//
// ========================================================================
// Copyright (c) 1995-2020 Mort Bay Consulting Pty Ltd and others.
//
// This program and the accompanying materials are made available under
// the terms of the Eclipse Public License 2.0 which is available at
// https://www.eclipse.org/legal/epl-2.0
//
// This Source Code may also be made available under the following
// Secondary Licenses when the conditions for such availability set
// forth in the Eclipse Public License, v. 2.0 are satisfied:
// the Apache License v2.0 which is available at
// https://www.apache.org/licenses/LICENSE-2.0
//
// SPDX-License-Identifier: EPL-2.0 OR Apache-2.0
// ========================================================================
//

package org.eclipse.jetty.util.thread.strategy;

import java.io.Closeable;
import java.time.ZonedDateTime;
import java.time.format.DateTimeFormatter;
import java.util.concurrent.Executor;
import java.util.concurrent.RejectedExecutionException;
import java.util.concurrent.atomic.LongAdder;

import org.eclipse.jetty.util.annotation.ManagedAttribute;
import org.eclipse.jetty.util.annotation.ManagedObject;
import org.eclipse.jetty.util.annotation.ManagedOperation;
import org.eclipse.jetty.util.component.ContainerLifeCycle;
import org.eclipse.jetty.util.thread.AutoLock;
import org.eclipse.jetty.util.thread.ExecutionStrategy;
import org.eclipse.jetty.util.thread.Invocable;
import org.eclipse.jetty.util.thread.TryExecutor;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

A strategy where the thread that produces will run the resulting task if it
is possible to do so without thread starvation.
This strategy preemptively dispatches a thread as a pending producer, so that
when a thread produces a task it can immediately run the task and let the pending
producer thread take over production. When operating in this way, the sub-strategy
is called Execute Produce Consume (EPC).
However, if the task produced uses the Invocable API to indicate that it will not block, then the strategy will run it directly, regardless of the presence of a pending producer thread and then resume production after the task has completed. When operating in this pattern, the sub-strategy is called ProduceConsume (PC).
If there is no pending producer thread available and if the task has not
indicated it is non-blocking, then this strategy will dispatch the execution of
the task and immediately continue production. When operating in this pattern, the
sub-strategy is called ProduceExecuteConsume (PEC).
/**
 * <p>A strategy where the thread that produces will run the resulting task if it
 * is possible to do so without thread starvation.</p>
 *
 * <p>This strategy preemptively dispatches a thread as a pending producer, so that
 * when a thread produces a task it can immediately run the task and let the pending
 * producer thread take over production. When operating in this way, the sub-strategy
 * is called Execute Produce Consume (EPC).</p>
 * <p>However, if the task produced uses the {@link Invocable} API to indicate that
 * it will not block, then the strategy will run it directly, regardless of the
 * presence of a pending producer thread and then resume production after the
 * task has completed. When operating in this pattern, the sub-strategy is called
 * ProduceConsume (PC).</p>
 * <p>If there is no pending producer thread available and if the task has not
 * indicated it is non-blocking, then this strategy will dispatch the execution of
 * the task and immediately continue production. When operating in this pattern, the
 * sub-strategy is called ProduceExecuteConsume (PEC).</p>
 */
@ManagedObject("eat what you kill execution strategy")
public class EatWhatYouKill extends ContainerLifeCycle implements ExecutionStrategy, Runnable
{
    private static final Logger LOG = LoggerFactory.getLogger(EatWhatYouKill.class);

    private enum State
    {
        IDLE, PRODUCING, REPRODUCING
    }

    /* The modes this strategy can work in */
    private enum Mode
    {
        PRODUCE_CONSUME,
        PRODUCE_INVOKE_CONSUME, // This is PRODUCE_CONSUME an EITHER task with NON_BLOCKING invocation
        PRODUCE_EXECUTE_CONSUME,
        EXECUTE_PRODUCE_CONSUME // Eat What You Kill!
    }

    private final AutoLock _lock = new AutoLock();
    private final LongAdder _pcMode = new LongAdder();
    private final LongAdder _picMode = new LongAdder();
    private final LongAdder _pecMode = new LongAdder();
    private final LongAdder _epcMode = new LongAdder();
    private final Producer _producer;
    private final Executor _executor;
    private final TryExecutor _tryExecutor;
    private State _state = State.IDLE;
    private boolean _pending;

    public EatWhatYouKill(Producer producer, Executor executor)
    {
        _producer = producer;
        _executor = executor;
        _tryExecutor = TryExecutor.asTryExecutor(executor);
        addBean(_producer);
        addBean(_tryExecutor);
        if (LOG.isDebugEnabled())
            LOG.debug("{} created", this);
    }

