Equivalence

/*
 * Copyright (C) 2010 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.base;

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

import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;

import java.io.Serializable;

import javax.annotation.Nullable;

/**
 * A strategy for determining whether two instances are considered equivalent. Examples of
 * equivalences are the {@linkplain #identity() identity equivalence} and {@linkplain #equals equals
 * equivalence}.
 *
 * @author Bob Lee
 * @author Ben Yu
 * @author Gregory Kick
 * @since 10.0 (<a href="http://code.google.com/p/guava-libraries/wiki/Compatibility"
 *        >mostly source-compatible</a> since 4.0)
 */
@GwtCompatible
public abstract class Equivalence<T> {
  /**
   * Constructor for use by subclasses.
   */
  protected Equivalence() {}

  /**
   * Returns {@code true} if the given objects are considered equivalent.
   *
   * <p>The {@code equivalent} method implements an equivalence relation on object references:
   *
   * <ul>
   * <li>It is <i>reflexive</i>: for any reference {@code x}, including null, {@code
   *     equivalent(x, x)} returns {@code true}.
   * <li>It is <i>symmetric</i>: for any references {@code x} and {@code y}, {@code
   *     equivalent(x, y) == equivalent(y, x)}.
   * <li>It is <i>transitive</i>: for any references {@code x}, {@code y}, and {@code z}, if
   *     {@code equivalent(x, y)} returns {@code true} and {@code equivalent(y, z)} returns {@code
   *     true}, then {@code equivalent(x, z)} returns {@code true}.
   * <li>It is <i>consistent</i>: for any references {@code x} and {@code y}, multiple invocations
   *     of {@code equivalent(x, y)} consistently return {@code true} or consistently return {@code
   *     false} (provided that neither {@code x} nor {@code y} is modified).
   * </ul>
   */
  public final boolean equivalent(@Nullable T a, @Nullable T b) {
    if (a == b) {
      return true;
    }
    if (a == null || b == null) {
      return false;
    }
    return doEquivalent(a, b);
  }

  /**
   * Returns {@code true} if {@code a} and {@code b} are considered equivalent.
   *
   * <p>Called by {@link #equivalent}. {@code a} and {@code b} are not the same
   * object and are not nulls.
   *
   * @since 10.0 (previously, subclasses would override equivalent())
   */
  protected abstract boolean doEquivalent(T a, T b);

  /**
   * Returns a hash code for {@code t}.
   *
   * <p>The {@code hash} has the following properties:
   * <ul>
   * <li>It is <i>consistent</i>: for any reference {@code x}, multiple invocations of
   *     {@code hash(x}} consistently return the same value provided {@code x} remains unchanged
   *     according to the definition of the equivalence. The hash need not remain consistent from
   *     one execution of an application to another execution of the same application.
   * <li>It is <i>distributable across equivalence</i>: for any references {@code x} and {@code y},
   *     if {@code equivalent(x, y)}, then {@code hash(x) == hash(y)}. It is <i>not</i> necessary
   *     that the hash be distributable across <i>inequivalence</i>. If {@code equivalence(x, y)}
   *     is false, {@code hash(x) == hash(y)} may still be true.
   * <li>{@code hash(null)} is {@code 0}.
   * </ul>
   */
  public final int hash(@Nullable T t) {
    if (t == null) {
      return 0;
    }
    return doHash(t);
  }

  /**
   * Returns a hash code for non-null object {@code t}.
   *
   * <p>Called by {@link #hash}.
   *
   * @since 10.0 (previously, subclasses would override hash())
   */
  protected abstract int doHash(T t);

  /**
   * Returns a new equivalence relation for {@code F} which evaluates equivalence by first applying
   * {@code function} to the argument, then evaluating using {@code this}. That is, for any pair of
   * non-null objects {@code x} and {@code y}, {@code
   * equivalence.onResultOf(function).equivalent(a, b)} is true if and only if {@code
   * equivalence.equivalent(function.apply(a), function.apply(b))} is true.
   *
   * <p>For example:
   *
   * <pre>   {@code
   *    Equivalence<Person> SAME_AGE = Equivalence.equals().onResultOf(GET_PERSON_AGE);}</pre>
   *
   * <p>{@code function} will never be invoked with a null value.
   *
   * <p>Note that {@code function} must be consistent according to {@code this} equivalence
   * relation. That is, invoking {@link Function#apply} multiple times for a given value must return
   * equivalent results.
   * For example, {@code Equivalence.identity().onResultOf(Functions.toStringFunction())} is broken
   * because it's not guaranteed that {@link Object#toString}) always returns the same string
   * instance.
   *
   * @since 10.0
   */
  public final <F> Equivalence<F> onResultOf(Function<F, ? extends T> function) {
    return new FunctionalEquivalence<F, T>(function, this);
  }

  /**
   * Returns a wrapper of {@code reference} that implements
   * {@link Wrapper#equals(Object) Object.equals()} such that
   * {@code wrap(a).equals(wrap(b))} if and only if {@code equivalent(a, b)}.
   *
   * @since 10.0
   */
  public final <S extends T> Wrapper<S> wrap(@Nullable S reference) {
    return new Wrapper<S>(this, reference);
  }

  /**
   * Wraps an object so that {@link #equals(Object)} and {@link #hashCode()} delegate to an
   * {@link Equivalence}.
   *
   * <p>For example, given an {@link Equivalence} for {@link String strings} named {@code equiv}
   * that tests equivalence using their lengths:
   *
   * <pre>   {@code
   *   equiv.wrap("a").equals(equiv.wrap("b")) // true
   *   equiv.wrap("a").equals(equiv.wrap("hello")) // false}</pre>
   *
   * <p>Note in particular that an equivalence wrapper is never equal to the object it wraps.
   *
   * <pre>   {@code
   *   equiv.wrap(obj).equals(obj) // always false}</pre>
   *
   * @since 10.0
   */
  public static final class Wrapper<T> implements Serializable {
    private final Equivalence<? super T> equivalence;
    @Nullable private final T reference;

    private Wrapper(Equivalence<? super T> equivalence, @Nullable T reference) {
      this.equivalence = checkNotNull(equivalence);
      this.reference = reference;
    }

