/*
 * 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 static com.google.common.collect.CollectPreconditions.checkNonnegative;

import com.google.common.annotations.GwtCompatible;
import com.google.common.annotations.VisibleForTesting;
import com.google.common.base.Function;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.Comparator;
import java.util.Iterator;
import java.util.List;
import java.util.Map.Entry;
import java.util.NoSuchElementException;
import java.util.SortedMap;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.atomic.AtomicInteger;
import org.checkerframework.checker.nullness.compatqual.NullableDecl;

/**
 * A comparator, with additional methods to support common operations. This is an "enriched" version
 * of {@code Comparator} for pre-Java-8 users, in the same sense that {@link FluentIterable} is an
 * enriched {@link Iterable} for pre-Java-8 users.
 *
 *
Three types of methods

 *
 * Like other fluent types, there are three types of methods present: methods for acquiring,
 * chaining, and using.
 *
 *
Acquiring

 *
 *
The common ways to get an instance of {@code Ordering} are:
 *
 *

 *
Subclass it and implement {@link #compare} instead of implementing {@link Comparator}
 *       directly
 *
Pass a pre-existing {@link Comparator} instance to {@link #from(Comparator)}
 *
Use the natural ordering, {@link Ordering#natural}
 *

 *
 *
Chaining

 *
 *
Then you can use the chaining methods to get an altered version of that {@code
 * Ordering}, including:
 *
 *

 *
{@link #reverse}
 *
{@link #compound(Comparator)}
 *
{@link #onResultOf(Function)}
 *
{@link #nullsFirst} / {@link #nullsLast}
 *

 *
 *
Using

 *
 *
Finally, use the resulting {@code Ordering} anywhere a {@link Comparator} is required, or use
 * any of its special operations, such as:
 *
 *

 *
{@link #immutableSortedCopy}
 *
{@link #isOrdered} / {@link #isStrictlyOrdered}
 *
{@link #min} / {@link #max}
 *

 *
 *
Understanding complex orderings

 *
 *
Complex chained orderings like the following example can be challenging to understand.
 *
 *
{@code
 * Ordering<Foo> ordering =
 *     Ordering.natural()
 *         .nullsFirst()
 *         .onResultOf(getBarFunction)
 *         .nullsLast();
 * }

 *
 * Note that each chaining method returns a new ordering instance which is backed by the previous
 * instance, but has the chance to act on values before handing off to that backing instance.
 * As a result, it usually helps to read chained ordering expressions backwards. For example,
 * when {@code compare} is called on the above ordering:
 *
 *

 *
First, if only one {@code Foo} is null, that null value is treated as greater
 *
Next, non-null {@code Foo} values are passed to {@code getBarFunction} (we will be
 *       comparing {@code Bar} values from now on)
 *
Next, if only one {@code Bar} is null, that null value is treated as lesser
 *
Finally, natural ordering is used (i.e. the result of {@code Bar.compareTo(Bar)} is
 *       returned)
 *

 *
 *
Alas, {@link #reverse} is a little different. As you read backwards through a chain and
 * encounter a call to {@code reverse}, continue working backwards until a result is determined, and
 * then reverse that result.
 *
 *
Additional notes

 *
 *
Except as noted, the orderings returned by the factory methods of this class are serializable
 * if and only if the provided instances that back them are. For example, if {@code ordering} and
 * {@code function} can themselves be serialized, then {@code ordering.onResultOf(function)} can as
 * well.
 *
 *
For Java 8 users

 *
 *
If you are using Java 8, this class is now obsolete. Most of its functionality is now provided
 * by {@link java.util.stream.Stream Stream} and by {@link Comparator} itself, and the rest can now
 * be found as static methods in our new {@link Comparators} class. See each method below for
 * further instructions. Whenever possible, you should change any references of type {@code
 * Ordering} to be of type {@code Comparator} instead. However, at this time we have no plan to
 * deprecate this class.
 *
 *
Many replacements involve adopting {@code Stream}, and these changes can sometimes make your
 * code verbose. Whenever following this advice, you should check whether {@code Stream} could be
 * adopted more comprehensively in your code; the end result may be quite a bit simpler.
 *
 *
See also

 *
 *
See the Guava User Guide article on {@code Ordering}.
 *
 * @author Jesse Wilson
 * @author Kevin Bourrillion
 * @since 2.0
 */
@GwtCompatible
public abstract class Ordering<T> implements Comparator<T> {
  // Natural order

  /**
   * Returns a serializable ordering that uses the natural order of the values. The ordering throws
   * a {@link NullPointerException} when passed a null parameter.
   *
   *
The type specification is {@code <C extends Comparable>}, instead of the technically correct
   * {@code <C extends Comparable<? super C>>}, to support legacy types from before Java 5.
   *
   *
Java 8 users: use {@link Comparator#naturalOrder} instead.
   */
  @GwtCompatible(serializable = true)
  @SuppressWarnings("unchecked") // TODO(kevinb): right way to explain this??
  public static <C extends Comparable> Ordering<C> natural() {
    return (Ordering<C>) NaturalOrdering.INSTANCE;
  }

  // Static factories

  /**
   * Returns an ordering based on an existing comparator instance. Note that it is
   * unnecessary to create a new anonymous inner class implementing {@code Comparator} just
   * to pass it in here. Instead, simply subclass {@code Ordering} and implement its {@code compare}
   * method directly.
   *
   *
Java 8 users: this class is now obsolete as explained in the class documentation, so
   * there is no need to use this method.
   *
   * @param comparator the comparator that defines the order
   * @return comparator itself if it is already an {@code Ordering}; otherwise an ordering that
   *     wraps that comparator
   */
  @GwtCompatible(serializable = true)
  public static <T> Ordering<T> from(Comparator<T> comparator) {
    return (comparator instanceof Ordering)
        ? (Ordering<T>) comparator
        : new ComparatorOrdering<T>(comparator);
  }

  /**
   * Simply returns its argument.
   *
   * @deprecated no need to use this
   */
  @GwtCompatible(serializable = true)
  @Deprecated
  public static <T> Ordering<T> from(Ordering<T> ordering) {
    return checkNotNull(ordering);
  }

  /**
   * Returns an ordering that compares objects according to the order in which they appear in the
   * given list. Only objects present in the list (according to {@link Object#equals}) may be
   * compared. This comparator imposes a "partial ordering" over the type {@code T}. Subsequent
   * changes to the {@code valuesInOrder} list will have no effect on the returned comparator. Null
   * values in the list are not supported.
   *
   *
The returned comparator throws a {@link ClassCastException} when it receives an input
   * parameter that isn't among the provided values.
   *
   *
The generated comparator is serializable if all the provided values are serializable.
   *
   * @param valuesInOrder the values that the returned comparator will be able to compare, in the
   *     order the comparator should induce
   * @return the comparator described above
   * @throws NullPointerException if any of the provided values is null
   * @throws IllegalArgumentException if {@code valuesInOrder} contains any duplicate values
   *     (according to {@link Object#equals})
   */
  // TODO(kevinb): provide replacement
  @GwtCompatible(serializable = true)
  public static <T> Ordering<T> explicit(List<T> valuesInOrder) {
    return new ExplicitOrdering<T>(valuesInOrder);
  }

  /**
   * Returns an ordering that compares objects according to the order in which they are given to
   * this method. Only objects present in the argument list (according to {@link Object#equals}) may
   * be compared. This comparator imposes a "partial ordering" over the type {@code T}. Null values
   * in the argument list are not supported.
   *
   *
The returned comparator throws a {@link ClassCastException} when it receives an input
   * parameter that isn't among the provided values.
   *
   *
The generated comparator is serializable if all the provided values are serializable.
   *
   * @param leastValue the value which the returned comparator should consider the "least" of all
   *     values
   * @param remainingValuesInOrder the rest of the values that the returned comparator will be able
   *     to compare, in the order the comparator should follow
   * @return the comparator described above
   * @throws NullPointerException if any of the provided values is null
   * @throws IllegalArgumentException if any duplicate values (according to {@link
   *     Object#equals(Object)}) are present among the method arguments
   */
  // TODO(kevinb): provide replacement
  @GwtCompatible(serializable = true)
  public static <T> Ordering<T> explicit(T leastValue, T... remainingValuesInOrder) {
    return explicit(Lists.asList(leastValue, remainingValuesInOrder));
  }

  // Ordering<Object> singletons

  /**
   * Returns an ordering which treats all values as equal, indicating "no ordering." Passing this
   * ordering to any stable sort algorithm results in no change to the order of elements.
   * Note especially that {@link #sortedCopy} and {@link #immutableSortedCopy} are stable, and in
   * the returned instance these are implemented by simply copying the source list.
   *
   *
Example:
   *
   *
{@code
   * Ordering.allEqual().nullsLast().sortedCopy(
   *     asList(t, null, e, s, null, t, null))
   * }

   *
   *
Assuming {@code t}, {@code e} and {@code s} are non-null, this returns {@code [t, e, s, t,
   * null, null, null]} regardless of the true comparison order of those three values (which might
   * not even implement {@link Comparable} at all).
   *
   *
Warning: by definition, this comparator is not consistent with equals (as
   * defined {@linkplain Comparator here}). Avoid its use in APIs, such as {@link
   * TreeSet#TreeSet(Comparator)}, where such consistency is expected.
   *
   *
The returned comparator is serializable.
   *
   *
Java 8 users: Use the lambda expression {@code (a, b) -> 0} instead (in certain cases
   * you may need to cast that to {@code Comparator<YourType>}).
   *
   * @since 13.0
   */
  @GwtCompatible(serializable = true)
  @SuppressWarnings("unchecked")
  public static Ordering<Object> allEqual() {
    return AllEqualOrdering.INSTANCE;
  }

