/**
 * Copyright 2016 LinkedIn Corp. 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.
 */
package com.github.ambry.router;

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


/**
 * A class that implements the future completion of a request.
 */
public class FutureResult<T> implements Future<T> {

  private final CountDownLatch latch = new CountDownLatch(1);
  private volatile Exception error;
  private volatile T result;

  /**
   * Mark this request as complete and unblock any threads waiting on its completion.
   * @param result The result for this request
   * @param error The error that occurred if there was one, or null.
   */
  public void done(T result, Exception error) {
    this.error = error;
    this.result = result;
    this.latch.countDown();
  }

  /**
   * Await the completion of this request
   */
  public void await() throws InterruptedException {
    latch.await();
  }

  /**
   * Await the completion of this request (up to the given time interval)
   * @param timeout The maximum time to wait
   * @param unit The unit for the max time
   * @return true if the request completed, false if we timed out
   */
  public boolean await(long timeout, TimeUnit unit) throws InterruptedException {
    return latch.await(timeout, unit);
  }

  /**
   * The result for the request
   */
  public T result() {
    return result;
  }

  /**
   * The error thrown (generally on the server) while processing this request
   */
  public Exception error() {
    return error;
  }

  /**
   * Has the request completed?
   */
  public boolean completed() {
    return this.latch.getCount() == 0L;
  }

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

  @Override
  public boolean isCancelled() {
    return false;
  }

  @Override
  public boolean isDone() {
    return completed();
  }

  @Override
  public T get() throws InterruptedException, ExecutionException {
    this.await();
    return resultOrThrow();
  }

  @Override
  public T get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException {
    boolean occurred = this.await(timeout, unit);
    if (!occurred) {
      throw new TimeoutException("Timeout after waiting for " + TimeUnit.MILLISECONDS.convert(timeout, unit) + " ms.");
    }
    return resultOrThrow();
  }

  private T resultOrThrow() throws ExecutionException {
    if (this.error() != null) {
      throw new ExecutionException(this.error());
    } else {
      return result();
    }
  }
}