/*
* 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 static com.google.common.collect.CollectPreconditions.checkNonnegative;
import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;
import java.util.Comparator;
import java.util.Iterator;
import java.util.List;
import java.util.Optional;
import java.util.stream.Collector;
/**
* 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. Because of
* this, and because it's awkward to have to convert comparators into {@code Ordering} instances,
* {@code Ordering} and its methods are planned for deletion. 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 a {@code Collector} that returns the {@code k} smallest (relative to the specified
* {@code Comparator}) input elements, in ascending order, as an unmodifiable {@code List}.
* Ties are broken arbitrarily.
*
* For example:
* <pre> {@code
*
* Stream.of("foo", "quux", "banana", "elephant")
* .collect(least(2, comparingInt(String::length)))
* // returns {"foo", "quux"}}</pre>
*
* <p>This {@code Collector} uses O(k) memory and takes expected time O(n)
* (worst-case O(n log k)), as opposed to e.g. {@code Stream.sorted(comparator).limit(k)}, which
* currently takes O(n log n) time and O(n) space.
*
* @throws IllegalArgumentException if {@code k < 0}
* @since 22.0
*/
public static <T> Collector<T, ?, List<T>> least(int k, Comparator<? super T> comparator) {
checkNonnegative(k, "k");
checkNotNull(comparator);
return Collector.of(
() -> TopKSelector.<T>least(k, comparator),
TopKSelector::offer,
TopKSelector::combine,
TopKSelector::topK,
Collector.Characteristics.UNORDERED);
}
/**
* Returns a {@code Collector} that returns the {@code k} greatest (relative to the specified
* {@code Comparator}) input elements, in descending order, as an unmodifiable {@code List}.
* Ties are broken arbitrarily.
*
* For example:
* <pre> {@code
*
* Stream.of("foo", "quux", "banana", "elephant")
* .collect(greatest(2, comparingInt(String::length)))
* // returns {"elephant", "banana"}}</pre>
*
* <p>This {@code Collector} uses O(k) memory and takes expected time O(n)
* (worst-case O(n log k)), as opposed to e.g.
* {@code Stream.sorted(comparator.reversed()).limit(k)}, which currently takes O(n log n) time
* and O(n) space.
*
* @throws IllegalArgumentException if {@code k < 0}
* @since 22.0
*/
public static <T> Collector<T, ?, List<T>> greatest(int k, Comparator<? super T> comparator) {
return least(k, comparator.reversed());
}
/**
* Returns a comparator of {@link Optional} values which treats {@link Optional#empty} as less
* than all other values, and orders the rest using {@code valueComparator} on the contained
* value.
*
* @since 22.0
*/
@Beta
public static <T> Comparator<Optional<T>> emptiesFirst(Comparator<T> valueComparator) {
checkNotNull(valueComparator);
return Comparator.comparing(o -> o.orElse(null), Comparator.nullsFirst(valueComparator));
}
/**
* Returns a comparator of {@link Optional} values which treats {@link Optional#empty} as greater
* than all other values, and orders the rest using {@code valueComparator} on the contained
* value.
*
* @since 22.0
*/
@Beta
public static <T> Comparator<Optional<T>> emptiesLast(Comparator<T> valueComparator) {
checkNotNull(valueComparator);
return Comparator.comparing(o -> o.orElse(null), Comparator.nullsLast(valueComparator));
}
}