  /**
   * Returns an ordering that compares objects by the natural ordering of their string
   * representations as returned by {@code toString()}. It does not support null values.
   *
   *
The comparator is serializable.
   *
   *
Java 8 users: Use {@code Comparator.comparing(Object::toString)} instead.
   */
  @GwtCompatible(serializable = true)
  public static Ordering<Object> usingToString() {
    return UsingToStringOrdering.INSTANCE;
  }

  /**
   * Returns an arbitrary ordering over all objects, for which {@code compare(a, b) == 0} implies
   * {@code a == b} (identity equality). There is no meaning whatsoever to the order imposed, but it
   * is constant for the life of the VM.
   *
   *
Because the ordering is identity-based, it is not "consistent with {@link
   * Object#equals(Object)}" as defined by {@link Comparator}. Use caution when building a {@link
   * SortedSet} or {@link SortedMap} from it, as the resulting collection will not behave exactly
   * according to spec.
   *
   *
This ordering is not serializable, as its implementation relies on {@link
   * System#identityHashCode(Object)}, so its behavior cannot be preserved across serialization.
   *
   * @since 2.0
   */
  // TODO(kevinb): copy to Comparators, etc.
  public static Ordering<Object> arbitrary() {
    return ArbitraryOrderingHolder.ARBITRARY_ORDERING;
  }

  private static class ArbitraryOrderingHolder {
    static final Ordering<Object> ARBITRARY_ORDERING = new ArbitraryOrdering();
  }

  @VisibleForTesting
  static class ArbitraryOrdering extends Ordering<Object> {

    private final AtomicInteger counter = new AtomicInteger(0);
    private final ConcurrentMap<Object, Integer> uids =
        Platform.tryWeakKeys(new MapMaker()).makeMap();

    private Integer getUid(Object obj) {
      Integer uid = uids.get(obj);
      if (uid == null) {
        // One or more integer values could be skipped in the event of a race
        // to generate a UID for the same object from multiple threads, but
        // that shouldn't be a problem.
        uid = counter.getAndIncrement();
        Integer alreadySet = uids.putIfAbsent(obj, uid);
        if (alreadySet != null) {
          uid = alreadySet;
        }
      }
      return uid;
    }

    @Override
    public int compare(Object left, Object right) {
      if (left == right) {
        return 0;
      } else if (left == null) {
        return -1;
      } else if (right == null) {
        return 1;
      }
      int leftCode = identityHashCode(left);
      int rightCode = identityHashCode(right);
      if (leftCode != rightCode) {
        return leftCode < rightCode ? -1 : 1;
      }

      // identityHashCode collision (rare, but not as rare as you'd think)
      int result = getUid(left).compareTo(getUid(right));
      if (result == 0) {
        throw new AssertionError(); // extremely, extremely unlikely.
      }
      return result;
    }

    @Override
    public String toString() {
      return "Ordering.arbitrary()";
    }

    /*
     * We need to be able to mock identityHashCode() calls for tests, because it
     * can take 1-10 seconds to find colliding objects. Mocking frameworks that
     * can do magic to mock static method calls still can't do so for a system
     * class, so we need the indirection. In production, Hotspot should still
     * recognize that the call is 1-morphic and should still be willing to
     * inline it if necessary.
     */
    int identityHashCode(Object object) {
      return System.identityHashCode(object);
    }
  }

  // Constructor

  /**
   * Constructs a new instance of this class (only invokable by the subclass constructor, typically
   * implicit).
   */
  protected Ordering() {}

  // Instance-based factories (and any static equivalents)

  /**
   * Returns the reverse of this ordering; the {@code Ordering} equivalent to {@link
   * Collections#reverseOrder(Comparator)}.
   *
   *
Java 8 users: Use {@code thisComparator.reversed()} instead.
   */
  // type parameter <S> lets us avoid the extra <String> in statements like:
  // Ordering<String> o = Ordering.<String>natural().reverse();
  @GwtCompatible(serializable = true)
  public <S extends T> Ordering<S> reverse() {
    return new ReverseOrdering<S>(this);
  }

  /**
   * Returns an ordering that treats {@code null} as less than all other values and uses {@code
   * this} to compare non-null values.
   *
   *
Java 8 users: Use {@code Comparator.nullsFirst(thisComparator)} instead.
   */
  // type parameter <S> lets us avoid the extra <String> in statements like:
  // Ordering<String> o = Ordering.<String>natural().nullsFirst();
  @GwtCompatible(serializable = true)
  public <S extends T> Ordering<S> nullsFirst() {
    return new NullsFirstOrdering<S>(this);
  }

  /**
   * Returns an ordering that treats {@code null} as greater than all other values and uses this
   * ordering to compare non-null values.
   *
   *
Java 8 users: Use {@code Comparator.nullsLast(thisComparator)} instead.
   */
  // type parameter <S> lets us avoid the extra <String> in statements like:
  // Ordering<String> o = Ordering.<String>natural().nullsLast();
  @GwtCompatible(serializable = true)
  public <S extends T> Ordering<S> nullsLast() {
    return new NullsLastOrdering<S>(this);
  }

  /**
   * Returns a new ordering on {@code F} which orders elements by first applying a function to them,
   * then comparing those results using {@code this}. For example, to compare objects by their
   * string forms, in a case-insensitive manner, use:
   *
   *
{@code
   * Ordering.from(String.CASE_INSENSITIVE_ORDER)
   *     .onResultOf(Functions.toStringFunction())
   * }

   *
   *
Java 8 users: Use {@code Comparator.comparing(function, thisComparator)} instead (you
   * can omit the comparator if it is the natural order).
   */
  @GwtCompatible(serializable = true)
  public <F> Ordering<F> onResultOf(Function<F, ? extends T> function) {
    return new ByFunctionOrdering<>(function, this);
  }

  <T2 extends T> Ordering<Entry<T2, ?>> onKeys() {
    return onResultOf(Maps.<T2>keyFunction());
  }

  /**
   * Returns an ordering which first uses the ordering {@code this}, but which in the event of a
   * "tie", then delegates to {@code secondaryComparator}. For example, to sort a bug list first by
   * status and second by priority, you might use {@code byStatus.compound(byPriority)}. For a
   * compound ordering with three or more components, simply chain multiple calls to this method.
   *
   *
An ordering produced by this method, or a chain of calls to this method, is equivalent to
   * one created using {@link Ordering#compound(Iterable)} on the same component comparators.
   *
   *
Java 8 users: Use {@code thisComparator.thenComparing(secondaryComparator)} instead.
   * Depending on what {@code secondaryComparator} is, one of the other overloads of {@code
   * thenComparing} may be even more useful.
   */
  @GwtCompatible(serializable = true)
  public  Ordering compound(Comparator<? super U> secondaryComparator) {
    return new CompoundOrdering(this, checkNotNull(secondaryComparator));
  }

  /**
   * Returns an ordering which tries each given comparator in order until a non-zero result is
   * found, returning that result, and returning zero only if all comparators return zero. The
   * returned ordering is based on the state of the {@code comparators} iterable at the time it was
   * provided to this method.
   *
   *
The returned ordering is equivalent to that produced using {@code
   * Ordering.from(comp1).compound(comp2).compound(comp3) . . .}.
   *
   *
Warning: Supplying an argument with undefined iteration order, such as a {@link
   * HashSet}, will produce non-deterministic results.
   *
   *
Java 8 users: Use a chain of calls to {@link Comparator#thenComparing(Comparator)},
   * or {@code comparatorCollection.stream().reduce(Comparator::thenComparing).get()} (if the
   * collection might be empty, also provide a default comparator as the {@code identity} parameter
   * to {@code reduce}).
   *
   * @param comparators the comparators to try in order
   */
  @GwtCompatible(serializable = true)
  public static <T> Ordering<T> compound(Iterable<? extends Comparator<? super T>> comparators) {
    return new CompoundOrdering<T>(comparators);
  }

  /**
   * Returns a new ordering which sorts iterables by comparing corresponding elements pairwise until
   * a nonzero result is found; imposes "dictionary order". If the end of one iterable is reached,
   * but not the other, the shorter iterable is considered to be less than the longer one. For
   * example, a lexicographical natural ordering over integers considers {@code [] < [1] < [1, 1] <
   * [1, 2] < [2]}.
   *
   *
Note that {@code ordering.lexicographical().reverse()} is not equivalent to {@code
   * ordering.reverse().lexicographical()} (consider how each would order {@code [1]} and {@code [1,
   * 1]}).
   *
   *
Java 8 users: Use {@link Comparators#lexicographical(Comparator)} instead.
   *
   * @since 2.0
   */
  @GwtCompatible(serializable = true)
  // type parameter <S> lets us avoid the extra <String> in statements like:
  // Ordering<Iterable<String>> o =
  //     Ordering.<String>natural().lexicographical();
  public <S extends T> Ordering<Iterable<S>> lexicographical() {
    /*
     * Note that technically the returned ordering should be capable of
     * handling not just {@code Iterable<S>} instances, but also any {@code
     * Iterable<? extends S>}. However, the need for this comes up so rarely
     * that it doesn't justify making everyone else deal with the very ugly
     * wildcard.
     */
    return new LexicographicalOrdering<S>(this);
  }

