/*
 * Copyright (c) 2016 Cisco Systems, Inc. and others.  All rights reserved.
 *
 * This program and the accompanying materials are made available under the
 * terms of the Eclipse Public License v1.0 which accompanies this distribution,
 * and is available at http://www.eclipse.org/legal/epl-v10.html
 */
package org.opendaylight.controller.cluster.access.client;

import akka.actor.ActorRef;
import com.google.common.annotations.VisibleForTesting;
import com.google.common.base.Preconditions;
import com.google.common.base.Verify;
import edu.umd.cs.findbugs.annotations.SuppressFBWarnings;
import java.util.ArrayDeque;
import java.util.Iterator;
import java.util.Map.Entry;
import java.util.Optional;
import java.util.Queue;
import java.util.concurrent.TimeUnit;
import java.util.function.Consumer;
import javax.annotation.Nonnull;
import javax.annotation.concurrent.GuardedBy;
import javax.annotation.concurrent.NotThreadSafe;
import org.opendaylight.controller.cluster.access.concepts.Request;
import org.opendaylight.controller.cluster.access.concepts.RequestEnvelope;
import org.opendaylight.controller.cluster.access.concepts.RequestException;
import org.opendaylight.controller.cluster.access.concepts.Response;
import org.opendaylight.controller.cluster.access.concepts.ResponseEnvelope;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import scala.concurrent.duration.FiniteDuration;

/**
 * Base class for a connection to the backend. Responsible to queueing and dispatch of requests toward the backend.
 * Can be in three conceptual states: Connecting, Connected and Reconnecting, which are represented by public final
 * classes exposed from this package.
 *
 * @author Robert Varga
 */
@NotThreadSafe
public abstract class AbstractClientConnection<T extends BackendInfo> {
    private static final Logger LOG = LoggerFactory.getLogger(AbstractClientConnection.class);

    // Keep these constants in nanoseconds, as that prevents unnecessary conversions in the fast path
    @VisibleForTesting
    static final long NO_PROGRESS_TIMEOUT_NANOS = TimeUnit.MINUTES.toNanos(15);
    @VisibleForTesting
    static final long REQUEST_TIMEOUT_NANOS = TimeUnit.SECONDS.toNanos(30);

    private final Queue<TransmittedConnectionEntry> inflight;
    private final Queue<ConnectionEntry> pending;

    private final ClientActorContext context;
    private final Long cookie;

    private volatile ReconnectForwarder successor;
    private volatile RequestException poisoned;
    private long lastProgress;

    private AbstractClientConnection(final ClientActorContext context, final Long cookie,
            final Queue<TransmittedConnectionEntry> inflight, final Queue<ConnectionEntry> pending) {
        this.context = Preconditions.checkNotNull(context);
        this.cookie = Preconditions.checkNotNull(cookie);
        this.inflight = Preconditions.checkNotNull(inflight);
        this.pending = Preconditions.checkNotNull(pending);
        this.lastProgress = readTime();
    }

    // Do not allow subclassing outside of this package
    AbstractClientConnection(final ClientActorContext context, final Long cookie) {
        this(context, cookie, new ArrayDeque<>(), new ArrayDeque<>(1));
    }

    // Do not allow subclassing outside of this package
    AbstractClientConnection(final AbstractClientConnection<T> oldConnection) {
        this(oldConnection.context, oldConnection.cookie, oldConnection.inflight, oldConnection.pending);
    }

    public final ClientActorContext context() {
        return context;
    }

    public final @Nonnull Long cookie() {
        return cookie;
    }

    public final ActorRef localActor() {
        return context.self();
    }

    /**
     * Send a request to the backend and invoke a specified callback when it finishes. This method is safe to invoke
     * from any thread.
     *
     * @param request Request to send
     * @param callback Callback to invoke
     */
    public final void sendRequest(final Request<?, ?> request, final Consumer<Response<?, ?>> callback) {
        Preconditions.checkState(poisoned == null, "Connection %s has been poisoned", this);

        final ReconnectForwarder beforeQueue = successor;
        final ConnectionEntry entry = new ConnectionEntry(request, callback, readTime());
        if (beforeQueue != null) {
            LOG.trace("Forwarding entry {} from {} to {}", entry, this, beforeQueue);
            beforeQueue.forwardEntry(entry);
            return;
        }

        enqueueEntry(entry);

        final ReconnectForwarder afterQueue = successor;
        if (afterQueue != null) {
            synchronized (this) {
                spliceToSuccessor(afterQueue);
            }
        }
    }

    public final synchronized void setForwarder(final ReconnectForwarder forwarder) {
        Verify.verify(successor == null, "Successor {} already set on connection {}", successor, this);
        successor = Preconditions.checkNotNull(forwarder);
        LOG.debug("Connection {} superseded by {}, splicing queue", this, successor);
        spliceToSuccessor(forwarder);
    }

    public abstract Optional<T> getBackendInfo();

    abstract ClientActorBehavior<T> reconnectConnection(ClientActorBehavior<T> current);

    abstract int remoteMaxMessages();

    abstract Entry<ActorRef, RequestEnvelope> prepareForTransmit(Request<?, ?> req);

