001/*
002 * Copyright (C) 2007 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License");
005 * you may not use this file except in compliance with the License.
006 * You may obtain a copy of the License at
007 *
008 * http://www.apache.org/licenses/LICENSE-2.0
009 *
010 * Unless required by applicable law or agreed to in writing, software
011 * distributed under the License is distributed on an "AS IS" BASIS,
012 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013 * See the License for the specific language governing permissions and
014 * limitations under the License.
015 */
016
017package com.google.common.collect;
018
019import static com.google.common.base.Preconditions.checkNotNull;
020import static com.google.common.collect.CollectPreconditions.checkNonnegative;
021
022import com.google.common.annotations.GwtCompatible;
023import com.google.common.annotations.J2ktIncompatible;
024import com.google.common.annotations.VisibleForTesting;
025import com.google.common.base.Function;
026import java.util.ArrayList;
027import java.util.Arrays;
028import java.util.Collection;
029import java.util.Collections;
030import java.util.Comparator;
031import java.util.HashSet;
032import java.util.Iterator;
033import java.util.List;
034import java.util.Map.Entry;
035import java.util.NoSuchElementException;
036import java.util.SortedMap;
037import java.util.SortedSet;
038import java.util.TreeSet;
039import java.util.concurrent.ConcurrentMap;
040import java.util.concurrent.atomic.AtomicInteger;
041import javax.annotation.CheckForNull;
042import org.checkerframework.checker.nullness.qual.NonNull;
043import org.checkerframework.checker.nullness.qual.Nullable;
044
045/**
046 * A comparator, with additional methods to support common operations. This is an "enriched" version
047 * of {@code Comparator} for pre-Java-8 users, in the same sense that {@link FluentIterable} is an
048 * enriched {@link Iterable} for pre-Java-8 users.
049 *
050 * <h3>Three types of methods</h3>
051 *
052 * Like other fluent types, there are three types of methods present: methods for <i>acquiring</i>,
053 * <i>chaining</i>, and <i>using</i>.
054 *
055 * <h4>Acquiring</h4>
056 *
057 * <p>The common ways to get an instance of {@code Ordering} are:
058 *
059 * <ul>
060 *   <li>Subclass it and implement {@link #compare} instead of implementing {@link Comparator}
061 *       directly
062 *   <li>Pass a <i>pre-existing</i> {@link Comparator} instance to {@link #from(Comparator)}
063 *   <li>Use the natural ordering, {@link Ordering#natural}
064 * </ul>
065 *
066 * <h4>Chaining</h4>
067 *
068 * <p>Then you can use the <i>chaining</i> methods to get an altered version of that {@code
069 * Ordering}, including:
070 *
071 * <ul>
072 *   <li>{@link #reverse}
073 *   <li>{@link #compound(Comparator)}
074 *   <li>{@link #onResultOf(Function)}
075 *   <li>{@link #nullsFirst} / {@link #nullsLast}
076 * </ul>
077 *
078 * <h4>Using</h4>
079 *
080 * <p>Finally, use the resulting {@code Ordering} anywhere a {@link Comparator} is required, or use
081 * any of its special operations, such as:
082 *
083 * <ul>
084 *   <li>{@link #immutableSortedCopy}
085 *   <li>{@link #isOrdered} / {@link #isStrictlyOrdered}
086 *   <li>{@link #min} / {@link #max}
087 * </ul>
088 *
089 * <h3>Understanding complex orderings</h3>
090 *
091 * <p>Complex chained orderings like the following example can be challenging to understand.
092 *
093 * <pre>{@code
094 * Ordering<Foo> ordering =
095 *     Ordering.natural()
096 *         .nullsFirst()
097 *         .onResultOf(getBarFunction)
098 *         .nullsLast();
099 * }</pre>
100 *
101 * Note that each chaining method returns a new ordering instance which is backed by the previous
102 * instance, but has the chance to act on values <i>before</i> handing off to that backing instance.
103 * As a result, it usually helps to read chained ordering expressions <i>backwards</i>. For example,
104 * when {@code compare} is called on the above ordering:
105 *
106 * <ol>
107 *   <li>First, if only one {@code Foo} is null, that null value is treated as <i>greater</i>
108 *   <li>Next, non-null {@code Foo} values are passed to {@code getBarFunction} (we will be
109 *       comparing {@code Bar} values from now on)
110 *   <li>Next, if only one {@code Bar} is null, that null value is treated as <i>lesser</i>
111 *   <li>Finally, natural ordering is used (i.e. the result of {@code Bar.compareTo(Bar)} is
112 *       returned)
113 * </ol>
114 *
115 * <p>Alas, {@link #reverse} is a little different. As you read backwards through a chain and
116 * encounter a call to {@code reverse}, continue working backwards until a result is determined, and
117 * then reverse that result.
118 *
119 * <h3>Additional notes</h3>
120 *
121 * <p>Except as noted, the orderings returned by the factory methods of this class are serializable
122 * if and only if the provided instances that back them are. For example, if {@code ordering} and
123 * {@code function} can themselves be serialized, then {@code ordering.onResultOf(function)} can as
124 * well.
