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/*
 * 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.annotations.VisibleForTesting;
import com.google.common.base.Equivalence;
import com.google.common.base.Function;
import com.google.common.collect.MapMakerInternalMap.InternalEntry;
import java.util.concurrent.ConcurrentMap;

/**
 * Contains static methods pertaining to instances of {@link Interner}.
 *
 * @author Kevin Bourrillion
 * @since 3.0
 */
@Beta
@GwtIncompatible
public final class Interners {
  private Interners() {}

  /**
   * Builder for {@link Interner} instances.
   *
   * @since 21.0
   */
  public static class InternerBuilder {
    private final MapMaker mapMaker = new MapMaker();
    private boolean strong = true;

    private InternerBuilder() {
    }

    /**
     * Instructs the {@link InternerBuilder} to build a strong interner.
     *
     * @see Interners#newStrongInterner()
     */
    public InternerBuilder strong() {
      this.strong = true;
      return this;
    }

    /**
     * Instructs the {@link InternerBuilder} to build a weak interner.
     *
     * @see Interners#newWeakInterner()
     */
    @GwtIncompatible("java.lang.ref.WeakReference")
    public InternerBuilder weak() {
      this.strong = false;
      return this;
    }

    /**
     * Sets the concurrency level that will be used by the to-be-built {@link Interner}.
     *
     * @see MapMaker#concurrencyLevel(int)
     */
    public InternerBuilder concurrencyLevel(int concurrencyLevel) {
      this.mapMaker.concurrencyLevel(concurrencyLevel);
      return this;
    }

    public  Interner build() {
      return strong ? new StrongInterner(mapMaker) : new WeakInterner(mapMaker);
    }
  }

  /** Returns a fresh {@link InternerBuilder} instance. */
  public static InternerBuilder newBuilder() {
    return new InternerBuilder();
  }

  /**
   * 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}.
   */
  public static  Interner newStrongInterner() {
    return newBuilder().strong().build();
  }

  /**
   * 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.
   */
  @GwtIncompatible("java.lang.ref.WeakReference")
  public static  Interner newWeakInterner() {
    return newBuilder().weak().build();
  }

  @VisibleForTesting
  static final class StrongInterner implements Interner {
    @VisibleForTesting
    final ConcurrentMap map;

    private StrongInterner(MapMaker mapMaker) {
      this.map = mapMaker.makeMap();
    }

    @Override
    public E intern(E sample) {
      E canonical = map.putIfAbsent(checkNotNull(sample), sample);
      return (canonical == null) ? sample : canonical;
    }
  }

  @VisibleForTesting
  static final class WeakInterner implements Interner {
    // MapMaker is our friend, we know about this type
    @VisibleForTesting
    final MapMakerInternalMap map;

    private WeakInterner(MapMaker mapMaker) {
      this.map = mapMaker.weakKeys().keyEquivalence(Equivalence.equals()).makeCustomMap();
    }

    @Override
    public E intern(E sample) {
      while (true) {
        // trying to read the canonical...
        InternalEntry 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  Function asFunction(Interner interner) {
    return new InternerFunction(checkNotNull(interner));
  }

  private static class InternerFunction implements Function {

    private final Interner interner;

    public InternerFunction(Interner 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;
    }
  }
}