  // Regular instance methods

  // Override to add @NullableDecl
  @CanIgnoreReturnValue // TODO(kak): Consider removing this
  @Override
  public abstract int compare(@NullableDecl T left, @NullableDecl T right);

  /**
   * Returns the least of the specified values according to this ordering. If there are multiple
   * least values, the first of those is returned. The iterator will be left exhausted: its {@code
   * hasNext()} method will return {@code false}.
   *
   *
Java 8 users: Use {@code Streams.stream(iterator).min(thisComparator).get()} instead
   * (but note that it does not guarantee which tied minimum element is returned).
   *
   * @param iterator the iterator whose minimum element is to be determined
   * @throws NoSuchElementException if {@code iterator} is empty
   * @throws ClassCastException if the parameters are not mutually comparable under this
   *     ordering.
   * @since 11.0
   */
  public <E extends T> E min(Iterator<E> iterator) {
    // let this throw NoSuchElementException as necessary
    E minSoFar = iterator.next();

    while (iterator.hasNext()) {
      minSoFar = min(minSoFar, iterator.next());
    }

    return minSoFar;
  }

  /**
   * Returns the least of the specified values according to this ordering. If there are multiple
   * least values, the first of those is returned.
   *
   *
Java 8 users: If {@code iterable} is a {@link Collection}, use {@code
   * Collections.min(collection, thisComparator)} instead. Otherwise, use {@code
   * Streams.stream(iterable).min(thisComparator).get()} instead. Note that these alternatives do
   * not guarantee which tied minimum element is returned)
   *
   * @param iterable the iterable whose minimum element is to be determined
   * @throws NoSuchElementException if {@code iterable} is empty
   * @throws ClassCastException if the parameters are not mutually comparable under this
   *     ordering.
   */
  public <E extends T> E min(Iterable<E> iterable) {
    return min(iterable.iterator());
  }

  /**
   * Returns the lesser of the two values according to this ordering. If the values compare as 0,
   * the first is returned.
   *
   *
Implementation note: this method is invoked by the default implementations of the
   * other {@code min} overloads, so overriding it will affect their behavior.
   *
   *
Java 8 users: Use {@code Collections.min(Arrays.asList(a, b), thisComparator)}
   * instead (but note that it does not guarantee which tied minimum element is returned).
   *
   * @param a value to compare, returned if less than or equal to b.
   * @param b value to compare.
   * @throws ClassCastException if the parameters are not mutually comparable under this
   *     ordering.
   */
  public <E extends T> E min(@NullableDecl E a, @NullableDecl E b) {
    return (compare(a, b) <= 0) ? a : b;
  }

  /**
   * Returns the least of the specified values according to this ordering. If there are multiple
   * least values, the first of those is returned.
   *
   *
Java 8 users: Use {@code Collections.min(Arrays.asList(a, b, c...), thisComparator)}
   * instead (but note that it does not guarantee which tied minimum element is returned).
   *
   * @param a value to compare, returned if less than or equal to the rest.
   * @param b value to compare
   * @param c value to compare
   * @param rest values to compare
   * @throws ClassCastException if the parameters are not mutually comparable under this
   *     ordering.
   */
  public <E extends T> E min(@NullableDecl E a, @NullableDecl E b, @NullableDecl E c, E... rest) {
    E minSoFar = min(min(a, b), c);

    for (E r : rest) {
      minSoFar = min(minSoFar, r);
    }

    return minSoFar;
  }

  /**
   * Returns the greatest of the specified values according to this ordering. If there are multiple
   * greatest values, the first of those is returned. The iterator will be left exhausted: its
   * {@code hasNext()} method will return {@code false}.
   *
   *
Java 8 users: Continue to use this method for now. Use {@code
   * Streams.stream(iterator).max(thisComparator).get()} instead (but note that it does not
   * guarantee which tied maximum element is returned).
   *
   * @param iterator the iterator whose maximum element is to be determined
   * @throws NoSuchElementException if {@code iterator} is empty
   * @throws ClassCastException if the parameters are not mutually comparable under this
   *     ordering.
   * @since 11.0
   */
  public <E extends T> E max(Iterator<E> iterator) {
    // let this throw NoSuchElementException as necessary
    E maxSoFar = iterator.next();

    while (iterator.hasNext()) {
      maxSoFar = max(maxSoFar, iterator.next());
    }

    return maxSoFar;
  }

  /**
   * Returns the greatest of the specified values according to this ordering. If there are multiple
   * greatest values, the first of those is returned.
   *
   *
Java 8 users: If {@code iterable} is a {@link Collection}, use {@code
   * Collections.max(collection, thisComparator)} instead. Otherwise, use {@code
   * Streams.stream(iterable).max(thisComparator).get()} instead. Note that these alternatives do
   * not guarantee which tied maximum element is returned)
   *
   * @param iterable the iterable whose maximum element is to be determined
   * @throws NoSuchElementException if {@code iterable} is empty
   * @throws ClassCastException if the parameters are not mutually comparable under this
   *     ordering.
   */
  public <E extends T> E max(Iterable<E> iterable) {
    return max(iterable.iterator());
  }

  /**
   * Returns the greater of the two values according to this ordering. If the values compare as 0,
   * the first is returned.
   *
   *
Implementation note: this method is invoked by the default implementations of the
   * other {@code max} overloads, so overriding it will affect their behavior.
   *
   *
Java 8 users: Use {@code Collections.max(Arrays.asList(a, b), thisComparator)}
   * instead (but note that it does not guarantee which tied maximum element is returned).
   *
   * @param a value to compare, returned if greater than or equal to b.
   * @param b value to compare.
   * @throws ClassCastException if the parameters are not mutually comparable under this
   *     ordering.
   */
  public <E extends T> E max(@NullableDecl E a, @NullableDecl E b) {
    return (compare(a, b) >= 0) ? a : b;
  }

  /**
   * Returns the greatest of the specified values according to this ordering. If there are multiple
   * greatest values, the first of those is returned.
   *
   *
Java 8 users: Use {@code Collections.max(Arrays.asList(a, b, c...), thisComparator)}
   * instead (but note that it does not guarantee which tied maximum element is returned).
   *
   * @param a value to compare, returned if greater than or equal to the rest.
   * @param b value to compare
   * @param c value to compare
   * @param rest values to compare
   * @throws ClassCastException if the parameters are not mutually comparable under this
   *     ordering.
   */
  public <E extends T> E max(@NullableDecl E a, @NullableDecl E b, @NullableDecl E c, E... rest) {
    E maxSoFar = max(max(a, b), c);

    for (E r : rest) {
      maxSoFar = max(maxSoFar, r);
    }

    return maxSoFar;
  }

  /**
   * Returns the {@code k} least elements of the given iterable according to this ordering, in order
   * from least to greatest. If there are fewer than {@code k} elements present, all will be
   * included.
   *
   *
The implementation does not necessarily use a stable sorting algorithm; when multiple
   * elements are equivalent, it is undefined which will come first.
   *
   *
Java 8 users: Continue to use this method for now. After the next release of Guava,
   * use {@code Streams.stream(iterable).collect(Comparators.least(k, thisComparator))} instead.
   *
   * @return an immutable {@code RandomAccess} list of the {@code k} least elements in ascending
   *     order
   * @throws IllegalArgumentException if {@code k} is negative
   * @since 8.0
   */
  public <E extends T> List<E> leastOf(Iterable<E> iterable, int k) {
    if (iterable instanceof Collection) {
      Collection<E> collection = (Collection<E>) iterable;
      if (collection.size() <= 2L * k) {
        // In this case, just dumping the collection to an array and sorting is
        // faster than using the implementation for Iterator, which is
        // specialized for k much smaller than n.

        @SuppressWarnings("unchecked") // c only contains E's and doesn't escape
        E[] array = (E[]) collection.toArray();
        Arrays.sort(array, this);
        if (array.length > k) {
          array = Arrays.copyOf(array, k);
        }
        return Collections.unmodifiableList(Arrays.asList(array));
      }
    }
    return leastOf(iterable.iterator(), k);
  }

  /**
   * Returns the {@code k} least elements from the given iterator according to this ordering, in
   * order from least to greatest. If there are fewer than {@code k} elements present, all will be
   * included.
   *
   *
The implementation does not necessarily use a stable sorting algorithm; when multiple
   * elements are equivalent, it is undefined which will come first.
   *
   *
Java 8 users: Use {@code Streams.stream(iterator).collect(Comparators.least(k,
   * thisComparator))} instead.
   *
   * @return an immutable {@code RandomAccess} list of the {@code k} least elements in ascending
   *     order
   * @throws IllegalArgumentException if {@code k} is negative
   * @since 14.0
   */
  public <E extends T> List<E> leastOf(Iterator<E> iterator, int k) {
    checkNotNull(iterator);
    checkNonnegative(k, "k");

    if (k == 0 || !iterator.hasNext()) {
      return Collections.emptyList();
    } else if (k >= Integer.MAX_VALUE / 2) {
      // k is really large; just do a straightforward sorted-copy-and-sublist
      ArrayList<E> list = Lists.newArrayList(iterator);
      Collections.sort(list, this);
      if (list.size() > k) {
        list.subList(k, list.size()).clear();
      }
      list.trimToSize();
      return Collections.unmodifiableList(list);
    } else {
      TopKSelector<E> selector = TopKSelector.least(k, this);
      selector.offerAll(iterator);
      return selector.topK();
    }
  }

