android/guava/src/com/google/common/collect/Comparators.java

/*
 * Copyright (C) 2016 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.collect;

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

import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;
import java.util.Comparator;
import java.util.Iterator;
import org.checkerframework.checker.nullness.compatqual.NullableDecl;

/**
 * Provides static methods for working with {@link Comparator} instances. For many other helpful
 * comparator utilities, see either {@code Comparator} itself (for Java 8 or later), or {@code
 * com.google.common.collect.Ordering} (otherwise).
 *
 * <h3>Relationship to {@code Ordering}</h3>
 *
 * <p>In light of the significant enhancements to {@code Comparator} in Java 8, the overwhelming
 * majority of usages of {@code Ordering} can be written using only built-in JDK APIs. This class is
 * intended to "fill the gap" and provide those features of {@code Ordering} not already provided by
 * the JDK.
 *
 * @since 21.0
 * @author Louis Wasserman
 */
@Beta
@GwtCompatible
public final class Comparators {
  private Comparators() {}

  /**
   * Returns a new comparator 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]}.
   *
   * <p>Note that {@code Collections.reverseOrder(lexicographical(comparator))} is not equivalent to
   * {@code lexicographical(Collections.reverseOrder(comparator))} (consider how each would order
   * {@code [1]} and {@code [1, 1]}).
   */
  // Note: 90% of the time we don't add type parameters or wildcards that serve only to "tweak" the
  // desired return type. However, *nested* generics introduce a special class of problems that we
  // think tip it over into being worthwhile.
  public static <T, S extends T> Comparator<Iterable<S>> lexicographical(Comparator<T> comparator) {
    return new LexicographicalOrdering<S>(checkNotNull(comparator));
  }

  /**
   * 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 the specified comparator. Note that this is
   * always true when the iterable has fewer than two elements.
   */
  public static <T> boolean isInOrder(Iterable<? extends T> iterable, Comparator<T> comparator) {
    checkNotNull(comparator);
    Iterator<? extends T> it = iterable.iterator();
    if (it.hasNext()) {
      T prev = it.next();
      while (it.hasNext()) {
        T next = it.next();
        if (comparator.compare(prev, next) > 0) {
          return false;
        }
        prev = next;
      }
    }
    return true;
  }

  /**
   * Returns {@code true} if each element in {@code iterable} after the first is <i>strictly</i>
   * greater than the element that preceded it, according to the specified comparator. Note that
   * this is always true when the iterable has fewer than two elements.
   */
  public static <T> boolean isInStrictOrder(
      Iterable<? extends T> iterable, Comparator<T> comparator) {
    checkNotNull(comparator);
    Iterator<? extends T> it = iterable.iterator();
    if (it.hasNext()) {
      T prev = it.next();
      while (it.hasNext()) {
        T next = it.next();
        if (comparator.compare(prev, next) >= 0) {
          return false;
        }
        prev = next;
      }
    }
    return true;
  }

  /**
   * Returns the minimum of the two values. If the values compare as 0, the first is returned.
   *
   * <p>The recommended solution for finding the {@code minimum} of some values depends on the type
   * of your data and the number of elements you have. Read more in the Guava User Guide article on
   * <a href="https://github.com/google/guava/wiki/CollectionUtilitiesExplained#comparators">{@code
   * Comparators}</a>.
   *
   * @param a first value to compare, returned if less than or equal to b.
   * @param b second value to compare.
   * @throws ClassCastException if the parameters are not <i>mutually comparable</i>.
   * @since 30.0
   */
  @Beta
  public static <T extends Comparable<? super T>> T min(T a, T b) {
    return (a.compareTo(b) <= 0) ? a : b;
  }

  /**
   * Returns the minimum of the two values, according to the given comparator. If the values compare
   * as equal, the first is returned.
   *
   * <p>The recommended solution for finding the {@code minimum} of some values depends on the type
   * of your data and the number of elements you have. Read more in the Guava User Guide article on
   * <a href="https://github.com/google/guava/wiki/CollectionUtilitiesExplained#comparators">{@code
   * Comparators}</a>.
   *
   * @param a first value to compare, returned if less than or equal to b
   * @param b second value to compare.
   * @throws ClassCastException if the parameters are not <i>mutually comparable</i> using the given
   *     comparator.
   * @since 30.0
   */
  @Beta
  public static <T> T min(@NullableDecl T a, @NullableDecl T b, Comparator<T> comparator) {
    return (comparator.compare(a, b) <= 0) ? a : b;
  }

  /**
   * Returns the maximum of the two values. If the values compare as 0, the first is returned.
   *
   * <p>The recommended solution for finding the {@code maximum} of some values depends on the type
   * of your data and the number of elements you have. Read more in the Guava User Guide article on
   * <a href="https://github.com/google/guava/wiki/CollectionUtilitiesExplained#comparators">{@code
   * Comparators}</a>.
   *
   * @param a first value to compare, returned if greater than or equal to b.
   * @param b second value to compare.
   * @throws ClassCastException if the parameters are not <i>mutually comparable</i>.
   * @since 30.0
   */
  @Beta
  public static <T extends Comparable<? super T>> T max(T a, T b) {
    return (a.compareTo(b) >= 0) ? a : b;
  }

  /**
   * Returns the maximum of the two values, according to the given comparator. If the values compare
   * as equal, the first is returned.
   *
   * <p>The recommended solution for finding the {@code maximum} of some values depends on the type
   * of your data and the number of elements you have. Read more in the Guava User Guide article on
   * <a href="https://github.com/google/guava/wiki/CollectionUtilitiesExplained#comparators">{@code
   * Comparators}</a>.
   *
   * @param a first value to compare, returned if greater than or equal to b.
   * @param b second value to compare.
   * @throws ClassCastException if the parameters are not <i>mutually comparable</i> using the given
   *     comparator.
   * @since 30.0
   */
  @Beta
  public static <T> T max(@NullableDecl T a, @NullableDecl T b, Comparator<T> comparator) {
    return (comparator.compare(a, b) >= 0) ? a : b;
  }
}