125 *
126 * <h3>For Java 8 users</h3>
127 *
128 * <p>If you are using Java 8, this class is now obsolete. Most of its functionality is now provided
129 * by {@link java.util.stream.Stream Stream} and by {@link Comparator} itself, and the rest can now
130 * be found as static methods in our new {@link Comparators} class. See each method below for
131 * further instructions. Whenever possible, you should change any references of type {@code
132 * Ordering} to be of type {@code Comparator} instead. However, at this time we have no plan to
133 * <i>deprecate</i> this class.
134 *
135 * <p>Many replacements involve adopting {@code Stream}, and these changes can sometimes make your
136 * code verbose. Whenever following this advice, you should check whether {@code Stream} could be
137 * adopted more comprehensively in your code; the end result may be quite a bit simpler.
138 *
139 * <h3>See also</h3>
140 *
141 * <p>See the Guava User Guide article on <a href=
142 * "https://github.com/google/guava/wiki/OrderingExplained">{@code Ordering}</a>.
143 *
144 * @author Jesse Wilson
145 * @author Kevin Bourrillion
146 * @since 2.0
147 */
148@GwtCompatible
149@ElementTypesAreNonnullByDefault
150public abstract class Ordering<T extends @Nullable Object> implements Comparator<T> {
151  // Natural order
152
153  /**
154   * Returns a serializable ordering that uses the natural order of the values. The ordering throws
155   * a {@link NullPointerException} when passed a null parameter.
156   *
157   * <p>The type specification is {@code <C extends Comparable>}, instead of the technically correct
158   * {@code <C extends Comparable<? super C>>}, to support legacy types from before Java 5.
159   *
160   * <p><b>Java 8 users:</b> use {@link Comparator#naturalOrder} instead.
161   */
162  @GwtCompatible(serializable = true)
163  @SuppressWarnings("unchecked") // TODO(kevinb): right way to explain this??
164  public static <C extends Comparable> Ordering<C> natural() {
165    return (Ordering<C>) NaturalOrdering.INSTANCE;
166  }
167
168  // Static factories
169
170  /**
171   * Returns an ordering based on an <i>existing</i> comparator instance. Note that it is
172   * unnecessary to create a <i>new</i> anonymous inner class implementing {@code Comparator} just
173   * to pass it in here. Instead, simply subclass {@code Ordering} and implement its {@code compare}
174   * method directly.
175   *
176   * <p>The returned object is serializable if {@code comparator} is serializable.
177   *
178   * <p><b>Java 8 users:</b> this class is now obsolete as explained in the class documentation, so
179   * there is no need to use this method.
180   *
181   * @param comparator the comparator that defines the order
182   * @return comparator itself if it is already an {@code Ordering}; otherwise an ordering that
183   *     wraps that comparator
184   */
185  @GwtCompatible(serializable = true)
186  public static <T extends @Nullable Object> Ordering<T> from(Comparator<T> comparator) {
187    return (comparator instanceof Ordering)
188        ? (Ordering<T>) comparator
189        : new ComparatorOrdering<T>(comparator);
190  }
191
192  /**
193   * Simply returns its argument.
194   *
195   * @deprecated no need to use this
196   */
197  @GwtCompatible(serializable = true)
198  @Deprecated
199  public static <T extends @Nullable Object> Ordering<T> from(Ordering<T> ordering) {
200    return checkNotNull(ordering);
201  }
202
203  /**
204   * Returns an ordering that compares objects according to the order in which they appear in the
205   * given list. Only objects present in the list (according to {@link Object#equals}) may be
206   * compared. This comparator imposes a "partial ordering" over the type {@code T}. Subsequent
207   * changes to the {@code valuesInOrder} list will have no effect on the returned comparator. Null
208   * values in the list are not supported.
209   *
210   * <p>The returned comparator throws a {@link ClassCastException} when it receives an input
211   * parameter that isn't among the provided values.
212   *
213   * <p>The generated comparator is serializable if all the provided values are serializable.
214   *
215   * @param valuesInOrder the values that the returned comparator will be able to compare, in the
216   *     order the comparator should induce
217   * @return the comparator described above
218   * @throws NullPointerException if any of the provided values is null
219   * @throws IllegalArgumentException if {@code valuesInOrder} contains any duplicate values
220   *     (according to {@link Object#equals})
221   */
222  // TODO(kevinb): provide replacement
223  @GwtCompatible(serializable = true)
224  public static <T> Ordering<T> explicit(List<T> valuesInOrder) {
225    return new ExplicitOrdering<T>(valuesInOrder);
226  }
227
228  /**
229   * Returns an ordering that compares objects according to the order in which they are given to
230   * this method. Only objects present in the argument list (according to {@link Object#equals}) may
231   * be compared. This comparator imposes a "partial ordering" over the type {@code T}. Null values
232   * in the argument list are not supported.
233   *
234   * <p>The returned comparator throws a {@link ClassCastException} when it receives an input
235   * parameter that isn't among the provided values.
236   *
237   * <p>The generated comparator is serializable if all the provided values are serializable.
