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