(n));

    }

    // Our implementation of `Subscription.cancel` sends a signal to the Subscription that the `Subscriber` is not interested in any more elements

    @Override public void cancel() {

      signal(Cancel.Instance);

    }

    // The reason for the `init` method is that we want to ensure the `SubscriptionImpl`

    // is completely constructed before it is exposed to the thread pool, therefor this

    // method is only intended to be invoked once, and immediately after the constructor has

    // finished.

    void init() {

      signal(Subscribe.Instance);

    }

  };

}