/* Copyright 2010 Google Inc.
 * 
 * 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.google.ie.common.openid.appengine;

import com.google.appengine.repackaged.com.google.common.collect.Lists;
import com.google.common.collect.ImmutableList;

import java.util.Collection;
import java.util.List;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
import java.util.concurrent.TimeUnit;

/**
 * Executes Callables in the current thread.
 */
public class SerialExecutorService implements ExecutorService {

    public boolean awaitTermination(long timeout, TimeUnit unit) {
        return false;
    }

    public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks) {

        List<Future<T>> futures = Lists.newArrayListWithCapacity(tasks.size());
        for (Callable<T> task : tasks) {
            futures.add(submit(task));
        }
        return futures;
    }

    public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks,
                    long timeout, TimeUnit unit) {
        return invokeAll(tasks);
    }

    public <T> T invokeAny(Collection<? extends Callable<T>> tasks)
                    throws ExecutionException {

        Exception lastException = null;
        for (Callable<T> task : tasks) {
            try {
                T result = task.call();
                return result;
            } catch (Exception e) {
                lastException = e;
                continue;
            }
        }

        throw new ExecutionException("none of the tasks completed successfully",
                        lastException);
    }

    public <T> T invokeAny(Collection<? extends Callable<T>> tasks, long timeout,
                    TimeUnit unit) throws ExecutionException {
        return invokeAny(tasks);
    }

    public boolean isShutdown() {
        return false;
    }

    public boolean isTerminated() {
        return false;
    }

    public void shutdown() {
    }

    public List<Runnable> shutdownNow() {
        return ImmutableList.of();
    }

    public <T> Future<T> submit(Callable<T> task) {
        try {
            T result = task.call();
            return new NoFuture<T>(result);
        } catch (Exception e) {
            return new NoFuture<T>(new ExecutionException(e));
        }
    }

    public Future<?> submit(Runnable task) {
        return submit(Executors.callable(task));
    }

    public <T> Future<T> submit(Runnable task, T result) {
        task.run();
        return new NoFuture<T>(result);
    }

    public void execute(Runnable command) {
        command.run();
    }

    private class NoFuture<T> implements Future<T> {

        private final T value;
        private final ExecutionException ex;

        public NoFuture(T value) {
            this.value = value;
            this.ex = null;
        }

        public NoFuture(ExecutionException ex) {
            this.value = null;
            this.ex = ex;
        }

        public boolean cancel(boolean mayInterruptIfRunning) {
            return false;
        }

        public T get() throws ExecutionException {
            if (ex == null) {
                return value;
            } else {
                throw ex;
            }
        }

        public T get(long timeout, TimeUnit unit) throws ExecutionException {
            return get();
        }

        public boolean isCancelled() {
            return false;
        }

        public boolean isDone() {
            return true;
        }
    }
}