/*
 * Copyright (c) OSGi Alliance (2014, 2016). All Rights Reserved.
 * 
 * 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 org.osgi.util.promise;

import static org.osgi.util.promise.PromiseImpl.requireNonNull;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.List;
import java.util.concurrent.atomic.AtomicInteger;

import org.osgi.util.promise.PromiseImpl.Result;

/**
 * Static helper methods for {@link Promise}s.
 * 
 * @ThreadSafe
 * @author $Id$
 */
public class Promises {
	private Promises() {
		// disallow object creation
	}

	/**
	 * Create a new Promise that has been resolved with the specified value.
	 * 
	 * @param <T> The value type associated with the returned Promise.
	 * @param value The value of the resolved Promise.
	 * @return A new Promise that has been resolved with the specified value.
	 */
	public static <T> Promise<T> resolved(T value) {
		return new PromiseImpl<>(value, null);
	}

	/**
	 * Create a new Promise that has been resolved with the specified failure.
	 * 
	 * @param <T> The value type associated with the returned Promise.
	 * @param failure The failure of the resolved Promise. Must not be
	 *        {@code null}.
	 * @return A new Promise that has been resolved with the specified failure.
	 */
	public static <T> Promise<T> failed(Throwable failure) {
		return new PromiseImpl<>(null, requireNonNull(failure));
	}

	/**
	 * Create a new Promise that is a latch on the resolution of the specified
	 * Promises.
	 * 
	 * <p>
	 * The new Promise acts as a gate and must be resolved after all of the
	 * specified Promises are resolved.
	 * 
	 * @param <T> The value type of the List value associated with the returned
	 *        Promise.
	 * @param <S> A subtype of the value type of the List value associated with
	 *        the returned Promise.
	 * @param promises The Promises which must be resolved before the returned
	 *        Promise must be resolved. Must not be {@code null} and all of the
	 *        elements in the collection must not be {@code null}.
	 * @return A Promise that is resolved only when all the specified Promises
	 *         are resolved. The returned Promise must be successfully resolved
	 *         with a List of the values in the order of the specified Promises
	 *         if all the specified Promises are successfully resolved. The List
	 *         in the returned Promise is the property of the caller and is
	 *         modifiable. The returned Promise must be resolved with a failure
	 *         of {@link FailedPromisesException} if any of the specified
	 *         Promises are resolved with a failure. The failure
	 *         {@link FailedPromisesException} must contain all of the specified
	 *         Promises which resolved with a failure.
	 */
	public static <T, S extends T> Promise<List<T>> all(Collection<Promise<S>> promises) {
		if (promises.isEmpty()) {
			List<T> result = new ArrayList<>();
			return resolved(result);
		}
		/* make a copy and capture the ordering */
		List<Promise< ? extends T>> list = new ArrayList<Promise< ? extends T>>(
				promises);
		PromiseImpl<List<T>> chained = new PromiseImpl<>();
		All<T> all = new All<>(chained, list);
		for (Promise<? extends T> promise : list) {
			promise.onResolve(all);
		}
		return chained;
	}

	/**
	 * Create a new Promise that is a latch on the resolution of the specified
	 * Promises.
	 * 
	 * <p>
	 * The new Promise acts as a gate and must be resolved after all of the
	 * specified Promises are resolved.
	 * 
	 * @param <T> The value type associated with the specified Promises.
	 * @param promises The Promises which must be resolved before the returned
	 *        Promise must be resolved. Must not be {@code null} and all of the
	 *        arguments must not be {@code null}.
	 * @return A Promise that is resolved only when all the specified Promises
	 *         are resolved. The returned Promise must be successfully resolved
	 *         with a List of the values in the order of the specified Promises
	 *         if all the specified Promises are successfully resolved. The List
	 *         in the returned Promise is the property of the caller and is
	 *         modifiable. The returned Promise must be resolved with a failure
	 *         of {@link FailedPromisesException} if any of the specified
	 *         Promises are resolved with a failure. The failure
	 *         {@link FailedPromisesException} must contain all of the specified
	 *         Promises which resolved with a failure.
	 */
	@SafeVarargs
	public static <T> Promise<List<T>> all(Promise<? extends T>... promises) {
		@SuppressWarnings("unchecked")
		List<Promise<T>> list = Arrays.asList((Promise<T>[]) promises);
		return all(list);
	}

	/**
	 * A callback used to resolve a Promise when the specified list of Promises
	 * are resolved for the {@link Promises#all(Collection)} method.
	 * 
	 * @ThreadSafe
	 */
	private static final class All<T> implements Runnable {
		private final PromiseImpl<List<T>>			chained;
		private final List<Promise<? extends T>>	promises;
		private final AtomicInteger					promiseCount;

		All(PromiseImpl<List<T>> chained, List<Promise<? extends T>> promises) {
			this.chained = chained;
			this.promises = promises;
			this.promiseCount = new AtomicInteger(promises.size());
		}

		@Override
		public void run() {
			if (promiseCount.decrementAndGet() != 0) {
				return;
			}
			List<T> value = new ArrayList<>(promises.size());
			List<Promise<?>> failed = new ArrayList<>(promises.size());
			Throwable cause = null;
			for (Promise<? extends T> promise : promises) {
				Result<T> result = Result.collect(promise);
				if (result.fail != null) {
					failed.add(promise);
					if (cause == null) {
						cause = result.fail;
					}
				} else {
					value.add(result.value);
				}
			}
			if (failed.isEmpty()) {
				chained.tryResolve(value, null);
			} else {
				chained.tryResolve(null,
						new FailedPromisesException(failed, cause));
			}
		}
	}
}