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/*
 * Copyright 2002-2013 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.xiongyingqi.util.concurrent;

import com.xiongyingqi.util.Assert;

import java.util.concurrent.ExecutionException;
import java.util.concurrent.Future;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;

/**
 * Abstract class that adapts a {@link java.util.concurrent.Future} parameterized over S into a {@code
 * Future} parameterized over T. All methods are delegated to the adaptee, where {@link
 * #get()} and {@link #get(long, java.util.concurrent.TimeUnit)} call {@link #adapt(Object)} on the adaptee's
 * result.
 *
 * @param  the type of this {@code Future}
 * @param  the type of the adaptee's {@code Future}
 * @author Arjen Poutsma
 * @since 4.0
 */
public abstract class FutureAdapter implements Future {

    private final Future adaptee;

    private Object result = null;

    private State state = State.NEW;

    private final Object mutex = new Object();

    /**
     * Constructs a new {@code FutureAdapter} with the given adaptee.
     *
     * @param adaptee the future to delegate to
     */
    protected FutureAdapter(Future adaptee) {
        Assert.notNull(adaptee, "'delegate' must not be null");
        this.adaptee = adaptee;
    }

    /**
     * Returns the adaptee.
     */
    protected Future getAdaptee() {
        return adaptee;
    }

    @Override
    public boolean cancel(boolean mayInterruptIfRunning) {
        return adaptee.cancel(mayInterruptIfRunning);
    }

    @Override
    public boolean isCancelled() {
        return adaptee.isCancelled();
    }

    @Override
    public boolean isDone() {
        return adaptee.isDone();
    }

    @Override
    public T get() throws InterruptedException, ExecutionException {
        return adaptInternal(adaptee.get());
    }

    @Override
    public T get(long timeout, TimeUnit unit)
            throws InterruptedException, ExecutionException, TimeoutException {
        return adaptInternal(adaptee.get(timeout, unit));
    }

    @SuppressWarnings("unchecked")
    final T adaptInternal(S adapteeResult) throws ExecutionException {
        synchronized (mutex) {
            switch (state) {
                case SUCCESS:
                    return (T) result;
                case FAILURE:
                    throw (ExecutionException) result;
                case NEW:
                    try {
                        T adapted = adapt(adapteeResult);
                        result = adapted;
                        state = State.SUCCESS;
                        return adapted;
                    } catch (ExecutionException ex) {
                        result = ex;
                        state = State.FAILURE;
                        throw ex;
                    }
                default:
                    throw new IllegalStateException();
            }
        }
    }

    /**
     * Adapts the given adaptee's result into T.
     *
     * @return the adapted result
     */
    protected abstract T adapt(S adapteeResult) throws ExecutionException;

    private enum State {NEW, SUCCESS, FAILURE}

}