  /**
   * Returns the {@code k} greatest elements of the given iterable according to this ordering, in
   * order from greatest to least. If there are fewer than {@code k} elements present, all will be
   * included.
   *
   *
The implementation does not necessarily use a stable sorting algorithm; when multiple
   * elements are equivalent, it is undefined which will come first.
   *
   *
Java 8 users: Continue to use this method for now. After the next release of Guava,
   * use {@code Streams.stream(iterable).collect(Comparators.greatest(k, thisComparator))} instead.
   *
   * @return an immutable {@code RandomAccess} list of the {@code k} greatest elements in
   *     descending order
   * @throws IllegalArgumentException if {@code k} is negative
   * @since 8.0
   */
  public <E extends T> List<E> greatestOf(Iterable<E> iterable, int k) {
    // TODO(kevinb): see if delegation is hurting performance noticeably
    // TODO(kevinb): if we change this implementation, add full unit tests.
    return reverse().leastOf(iterable, k);
  }

  /**
   * Returns the {@code k} greatest elements from the given iterator according to this ordering, in
   * order from greatest to least. If there are fewer than {@code k} elements present, all will be
   * included.
   *
   *
The implementation does not necessarily use a stable sorting algorithm; when multiple
   * elements are equivalent, it is undefined which will come first.
   *
   *
Java 8 users: Use {@code Streams.stream(iterator).collect(Comparators.greatest(k,
   * thisComparator))} instead.
   *
   * @return an immutable {@code RandomAccess} list of the {@code k} greatest elements in
   *     descending order
   * @throws IllegalArgumentException if {@code k} is negative
   * @since 14.0
   */
  public <E extends T> List<E> greatestOf(Iterator<E> iterator, int k) {
    return reverse().leastOf(iterator, k);
  }

  /**
   * Returns a mutable list containing {@code elements} sorted by this ordering; use this
   * only when the resulting list may need further modification, or may contain {@code null}. The
   * input is not modified. The returned list is serializable and has random access.
   *
   *
Unlike {@link Sets#newTreeSet(Iterable)}, this method does not discard elements that are
   * duplicates according to the comparator. The sort performed is stable, meaning that such
   * elements will appear in the returned list in the same order they appeared in {@code elements}.
   *
   *
Performance note: According to our
   * benchmarking
   * on Open JDK 7, {@link #immutableSortedCopy} generally performs better (in both time and space)
   * than this method, and this method in turn generally performs better than copying the list and
   * calling {@link Collections#sort(List)}.
   */
  // TODO(kevinb): rerun benchmarks including new options
  public <E extends T> List<E> sortedCopy(Iterable<E> elements) {
    @SuppressWarnings("unchecked") // does not escape, and contains only E's
    E[] array = (E[]) Iterables.toArray(elements);
    Arrays.sort(array, this);
    return Lists.newArrayList(Arrays.asList(array));
  }

  /**
   * Returns an immutable list containing {@code elements} sorted by this ordering. The input
   * is not modified.
   *
   *
Unlike {@link Sets#newTreeSet(Iterable)}, this method does not discard elements that are
   * duplicates according to the comparator. The sort performed is stable, meaning that such
   * elements will appear in the returned list in the same order they appeared in {@code elements}.
   *
   *
Performance note: According to our
   * benchmarking
   * on Open JDK 7, this method is the most efficient way to make a sorted copy of a collection.
   *
   * @throws NullPointerException if any element of {@code elements} is {@code null}
   * @since 3.0
   */
  // TODO(kevinb): rerun benchmarks including new options
  public <E extends T> ImmutableList<E> immutableSortedCopy(Iterable<E> elements) {
    return ImmutableList.sortedCopyOf(this, elements);
  }

  /**
   * Returns {@code true} if each element in {@code iterable} after the first is greater than or
   * equal to the element that preceded it, according to this ordering. Note that this is always
   * true when the iterable has fewer than two elements.
   *
   *
Java 8 users: Use the equivalent {@link Comparators#isInOrder(Iterable, Comparator)}
   * instead, since the rest of {@code Ordering} is mostly obsolete (as explained in the class
   * documentation).
   */
  public boolean isOrdered(Iterable<? extends T> iterable) {
    Iterator<? extends T> it = iterable.iterator();
    if (it.hasNext()) {
      T prev = it.next();
      while (it.hasNext()) {
        T next = it.next();
        if (compare(prev, next) > 0) {
          return false;
        }
        prev = next;
      }
    }
    return true;
  }

  /**
   * Returns {@code true} if each element in {@code iterable} after the first is strictly
   * greater than the element that preceded it, according to this ordering. Note that this is always
   * true when the iterable has fewer than two elements.
   *
   *
Java 8 users: Use the equivalent {@link Comparators#isInStrictOrder(Iterable,
   * Comparator)} instead, since the rest of {@code Ordering} is mostly obsolete (as explained in
   * the class documentation).
   */
  public boolean isStrictlyOrdered(Iterable<? extends T> iterable) {
    Iterator<? extends T> it = iterable.iterator();
    if (it.hasNext()) {
      T prev = it.next();
      while (it.hasNext()) {
        T next = it.next();
        if (compare(prev, next) >= 0) {
          return false;
        }
        prev = next;
      }
    }
    return true;
  }

  /**
   * {@link Collections#binarySearch(List, Object, Comparator) Searches} {@code sortedList} for
   * {@code key} using the binary search algorithm. The list must be sorted using this ordering.
   *
   * @param sortedList the list to be searched
   * @param key the key to be searched for
   * @deprecated Use {@link Collections#binarySearch(List, Object, Comparator)} directly.
   */
  @Deprecated
  public int binarySearch(List<? extends T> sortedList, @NullableDecl T key) {
    return Collections.binarySearch(sortedList, key, this);
  }

  /**
   * Exception thrown by a {@link Ordering#explicit(List)} or {@link Ordering#explicit(Object,
   * Object[])} comparator when comparing a value outside the set of values it can compare.
   * Extending {@link ClassCastException} may seem odd, but it is required.
   */
  @VisibleForTesting
  static class IncomparableValueException extends ClassCastException {
    final Object value;

    IncomparableValueException(Object value) {
      super("Cannot compare value: " + value);
      this.value = value;
    }

    private static final long serialVersionUID = 0;
  }

  // Never make these public
  static final int LEFT_IS_GREATER = 1;
  static final int RIGHT_IS_GREATER = -1;
}


/*
 * 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.util.concurrent;

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

import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtIncompatible;
import com.google.errorprone.annotations.concurrent.GuardedBy;
import com.google.j2objc.annotations.Weak;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;
import org.checkerframework.checker.nullness.compatqual.NullableDecl;

/**
 * A synchronization abstraction supporting waiting on arbitrary boolean conditions.
 *
 *
This class is intended as a replacement for {@link ReentrantLock}. Code using {@code Monitor}
 * is less error-prone and more readable than code using {@code ReentrantLock}, without significant
 * performance loss. {@code Monitor} even has the potential for performance gain by optimizing the
 * evaluation and signaling of conditions. Signaling is entirely implicit. By
 * eliminating explicit signaling, this class can guarantee that only one thread is awakened when a
 * condition becomes true (no "signaling storms" due to use of {@link
 * java.util.concurrent.locks.Condition#signalAll Condition.signalAll}) and that no signals are lost
 * (no "hangs" due to incorrect use of {@link java.util.concurrent.locks.Condition#signal
 * Condition.signal}).
 *
 *
A thread is said to occupy a monitor if it has entered the monitor but not yet
 * left. Only one thread may occupy a given monitor at any moment. A monitor is also
 * reentrant, so a thread may enter a monitor any number of times, and then must leave the same
 * number of times. The enter and leave operations have the same synchronization
 * semantics as the built-in Java language synchronization primitives.
 *
 *
A call to any of the enter methods with void return type should always be
 * followed immediately by a try/finally block to ensure that the current thread leaves the
 * monitor cleanly:
 *
 *
{@code
 * monitor.enter();
 * try {
 *   // do things while occupying the monitor
 * } finally {
 *   monitor.leave();
 * }
 * }

 *
 *
A call to any of the enter methods with boolean return type should always appear
 * as the condition of an if statement containing a try/finally block to ensure that
 * the current thread leaves the monitor cleanly:
 *
 *
{@code
 * if (monitor.tryEnter()) {
 *   try {
 *     // do things while occupying the monitor
 *   } finally {
 *     monitor.leave();
 *   }
 * } else {
 *   // do other things since the monitor was not available
 * }
 * }

 *
 *
Comparison with {@code synchronized} and {@code ReentrantLock}

 *
 *
The following examples show a simple threadsafe holder expressed using {@code synchronized},
 * {@link ReentrantLock}, and {@code Monitor}.
 *
 *
{@code synchronized}

 *
 *
This version is the fewest lines of code, largely because the synchronization mechanism used
 * is built into the language and runtime. But the programmer has to remember to avoid a couple of
 * common bugs: The {@code wait()} must be inside a {@code while} instead of an {@code if}, and
 * {@code notifyAll()} must be used instead of {@code notify()} because there are two different
 * logical conditions being awaited.
 *
 *
{@code
 * public class SafeBox<V> {
 *   private V value;
 *
 *   public synchronized V get() throws InterruptedException {
 *     while (value == null) {
 *       wait();
 *     }
 *     V result = value;
 *     value = null;
 *     notifyAll();
 *     return result;
 *   }
 *
 *   public synchronized void set(V newValue) throws InterruptedException {
 *     while (value != null) {
 *       wait();
 *     }
 *     value = newValue;
 *     notifyAll();
 *   }
 * }
 * }