238   *
239   * @param leastValue the value which the returned comparator should consider the "least" of all
240   *     values
241   * @param remainingValuesInOrder the rest of the values that the returned comparator will be able
242   *     to compare, in the order the comparator should follow
243   * @return the comparator described above
244   * @throws NullPointerException if any of the provided values is null
245   * @throws IllegalArgumentException if any duplicate values (according to {@link
246   *     Object#equals(Object)}) are present among the method arguments
247   */
248  // TODO(kevinb): provide replacement
249  @GwtCompatible(serializable = true)
250  public static <T> Ordering<T> explicit(T leastValue, T... remainingValuesInOrder) {
251    return explicit(Lists.asList(leastValue, remainingValuesInOrder));
252  }
253
254  // Ordering<Object> singletons
255
256  /**
257   * Returns an ordering which treats all values as equal, indicating "no ordering." Passing this
258   * ordering to any <i>stable</i> sort algorithm results in no change to the order of elements.
259   * Note especially that {@link #sortedCopy} and {@link #immutableSortedCopy} are stable, and in
260   * the returned instance these are implemented by simply copying the source list.
261   *
262   * <p>Example:
263   *
264   * <pre>{@code
265   * Ordering.allEqual().nullsLast().sortedCopy(
266   *     asList(t, null, e, s, null, t, null))
267   * }</pre>
268   *
269   * <p>Assuming {@code t}, {@code e} and {@code s} are non-null, this returns {@code [t, e, s, t,
270   * null, null, null]} regardless of the true comparison order of those three values (which might
271   * not even implement {@link Comparable} at all).
272   *
273   * <p><b>Warning:</b> by definition, this comparator is not <i>consistent with equals</i> (as
274   * defined {@linkplain Comparator here}). Avoid its use in APIs, such as {@link
275   * TreeSet#TreeSet(Comparator)}, where such consistency is expected.
276   *
277   * <p>The returned comparator is serializable.
278   *
279   * <p><b>Java 8 users:</b> Use the lambda expression {@code (a, b) -> 0} instead (in certain cases
280   * you may need to cast that to {@code Comparator<YourType>}).
281   *
282   * @since 13.0
283   */
284  @GwtCompatible(serializable = true)
285  @SuppressWarnings("unchecked")
286  public static Ordering<@Nullable Object> allEqual() {
287    return AllEqualOrdering.INSTANCE;
288  }
289
290  /**
291   * Returns an ordering that compares objects by the natural ordering of their string
292   * representations as returned by {@code toString()}. It does not support null values.
293   *
294   * <p>The comparator is serializable.
295   *
296   * <p><b>Java 8 users:</b> Use {@code Comparator.comparing(Object::toString)} instead.
297   */
298  @GwtCompatible(serializable = true)
299  public static Ordering<Object> usingToString() {
300    return UsingToStringOrdering.INSTANCE;
301  }
302
303  /**
304   * Returns an arbitrary ordering over all objects, for which {@code compare(a, b) == 0} implies
305   * {@code a == b} (identity equality). There is no meaning whatsoever to the order imposed, but it
306   * is constant for the life of the VM.
307   *
308   * <p>Because the ordering is identity-based, it is not "consistent with {@link
309   * Object#equals(Object)}" as defined by {@link Comparator}. Use caution when building a {@link
310   * SortedSet} or {@link SortedMap} from it, as the resulting collection will not behave exactly
311   * according to spec.
312   *
313   * <p>This ordering is not serializable, as its implementation relies on {@link
314   * System#identityHashCode(Object)}, so its behavior cannot be preserved across serialization.
315   *
316   * @since 2.0
317   */
318  // TODO(kevinb): copy to Comparators, etc.
319  @J2ktIncompatible // MapMaker
320  public static Ordering<@Nullable Object> arbitrary() {
321    return ArbitraryOrderingHolder.ARBITRARY_ORDERING;
322  }
323
324  @J2ktIncompatible // MapMaker
325  private static class ArbitraryOrderingHolder {
326    static final Ordering<@Nullable Object> ARBITRARY_ORDERING = new ArbitraryOrdering();
327  }
328
329  @J2ktIncompatible // MapMaker
330  @VisibleForTesting
331  static class ArbitraryOrdering extends Ordering<@Nullable Object> {
332
333    private final AtomicInteger counter = new AtomicInteger(0);
334    private final ConcurrentMap<Object, Integer> uids =
335        Platform.tryWeakKeys(new MapMaker()).makeMap();
336
337    private Integer getUid(Object obj) {
338      Integer uid = uids.get(obj);
339      if (uid == null) {
340        // One or more integer values could be skipped in the event of a race
341        // to generate a UID for the same object from multiple threads, but
342        // that shouldn't be a problem.
343        uid = counter.getAndIncrement();
344        Integer alreadySet = uids.putIfAbsent(obj, uid);
345        if (alreadySet != null) {
346          uid = alreadySet;
347        }
348      }
349      return uid;
350    }
351
352    @Override
353    public int compare(@CheckForNull Object left, @CheckForNull Object right) {
354      if (left == right) {
355        return 0;
356      } else if (left == null) {
357        return -1;
358      } else if (right == null) {
359        return 1;
360      }
361      int leftCode = identityHashCode(left);
362      int rightCode = identityHashCode(right);
363      if (leftCode != rightCode) {
364        return leftCode < rightCode ? -1 : 1;
365      }
366
367      // identityHashCode collision (rare, but not as rare as you'd think)
368      int result = getUid(left).compareTo(getUid(right));
369      if (result == 0) {
370        throw new AssertionError(); // extremely, extremely unlikely.