    @GuardedBy("this")
    final void spliceToSuccessor(final ReconnectForwarder successor) {
        ConnectionEntry entry = inflight.poll();
        while (entry != null) {
            successor.forwardEntry(entry);
            entry = inflight.poll();
        }

        entry = pending.poll();
        while (entry != null) {
            successor.forwardEntry(entry);
            entry = pending.poll();
        }
    }

    private long readTime() {
        return context.ticker().read();
    }

    private void transmit(final ConnectionEntry entry) {
        final Entry<ActorRef, RequestEnvelope> tuple = prepareForTransmit(entry.getRequest());
        final RequestEnvelope req = tuple.getValue();

        // We need to enqueue the request before we send it to the actor, as we may be executing on a different thread
        // than the client actor thread, in which case the round-trip could be made faster than we can enqueue --
        // in which case the receive routine would not find the entry.
        final TransmittedConnectionEntry txEntry = new TransmittedConnectionEntry(entry, req.getSessionId(),
            req.getTxSequence(), readTime());
        inflight.add(txEntry);

        final ActorRef actor = tuple.getKey();
        LOG.trace("Transmitting request {} as {} to {}", entry.getRequest(), req, actor);
        actor.tell(req, ActorRef.noSender());
    }

    final void enqueueEntry(final ConnectionEntry entry) {
        if (inflight.size() < remoteMaxMessages()) {
            transmit(entry);
            LOG.debug("Enqueued request {} to queue {}", entry.getRequest(), this);
        } else {
            LOG.debug("Queue is at capacity, delayed sending of request {}", entry.getRequest());
            pending.add(entry);
        }
    }

    /**
     * Schedule a timer to fire on the actor thread after a delay.
     *
     * @param delay Delay, in nanoseconds
     */
    private void scheduleTimer(final FiniteDuration delay) {
        LOG.debug("{}: scheduling timeout in {}", context.persistenceId(), delay);
        context.executeInActor(this::runTimer, delay);
    }

    /**
     * Check this queue for timeout and initiate reconnection if that happened. If the queue has not made progress
     * in {@link #NO_PROGRESS_TIMEOUT_NANOS} nanoseconds, it will be aborted.
     *
     * @param current Current behavior
     * @return Next behavior to use
     */
    @VisibleForTesting
    final ClientActorBehavior<T> runTimer(final ClientActorBehavior<T> current) {
        final long now = readTime();

        if (!inflight.isEmpty() || !pending.isEmpty()) {
            final long ticksSinceProgress = now - lastProgress;
            if (ticksSinceProgress >= NO_PROGRESS_TIMEOUT_NANOS) {
                LOG.error("Queue {} has not seen progress in {} seconds, failing all requests", this,
                    TimeUnit.NANOSECONDS.toSeconds(ticksSinceProgress));

                poison(new NoProgressException(ticksSinceProgress));
                current.removeConnection(this);
                return current;
            }
        }

        // Requests are always scheduled in sequence, hence checking for timeout is relatively straightforward.
        // Note we use also inquire about the delay, so we can re-schedule if needed, hence the unusual tri-state
        // return convention.
        final Optional<FiniteDuration> delay = checkTimeout(now);
        if (delay == null) {
            // We have timed out. There is no point in scheduling a timer
            return reconnectConnection(current);
        }

        if (delay.isPresent()) {
            // If there is new delay, schedule a timer
            scheduleTimer(delay.get());
        }

        return current;
    }

    /*
     * We are using tri-state return here to indicate one of three conditions:
     * - if there is no timeout to schedule, return Optional.empty()
     * - if there is a timeout to schedule, return a non-empty optional
     * - if this connections has timed out, return null
     */
    @SuppressFBWarnings(value = "NP_OPTIONAL_RETURN_NULL",
            justification = "Returning null Optional is documented in the API contract.")
    private Optional<FiniteDuration> checkTimeout(final ConnectionEntry head, final long now) {
        if (head == null) {
            return Optional.empty();
        }

        final long delay = head.getEnqueuedTicks() - now + REQUEST_TIMEOUT_NANOS;
        if (delay <= 0) {
            LOG.debug("Connection {} timed out", this);
            return null;
        }