 *
 *
{@code ReentrantLock}

 *
 *
This version is much more verbose than the {@code synchronized} version, and still suffers
 * from the need for the programmer to remember to use {@code while} instead of {@code if}. However,
 * one advantage is that we can introduce two separate {@code Condition} objects, which allows us to
 * use {@code signal()} instead of {@code signalAll()}, which may be a performance benefit.
 *
 *
{@code
 * public class SafeBox<V> {
 *   private V value;
 *   private final ReentrantLock lock = new ReentrantLock();
 *   private final Condition valuePresent = lock.newCondition();
 *   private final Condition valueAbsent = lock.newCondition();
 *
 *   public V get() throws InterruptedException {
 *     lock.lock();
 *     try {
 *       while (value == null) {
 *         valuePresent.await();
 *       }
 *       V result = value;
 *       value = null;
 *       valueAbsent.signal();
 *       return result;
 *     } finally {
 *       lock.unlock();
 *     }
 *   }
 *
 *   public void set(V newValue) throws InterruptedException {
 *     lock.lock();
 *     try {
 *       while (value != null) {
 *         valueAbsent.await();
 *       }
 *       value = newValue;
 *       valuePresent.signal();
 *     } finally {
 *       lock.unlock();
 *     }
 *   }
 * }
 * }

 *
 *
{@code Monitor}

 *
 *
This version adds some verbosity around the {@code Guard} objects, but removes that same
 * verbosity, and more, from the {@code get} and {@code set} methods. {@code Monitor} implements the
 * same efficient signaling as we had to hand-code in the {@code ReentrantLock} version above.
 * Finally, the programmer no longer has to hand-code the wait loop, and therefore doesn't have to
 * remember to use {@code while} instead of {@code if}.
 *
 *
{@code
 * public class SafeBox<V> {
 *   private V value;
 *   private final Monitor monitor = new Monitor();
 *   private final Monitor.Guard valuePresent = monitor.newGuard(() -> value != null);
 *   private final Monitor.Guard valueAbsent = monitor.newGuard(() -> value == null);
 *
 *   public V get() throws InterruptedException {
 *     monitor.enterWhen(valuePresent);
 *     try {
 *       V result = value;
 *       value = null;
 *       return result;
 *     } finally {
 *       monitor.leave();
 *     }
 *   }
 *
 *   public void set(V newValue) throws InterruptedException {
 *     monitor.enterWhen(valueAbsent);
 *     try {
 *       value = newValue;
 *     } finally {
 *       monitor.leave();
 *     }
 *   }
 * }
 * }

 *
 * @author Justin T. Sampson
 * @author Martin Buchholz
 * @since 10.0
 */
@Beta
@GwtIncompatible
@SuppressWarnings("GuardedBy") // TODO(b/35466881): Fix or suppress.
public final class Monitor {
  // TODO(user): Use raw LockSupport or AbstractQueuedSynchronizer instead of ReentrantLock.
  // TODO(user): "Port" jsr166 tests for ReentrantLock.
  //
  // TODO(user): Change API to make it impossible to use a Guard with the "wrong" monitor,
  //    by making the monitor implicit, and to eliminate other sources of IMSE.
  //    Imagine:
  //    guard.lock();
  //    try { /* monitor locked and guard satisfied here */ }
  //    finally { guard.unlock(); }
  // Here are Justin's design notes about this:
  //
  // This idea has come up from time to time, and I think one of my
  // earlier versions of Monitor even did something like this. I ended
  // up strongly favoring the current interface.
  //
  // I probably can't remember all the reasons (it's possible you
  // could find them in the code review archives), but here are a few:
  //
  // 1. What about leaving/unlocking? Are you going to do
  //    guard.enter() paired with monitor.leave()? That might get
  //    confusing. It's nice for the finally block to look as close as
  //    possible to the thing right before the try. You could have
  //    guard.leave(), but that's a little odd as well because the
  //    guard doesn't have anything to do with leaving. You can't
  //    really enforce that the guard you're leaving is the same one
  //    you entered with, and it doesn't actually matter.
  //
  // 2. Since you can enter the monitor without a guard at all, some
  //    places you'll have monitor.enter()/monitor.leave() and other
  //    places you'll have guard.enter()/guard.leave() even though
  //    it's the same lock being acquired underneath. Always using
  //    monitor.enterXXX()/monitor.leave() will make it really clear
  //    which lock is held at any point in the code.
  //
  // 3. I think "enterWhen(notEmpty)" reads better than "notEmpty.enter()".
  //
  // TODO(user): Implement ReentrantLock features:
  //    - toString() method
  //    - getOwner() method
  //    - getQueuedThreads() method
  //    - getWaitingThreads(Guard) method
  //    - implement Serializable
  //    - redo the API to be as close to identical to ReentrantLock as possible,
  //      since, after all, this class is also a reentrant mutual exclusion lock!?

  /*
   * One of the key challenges of this class is to prevent lost signals, while trying hard to
   * minimize unnecessary signals. One simple and correct algorithm is to signal some other waiter
   * with a satisfied guard (if one exists) whenever any thread occupying the monitor exits the
   * monitor, either by unlocking all of its held locks, or by starting to wait for a guard. This
   * includes exceptional exits, so all control paths involving signalling must be protected by a
   * finally block.
   *
   * Further optimizations of this algorithm become increasingly subtle. A wait that terminates
   * without the guard being satisfied (due to timeout, but not interrupt) can then immediately exit
   * the monitor without signalling. If it timed out without being signalled, it does not need to
   * "pass on" the signal to another thread. If it *was* signalled, then its guard must have been
   * satisfied at the time of signal, and has since been modified by some other thread to be
   * non-satisfied before reacquiring the lock, and that other thread takes over the responsibility
   * of signaling the next waiter.
   *
   * Unlike the underlying Condition, if we are not careful, an interrupt *can* cause a signal to be
   * lost, because the signal may be sent to a condition whose sole waiter has just been
   * interrupted.
   *
   * Imagine a monitor with multiple guards. A thread enters the monitor, satisfies all the guards,
   * and leaves, calling signalNextWaiter. With traditional locks and conditions, all the conditions
   * need to be signalled because it is not known which if any of them have waiters (and hasWaiters
   * can't be used reliably because of a check-then-act race). With our Monitor guards, we only
   * signal the first active guard that is satisfied. But the corresponding thread may have already
   * been interrupted and is waiting to reacquire the lock while still registered in activeGuards,
   * in which case the signal is a no-op, and the bigger-picture signal is lost unless interrupted
   * threads take special action by participating in the signal-passing game.
   */

  /*
   * Timeout handling is intricate, especially given our ambitious goals:
   * - Avoid underflow and overflow of timeout values when specified timeouts are close to
   *   Long.MIN_VALUE or Long.MAX_VALUE.
   * - Favor responding to interrupts over timeouts.
   * - System.nanoTime() is expensive enough that we want to call it the minimum required number of
   *   times, typically once before invoking a blocking method. This often requires keeping track of
   *   the first time in a method that nanoTime() has been invoked, for which the special value 0L
   *   is reserved to mean "uninitialized". If timeout is non-positive, then nanoTime need never be
   *   called.
   * - Keep behavior of fair and non-fair instances consistent.
   */

  /**
   * A boolean condition for which a thread may wait. A {@code Guard} is associated with a single
   * {@code Monitor}. The monitor may check the guard at arbitrary times from any thread occupying
   * the monitor, so code should not be written to rely on how often a guard might or might not be
   * checked.
   *
   *
If a {@code Guard} is passed into any method of a {@code Monitor} other than the one it is
   * associated with, an {@link IllegalMonitorStateException} is thrown.
   *
   * @since 10.0
   */
  @Beta
  public abstract static class Guard {

    @Weak final Monitor monitor;
    final Condition condition;

    @GuardedBy("monitor.lock")
    int waiterCount = 0;

    /** The next active guard */
    @GuardedBy("monitor.lock")
    @NullableDecl
    Guard next;

    protected Guard(Monitor monitor) {
      this.monitor = checkNotNull(monitor, "monitor");
      this.condition = monitor.lock.newCondition();
    }

    /**
     * Evaluates this guard's boolean condition. This method is always called with the associated
     * monitor already occupied. Implementations of this method must depend only on state protected
     * by the associated monitor, and must not modify that state.
     */
    public abstract boolean isSatisfied();
  }

  /** Whether this monitor is fair. */
  private final boolean fair;

  /** The lock underlying this monitor. */
  private final ReentrantLock lock;

  /**
   * The guards associated with this monitor that currently have waiters ({@code waiterCount > 0}).
   * A linked list threaded through the Guard.next field.
   */
  @GuardedBy("lock")
  private Guard activeGuards = null;

  /**
   * Creates a monitor with a non-fair (but fast) ordering policy. Equivalent to {@code
   * Monitor(false)}.
   */
  public Monitor() {
    this(false);
  }

  /**
   * Creates a monitor with the given ordering policy.
   *
   * @param fair whether this monitor should use a fair ordering policy rather than a non-fair (but
   *     fast) one
   */
  public Monitor(boolean fair) {
    this.fair = fair;
    this.lock = new ReentrantLock(fair);
  }

