/*
* Copyright (C) 2007 The Guava Authors
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.google.common.collect;
import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.collect.CollectPreconditions.checkNonnegative;
import com.google.common.annotations.GwtCompatible;
import com.google.common.annotations.VisibleForTesting;
import com.google.common.base.Function;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashSet;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.SortedMap;
import java.util.SortedSet;
import java.util.TreeSet;
import java.util.concurrent.atomic.AtomicInteger;
import javax.annotation.Nullable;
/**
* A comparator, with additional methods to support common operations. This is
* an "enriched" version of {@code Comparator}, in the same sense that {@link
* FluentIterable} is an enriched {@link Iterable}.
*
* <p>The common ways to get an instance of {@code Ordering} are:
*
* <ul>
* <li>Subclass it and implement {@link #compare} instead of implementing
* {@link Comparator} directly
* <li>Pass a <i>pre-existing</i> {@link Comparator} instance to {@link
* #from(Comparator)}
* <li>Use the natural ordering, {@link Ordering#natural}
* </ul>
*
* <p>Then you can use the <i>chaining</i> methods to get an altered version of
* that {@code Ordering}, including:
*
* <ul>
* <li>{@link #reverse}
* <li>{@link #compound(Comparator)}
* <li>{@link #onResultOf(Function)}
* <li>{@link #nullsFirst} / {@link #nullsLast}
* </ul>
*
* <p>Finally, use the resulting {@code Ordering} anywhere a {@link Comparator}
* is required, or use any of its special operations, such as:</p>
*
* <ul>
* <li>{@link #immutableSortedCopy}
* <li>{@link #isOrdered} / {@link #isStrictlyOrdered}
* <li>{@link #min} / {@link #max}
* </ul>
*
* <p>Except as noted, the orderings returned by the factory methods of this
* class are serializable if and only if the provided instances that back them
* are. For example, if {@code ordering} and {@code function} can themselves be
* serialized, then {@code ordering.onResultOf(function)} can as well.
*
* <p>See the Guava User Guide article on <a href=
* "http://code.google.com/p/guava-libraries/wiki/OrderingExplained">
* {@code Ordering}</a>.
*
* @author Jesse Wilson
* @author Kevin Bourrillion
* @since 2.0 (imported from Google Collections Library)
*/
@GwtCompatible
public abstract class Ordering<T> implements Comparator<T> {
// Natural order
/**
* Returns a serializable ordering that uses the natural order of the values.
* The ordering throws a {@link NullPointerException} when passed a null
* parameter.
*
* <p>The type specification is {@code <C extends Comparable>}, instead of
* the technically correct {@code <C extends Comparable<? super C>>}, to
* support legacy types from before Java 5.
*/
@GwtCompatible(serializable = true)
@SuppressWarnings("unchecked") // TODO(kevinb): right way to explain this??
public static <C extends Comparable> Ordering<C> natural() {
return (Ordering<C>) NaturalOrdering.INSTANCE;
}
// Static factories
/**
* Returns an ordering based on an <i>existing</i> comparator instance. Note
* that it is unnecessary to create a <i>new</i> anonymous inner class
* implementing {@code Comparator} just to pass it in here. Instead, simply
* subclass {@code Ordering} and implement its {@code compare} method
* directly.
*
* @param comparator the comparator that defines the order
* @return comparator itself if it is already an {@code Ordering}; otherwise
* an ordering that wraps that comparator
*/
@GwtCompatible(serializable = true)
public static <T> Ordering<T> from(Comparator<T> comparator) {
return (comparator instanceof Ordering)
? (Ordering<T>) comparator
: new ComparatorOrdering<T>(comparator);
}
/**
* Simply returns its argument.
*
* @deprecated no need to use this
*/
@GwtCompatible(serializable = true)
@Deprecated public static <T> Ordering<T> from(Ordering<T> ordering) {
return checkNotNull(ordering);
}
/**
* Returns an ordering that compares objects according to the order in
* which they appear in the given list. Only objects present in the list
* (according to {@link Object#equals}) may be compared. This comparator
* imposes a "partial ordering" over the type {@code T}. Subsequent changes
* to the {@code valuesInOrder} list will have no effect on the returned
* comparator. Null values in the list are not supported.
*
* <p>The returned comparator throws an {@link ClassCastException} when it
* receives an input parameter that isn't among the provided values.
*
* <p>The generated comparator is serializable if all the provided values are
* serializable.
