/*
    * 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 static com.google.common.base.Preconditions.checkNotNull;
  
  import com.google.common.annotations.Beta;
  import com.google.common.annotations.GwtIncompatible;
  import com.google.common.base.Equivalence;
  import com.google.common.base.Function;
  import com.google.common.collect.MapMakerInternalMap.ReferenceEntry;
  
  import java.util.concurrent.ConcurrentMap;
  
  /**
   * Contains static methods pertaining to instances of {@link Interner}.
   *
   * @author Kevin Bourrillion
   * @since 3.0
   */
  @Beta
  public final class Interners {
    private Interners() {}
  
    /**
     * Returns a new thread-safe interner which retains a strong reference to each instance it has
     * interned, thus preventing these instances from being garbage-collected. If this retention is
     * acceptable, this implementation may perform better than {@link #newWeakInterner}. Note that
     * unlike {@link String#intern}, using this interner does not consume memory in the permanent
     * generation.
     */
    public static <E> Interner<E> newStrongInterner() {
      final ConcurrentMap<E, E> map = new MapMaker().makeMap();
      return new Interner<E>() {
        @Override public E intern(E sample) {
          E canonical = map.putIfAbsent(checkNotNull(sample), sample);
          return (canonical == null) ? sample : canonical;
        }
      };
    }
  
    /**
     * Returns a new thread-safe interner which retains a weak reference to each instance it has
     * interned, and so does not prevent these instances from being garbage-collected. This most
     * likely does not perform as well as {@link #newStrongInterner}, but is the best alternative
     * when the memory usage of that implementation is unacceptable. Note that unlike {@link
     * String#intern}, using this interner does not consume memory in the permanent generation.
     */
    @GwtIncompatible("java.lang.ref.WeakReference")
    public static <E> Interner<E> newWeakInterner() {
      return new WeakInterner<E>();
    }
  
    private static class WeakInterner<E> implements Interner<E> {
      // MapMaker is our friend, we know about this type
      private final MapMakerInternalMap<E, Dummy> map = new MapMaker()
            .weakKeys()
            .keyEquivalence(Equivalence.equals())
            .makeCustomMap();
  
      @Override public E intern(E sample) {
        while (true) {
          // trying to read the canonical...
          ReferenceEntry<E, Dummy> entry = map.getEntry(sample);
          if (entry != null) {
            E canonical = entry.getKey();
            if (canonical != null) { // only matters if weak/soft keys are used
              return canonical;
            }
          }
  
          // didn't see it, trying to put it instead...
          Dummy sneaky = map.putIfAbsent(sample, Dummy.VALUE);
          if (sneaky == null) {
            return sample;
          } else {
            /* Someone beat us to it! Trying again...
             *
             * Technically this loop not guaranteed to terminate, so theoretically (extremely
             * unlikely) this thread might starve, but even then, there is always going to be another
             * thread doing progress here.
             */
          }
        }
      }
  
      private enum Dummy { VALUE }
   }
 
   /**
    * Returns a function that delegates to the {@link Interner#intern} method of the given interner.
    *
    * @since 8.0
    */
   public static <E> Function<E, E> asFunction(Interner<E> interner) {
     return new InternerFunction<E>(checkNotNull(interner));
   }
 
   private static class InternerFunction<E> implements Function<E, E> {
 
     private final Interner<E> interner;
 
     public InternerFunction(Interner<E> interner) {
       this.interner = interner;
     }
 
     @Override public E apply(E input) {
       return interner.intern(input);
     }
 
     @Override public int hashCode() {
       return interner.hashCode();
     }
 
     @Override public boolean equals(Object other) {
       if (other instanceof InternerFunction) {
         InternerFunction<?> that = (InternerFunction<?>) other;
         return interner.equals(that.interner);
       }
 
       return false;
     }
   }
 }