371      }
372      return result;
373    }
374
375    @Override
376    public String toString() {
377      return "Ordering.arbitrary()";
378    }
379
380    /*
381     * We need to be able to mock identityHashCode() calls for tests, because it
382     * can take 1-10 seconds to find colliding objects. Mocking frameworks that
383     * can do magic to mock static method calls still can't do so for a system
384     * class, so we need the indirection. In production, Hotspot should still
385     * recognize that the call is 1-morphic and should still be willing to
386     * inline it if necessary.
387     */
388    int identityHashCode(Object object) {
389      return System.identityHashCode(object);
390    }
391  }
392
393  // Constructor
394
395  /**
396   * Constructs a new instance of this class (only invokable by the subclass constructor, typically
397   * implicit).
398   */
399  protected Ordering() {}
400
401  // Instance-based factories (and any static equivalents)
402
403  /**
404   * Returns the reverse of this ordering; the {@code Ordering} equivalent to {@link
405   * Collections#reverseOrder(Comparator)}.
406   *
407   * <p><b>Java 8 users:</b> Use {@code thisComparator.reversed()} instead.
408   */
409  // type parameter <S> lets us avoid the extra <String> in statements like:
410  // Ordering<String> o = Ordering.<String>natural().reverse();
411  @GwtCompatible(serializable = true)
412  public <S extends T> Ordering<S> reverse() {
413    return new ReverseOrdering<S>(this);
414  }
415
416  /**
417   * Returns an ordering that treats {@code null} as less than all other values and uses {@code
418   * this} to compare non-null values.
419   *
420   * <p>The returned object is serializable if this object is serializable.
421   *
422   * <p><b>Java 8 users:</b> Use {@code Comparator.nullsFirst(thisComparator)} instead.
423   */
424  // type parameter <S> lets us avoid the extra <String> in statements like:
425  // Ordering<String> o = Ordering.<String>natural().nullsFirst();
426  @GwtCompatible(serializable = true)
427  public <S extends T> Ordering<@Nullable S> nullsFirst() {
428    return new NullsFirstOrdering<S>(this);
429  }
430
431  /**
432   * Returns an ordering that treats {@code null} as greater than all other values and uses this
433   * ordering to compare non-null values.
434   *
435   * <p>The returned object is serializable if this object is serializable.
436   *
437   * <p><b>Java 8 users:</b> Use {@code Comparator.nullsLast(thisComparator)} instead.
438   */
439  // type parameter <S> lets us avoid the extra <String> in statements like:
440  // Ordering<String> o = Ordering.<String>natural().nullsLast();
441  @GwtCompatible(serializable = true)
442  public <S extends T> Ordering<@Nullable S> nullsLast() {
443    return new NullsLastOrdering<S>(this);
444  }
445
446  /**
447   * Returns a new ordering on {@code F} which orders elements by first applying a function to them,
448   * then comparing those results using {@code this}. For example, to compare objects by their
449   * string forms, in a case-insensitive manner, use:
450   *
451   * <pre>{@code
452   * Ordering.from(String.CASE_INSENSITIVE_ORDER)
453   *     .onResultOf(Functions.toStringFunction())
454   * }</pre>
455   *
456   * <p><b>Java 8 users:</b> Use {@code Comparator.comparing(function, thisComparator)} instead (you
457   * can omit the comparator if it is the natural order).
458   */
459  @GwtCompatible(serializable = true)
460  public <F extends @Nullable Object> Ordering<F> onResultOf(Function<F, ? extends T> function) {
461    return new ByFunctionOrdering<>(function, this);
462  }
463
464  <T2 extends T> Ordering<Entry<T2, ?>> onKeys() {
465    return onResultOf(Maps.<T2>keyFunction());
466  }
467
468  /**
469   * Returns an ordering which first uses the ordering {@code this}, but which in the event of a
470   * "tie", then delegates to {@code secondaryComparator}. For example, to sort a bug list first by
471   * status and second by priority, you might use {@code byStatus.compound(byPriority)}. For a
472   * compound ordering with three or more components, simply chain multiple calls to this method.
473   *
474   * <p>An ordering produced by this method, or a chain of calls to this method, is equivalent to
475   * one created using {@link Ordering#compound(Iterable)} on the same component comparators.
476   *
477   * <p>The returned object is serializable if this object and {@code secondaryComparator} are both
478   * serializable.
479   *
480   * <p><b>Java 8 users:</b> Use {@code thisComparator.thenComparing(secondaryComparator)} instead.
481   * Depending on what {@code secondaryComparator} is, one of the other overloads of {@code
482   * thenComparing} may be even more useful.
483   */
484  @GwtCompatible(serializable = true)
485  public <U extends T> Ordering<U> compound(Comparator<? super U> secondaryComparator) {
486    return new CompoundOrdering<U>(this, checkNotNull(secondaryComparator));
487  }
488
489  /**
490   * Returns an ordering which tries each given comparator in order until a non-zero result is
491   * found, returning that result, and returning zero only if all comparators return zero. The
492   * returned ordering is based on the state of the {@code comparators} iterable at the time it was
493   * provided to this method.