    @Override
    public void dispatch()
    {
        boolean execute = false;
        try (AutoLock l = _lock.lock())
        {
            switch (_state)
            {
                case IDLE:
                    if (!_pending)
                    {
                        _pending = true;
                        execute = true;
                    }
                    break;

                case PRODUCING:
                    _state = State.REPRODUCING;
                    break;

                default:
                    break;
            }
        }
        if (LOG.isDebugEnabled())
            LOG.debug("{} dispatch {}", this, execute);
        if (execute)
            _executor.execute(this);
    }

    @Override
    public void run()
    {
        tryProduce(true);
    }

    @Override
    public void produce()
    {
        tryProduce(false);
    }

    private void tryProduce(boolean wasPending)
    {
        if (LOG.isDebugEnabled())
            LOG.debug("{} tryProduce {}", this, wasPending);

        try (AutoLock l = _lock.lock())
        {
            if (wasPending)
                _pending = false;

            switch (_state)
            {
                case IDLE:
                    // Enter PRODUCING
                    _state = State.PRODUCING;
                    break;

                case PRODUCING:
                    // Keep other Thread producing
                    _state = State.REPRODUCING;
                    return;

                default:
                    return;
            }
        }

        boolean nonBlocking = Invocable.isNonBlockingInvocation();

        while (isRunning())
        {
            try
            {
                if (doProduce(nonBlocking))
                    continue;
                return;
            }
            catch (Throwable th)
            {
                LOG.warn("Unable to produce", th);
            }
        }
    }

    private boolean doProduce(boolean nonBlocking)
    {
        Runnable task = produceTask();

        if (task == null)
        {
            try (AutoLock l = _lock.lock())
            {
                // Could another task just have been queued with a produce call?
                switch (_state)
                {
                    case PRODUCING:
                        _state = State.IDLE;
                        return false;

                    case REPRODUCING:
                        _state = State.PRODUCING;
                        return true;

                    default:
                        throw new IllegalStateException(toStringLocked());
                }
            }
        }

        Mode mode;
        if (nonBlocking)
        {
            // The calling thread cannot block, so we only have a choice between PC and PEC modes,
            // based on the invocation type of the task
            switch (Invocable.getInvocationType(task))
            {
                case NON_BLOCKING:
                    mode = Mode.PRODUCE_CONSUME;
                    break;

                case EITHER:
                    mode = Mode.PRODUCE_INVOKE_CONSUME;
                    break;

                default:
                    mode = Mode.PRODUCE_EXECUTE_CONSUME;
                    break;
            }
        }
        else
        {
            // The calling thread can block, so we can choose between PC, PEC and EPC modes,
            // based on the invocation type of the task and if a reserved thread is available
            switch (Invocable.getInvocationType(task))
            {
                case NON_BLOCKING:
                    mode = Mode.PRODUCE_CONSUME;
                    break;

                case BLOCKING:
                    // The task is blocking, so PC is not an option. Thus we choose
                    // between EPC and PEC based on the availability of a reserved thread.
                    try (AutoLock l = _lock.lock())
                    {
                        if (_pending)
                        {
                            _state = State.IDLE;
                            mode = Mode.EXECUTE_PRODUCE_CONSUME;
                        }
                        else if (_tryExecutor.tryExecute(this))
                        {
                            _pending = true;
                            _state = State.IDLE;
                            mode = Mode.EXECUTE_PRODUCE_CONSUME;
                        }
                        else
                        {
                            mode = Mode.PRODUCE_EXECUTE_CONSUME;
                        }
                    }
                    break;

                case EITHER:
                    // The task may be non blocking, so PC is an option. Thus we choose
                    // between EPC and PC based on the availability of a reserved thread.
                    try (AutoLock l = _lock.lock())
                    {
                        if (_pending)
                        {
                            _state = State.IDLE;
                            mode = Mode.EXECUTE_PRODUCE_CONSUME;
                        }
                        else if (_tryExecutor.tryExecute(this))
                        {
                            _pending = true;
                            _state = State.IDLE;
                            mode = Mode.EXECUTE_PRODUCE_CONSUME;
                        }
                        else
                        {
                            // PC mode, but we must consume with non-blocking invocation
                            // as we may be the last thread and we cannot block
                            mode = Mode.PRODUCE_INVOKE_CONSUME;
                        }
                    }
                    break;

                default:
                    throw new IllegalStateException(toString());
            }
        }

        if (LOG.isDebugEnabled())
            LOG.debug("{} m={} t={}/{}", this, mode, task, Invocable.getInvocationType(task));