*
* @param valuesInOrder the values that the returned comparator will be able
* to compare, in the order the comparator should induce
* @return the comparator described above
* @throws NullPointerException if any of the provided values is null
* @throws IllegalArgumentException if {@code valuesInOrder} contains any
* duplicate values (according to {@link Object#equals})
*/
@GwtCompatible(serializable = true)
public static <T> Ordering<T> explicit(List<T> valuesInOrder) {
return new ExplicitOrdering<T>(valuesInOrder);
}
/**
* Returns an ordering that compares objects according to the order in
* which they are given to this method. Only objects present in the argument
* list (according to {@link Object#equals}) may be compared. This comparator
* imposes a "partial ordering" over the type {@code T}. Null values in the
* argument list are not supported.
*
* <p>The returned comparator throws a {@link ClassCastException} when it
* receives an input parameter that isn't among the provided values.
*
* <p>The generated comparator is serializable if all the provided values are
* serializable.
*
* @param leastValue the value which the returned comparator should consider
* the "least" of all values
* @param remainingValuesInOrder the rest of the values that the returned
* comparator will be able to compare, in the order the comparator should
* follow
* @return the comparator described above
* @throws NullPointerException if any of the provided values is null
* @throws IllegalArgumentException if any duplicate values (according to
* {@link Object#equals(Object)}) are present among the method arguments
*/
@GwtCompatible(serializable = true)
public static <T> Ordering<T> explicit(
T leastValue, T... remainingValuesInOrder) {
return explicit(Lists.asList(leastValue, remainingValuesInOrder));
}
// Ordering<Object> singletons
/**
* Returns an ordering which treats all values as equal, indicating "no
* ordering." Passing this ordering to any <i>stable</i> sort algorithm
* results in no change to the order of elements. Note especially that {@link
* #sortedCopy} and {@link #immutableSortedCopy} are stable, and in the
* returned instance these are implemented by simply copying the source list.
*
* <p>Example: <pre> {@code
*
* Ordering.allEqual().nullsLast().sortedCopy(
* asList(t, null, e, s, null, t, null))}</pre>
*
* <p>Assuming {@code t}, {@code e} and {@code s} are non-null, this returns
* {@code [t, e, s, t, null, null, null]} regardlesss of the true comparison
* order of those three values (which might not even implement {@link
* Comparable} at all).
*
* <p><b>Warning:</b> by definition, this comparator is not <i>consistent with
* equals</i> (as defined {@linkplain Comparator here}). Avoid its use in
* APIs, such as {@link TreeSet#TreeSet(Comparator)}, where such consistency
* is expected.
*
* <p>The returned comparator is serializable.
*
* @since 13.0
*/
@GwtCompatible(serializable = true)
@SuppressWarnings("unchecked")
public static Ordering<Object> allEqual() {
return AllEqualOrdering.INSTANCE;
}
/**
* Returns an ordering that compares objects by the natural ordering of their
* string representations as returned by {@code toString()}. It does not
* support null values.
*
* <p>The comparator is serializable.
*/
@GwtCompatible(serializable = true)
public static Ordering<Object> usingToString() {
return UsingToStringOrdering.INSTANCE;
}
/**
* Returns an arbitrary ordering over all objects, for which {@code compare(a,
* b) == 0} implies {@code a == b} (identity equality). There is no meaning
* whatsoever to the order imposed, but it is constant for the life of the VM.
*
* <p>Because the ordering is identity-based, it is not "consistent with
* {@link Object#equals(Object)}" as defined by {@link Comparator}. Use
* caution when building a {@link SortedSet} or {@link SortedMap} from it, as
* the resulting collection will not behave exactly according to spec.
*
* <p>This ordering is not serializable, as its implementation relies on
* {@link System#identityHashCode(Object)}, so its behavior cannot be
* preserved across serialization.
*
* @since 2.0
*/
public static Ordering<Object> arbitrary() {
return ArbitraryOrderingHolder.ARBITRARY_ORDERING;
}
private static class ArbitraryOrderingHolder {
static final Ordering<Object> ARBITRARY_ORDERING = new ArbitraryOrdering();
}
@VisibleForTesting static class ArbitraryOrdering extends Ordering<Object> {
@SuppressWarnings("deprecation") // TODO(kevinb): ?
private Map<Object, Integer> uids =
Platform.tryWeakKeys(new MapMaker()).makeComputingMap(
new Function<Object, Integer>() {
final AtomicInteger counter = new AtomicInteger(0);
@Override
public Integer apply(Object from) {
return counter.getAndIncrement();
}
});
@Override public int compare(Object left, Object right) {
if (left == right) {
return 0;
} else if (left == null) {
return -1;
} else if (right == null) {
return 1;
}
int leftCode = identityHashCode(left);
int rightCode = identityHashCode(right);
if (leftCode != rightCode) {
return leftCode < rightCode ? -1 : 1;
}
// identityHashCode collision (rare, but not as rare as you'd think)
int result = uids.get(left).compareTo(uids.get(right));
if (result == 0) {
throw new AssertionError(); // extremely, extremely unlikely.
