/*
    * Copyright (C) 2007 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.collect;
  
  import com.google.common.annotations.GwtCompatible;
  import com.google.common.base.Preconditions;
  
  import java.util.Set;
  
  import javax.annotation.Nullable;
  
  /**
   * Implementation of {@link ImmutableSet} with exactly one element.
   *
   * @author Kevin Bourrillion
   * @author Nick Kralevich
   */
  @GwtCompatible(serializable = true, emulated = true)
  @SuppressWarnings("serial") // uses writeReplace(), not default serialization
  final class SingletonImmutableSet<E> extends ImmutableSet<E> {
  
    final transient E element;
    // This is transient because it will be recalculated on the first
    // call to hashCode().
    //
    // A race condition is avoided since threads will either see that the value
    // is zero and recalculate it themselves, or two threads will see it at
    // the same time, and both recalculate it.  If the cachedHashCode is 0,
    // it will always be recalculated, unfortunately.
    private transient int cachedHashCode;
  
    SingletonImmutableSet(E element) {
      this.element = Preconditions.checkNotNull(element);
    }
  
    SingletonImmutableSet(E element, int hashCode) {
      // Guaranteed to be non-null by the presence of the pre-computed hash code.
      this.element = element;
      cachedHashCode = hashCode;
    }
  
    @Override
    public int size() {
      return 1;
    }
  
    @Override public boolean isEmpty() {
      return false;
    }
  
    @Override public boolean contains(Object target) {
      return element.equals(target);
    }
  
    @Override public UnmodifiableIterator<E> iterator() {
      return Iterators.singletonIterator(element);
    }
  
    @Override boolean isPartialView() {
      return false;
    }
  
    @Override
    int copyIntoArray(Object[] dst, int offset) {
      dst[offset] = element;
      return offset + 1;
    }
  
    @Override public boolean equals(@Nullable Object object) {
      if (object == this) {
        return true;
      }
      if (object instanceof Set) {
        Set<?> that = (Set<?>) object;
        return that.size() == 1 && element.equals(that.iterator().next());
      }
      return false;
    }
  
    @Override public final int hashCode() {
      // Racy single-check.
      int code = cachedHashCode;
      if (code == 0) {
        cachedHashCode = code = element.hashCode();
      }
     return code;
   }
 
   @Override boolean isHashCodeFast() {
     return cachedHashCode != 0;
   }
 
   @Override public String toString() {
     String elementToString = element.toString();
     return new StringBuilder(elementToString.length() + 2)
         .append('[')
         .append(elementToString)
         .append(']')
         .toString();
   }
 }