494   *
495   * <p>The returned ordering is equivalent to that produced using {@code
496   * Ordering.from(comp1).compound(comp2).compound(comp3) . . .}.
497   *
498   * <p>The returned object is serializable if each of the {@code comparators} is serializable.
499   *
500   * <p><b>Warning:</b> Supplying an argument with undefined iteration order, such as a {@link
501   * HashSet}, will produce non-deterministic results.
502   *
503   * <p><b>Java 8 users:</b> Use a chain of calls to {@link Comparator#thenComparing(Comparator)},
504   * or {@code comparatorCollection.stream().reduce(Comparator::thenComparing).get()} (if the
505   * collection might be empty, also provide a default comparator as the {@code identity} parameter
506   * to {@code reduce}).
507   *
508   * @param comparators the comparators to try in order
509   */
510  @GwtCompatible(serializable = true)
511  public static <T extends @Nullable Object> Ordering<T> compound(
512      Iterable<? extends Comparator<? super T>> comparators) {
513    return new CompoundOrdering<T>(comparators);
514  }
515
516  /**
517   * Returns a new ordering which sorts iterables by comparing corresponding elements pairwise until
518   * a nonzero result is found; imposes "dictionary order". If the end of one iterable is reached,
519   * but not the other, the shorter iterable is considered to be less than the longer one. For
520   * example, a lexicographical natural ordering over integers considers {@code [] < [1] < [1, 1] <
521   * [1, 2] < [2]}.
522   *
523   * <p>Note that {@code ordering.lexicographical().reverse()} is not equivalent to {@code
524   * ordering.reverse().lexicographical()} (consider how each would order {@code [1]} and {@code [1,
525   * 1]}).
526   *
527   * <p><b>Java 8 users:</b> Use {@link Comparators#lexicographical(Comparator)} instead.
528   *
529   * @since 2.0
530   */
531  @GwtCompatible(serializable = true)
532  // type parameter <S> lets us avoid the extra <String> in statements like:
533  // Ordering<Iterable<String>> o =
534  //     Ordering.<String>natural().lexicographical();
535  public <S extends T> Ordering<Iterable<S>> lexicographical() {
536    /*
537     * Note that technically the returned ordering should be capable of
538     * handling not just {@code Iterable<S>} instances, but also any {@code
539     * Iterable<? extends S>}. However, the need for this comes up so rarely
540     * that it doesn't justify making everyone else deal with the very ugly
541     * wildcard.
542     */
543    return new LexicographicalOrdering<S>(this);
544  }
545
546  // Regular instance methods
547
548  @Override
549  public abstract int compare(@ParametricNullness T left, @ParametricNullness T right);
550
551  /**
552   * Returns the least of the specified values according to this ordering. If there are multiple
553   * least values, the first of those is returned. The iterator will be left exhausted: its {@code
554   * hasNext()} method will return {@code false}.
555   *
556   * <p><b>Java 8 users:</b> Use {@code Streams.stream(iterator).min(thisComparator).get()} instead
557   * (but note that it does not guarantee which tied minimum element is returned).
558   *
559   * @param iterator the iterator whose minimum element is to be determined
560   * @throws NoSuchElementException if {@code iterator} is empty
561   * @throws ClassCastException if the parameters are not <i>mutually comparable</i> under this
562   *     ordering.
563   * @since 11.0
564   */
565  @ParametricNullness
566  public <E extends T> E min(Iterator<E> iterator) {
567    // let this throw NoSuchElementException as necessary
568    E minSoFar = iterator.next();
569
570    while (iterator.hasNext()) {
571      minSoFar = min(minSoFar, iterator.next());
572    }
573
574    return minSoFar;
575  }
576
577  /**
578   * Returns the least of the specified values according to this ordering. If there are multiple
579   * least values, the first of those is returned.
580   *
581   * <p><b>Java 8 users:</b> If {@code iterable} is a {@link Collection}, use {@code
582   * Collections.min(collection, thisComparator)} instead. Otherwise, use {@code
583   * Streams.stream(iterable).min(thisComparator).get()} instead. Note that these alternatives do
584   * not guarantee which tied minimum element is returned.
585   *
586   * @param iterable the iterable whose minimum element is to be determined
587   * @throws NoSuchElementException if {@code iterable} is empty
588   * @throws ClassCastException if the parameters are not <i>mutually comparable</i> under this
589   *     ordering.
590   */
591  @ParametricNullness
592  public <E extends T> E min(Iterable<E> iterable) {
593    return min(iterable.iterator());
594  }
595
596  /**
597   * Returns the lesser of the two values according to this ordering. If the values compare as 0,
598   * the first is returned.
599   *
600   * <p><b>Implementation note:</b> this method is invoked by the default implementations of the
601   * other {@code min} overloads, so overriding it will affect their behavior.
602   *
603   * <p><b>Note:</b> Consider using {@code Comparators.min(a, b, thisComparator)} instead. If {@code
604   * thisComparator} is {@link Ordering#natural}, then use {@code Comparators.min(a, b)}.
