/*
 * Copyright (C) 2009 The Guava 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.google.common.util.concurrent;

import static com.google.common.base.Preconditions.checkNotNull;

import com.google.common.base.Supplier;

import java.util.concurrent.Callable;

import javax.annotation.Nullable;

/**
 * Static utility methods pertaining to the {@link Callable} interface.
 *
 * @author Isaac Shum
 * @since 1.0
 */
public final class Callables {
  private Callables() {}

  /**
   * Creates a {@code Callable} which immediately returns a preset value each
   * time it is called.
   */
  public static <T> Callable<T> returning(final @Nullable T value) {
    return new Callable<T>() {
      @Override public T call() {
        return value;
      }
    };
  }

  /**
   * Wraps the given callable such that for the duration of {@link Callable#call} the thread that is
   * running will have the given name.
   *
   *
   * @param callable The callable to wrap
   * @param nameSupplier The supplier of thread names, {@link Supplier#get get} will be called once
   *     for each invocation of the wrapped callable.
   */
  static <T> Callable<T> threadRenaming(final Callable<T> callable,
      final Supplier<String> nameSupplier) {
    checkNotNull(nameSupplier);
    checkNotNull(callable);
    return new Callable<T>() {
      @Override public T call() throws Exception {
        Thread currentThread = Thread.currentThread();
        String oldName = currentThread.getName();
        boolean restoreName = trySetName(nameSupplier.get(), currentThread);
        try {
          return callable.call();
        } finally {
          if (restoreName) {
            trySetName(oldName, currentThread);
          }
        }
      }
    };
  }

  /**
   * Wraps the given runnable such that for the duration of {@link Runnable#run} the thread that is
   * running with have the given name.
   *
   *
   * @param task The Runnable to wrap
   * @param nameSupplier The supplier of thread names, {@link Supplier#get get} will be called once
   *     for each invocation of the wrapped callable.
   */
  static Runnable threadRenaming(final Runnable task, final Supplier<String> nameSupplier) {
    checkNotNull(nameSupplier);
    checkNotNull(task);
    return new Runnable() {
      @Override public void run() {
        Thread currentThread = Thread.currentThread();
        String oldName = currentThread.getName();
        boolean restoreName = trySetName(nameSupplier.get(), currentThread);
        try {
          task.run();
        } finally {
          if (restoreName) {
            trySetName(oldName, currentThread);
          }
        }
      }
    };
  }

  /** Tries to set name of the given {@link Thread}, returns true if successful. */
  private static boolean trySetName(final String threadName, Thread currentThread) {
    // In AppEngine this will always fail, should we test for that explicitly using
    // MoreExecutors.isAppEngine.  More generally, is there a way to see if we have the modifyThread
    // permission without catching an exception?
    try {
      currentThread.setName(threadName);
      return true;
    } catch (SecurityException e) {
      return false;
    }
  }
}