  /** Enters this monitor. Blocks indefinitely. */
  public void enter() {
    lock.lock();
  }

  /**
   * Enters this monitor. Blocks at most the given time.
   *
   * @return whether the monitor was entered
   */
  @SuppressWarnings("GoodTime") // should accept a java.time.Duration
  public boolean enter(long time, TimeUnit unit) {
    final long timeoutNanos = toSafeNanos(time, unit);
    final ReentrantLock lock = this.lock;
    if (!fair && lock.tryLock()) {
      return true;
    }
    boolean interrupted = Thread.interrupted();
    try {
      final long startTime = System.nanoTime();
      for (long remainingNanos = timeoutNanos; ; ) {
        try {
          return lock.tryLock(remainingNanos, TimeUnit.NANOSECONDS);
        } catch (InterruptedException interrupt) {
          interrupted = true;
          remainingNanos = remainingNanos(startTime, timeoutNanos);
        }
      }
    } finally {
      if (interrupted) {
        Thread.currentThread().interrupt();
      }
    }
  }

  /**
   * Enters this monitor. Blocks indefinitely, but may be interrupted.
   *
   * @throws InterruptedException if interrupted while waiting
   */
  public void enterInterruptibly() throws InterruptedException {
    lock.lockInterruptibly();
  }

  /**
   * Enters this monitor. Blocks at most the given time, and may be interrupted.
   *
   * @return whether the monitor was entered
   * @throws InterruptedException if interrupted while waiting
   */
  @SuppressWarnings("GoodTime") // should accept a java.time.Duration
  public boolean enterInterruptibly(long time, TimeUnit unit) throws InterruptedException {
    return lock.tryLock(time, unit);
  }

  /**
   * Enters this monitor if it is possible to do so immediately. Does not block.
   *
   *
Note: This method disregards the fairness setting of this monitor.
   *
   * @return whether the monitor was entered
   */
  public boolean tryEnter() {
    return lock.tryLock();
  }

  /**
   * Enters this monitor when the guard is satisfied. Blocks indefinitely, but may be interrupted.
   *
   * @throws InterruptedException if interrupted while waiting
   */
  public void enterWhen(Guard guard) throws InterruptedException {
    if (guard.monitor != this) {
      throw new IllegalMonitorStateException();
    }
    final ReentrantLock lock = this.lock;
    boolean signalBeforeWaiting = lock.isHeldByCurrentThread();
    lock.lockInterruptibly();

    boolean satisfied = false;
    try {
      if (!guard.isSatisfied()) {
        await(guard, signalBeforeWaiting);
      }
      satisfied = true;
    } finally {
      if (!satisfied) {
        leave();
      }
    }
  }

  /**
   * Enters this monitor when the guard is satisfied. Blocks at most the given time, including both
   * the time to acquire the lock and the time to wait for the guard to be satisfied, and may be
   * interrupted.
   *
   * @return whether the monitor was entered, which guarantees that the guard is now satisfied
   * @throws InterruptedException if interrupted while waiting
   */
  @SuppressWarnings("GoodTime") // should accept a java.time.Duration
  public boolean enterWhen(Guard guard, long time, TimeUnit unit) throws InterruptedException {
    final long timeoutNanos = toSafeNanos(time, unit);
    if (guard.monitor != this) {
      throw new IllegalMonitorStateException();
    }
    final ReentrantLock lock = this.lock;
    boolean reentrant = lock.isHeldByCurrentThread();
    long startTime = 0L;

    locked:
    {
      if (!fair) {
        // Check interrupt status to get behavior consistent with fair case.
        if (Thread.interrupted()) {
          throw new InterruptedException();
        }
        if (lock.tryLock()) {
          break locked;
        }
      }
      startTime = initNanoTime(timeoutNanos);
      if (!lock.tryLock(time, unit)) {
        return false;
      }
    }

    boolean satisfied = false;
    boolean threw = true;
    try {
      satisfied =
          guard.isSatisfied()
              || awaitNanos(
                  guard,
                  (startTime == 0L) ? timeoutNanos : remainingNanos(startTime, timeoutNanos),
                  reentrant);
      threw = false;
      return satisfied;
    } finally {
      if (!satisfied) {
        try {
          // Don't need to signal if timed out, but do if interrupted
          if (threw && !reentrant) {
            signalNextWaiter();
          }
        } finally {
          lock.unlock();
        }
      }
    }
  }

  /** Enters this monitor when the guard is satisfied. Blocks indefinitely. */
  public void enterWhenUninterruptibly(Guard guard) {
    if (guard.monitor != this) {
      throw new IllegalMonitorStateException();
    }
    final ReentrantLock lock = this.lock;
    boolean signalBeforeWaiting = lock.isHeldByCurrentThread();
    lock.lock();

    boolean satisfied = false;
    try {
      if (!guard.isSatisfied()) {
        awaitUninterruptibly(guard, signalBeforeWaiting);
      }
      satisfied = true;
    } finally {
      if (!satisfied) {
        leave();
      }
    }
  }

  /**
   * Enters this monitor when the guard is satisfied. Blocks at most the given time, including both
   * the time to acquire the lock and the time to wait for the guard to be satisfied.
   *
   * @return whether the monitor was entered, which guarantees that the guard is now satisfied
   */
  @SuppressWarnings("GoodTime") // should accept a java.time.Duration
  public boolean enterWhenUninterruptibly(Guard guard, long time, TimeUnit unit) {
    final long timeoutNanos = toSafeNanos(time, unit);
    if (guard.monitor != this) {
      throw new IllegalMonitorStateException();
    }
    final ReentrantLock lock = this.lock;
    long startTime = 0L;
    boolean signalBeforeWaiting = lock.isHeldByCurrentThread();
    boolean interrupted = Thread.interrupted();
    try {
      if (fair || !lock.tryLock()) {
        startTime = initNanoTime(timeoutNanos);
        for (long remainingNanos = timeoutNanos; ; ) {
          try {
            if (lock.tryLock(remainingNanos, TimeUnit.NANOSECONDS)) {
              break;
            } else {
              return false;
            }
          } catch (InterruptedException interrupt) {
            interrupted = true;
            remainingNanos = remainingNanos(startTime, timeoutNanos);
          }
        }
      }

      boolean satisfied = false;
      try {
        while (true) {
          try {
            if (guard.isSatisfied()) {
              satisfied = true;
            } else {
              final long remainingNanos;
              if (startTime == 0L) {
                startTime = initNanoTime(timeoutNanos);
                remainingNanos = timeoutNanos;
              } else {
                remainingNanos = remainingNanos(startTime, timeoutNanos);
              }
              satisfied = awaitNanos(guard, remainingNanos, signalBeforeWaiting);
            }
            return satisfied;
          } catch (InterruptedException interrupt) {
            interrupted = true;
            signalBeforeWaiting = false;
          }
        }
      } finally {
        if (!satisfied) {
          lock.unlock(); // No need to signal if timed out
        }
      }
    } finally {
      if (interrupted) {
        Thread.currentThread().interrupt();
      }
    }
  }

  /**
   * Enters this monitor if the guard is satisfied. Blocks indefinitely acquiring the lock, but does
   * not wait for the guard to be satisfied.
   *
   * @return whether the monitor was entered, which guarantees that the guard is now satisfied
   */
  public boolean enterIf(Guard guard) {
    if (guard.monitor != this) {
      throw new IllegalMonitorStateException();
    }
    final ReentrantLock lock = this.lock;
    lock.lock();

    boolean satisfied = false;
    try {
      return satisfied = guard.isSatisfied();
    } finally {
      if (!satisfied) {
        lock.unlock();
      }
    }
  }

  /**
   * Enters this monitor if the guard is satisfied. Blocks at most the given time acquiring the
   * lock, but does not wait for the guard to be satisfied.
   *
   * @return whether the monitor was entered, which guarantees that the guard is now satisfied
   */
  @SuppressWarnings("GoodTime") // should accept a java.time.Duration
  public boolean enterIf(Guard guard, long time, TimeUnit unit) {
    if (guard.monitor != this) {
      throw new IllegalMonitorStateException();
    }
    if (!enter(time, unit)) {
      return false;
    }

    boolean satisfied = false;
    try {
      return satisfied = guard.isSatisfied();
    } finally {
      if (!satisfied) {
        lock.unlock();
      }
    }
  }

  /**
   * Enters this monitor if the guard is satisfied. Blocks indefinitely acquiring the lock, but does
   * not wait for the guard to be satisfied, and may be interrupted.
   *
   * @return whether the monitor was entered, which guarantees that the guard is now satisfied
   * @throws InterruptedException if interrupted while waiting
   */
  public boolean enterIfInterruptibly(Guard guard) throws InterruptedException {
    if (guard.monitor != this) {
      throw new IllegalMonitorStateException();
    }
    final ReentrantLock lock = this.lock;
    lock.lockInterruptibly();

    boolean satisfied = false;
    try {
      return satisfied = guard.isSatisfied();
    } finally {
      if (!satisfied) {
        lock.unlock();
      }
    }
  }