605   *
606   * @param a value to compare, returned if less than or equal to b.
607   * @param b value to compare.
608   * @throws ClassCastException if the parameters are not <i>mutually comparable</i> under this
609   *     ordering.
610   */
611  @ParametricNullness
612  public <E extends T> E min(@ParametricNullness E a, @ParametricNullness E b) {
613    return (compare(a, b) <= 0) ? a : b;
614  }
615
616  /**
617   * Returns the least of the specified values according to this ordering. If there are multiple
618   * least values, the first of those is returned.
619   *
620   * <p><b>Java 8 users:</b> Use {@code Collections.min(Arrays.asList(a, b, c...), thisComparator)}
621   * instead (but note that it does not guarantee which tied minimum element is returned).
622   *
623   * @param a value to compare, returned if less than or equal to the rest.
624   * @param b value to compare
625   * @param c value to compare
626   * @param rest values to compare
627   * @throws ClassCastException if the parameters are not <i>mutually comparable</i> under this
628   *     ordering.
629   */
630  @ParametricNullness
631  public <E extends T> E min(
632      @ParametricNullness E a, @ParametricNullness E b, @ParametricNullness E c, E... rest) {
633    E minSoFar = min(min(a, b), c);
634
635    for (E r : rest) {
636      minSoFar = min(minSoFar, r);
637    }
638
639    return minSoFar;
640  }
641
642  /**
643   * Returns the greatest of the specified values according to this ordering. If there are multiple
644   * greatest values, the first of those is returned. The iterator will be left exhausted: its
645   * {@code hasNext()} method will return {@code false}.
646   *
647   * <p><b>Java 8 users:</b> Use {@code Streams.stream(iterator).max(thisComparator).get()} instead
648   * (but note that it does not guarantee which tied maximum element is returned).
649   *
650   * @param iterator the iterator whose maximum element is to be determined
651   * @throws NoSuchElementException if {@code iterator} is empty
652   * @throws ClassCastException if the parameters are not <i>mutually comparable</i> under this
653   *     ordering.
654   * @since 11.0
655   */
656  @ParametricNullness
657  public <E extends T> E max(Iterator<E> iterator) {
658    // let this throw NoSuchElementException as necessary
659    E maxSoFar = iterator.next();
660
661    while (iterator.hasNext()) {
662      maxSoFar = max(maxSoFar, iterator.next());
663    }
664
665    return maxSoFar;
666  }
667
668  /**
669   * Returns the greatest of the specified values according to this ordering. If there are multiple
670   * greatest values, the first of those is returned.
671   *
672   * <p><b>Java 8 users:</b> If {@code iterable} is a {@link Collection}, use {@code
673   * Collections.max(collection, thisComparator)} instead. Otherwise, use {@code
674   * Streams.stream(iterable).max(thisComparator).get()} instead. Note that these alternatives do
675   * not guarantee which tied maximum element is returned.
676   *
677   * @param iterable the iterable whose maximum element is to be determined
678   * @throws NoSuchElementException if {@code iterable} is empty
679   * @throws ClassCastException if the parameters are not <i>mutually comparable</i> under this
680   *     ordering.
681   */
682  @ParametricNullness
683  public <E extends T> E max(Iterable<E> iterable) {
684    return max(iterable.iterator());
685  }
686
687  /**
688   * Returns the greater of the two values according to this ordering. If the values compare as 0,
689   * the first is returned.
690   *
691   * <p><b>Implementation note:</b> this method is invoked by the default implementations of the
692   * other {@code max} overloads, so overriding it will affect their behavior.
693   *
694   * <p><b>Note:</b> Consider using {@code Comparators.max(a, b, thisComparator)} instead. If {@code
695   * thisComparator} is {@link Ordering#natural}, then use {@code Comparators.max(a, b)}.
696   *
697   * @param a value to compare, returned if greater than or equal to b.
698   * @param b value to compare.
699   * @throws ClassCastException if the parameters are not <i>mutually comparable</i> under this
700   *     ordering.
701   */
702  @ParametricNullness
703  public <E extends T> E max(@ParametricNullness E a, @ParametricNullness E b) {
704    return (compare(a, b) >= 0) ? a : b;
705  }
706
707  /**
708   * Returns the greatest of the specified values according to this ordering. If there are multiple
709   * greatest values, the first of those is returned.
710   *
711   * <p><b>Java 8 users:</b> Use {@code Collections.max(Arrays.asList(a, b, c...), thisComparator)}
712   * instead (but note that it does not guarantee which tied maximum element is returned).
713   *
714   * @param a value to compare, returned if greater than or equal to the rest.
715   * @param b value to compare
716   * @param c value to compare
717   * @param rest values to compare
718   * @throws ClassCastException if the parameters are not <i>mutually comparable</i> under this
719   *     ordering.