        // Consume or execute task
        switch (mode)
        {
            case PRODUCE_CONSUME:
                _pcMode.increment();
                runTask(task);
                return true;

            case PRODUCE_INVOKE_CONSUME:
                _picMode.increment();
                invokeTask(task);
                return true;

            case PRODUCE_EXECUTE_CONSUME:
                _pecMode.increment();
                execute(task);
                return true;

            case EXECUTE_PRODUCE_CONSUME:
                _epcMode.increment();
                runTask(task);

                // Try to produce again?
                try (AutoLock l = _lock.lock())
                {
                    if (_state == State.IDLE)
                    {
                        // We beat the pending producer, so we will become the producer instead
                        _state = State.PRODUCING;
                        return true;
                    }
                }
                return false;

            default:
                throw new IllegalStateException(toString());
        }
    }

    private void runTask(Runnable task)
    {
        try
        {
            task.run();
        }
        catch (Throwable x)
        {
            LOG.warn("Task run failed", x);
        }
    }

    private void invokeTask(Runnable task)
    {
        try
        {
            Invocable.invokeNonBlocking(task);
        }
        catch (Throwable x)
        {
            LOG.warn("Task invoke failed", x);
        }
    }

    private Runnable produceTask()
    {
        try
        {
            return _producer.produce();
        }
        catch (Throwable e)
        {
            LOG.warn("Task produce failed", e);
            return null;
        }
    }

    private void execute(Runnable task)
    {
        try
        {
            _executor.execute(task);
        }
        catch (RejectedExecutionException e)
        {
            if (isRunning())
                LOG.warn("Execute failed", e);
            else
                LOG.trace("IGNORED", e);

            if (task instanceof Closeable)
            {
                try
                {
                    ((Closeable)task).close();
                }
                catch (Throwable e2)
                {
                    LOG.trace("IGNORED", e2);
                }
            }
        }
    }

    @ManagedAttribute(value = "number of tasks consumed with PC mode", readonly = true)
    public long getPCTasksConsumed()
    {
        return _pcMode.longValue();
    }

    @ManagedAttribute(value = "number of tasks executed with PIC mode", readonly = true)
    public long getPICTasksExecuted()
    {
        return _picMode.longValue();
    }

    @ManagedAttribute(value = "number of tasks executed with PEC mode", readonly = true)
    public long getPECTasksExecuted()
    {
        return _pecMode.longValue();
    }

    @ManagedAttribute(value = "number of tasks consumed with EPC mode", readonly = true)
    public long getEPCTasksConsumed()
    {
        return _epcMode.longValue();
    }

    @ManagedAttribute(value = "whether this execution strategy is idle", readonly = true)
    public boolean isIdle()
    {
        try (AutoLock l = _lock.lock())
        {
            return _state == State.IDLE;
        }
    }

    @ManagedOperation(value = "resets the task counts", impact = "ACTION")
    public void reset()
    {
        _pcMode.reset();
        _epcMode.reset();
        _pecMode.reset();
        _picMode.reset();
    }

    @Override
    public String toString()
    {
        try (AutoLock l = _lock.lock())
        {
            return toStringLocked();
        }
    }

    public String toStringLocked()
    {
        StringBuilder builder = new StringBuilder();
        getString(builder);
        getState(builder);
        return builder.toString();
    }

    private void getString(StringBuilder builder)
    {
        builder.append(getClass().getSimpleName());
        builder.append('@');
        builder.append(Integer.toHexString(hashCode()));
        builder.append('/');
        builder.append(_producer);
        builder.append('/');
    }

    private void getState(StringBuilder builder)
    {
        builder.append(_state);
        builder.append("/p=");
        builder.append(_pending);
        builder.append('/');
        builder.append(_tryExecutor);
        builder.append("[pc=");
        builder.append(getPCTasksConsumed());
        builder.append(",pic=");
        builder.append(getPICTasksExecuted());
        builder.append(",pec=");
        builder.append(getPECTasksExecuted());
        builder.append(",epc=");
        builder.append(getEPCTasksConsumed());
        builder.append("]");
        builder.append("@");
        builder.append(DateTimeFormatter.ISO_OFFSET_DATE_TIME.format(ZonedDateTime.now()));
    }
}