  /**
   * Enters this monitor if the guard is satisfied. Blocks at most the given time acquiring the
   * lock, but does not wait for the guard to be satisfied, and may be interrupted.
   *
   * @return whether the monitor was entered, which guarantees that the guard is now satisfied
   */
  @SuppressWarnings("GoodTime") // should accept a java.time.Duration
  public boolean enterIfInterruptibly(Guard guard, long time, TimeUnit unit)
      throws InterruptedException {
    if (guard.monitor != this) {
      throw new IllegalMonitorStateException();
    }
    final ReentrantLock lock = this.lock;
    if (!lock.tryLock(time, unit)) {
      return false;
    }

    boolean satisfied = false;
    try {
      return satisfied = guard.isSatisfied();
    } finally {
      if (!satisfied) {
        lock.unlock();
      }
    }
  }

  /**
   * Enters this monitor if it is possible to do so immediately and the guard is satisfied. Does not
   * block acquiring the lock and does not wait for the guard to be satisfied.
   *
   *
Note: This method disregards the fairness setting of this monitor.
   *
   * @return whether the monitor was entered, which guarantees that the guard is now satisfied
   */
  public boolean tryEnterIf(Guard guard) {
    if (guard.monitor != this) {
      throw new IllegalMonitorStateException();
    }
    final ReentrantLock lock = this.lock;
    if (!lock.tryLock()) {
      return false;
    }

    boolean satisfied = false;
    try {
      return satisfied = guard.isSatisfied();
    } finally {
      if (!satisfied) {
        lock.unlock();
      }
    }
  }

  /**
   * Waits for the guard to be satisfied. Waits indefinitely, but may be interrupted. May be called
   * only by a thread currently occupying this monitor.
   *
   * @throws InterruptedException if interrupted while waiting
   */
  public void waitFor(Guard guard) throws InterruptedException {
    if (!((guard.monitor == this) & lock.isHeldByCurrentThread())) {
      throw new IllegalMonitorStateException();
    }
    if (!guard.isSatisfied()) {
      await(guard, true);
    }
  }

  /**
   * Waits for the guard to be satisfied. Waits at most the given time, and may be interrupted. May
   * be called only by a thread currently occupying this monitor.
   *
   * @return whether the guard is now satisfied
   * @throws InterruptedException if interrupted while waiting
   */
  @SuppressWarnings("GoodTime") // should accept a java.time.Duration
  public boolean waitFor(Guard guard, long time, TimeUnit unit) throws InterruptedException {
    final long timeoutNanos = toSafeNanos(time, unit);
    if (!((guard.monitor == this) & lock.isHeldByCurrentThread())) {
      throw new IllegalMonitorStateException();
    }
    if (guard.isSatisfied()) {
      return true;
    }
    if (Thread.interrupted()) {
      throw new InterruptedException();
    }
    return awaitNanos(guard, timeoutNanos, true);
  }

  /**
   * Waits for the guard to be satisfied. Waits indefinitely. May be called only by a thread
   * currently occupying this monitor.
   */
  public void waitForUninterruptibly(Guard guard) {
    if (!((guard.monitor == this) & lock.isHeldByCurrentThread())) {
      throw new IllegalMonitorStateException();
    }
    if (!guard.isSatisfied()) {
      awaitUninterruptibly(guard, true);
    }
  }

  /**
   * Waits for the guard to be satisfied. Waits at most the given time. May be called only by a
   * thread currently occupying this monitor.
   *
   * @return whether the guard is now satisfied
   */
  @SuppressWarnings("GoodTime") // should accept a java.time.Duration
  public boolean waitForUninterruptibly(Guard guard, long time, TimeUnit unit) {
    final long timeoutNanos = toSafeNanos(time, unit);
    if (!((guard.monitor == this) & lock.isHeldByCurrentThread())) {
      throw new IllegalMonitorStateException();
    }
    if (guard.isSatisfied()) {
      return true;
    }
    boolean signalBeforeWaiting = true;
    final long startTime = initNanoTime(timeoutNanos);
    boolean interrupted = Thread.interrupted();
    try {
      for (long remainingNanos = timeoutNanos; ; ) {
        try {
          return awaitNanos(guard, remainingNanos, signalBeforeWaiting);
        } catch (InterruptedException interrupt) {
          interrupted = true;
          if (guard.isSatisfied()) {
            return true;
          }
          signalBeforeWaiting = false;
          remainingNanos = remainingNanos(startTime, timeoutNanos);
        }
      }
    } finally {
      if (interrupted) {
        Thread.currentThread().interrupt();
      }
    }
  }

  /** Leaves this monitor. May be called only by a thread currently occupying this monitor. */
  public void leave() {
    final ReentrantLock lock = this.lock;
    try {
      // No need to signal if we will still be holding the lock when we return
      if (lock.getHoldCount() == 1) {
        signalNextWaiter();
      }
    } finally {
      lock.unlock(); // Will throw IllegalMonitorStateException if not held
    }
  }

  /** Returns whether this monitor is using a fair ordering policy. */
  public boolean isFair() {
    return fair;
  }

  /**
   * Returns whether this monitor is occupied by any thread. This method is designed for use in
   * monitoring of the system state, not for synchronization control.
   */
  public boolean isOccupied() {
    return lock.isLocked();
  }

  /**
   * Returns whether the current thread is occupying this monitor (has entered more times than it
   * has left).
   */
  public boolean isOccupiedByCurrentThread() {
    return lock.isHeldByCurrentThread();
  }

  /**
   * Returns the number of times the current thread has entered this monitor in excess of the number
   * of times it has left. Returns 0 if the current thread is not occupying this monitor.
   */
  public int getOccupiedDepth() {
    return lock.getHoldCount();
  }

  /**
   * Returns an estimate of the number of threads waiting to enter this monitor. The value is only
   * an estimate because the number of threads may change dynamically while this method traverses
   * internal data structures. This method is designed for use in monitoring of the system state,
   * not for synchronization control.
   */
  public int getQueueLength() {
    return lock.getQueueLength();
  }

  /**
   * Returns whether any threads are waiting to enter this monitor. Note that because cancellations
   * may occur at any time, a {@code true} return does not guarantee that any other thread will ever
   * enter this monitor. This method is designed primarily for use in monitoring of the system
   * state.
   */
  public boolean hasQueuedThreads() {
    return lock.hasQueuedThreads();
  }

  /**
   * Queries whether the given thread is waiting to enter this monitor. Note that because
   * cancellations may occur at any time, a {@code true} return does not guarantee that this thread
   * will ever enter this monitor. This method is designed primarily for use in monitoring of the
   * system state.
   */
  public boolean hasQueuedThread(Thread thread) {
    return lock.hasQueuedThread(thread);
  }

  /**
   * Queries whether any threads are waiting for the given guard to become satisfied. Note that
   * because timeouts and interrupts may occur at any time, a {@code true} return does not guarantee
   * that the guard becoming satisfied in the future will awaken any threads. This method is
   * designed primarily for use in monitoring of the system state.
   */
  public boolean hasWaiters(Guard guard) {
    return getWaitQueueLength(guard) > 0;
  }

  /**
   * Returns an estimate of the number of threads waiting for the given guard to become satisfied.
   * Note that because timeouts and interrupts may occur at any time, the estimate serves only as an
   * upper bound on the actual number of waiters. This method is designed for use in monitoring of
   * the system state, not for synchronization control.
   */
  public int getWaitQueueLength(Guard guard) {
    if (guard.monitor != this) {
      throw new IllegalMonitorStateException();
    }
    lock.lock();
    try {
      return guard.waiterCount;
    } finally {
      lock.unlock();
    }
  }

  /**
   * Returns unit.toNanos(time), additionally ensuring the returned value is not at risk of
   * overflowing or underflowing, by bounding the value between 0 and (Long.MAX_VALUE / 4) * 3.
   * Actually waiting for more than 219 years is not supported!
   */
  private static long toSafeNanos(long time, TimeUnit unit) {
    long timeoutNanos = unit.toNanos(time);
    return (timeoutNanos <= 0L)
        ? 0L
        : (timeoutNanos > (Long.MAX_VALUE / 4) * 3) ? (Long.MAX_VALUE / 4) * 3 : timeoutNanos;
  }

  /**
   * Returns System.nanoTime() unless the timeout has already elapsed. Returns 0L if and only if the
   * timeout has already elapsed.
   */
  private static long initNanoTime(long timeoutNanos) {
    if (timeoutNanos <= 0L) {
      return 0L;
    } else {
      long startTime = System.nanoTime();
      return (startTime == 0L) ? 1L : startTime;
    }
  }

  /**
   * Returns the remaining nanos until the given timeout, or 0L if the timeout has already elapsed.
   * Caller must have previously sanitized timeoutNanos using toSafeNanos.
   */
  private static long remainingNanos(long startTime, long timeoutNanos) {
    // assert timeoutNanos == 0L || startTime != 0L;

    // TODO : NOT CORRECT, BUT TESTS PASS ANYWAYS!
    // if (true) return timeoutNanos;
    // ONLY 2 TESTS FAIL IF WE DO:
    // if (true) return 0;

    return (timeoutNanos <= 0L) ? 0L : timeoutNanos - (System.nanoTime() - startTime);
  }