720   */
721  @ParametricNullness
722  public <E extends T> E max(
723      @ParametricNullness E a, @ParametricNullness E b, @ParametricNullness E c, E... rest) {
724    E maxSoFar = max(max(a, b), c);
725
726    for (E r : rest) {
727      maxSoFar = max(maxSoFar, r);
728    }
729
730    return maxSoFar;
731  }
732
733  /**
734   * Returns the {@code k} least elements of the given iterable according to this ordering, in order
735   * from least to greatest. If there are fewer than {@code k} elements present, all will be
736   * included.
737   *
738   * <p>The implementation does not necessarily use a <i>stable</i> sorting algorithm; when multiple
739   * elements are equivalent, it is undefined which will come first.
740   *
741   * <p><b>Java 8 users:</b> Use {@code Streams.stream(iterable).collect(Comparators.least(k,
742   * thisComparator))} instead.
743   *
744   * @return an immutable {@code RandomAccess} list of the {@code k} least elements in ascending
745   *     order
746   * @throws IllegalArgumentException if {@code k} is negative
747   * @since 8.0
748   */
749  public <E extends T> List<E> leastOf(Iterable<E> iterable, int k) {
750    if (iterable instanceof Collection) {
751      Collection<E> collection = (Collection<E>) iterable;
752      if (collection.size() <= 2L * k) {
753        // In this case, just dumping the collection to an array and sorting is
754        // faster than using the implementation for Iterator, which is
755        // specialized for k much smaller than n.
756
757        @SuppressWarnings("unchecked") // c only contains E's and doesn't escape
758        E[] array = (E[]) collection.toArray();
759        Arrays.sort(array, this);
760        if (array.length > k) {
761          array = Arrays.copyOf(array, k);
762        }
763        return Collections.unmodifiableList(Arrays.asList(array));
764      }
765    }
766    return leastOf(iterable.iterator(), k);
767  }
768
769  /**
770   * Returns the {@code k} least elements from the given iterator according to this ordering, in
771   * order from least to greatest. If there are fewer than {@code k} elements present, all will be
772   * included.
773   *
774   * <p>The implementation does not necessarily use a <i>stable</i> sorting algorithm; when multiple
775   * elements are equivalent, it is undefined which will come first.
776   *
777   * <p><b>Java 8 users:</b> Use {@code Streams.stream(iterator).collect(Comparators.least(k,
778   * thisComparator))} instead.
779   *
780   * @return an immutable {@code RandomAccess} list of the {@code k} least elements in ascending
781   *     order
782   * @throws IllegalArgumentException if {@code k} is negative
783   * @since 14.0
784   */
785  public <E extends T> List<E> leastOf(Iterator<E> iterator, int k) {
786    checkNotNull(iterator);
787    checkNonnegative(k, "k");
788
789    if (k == 0 || !iterator.hasNext()) {
790      return Collections.emptyList();
791    } else if (k >= Integer.MAX_VALUE / 2) {
792      // k is really large; just do a straightforward sorted-copy-and-sublist
793      ArrayList<E> list = Lists.newArrayList(iterator);
794      Collections.sort(list, this);
795      if (list.size() > k) {
796        list.subList(k, list.size()).clear();
797      }
798      list.trimToSize();
799      return Collections.unmodifiableList(list);
800    } else {
801      TopKSelector<E> selector = TopKSelector.least(k, this);
802      selector.offerAll(iterator);
803      return selector.topK();
804    }
805  }
806
807  /**
808   * Returns the {@code k} greatest elements of the given iterable according to this ordering, in
809   * order from greatest to least. If there are fewer than {@code k} elements present, all will be
810   * included.
811   *
812   * <p>The implementation does not necessarily use a <i>stable</i> sorting algorithm; when multiple
813   * elements are equivalent, it is undefined which will come first.
814   *
815   * <p><b>Java 8 users:</b> Use {@code Streams.stream(iterable).collect(Comparators.greatest(k,
816   * thisComparator))} instead.
817   *
818   * @return an immutable {@code RandomAccess} list of the {@code k} greatest elements in
819   *     <i>descending order</i>
820   * @throws IllegalArgumentException if {@code k} is negative
821   * @since 8.0
822   */
823  public <E extends T> List<E> greatestOf(Iterable<E> iterable, int k) {
824    // TODO(kevinb): see if delegation is hurting performance noticeably
825    // TODO(kevinb): if we change this implementation, add full unit tests.
826    return this.<E>reverse().leastOf(iterable, k);
827  }
828
829  /**
830   * Returns the {@code k} greatest elements from the given iterator according to this ordering, in
831   * order from greatest to least. If there are fewer than {@code k} elements present, all will be
832   * included.
833   *
834   * <p>The implementation does not necessarily use a <i>stable</i> sorting algorithm; when multiple
835   * elements are equivalent, it is undefined which will come first.
836   *
837   * <p><b>Java 8 users:</b> Use {@code Streams.stream(iterator).collect(Comparators.greatest(k,
838   * thisComparator))} instead.
839   *
840   * @return an immutable {@code RandomAccess} list of the {@code k} greatest elements in
841   *     <i>descending order</i>
842   * @throws IllegalArgumentException if {@code k} is negative
843   * @since 14.0
844   */
845  public <E extends T> List<E> greatestOf(Iterator<E> iterator, int k) {
846    return this.<E>reverse().leastOf(iterator, k);
847  }
848
849  /**
850   * Returns a <b>mutable</b> list containing {@code elements} sorted by this ordering; use this
851   * only when the resulting list may need further modification, or may contain {@code null}. The
852   * input is not modified. The returned list is serializable and has random access.
