/*
 * 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.GwtCompatible;
import com.google.errorprone.annotations.ForOverride;
import java.io.Serializable;
import java.util.function.BiPredicate;
import org.checkerframework.checker.nullness.qual.Nullable;

A strategy for determining whether two instances are considered equivalent, and for computing hash codes in a manner consistent with that equivalence. Two examples of equivalences are the identity equivalence and the "equals" equivalence. 
Author:
Bob Lee, Ben Yu, Gregory Kick
Since:
10.0 (mostly
    source-compatible since 4.0)/**
 * A strategy for determining whether two instances are considered equivalent, and for computing
 * hash codes in a manner consistent with that equivalence. Two examples of equivalences are the
 * {@linkplain #identity() identity equivalence} and the {@linkplain #equals "equals" equivalence}.
 *
 * @author Bob Lee
 * @author Ben Yu
 * @author Gregory Kick
 * @since 10.0 (<a href="https://github.com/google/guava/wiki/Compatibility">mostly
 *     source-compatible</a> since 4.0)
 */
@GwtCompatible
public abstract class Equivalence<T> implements BiPredicate<T, T> {
  
Constructor for use by subclasses. /** Constructor for use by subclasses. */
  protected Equivalence() {}

  
Returns true if the given objects are considered equivalent. 
This method describes an equivalence relation on object references, meaning that for all references x, y, and z (any of which may be null): 

  
equivalent(x, x) is true (reflexive property)
  
equivalent(x, y) and equivalent(y, x) each return the same result (symmetric property)
  
If equivalent(x, y) and equivalent(y, z) are both true, then 
      equivalent(x, z) is also true (transitive property)

Note that all calls to equivalent(x, y) are expected to return the same result as long as neither x nor y is modified. /**
   * Returns {@code true} if the given objects are considered equivalent.
   *
   * <p>This method describes an <i>equivalence relation</i> on object references, meaning that for
   * all references {@code x}, {@code y}, and {@code z} (any of which may be null):
   *
   * <ul>
   *   <li>{@code equivalent(x, x)} is true (<i>reflexive</i> property)
   *   <li>{@code equivalent(x, y)} and {@code equivalent(y, x)} each return the same result
   *       (<i>symmetric</i> property)
   *   <li>If {@code equivalent(x, y)} and {@code equivalent(y, z)} are both true, then {@code
   *       equivalent(x, z)} is also true (<i>transitive</i> property)
   * </ul>
   *
   * <p>Note that all calls to {@code equivalent(x, y)} are expected to return the same result as
   * long as neither {@code x} nor {@code y} is modified.
   */
  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);
  }

  
Deprecated:
Provided only to satisfy the BiPredicate interface; use equivalent instead.
Since:
21.0/**
   * @deprecated Provided only to satisfy the {@link BiPredicate} interface; use {@link #equivalent}
   *     instead.
   * @since 21.0
   */
  @Deprecated
  @Override
  public final boolean test(@Nullable T t, @Nullable T u) {
    return equivalent(t, u);
  }

  
Implemented by the user to determine whether a and b are considered equivalent, subject to the requirements specified in equivalent. 
This method should not be called except by equivalent. When equivalent calls this method, a and b are guaranteed to be distinct, non-null instances. 
Since:
10.0 (previously, subclasses would override equivalent())/**
   * Implemented by the user to determine whether {@code a} and {@code b} are considered equivalent,
   * subject to the requirements specified in {@link #equivalent}.
   *
   * <p>This method should not be called except by {@link #equivalent}. When {@link #equivalent}
   * calls this method, {@code a} and {@code b} are guaranteed to be distinct, non-null instances.
   *
   * @since 10.0 (previously, subclasses would override equivalent())
   */
  @ForOverride
  protected abstract boolean doEquivalent(T a, T b);

  
Returns a hash code for t. 
The hash has the following properties: 

  
It is consistent: for any reference x, multiple invocations of 
      hash(x} consistently return the same value provided 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. 
It is distributable across equivalence: for any references x and 
      y, if equivalent(x, y), then hash(x) == hash(y). It is not
      necessary that the hash be distributable across inequivalence. If 
      equivalence(x, y) is false, hash(x) == hash(y) may still be true. 
hash(null) is 0. 
/**
   * 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);
  }

  
Implemented by the user to return a hash code for t, subject to the requirements specified in hash. 
This method should not be called except by hash. When hash calls this method, t is guaranteed to be non-null. 
Since:
10.0 (previously, subclasses would override hash())/**
   * Implemented by the user to return a hash code for {@code t}, subject to the requirements
   * specified in {@link #hash}.
   *
   * <p>This method should not be called except by {@link #hash}. When {@link #hash} calls this
   * method, {@code t} is guaranteed to be non-null.
   *
   * @since 10.0 (previously, subclasses would override hash())
   */
  @ForOverride
  protected abstract int doHash(T t);

  
Returns a new equivalence relation for F which evaluates equivalence by first applying function to the argument, then evaluating using this. That is, for any pair of non-null objects x and y, equivalence.onResultOf(function).equivalent(a,
b) is true if and only if equivalence.equivalent(function.apply(a), function.apply(b)) is true. 
For example:

Equivalence<Person> SAME_AGE = Equivalence.equals().onResultOf(GET_PERSON_AGE);

function will never be invoked with a null value. 
Note that function must be consistent according to this equivalence relation. That is, invoking Function.apply multiple times for a given value must return equivalent results. For example, 
Equivalence.identity().onResultOf(Functions.toStringFunction()) is broken because it's not guaranteed that Object.toString) always returns the same string instance. 
Since:
10.0/**
   * 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<>(function, this);
  }

  
Returns a wrapper of reference that implements 
Object.equals() such that wrap(a).equals(wrap(b)) if and only if equivalent(a,
b). 
Since:
10.0/**
   * 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 equals(Object) and hashCode() delegate to an Equivalence. 
For example, given an Equivalence for strings named equiv that tests equivalence using their lengths: 

equiv.wrap("a").equals(equiv.wrap("b")) // true
equiv.wrap("a").equals(equiv.wrap("hello")) // false

Note in particular that an equivalence wrapper is never equal to the object it wraps.

equiv.wrap(obj).equals(obj) // always false

Since:
10.0/**
   * 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;
    private final @Nullable 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. /** Returns the (possibly null) reference wrapped by this instance. */
    public @Nullable T get() {
      return reference;
    }

    
Returns true if Equivalence.equivalent(Object, Object) applied to the wrapped references is true and both wrappers use the same equivalence. /**
     * 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 Equivalence.hash(Object) applied to the wrapped reference. /** 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.
/**
     * 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 this. Null iterables are equivalent to one another. 
Note that this method performs a similar function for equivalences as Ordering.lexicographical does for orderings. 
Since:
10.0/**
   * 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 
target according to this equivalence relation. 
Since:
10.0/**
   * 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
   */
  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;
    private final @Nullable 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 Object.equals and Object.hashCode. equivalent returns true if both values are null, or if neither value is null and Object.equals returns true. hash returns 0 if passed a null value. 
Since:
13.0
Since:
8.0 (in Equivalences with null-friendly behavior)
Since:
4.0 (in Equivalences)/**
   * 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 == to compare values and System.identityHashCode(Object) to compute the hash code. equivalent returns true if a == b, including in the case that a and b are both null. 
Since:
13.0
Since:
4.0 (in Equivalences)/**
   * 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
    protected 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;
  }
}