  /**
   * Signals some other thread waiting on a satisfied guard, if one exists.
   *
   *
We manage calls to this method carefully, to signal only when necessary, but never losing a
   * signal, which is the classic problem of this kind of concurrency construct. We must signal if
   * the current thread is about to relinquish the lock and may have changed the state protected by
   * the monitor, thereby causing some guard to be satisfied.
   *
   *
In addition, any thread that has been signalled when its guard was satisfied acquires the
   * responsibility of signalling the next thread when it again relinquishes the lock. Unlike a
   * normal Condition, there is no guarantee that an interrupted thread has not been signalled,
   * since the concurrency control must manage multiple Conditions. So this method must generally be
   * called when waits are interrupted.
   *
   *
On the other hand, if a signalled thread wakes up to discover that its guard is still not
   * satisfied, it does *not* need to call this method before returning to wait. This can only
   * happen due to spurious wakeup (ignorable) or another thread acquiring the lock before the
   * current thread can and returning the guard to the unsatisfied state. In the latter case the
   * other thread (last thread modifying the state protected by the monitor) takes over the
   * responsibility of signalling the next waiter.
   *
   *
This method must not be called from within a beginWaitingFor/endWaitingFor block, or else
   * the current thread's guard might be mistakenly signalled, leading to a lost signal.
   */
  @GuardedBy("lock")
  private void signalNextWaiter() {
    for (Guard guard = activeGuards; guard != null; guard = guard.next) {
      if (isSatisfied(guard)) {
        guard.condition.signal();
        break;
      }
    }
  }

  /**
   * Exactly like signalNextWaiter, but caller guarantees that guardToSkip need not be considered,
   * because caller has previously checked that guardToSkip.isSatisfied() returned false. An
   * optimization for the case that guardToSkip.isSatisfied() may be expensive.
   *
   *
We decided against using this method, since in practice, isSatisfied() is likely to be very
   * cheap (typically one field read). Resurrect this method if you find that not to be true.
   */
  //   @GuardedBy("lock")
  //   private void signalNextWaiterSkipping(Guard guardToSkip) {
  //     for (Guard guard = activeGuards; guard != null; guard = guard.next) {
  //       if (guard != guardToSkip && isSatisfied(guard)) {
  //         guard.condition.signal();
  //         break;
  //       }
  //     }
  //   }

  /**
   * Exactly like guard.isSatisfied(), but in addition signals all waiting threads in the (hopefully
   * unlikely) event that isSatisfied() throws.
   */
  @GuardedBy("lock")
  private boolean isSatisfied(Guard guard) {
    try {
      return guard.isSatisfied();
    } catch (Throwable throwable) {
      signalAllWaiters();
      throw throwable;
    }
  }

  /** Signals all threads waiting on guards. */
  @GuardedBy("lock")
  private void signalAllWaiters() {
    for (Guard guard = activeGuards; guard != null; guard = guard.next) {
      guard.condition.signalAll();
    }
  }

  /** Records that the current thread is about to wait on the specified guard. */
  @GuardedBy("lock")
  private void beginWaitingFor(Guard guard) {
    int waiters = guard.waiterCount++;
    if (waiters == 0) {
      // push guard onto activeGuards
      guard.next = activeGuards;
      activeGuards = guard;
    }
  }

  /** Records that the current thread is no longer waiting on the specified guard. */
  @GuardedBy("lock")
  private void endWaitingFor(Guard guard) {
    int waiters = --guard.waiterCount;
    if (waiters == 0) {
      // unlink guard from activeGuards
      for (Guard p = activeGuards, pred = null; ; pred = p, p = p.next) {
        if (p == guard) {
          if (pred == null) {
            activeGuards = p.next;
          } else {
            pred.next = p.next;
          }
          p.next = null; // help GC
          break;
        }
      }
    }
  }

  /*
   * Methods that loop waiting on a guard's condition until the guard is satisfied, while recording
   * this fact so that other threads know to check our guard and signal us. It's caller's
   * responsibility to ensure that the guard is *not* currently satisfied.
   */

  @GuardedBy("lock")
  private void await(Guard guard, boolean signalBeforeWaiting) throws InterruptedException {
    if (signalBeforeWaiting) {
      signalNextWaiter();
    }
    beginWaitingFor(guard);
    try {
      do {
        guard.condition.await();
      } while (!guard.isSatisfied());
    } finally {
      endWaitingFor(guard);
    }
  }

  @GuardedBy("lock")
  private void awaitUninterruptibly(Guard guard, boolean signalBeforeWaiting) {
    if (signalBeforeWaiting) {
      signalNextWaiter();
    }
    beginWaitingFor(guard);
    try {
      do {
        guard.condition.awaitUninterruptibly();
      } while (!guard.isSatisfied());
    } finally {
      endWaitingFor(guard);
    }
  }

  /** Caller should check before calling that guard is not satisfied. */
  @GuardedBy("lock")
  private boolean awaitNanos(Guard guard, long nanos, boolean signalBeforeWaiting)
      throws InterruptedException {
    boolean firstTime = true;
    try {
      do {
        if (nanos <= 0L) {
          return false;
        }
        if (firstTime) {
          if (signalBeforeWaiting) {
            signalNextWaiter();
          }
          beginWaitingFor(guard);
          firstTime = false;
        }
        nanos = guard.condition.awaitNanos(nanos);
      } while (!guard.isSatisfied());
      return true;
    } finally {
      if (!firstTime) {
        endWaitingFor(guard);
      }
    }
  }
}


/*
 * Copyright (C) 2011 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.common.annotations.GwtIncompatible;
import java.io.Serializable;
import java.lang.ref.WeakReference;
import java.lang.reflect.Field;
import java.util.EnumSet;
import java.util.HashMap;
import java.util.Map;
import java.util.WeakHashMap;
import org.checkerframework.checker.nullness.compatqual.NullableDecl;

/**
 * Utility methods for working with {@link Enum} instances.
 *
 * @author Steve McKay
 * @since 9.0
 */
@GwtCompatible(emulated = true)
public final class Enums {

  private Enums() {}

  /**
   * Returns the {@link Field} in which {@code enumValue} is defined. For example, to get the {@code
   * Description} annotation on the {@code GOLF} constant of enum {@code Sport}, use {@code
   * Enums.getField(Sport.GOLF).getAnnotation(Description.class)}.
   *
   * @since 12.0
   */
  @GwtIncompatible // reflection
  public static Field getField(Enum<?> enumValue) {
    Class<?> clazz = enumValue.getDeclaringClass();
    try {
      return clazz.getDeclaredField(enumValue.name());
    } catch (NoSuchFieldException impossible) {
      throw new AssertionError(impossible);
    }
  }

  /**
   * Returns an optional enum constant for the given type, using {@link Enum#valueOf}. If the
   * constant does not exist, {@link Optional#absent} is returned. A common use case is for parsing
   * user input or falling back to a default enum constant. For example, {@code
   * Enums.getIfPresent(Country.class, countryInput).or(Country.DEFAULT);}
   *
   * @since 12.0
   */
  public static <T extends Enum<T>> Optional<T> getIfPresent(Class<T> enumClass, String value) {
    checkNotNull(enumClass);
    checkNotNull(value);
    return Platform.getEnumIfPresent(enumClass, value);
  }

  @GwtIncompatible // java.lang.ref.WeakReference
  private static final Map<Class<? extends Enum<?>>, Map<String, WeakReference<? extends Enum<?>>>>
      enumConstantCache = new WeakHashMap<>();

  @GwtIncompatible // java.lang.ref.WeakReference
  private static <T extends Enum<T>> Map<String, WeakReference<? extends Enum<?>>> populateCache(
      Class<T> enumClass) {
    Map<String, WeakReference<? extends Enum<?>>> result = new HashMap<>();
    for (T enumInstance : EnumSet.allOf(enumClass)) {
      result.put(enumInstance.name(), new WeakReference<Enum<?>>(enumInstance));
    }
    enumConstantCache.put(enumClass, result);
    return result;
  }

  @GwtIncompatible // java.lang.ref.WeakReference
  static <T extends Enum<T>> Map<String, WeakReference<? extends Enum<?>>> getEnumConstants(
      Class<T> enumClass) {
    synchronized (enumConstantCache) {
      Map<String, WeakReference<? extends Enum<?>>> constants = enumConstantCache.get(enumClass);
      if (constants == null) {
        constants = populateCache(enumClass);
      }
      return constants;
    }
  }

  /**
   * Returns a converter that converts between strings and {@code enum} values of type {@code
   * enumClass} using {@link Enum#valueOf(Class, String)} and {@link Enum#name()}. The converter
   * will throw an {@code IllegalArgumentException} if the argument is not the name of any enum
   * constant in the specified enum.
   *
   * @since 16.0
   */
  public static <T extends Enum<T>> Converter<String, T> stringConverter(final Class<T> enumClass) {
    return new StringConverter<T>(enumClass);
  }

  private static final class StringConverter<T extends Enum<T>> extends Converter<String, T>
      implements Serializable {

    private final Class<T> enumClass;

    StringConverter(Class<T> enumClass) {
      this.enumClass = checkNotNull(enumClass);
    }

    @Override
    protected T doForward(String value) {
      return Enum.valueOf(enumClass, value);
    }

    @Override
    protected String doBackward(T enumValue) {
      return enumValue.name();
    }

    @Override
    public boolean equals(@NullableDecl Object object) {
      if (object instanceof StringConverter) {
        StringConverter<?> that = (StringConverter<?>) object;
        return this.enumClass.equals(that.enumClass);
      }
      return false;
    }

    @Override
    public int hashCode() {
      return enumClass.hashCode();
    }

    @Override
    public String toString() {
      return "Enums.stringConverter(" + enumClass.getName() + ".class)";
    }

    private static final long serialVersionUID = 0L;
  }
}