        return Optional.of(FiniteDuration.apply(delay, TimeUnit.NANOSECONDS));
    }

    /*
     * We are using tri-state return here to indicate one of three conditions:
     * - if there is no timeout to schedule, return Optional.empty()
     * - if there is a timeout to schedule, return a non-empty optional
     * - if this connections has timed out, return null
     */
    @SuppressFBWarnings(value = "NP_OPTIONAL_RETURN_NULL",
            justification = "Returning null Optional is documented in the API contract.")
    @VisibleForTesting
    final Optional<FiniteDuration> checkTimeout(final long now) {
        final Optional<FiniteDuration> xmit = checkTimeout(inflight.peek(), now);
        if (xmit == null) {
            return null;
        }
        final Optional<FiniteDuration> pend = checkTimeout(pending.peek(), now);
        if (pend == null) {
            return null;
        }
        if (!xmit.isPresent()) {
            return pend;
        }
        if (!pend.isPresent()) {
            return xmit;
        }

        return Optional.of(xmit.get().min(pend.get()));
    }

    final void poison(final RequestException cause) {
        poisoned = cause;

        poisonQueue(inflight, cause);
        poisonQueue(pending, cause);
    }

    @VisibleForTesting
    final RequestException poisoned() {
        return poisoned;
    }

    final void receiveResponse(final ResponseEnvelope<?> envelope) {
        Optional<TransmittedConnectionEntry> maybeEntry = findMatchingEntry(inflight, envelope);
        if (maybeEntry == null) {
            LOG.debug("Request for {} not found in inflight queue, checking pending queue", envelope);
            maybeEntry = findMatchingEntry(pending, envelope);
        }

        if (maybeEntry == null || !maybeEntry.isPresent()) {
            LOG.warn("No request matching {} found, ignoring response", envelope);
            return;
        }

        final TransmittedConnectionEntry entry = maybeEntry.get();
        LOG.debug("Completing {} with {}", entry, envelope);
        entry.complete(envelope.getMessage());

        // We have freed up a slot, try to transmit something
        int toSend = remoteMaxMessages() - inflight.size();
        while (toSend > 0) {
            final ConnectionEntry e = pending.poll();
            if (e == null) {
                break;
            }

            LOG.debug("Transmitting entry {}", e);
            transmit(e);
            toSend--;
        }

        lastProgress = readTime();
    }

    private static void poisonQueue(final Queue<? extends ConnectionEntry> queue, final RequestException cause) {
        for (ConnectionEntry e : queue) {
            final Request<?, ?> request = e.getRequest();
            LOG.trace("Poisoning request {}", request, cause);
            e.complete(request.toRequestFailure(cause));
        }
        queue.clear();
    }

    /*
     * We are using tri-state return here to indicate one of three conditions:
     * - if a matching entry is found, return an Optional containing it
     * - if a matching entry is not found, but it makes sense to keep looking at other queues, return null
     * - if a conflicting entry is encountered, indicating we should ignore this request, return an empty Optional
     */
    @SuppressFBWarnings(value = "NP_OPTIONAL_RETURN_NULL",
            justification = "Returning null Optional is documented in the API contract.")
    private static Optional<TransmittedConnectionEntry> findMatchingEntry(final Queue<? extends ConnectionEntry> queue,
            final ResponseEnvelope<?> envelope) {
        // Try to find the request in a queue. Responses may legally come back in a different order, hence we need
        // to use an iterator
        final Iterator<? extends ConnectionEntry> it = queue.iterator();
        while (it.hasNext()) {
            final ConnectionEntry e = it.next();
            final Request<?, ?> request = e.getRequest();
            final Response<?, ?> response = envelope.getMessage();

            // First check for matching target, or move to next entry
            if (!request.getTarget().equals(response.getTarget())) {
                continue;
            }

            // Sanity-check logical sequence, ignore any out-of-order messages
            if (request.getSequence() != response.getSequence()) {
                LOG.debug("Expecting sequence {}, ignoring response {}", request.getSequence(), envelope);
                return Optional.empty();
            }

            // Check if the entry has (ever) been transmitted
            if (!(e instanceof TransmittedConnectionEntry)) {
                return Optional.empty();
            }

            final TransmittedConnectionEntry te = (TransmittedConnectionEntry) e;

            // Now check session match
            if (envelope.getSessionId() != te.getSessionId()) {
                LOG.debug("Expecting session {}, ignoring response {}", te.getSessionId(), envelope);
                return Optional.empty();
            }
            if (envelope.getTxSequence() != te.getTxSequence()) {
                LOG.warn("Expecting txSequence {}, ignoring response {}", te.getTxSequence(), envelope);
                return Optional.empty();
            }

            LOG.debug("Completing request {} with {}", request, envelope);
            it.remove();
            return Optional.of(te);
        }

        return null;
    }
}