853   *
854   * <p>Unlike {@link Sets#newTreeSet(Iterable)}, this method does not discard elements that are
855   * duplicates according to the comparator. The sort performed is <i>stable</i>, meaning that such
856   * elements will appear in the returned list in the same order they appeared in {@code elements}.
857   *
858   * <p><b>Performance note:</b> According to our
859   * benchmarking
860   * on Open JDK 7, {@link #immutableSortedCopy} generally performs better (in both time and space)
861   * than this method, and this method in turn generally performs better than copying the list and
862   * calling {@link Collections#sort(List)}.
863   */
864  // TODO(kevinb): rerun benchmarks including new options
865  public <E extends T> List<E> sortedCopy(Iterable<E> elements) {
866    @SuppressWarnings("unchecked") // does not escape, and contains only E's
867    E[] array = (E[]) Iterables.toArray(elements);
868    Arrays.sort(array, this);
869    return Lists.newArrayList(Arrays.asList(array));
870  }
871
872  /**
873   * Returns an <b>immutable</b> list containing {@code elements} sorted by this ordering. The input
874   * is not modified.
875   *
876   * <p>Unlike {@link Sets#newTreeSet(Iterable)}, this method does not discard elements that are
877   * duplicates according to the comparator. The sort performed is <i>stable</i>, meaning that such
878   * elements will appear in the returned list in the same order they appeared in {@code elements}.
879   *
880   * <p><b>Performance note:</b> According to our
881   * benchmarking
882   * on Open JDK 7, this method is the most efficient way to make a sorted copy of a collection.
883   *
884   * @throws NullPointerException if any element of {@code elements} is {@code null}
885   * @since 3.0
886   */
887  // TODO(kevinb): rerun benchmarks including new options
888  public <E extends @NonNull T> ImmutableList<E> immutableSortedCopy(Iterable<E> elements) {
889    return ImmutableList.sortedCopyOf(this, elements);
890  }
891
892  /**
893   * Returns {@code true} if each element in {@code iterable} after the first is greater than or
894   * equal to the element that preceded it, according to this ordering. Note that this is always
895   * true when the iterable has fewer than two elements.
896   *
897   * <p><b>Java 8 users:</b> Use the equivalent {@link Comparators#isInOrder(Iterable, Comparator)}
898   * instead, since the rest of {@code Ordering} is mostly obsolete (as explained in the class
899   * documentation).
900   */
901  public boolean isOrdered(Iterable<? extends T> iterable) {
902    Iterator<? extends T> it = iterable.iterator();
903    if (it.hasNext()) {
904      T prev = it.next();
905      while (it.hasNext()) {
906        T next = it.next();
907        if (compare(prev, next) > 0) {
908          return false;
909        }
910        prev = next;
911      }
912    }
913    return true;
914  }
915
916  /**
917   * Returns {@code true} if each element in {@code iterable} after the first is <i>strictly</i>
918   * greater than the element that preceded it, according to this ordering. Note that this is always
919   * true when the iterable has fewer than two elements.
920   *
921   * <p><b>Java 8 users:</b> Use the equivalent {@link Comparators#isInStrictOrder(Iterable,
922   * Comparator)} instead, since the rest of {@code Ordering} is mostly obsolete (as explained in
923   * the class documentation).
924   */
925  public boolean isStrictlyOrdered(Iterable<? extends T> iterable) {
926    Iterator<? extends T> it = iterable.iterator();
927    if (it.hasNext()) {
928      T prev = it.next();
929      while (it.hasNext()) {
930        T next = it.next();
931        if (compare(prev, next) >= 0) {
932          return false;
933        }
934        prev = next;
935      }
936    }
937    return true;
938  }
939
940  /**
941   * {@link Collections#binarySearch(List, Object, Comparator) Searches} {@code sortedList} for
942   * {@code key} using the binary search algorithm. The list must be sorted using this ordering.
943   *
944   * @param sortedList the list to be searched
945   * @param key the key to be searched for
946   * @deprecated Use {@link Collections#binarySearch(List, Object, Comparator)} directly.
947   */
948  @Deprecated
949  public int binarySearch(
950      List<? extends T> sortedList, @ParametricNullness T key) {
951    return Collections.binarySearch(sortedList, key, this);
952  }
953
954  /**
955   * Exception thrown by a {@link Ordering#explicit(List)} or {@link Ordering#explicit(Object,
956   * Object[])} comparator when comparing a value outside the set of values it can compare.
957   * Extending {@link ClassCastException} may seem odd, but it is required.
958   */
959  @VisibleForTesting
960  static class IncomparableValueException extends ClassCastException {
961    final Object value;
962
963    IncomparableValueException(Object value) {
964      super("Cannot compare value: " + value);
965      this.value = value;
966    }
967
968    private static final long serialVersionUID = 0;
969  }
970
971  // Never make these public
972  static final int LEFT_IS_GREATER = 1;
973  static final int RIGHT_IS_GREATER = -1;
974}