/**
    * Copyright 2008 Atlassian Pty Ltd 
    * 
    * 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.atlassian.util.concurrent;
  
  import java.util.concurrent.Callable;
  import java.util.concurrent.CancellationException;
  import java.util.concurrent.CountDownLatch;
  import java.util.concurrent.ExecutionException;
  import java.util.concurrent.Future;
  import java.util.concurrent.FutureTask;
  import java.util.concurrent.TimeUnit;
  import java.util.concurrent.TimeoutException;
  import java.util.concurrent.atomic.AtomicReference;
  
  import net.jcip.annotations.ThreadSafe;
  
  /**
   * {@link SettableFuture} is a {@link Future} implementation where the
   * responsibility for producing the result is external to the future instance,
   * unlike {@link FutureTask} where the future holds the operation (a
   * {@link Callable} or {@link Runnable} instance) and the first thread that
   * calls {@link FutureTask#run()} executes the operation.
   * <p>
   * This is useful in situations where all the inputs may not be available at
   * construction time.
   */
  @ThreadSafe
  public class SettableFuture<T> implements Future<T> {
      private final AtomicReference<Value<T>> ref = new AtomicReference<Value<T>>();
      private final CountDownLatch latch = new CountDownLatch(1);
  
      /**
       * Set the value returned by {@link #get()} and {@link #get(long, TimeUnit)}
       * <p>
       * Note that this can only be done once unless the value of the second set
       * equals the first value otherwise an exception will be thrown. It also
       * cannot be set if this future has been cancelled or an exception has been
       * set.
       * 
       * @param value the value to be set.
       */
      public void set(final T value) {
          setAndCheckValue(new ReferenceValue<T>(value));
      }
  
      /**
       * Set the exception thrown as the causal exception of an ExecutionException
       * by {@link #get()} and {@link #get(long, TimeUnit)}
       * <p>
       * Note that this can only be done once unless the value of the second
       * {@link #setException(Throwable)} equals the first value otherwise an
       * exception will be thrown (as most exceptions do not implement equals this
       * effectively means the same reference). It also cannot be set if this
       * future has been cancelled or a a value has been set.
       * 
       * @param value the value to be set.
       */
      public void setException(final Throwable throwable) {
          setAndCheckValue(new ThrowableValue<T>(throwable));
      }
  
      public T get() throws InterruptedException, ExecutionException {
          latch.await();
          return ref.get().get();
      }
  
      public T get(final long timeout, final TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException {
          if (!latch.await(timeout, unit)) {
              throw new TimedOutException(timeout, unit);
          }
          return ref.get().get();
      }
  
      public boolean isDone() {
          return ref.get() != null;
      }
  
      public boolean isCancelled() {
          return isDone() && (ref.get() instanceof CancelledValue);
      }
  
      public boolean cancel(final boolean mayInterruptIfRunning) {
          return setValue(new CancelledValue<T>()) == null;
      }
  
     /**
      * Set the inner value and check that if there is an old value it equals the
      * one we are setting.
      * 
      * @param value to set.
      * @return the old value if set or null.
      */
     private void setAndCheckValue(final Value<T> value) {
         final Value<T> oldValue = setValue(value);
         if ((oldValue != null) && !value.equals(oldValue)) {
             throw new IllegalStateException("cannot change value after it has been set");
         }
     }
 
     /**
      * Set the inner value.
      * 
      * @param value to set.
      * @return the old value if set or null.
      */
     private Value<T> setValue(final Value<T> value) {
         while (true) {
             final Value<T> oldValue = ref.get();
             if (oldValue != null) {
                 return oldValue;
             }
             // /CLOVER:OFF
             if (!ref.compareAndSet(null, value)) {
                 continue;
             }
             // /CLOVER:ON
             latch.countDown();
             return null;
         }
     }
 
     /** the inner value */
     private static interface Value<T> {
         T get() throws ExecutionException;
     }
 
     /** holds a reference */
     private static class ReferenceValue<T> implements Value<T> {
         private final T value;
 
         ReferenceValue(final T value) {
             this.value = value;
         }
 
         public T get() {
             return value;
         }
 
         @Override
         public boolean equals(final Object obj) {
             // no need to check for reference equality, not possible
             if (!(obj instanceof ReferenceValue)) {
                 return false;
             }
             final ReferenceValue<?> other = (ReferenceValue<?>) obj;
             return (value == null) ? (other.value == null) : value.equals(other.value);
         }
 
         // /CLOVER:OFF
         @Override
         public int hashCode() {
             throw new UnsupportedOperationException();
         }
         // /CLOVER:ON
     }
 
     /** holds an exception */
     private static class ThrowableValue<T> implements Value<T> {
         private final Throwable throwable;
 
         ThrowableValue(final Throwable throwable) {
             this.throwable = throwable;
         }
 
         public T get() throws ExecutionException {
             throw new ExecutionException(throwable);
         }
 
         @Override
         public boolean equals(final Object obj) {
             // no need to check for reference equality, not possible
             if (!(obj instanceof ThrowableValue)) {
                 return false;
             }
             return throwable.equals(((ThrowableValue<?>) obj).throwable);
         }
 
         // /CLOVER:OFF
         @Override
         public int hashCode() {
             throw new UnsupportedOperationException();
         }
         // /CLOVER:ON
     }
 
     // doesn't need to implement equals as cancel doesn't check
     private static class CancelledValue<T> implements Value<T> {
         public T get() throws ExecutionException {
             throw new CancellationException();
         }
     }
 }