/*

 * Copyright (c) 2016, Oracle and/or its affiliates. All rights reserved.

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

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General  License version 2 only, as

 * published by the Free Software Foundation.  Oracle designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Oracle in the LICENSE file that accompanied this code.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

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

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

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package java.net.http;

import java.util.concurrent.Executor;

import java.util.concurrent.RejectedExecutionException;

import java.util.concurrent.atomic.AtomicInteger;

import static java.util.Objects.requireNonNull;

//

// The problem:

// ------------

//   1. For every invocation of 'signal()' there must be at least

//      1 invocation of 'handler.run()' that goes after

//   2. There must be no more than 1 thread running the 'handler.run()'

//      at any given time

//

// For example, imagine each signal increments (+1) some number. Then the

// handler responds (eventually) the way that makes the number 0.

//

// For each signal there's a response. Several signals may be handled by a

// single response.

//

final class WSSignalHandler {

    // In this state the task is neither submitted nor running.

    // No one is handling signals. If a new signal has been received, the task

    // has to be submitted to the executor in order to handle this signal.

    private static final int DONE    = 0;

    // In this state the task is running.

    // * If the signaller has found the task in this state it will try to change

    //   the state to RERUN in order to make the already running task to handle

    //   the new signal before exiting.

    // * If the task has found itself in this state it will exit.

    private static final int RUNNING = 1;

    // A signal to the task, that it must rerun on the spot (without being

    // resubmitted to the executor).

    // If the task has found itself in this state it resets the state to

    // RUNNING and repeats the pass.

    private static final int RERUN   = 2;

    private final AtomicInteger state = new AtomicInteger(DONE);

    private final Executor executor;

    private final Runnable task;

    WSSignalHandler(Executor executor, Runnable handler) {

        this.executor = requireNonNull(executor);

        requireNonNull(handler);

        task = () -> {

            while (!Thread.currentThread().isInterrupted()) {

                try {

                    handler.run();

                } catch (Exception e) {

                    // Sorry, the task won't be automatically retried;

                    // hope next signals (if any) will kick off the handling

                    state.set(DONE);

                    throw e;

                }

                int prev = state.getAndUpdate(s -> {

                    if (s == RUNNING) {

                        return DONE;

                    } else {

                        return RUNNING;

                    }

                });

                // Can't be DONE, since only the task itself may transit state

                // into DONE (with one exception: RejectedExecution in signal();

                // but in that case we couldn't be here at all)

                assert prev == RUNNING || prev == RERUN;

                if (prev == RUNNING) {

                    break;

                }

            }

        };

    }

    // Invoked by outer code to signal

    void signal() {

        int prev = state.getAndUpdate(s -> {

            switch (s) {

                case RUNNING:

                    return RERUN;

                case DONE:

                    return RUNNING;

                case RERUN:

                    return RERUN;

                default:

                    throw new InternalError(String.valueOf(s));

            }

        });

        if (prev != DONE) {

            // Nothing to do! piggybacking on previous signal

            return;

        }

        try {

            executor.execute(task);

        } catch (RejectedExecutionException e) {

            // Sorry some signal() invocations may have been accepted, but won't

            // be done, since the 'task' couldn't be submitted

            state.set(DONE);

            throw e;

        }

    }

}