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Utility classes for Jetty
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//
// ========================================================================
// Copyright (c) 1995-2021 Mort Bay Consulting Pty Ltd and others.
//
// This program and the accompanying materials are made available under the
// terms of the Eclipse Public License v. 2.0 which is available at
// https://www.eclipse.org/legal/epl-2.0, or the Apache License, Version 2.0
// 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;

import java.util.Objects;
import java.util.concurrent.CompletableFuture;
import java.util.function.Consumer;

import org.slf4j.LoggerFactory;

/**
 * A callback abstraction that handles completed/failed events of asynchronous operations.
 *
 * @param  the type of the context object
 */
public interface Promise
{
    /**
     * Callback invoked when the operation completes.
     *
     * @param result the context
     * @see #failed(Throwable)
     */
    default void succeeded(C result)
    {
    }

    /**
     * Callback invoked when the operation fails.
     *
     * @param x the reason for the operation failure
     */
    default void failed(Throwable x)
    {
    }

    /**
     * Empty implementation of {@link Promise}.
     *
     * @param  the type of the result
     */
    class Adapter implements Promise
    {
        @Override
        public void failed(Throwable x)
        {
            LoggerFactory.getLogger(this.getClass()).warn("Failed", x);
        }
    }

    /**
     * Creates a Promise from the given success and failure consumers.
     *
     * @param success the consumer invoked when the promise is succeeded
     * @param failure the consumer invoked when the promise is failed
     * @param  the type of the result
     * @return a new Promise wrapping the success and failure consumers.
     */
    static  Promise from(Consumer success, Consumer failure)
    {
        return new Promise<>()
        {
            @Override
            public void succeeded(T result)
            {
                success.accept(result);
            }

            @Override
            public void failed(Throwable x)
            {
                failure.accept(x);
            }
        };
    }

    /**
     * Creates a promise from the given incomplete CompletableFuture.
     * When the promise completes, either succeeding or failing, the
     * CompletableFuture is also completed, respectively via
     * {@link CompletableFuture#complete(Object)} or
     * {@link CompletableFuture#completeExceptionally(Throwable)}.
     *
     * @param completable the CompletableFuture to convert into a promise
     * @param  the type of the result
     * @return a promise that when completed, completes the given CompletableFuture
     */
    static  Promise from(CompletableFuture completable)
    {
        if (completable instanceof Promise)
            return (Promise)completable;

        return new Promise()
        {
            @Override
            public void succeeded(T result)
            {
                completable.complete(result);
            }

            @Override
            public void failed(Throwable x)
            {
                completable.completeExceptionally(x);
            }
        };
    }

    /**
     * A CompletableFuture that is also a Promise.
     *
     * @param  the type of the result
     */
    class Completable extends CompletableFuture implements Promise
    {
        @Override
        public void succeeded(S result)
        {
            complete(result);
        }

        @Override
        public void failed(Throwable x)
        {
            completeExceptionally(x);
        }
    }

    class Wrapper implements Promise
    {
        private final Promise promise;

        public Wrapper(Promise promise)
        {
            this.promise = Objects.requireNonNull(promise);
        }

        @Override
        public void succeeded(W result)
        {
            promise.succeeded(result);
        }

        @Override
        public void failed(Throwable x)
        {
            promise.failed(x);
        }

        public Promise getPromise()
        {
            return promise;
        }

        public Promise unwrap()
        {
            Promise result = promise;
            while (true)
            {
                if (result instanceof Wrapper)
                    result = ((Wrapper)result).unwrap();
                else
                    break;
            }
            return result;
        }
    }
}