    /** Returns the (possibly null) reference wrapped by this instance. */
    @Nullable public T get() {
      return reference;
    }

    /**
     * Returns {@code true} if {@link Equivalence#equivalent(Object, Object)} applied to the wrapped
     * references is {@code true} and both wrappers use the {@link Object#equals(Object) same}
     * equivalence.
     */
    @Override public boolean equals(@Nullable Object obj) {
      if (obj == this) {
        return true;
      }
      if (obj instanceof Wrapper) {
        Wrapper<?> that = (Wrapper<?>) obj; // note: not necessarily a Wrapper<T>

        if (this.equivalence.equals(that.equivalence)) {
          /*
           * We'll accept that as sufficient "proof" that either equivalence should be able to
           * handle either reference, so it's safe to circumvent compile-time type checking.
           */
          @SuppressWarnings("unchecked")
          Equivalence<Object> equivalence = (Equivalence<Object>) this.equivalence;
          return equivalence.equivalent(this.reference, that.reference);
        }
      }
      return false;
    }

    /**
     * Returns the result of {@link Equivalence#hash(Object)} applied to the wrapped reference.
     */
    @Override public int hashCode() {
      return equivalence.hash(reference);
    }

    /**
     * Returns a string representation for this equivalence wrapper. The form of this string
     * representation is not specified.
     */
    @Override public String toString() {
      return equivalence + ".wrap(" + reference + ")";
    }

    private static final long serialVersionUID = 0;
  }

  /**
   * Returns an equivalence over iterables based on the equivalence of their elements.  More
   * specifically, two iterables are considered equivalent if they both contain the same number of
   * elements, and each pair of corresponding elements is equivalent according to
   * {@code this}.  Null iterables are equivalent to one another.
   *
   * <p>Note that this method performs a similar function for equivalences as {@link
   * com.google.common.collect.Ordering#lexicographical} does for orderings.
   *
   * @since 10.0
   */
  @GwtCompatible(serializable = true)
  public final <S extends T> Equivalence<Iterable<S>> pairwise() {
    // Ideally, the returned equivalence would support Iterable<? extends T>. However,
    // the need for this is so rare that it's not worth making callers deal with the ugly wildcard.
    return new PairwiseEquivalence<S>(this);
  }

  /**
   * Returns a predicate that evaluates to true if and only if the input is
   * equivalent to {@code target} according to this equivalence relation.
   *
   * @since 10.0
   */
  @Beta
  public final Predicate<T> equivalentTo(@Nullable T target) {
    return new EquivalentToPredicate<T>(this, target);
  }

  private static final class EquivalentToPredicate<T> implements Predicate<T>, Serializable {

    private final Equivalence<T> equivalence;
    @Nullable private final T target;

    EquivalentToPredicate(Equivalence<T> equivalence, @Nullable T target) {
      this.equivalence = checkNotNull(equivalence);
      this.target = target;
    }

    @Override public boolean apply(@Nullable T input) {
      return equivalence.equivalent(input, target);
    }

    @Override public boolean equals(@Nullable Object obj) {
      if (this == obj) {
        return true;
      }
      if (obj instanceof EquivalentToPredicate) {
        EquivalentToPredicate<?> that = (EquivalentToPredicate<?>) obj;
        return equivalence.equals(that.equivalence)
            && Objects.equal(target, that.target);
      }
      return false;
    }

    @Override public int hashCode() {
      return Objects.hashCode(equivalence, target);
    }

    @Override public String toString() {
      return equivalence + ".equivalentTo(" + target + ")";
    }

    private static final long serialVersionUID = 0;
  }

  /**
   * Returns an equivalence that delegates to {@link Object#equals} and {@link Object#hashCode}.
   * {@link Equivalence#equivalent} returns {@code true} if both values are null, or if neither
   * value is null and {@link Object#equals} returns {@code true}. {@link Equivalence#hash} returns
   * {@code 0} if passed a null value.
   *
   * @since 13.0
   * @since 8.0 (in Equivalences with null-friendly behavior)
   * @since 4.0 (in Equivalences)
   */
  public static Equivalence<Object> equals() {
    return Equals.INSTANCE;
  }

  /**
   * Returns an equivalence that uses {@code ==} to compare values and {@link
   * System#identityHashCode(Object)} to compute the hash code.  {@link Equivalence#equivalent}
   * returns {@code true} if {@code a == b}, including in the case that a and b are both null.
   *
   * @since 13.0
   * @since 4.0 (in Equivalences)
   */
  public static Equivalence<Object> identity() {
    return Identity.INSTANCE;
  }

  static final class Equals extends Equivalence<Object>
      implements Serializable {

    static final Equals INSTANCE = new Equals();

    @Override protected boolean doEquivalent(Object a, Object b) {
      return a.equals(b);
    }
    @Override public int doHash(Object o) {
      return o.hashCode();
    }

    private Object readResolve() {
      return INSTANCE;
    }
    private static final long serialVersionUID = 1;
  }

  static final class Identity extends Equivalence<Object>
      implements Serializable {

    static final Identity INSTANCE = new Identity();

    @Override protected boolean doEquivalent(Object a, Object b) {
      return false;
    }

    @Override protected int doHash(Object o) {
      return System.identityHashCode(o);
    }

    private Object readResolve() {
      return INSTANCE;
    }
    private static final long serialVersionUID = 1;
  }
}