}
return result;
}
@Override public String toString() {
return "Ordering.arbitrary()";
}
/*
* We need to be able to mock identityHashCode() calls for tests, because it
* can take 1-10 seconds to find colliding objects. Mocking frameworks that
* can do magic to mock static method calls still can't do so for a system
* class, so we need the indirection. In production, Hotspot should still
* recognize that the call is 1-morphic and should still be willing to
* inline it if necessary.
*/
int identityHashCode(Object object) {
return System.identityHashCode(object);
}
}
// Constructor
/**
* Constructs a new instance of this class (only invokable by the subclass
* constructor, typically implicit).
*/
protected Ordering() {}
// Instance-based factories (and any static equivalents)
/**
* Returns the reverse of this ordering; the {@code Ordering} equivalent to
* {@link Collections#reverseOrder(Comparator)}.
*/
// type parameter <S> lets us avoid the extra <String> in statements like:
// Ordering<String> o = Ordering.<String>natural().reverse();
@GwtCompatible(serializable = true)
public <S extends T> Ordering<S> reverse() {
return new ReverseOrdering<S>(this);
}
/**
* Returns an ordering that treats {@code null} as less than all other values
* and uses {@code this} to compare non-null values.
*/
// type parameter <S> lets us avoid the extra <String> in statements like:
// Ordering<String> o = Ordering.<String>natural().nullsFirst();
@GwtCompatible(serializable = true)
public <S extends T> Ordering<S> nullsFirst() {
return new NullsFirstOrdering<S>(this);
}
/**
* Returns an ordering that treats {@code null} as greater than all other
* values and uses this ordering to compare non-null values.
*/
// type parameter <S> lets us avoid the extra <String> in statements like:
// Ordering<String> o = Ordering.<String>natural().nullsLast();
@GwtCompatible(serializable = true)
public <S extends T> Ordering<S> nullsLast() {
return new NullsLastOrdering<S>(this);
}
/**
* Returns a new ordering on {@code F} which orders elements by first applying
* a function to them, then comparing those results using {@code this}. For
* example, to compare objects by their string forms, in a case-insensitive
* manner, use: <pre> {@code
*
* Ordering.from(String.CASE_INSENSITIVE_ORDER)
* .onResultOf(Functions.toStringFunction())}</pre>
*/
@GwtCompatible(serializable = true)
public <F> Ordering<F> onResultOf(Function<F, ? extends T> function) {
return new ByFunctionOrdering<F, T>(function, this);
}
<T2 extends T> Ordering<Map.Entry<T2, ?>> onKeys() {
return onResultOf(Maps.<T2>keyFunction());
}
/**
* Returns an ordering which first uses the ordering {@code this}, but which
* in the event of a "tie", then delegates to {@code secondaryComparator}.
* For example, to sort a bug list first by status and second by priority, you
* might use {@code byStatus.compound(byPriority)}. For a compound ordering
* with three or more components, simply chain multiple calls to this method.
*
* <p>An ordering produced by this method, or a chain of calls to this method,
* is equivalent to one created using {@link Ordering#compound(Iterable)} on
* the same component comparators.
*/
@GwtCompatible(serializable = true)
public <U extends T> Ordering<U> compound(
Comparator<? super U> secondaryComparator) {
return new CompoundOrdering<U>(this, checkNotNull(secondaryComparator));
}
/**
* Returns an ordering which tries each given comparator in order until a
* non-zero result is found, returning that result, and returning zero only if
* all comparators return zero. The returned ordering is based on the state of
* the {@code comparators} iterable at the time it was provided to this
* method.
*
* <p>The returned ordering is equivalent to that produced using {@code
* Ordering.from(comp1).compound(comp2).compound(comp3) . . .}.
*
* <p><b>Warning:</b> Supplying an argument with undefined iteration order,
* such as a {@link HashSet}, will produce non-deterministic results.
*
* @param comparators the comparators to try in order
*/
@GwtCompatible(serializable = true)
public static <T> Ordering<T> compound(
Iterable<? extends Comparator<? super T>> comparators) {
return new CompoundOrdering<T>(comparators);
}
/**
* Returns a new ordering which sorts iterables by comparing corresponding
* elements pairwise until a nonzero result is found; imposes "dictionary
* order". If the end of one iterable is reached, but not the other, the
* shorter iterable is considered to be less than the longer one. For example,
* a lexicographical natural ordering over integers considers {@code
* [] < [1] < [1, 1] < [1, 2] < [2]}.
*
* <p>Note that {@code ordering.lexicographical().reverse()} is not
* equivalent to {@code ordering.reverse().lexicographical()} (consider how
* each would order {@code [1]} and {@code [1, 1]}).
*
* @since 2.0
*/
@GwtCompatible(serializable = true)
// type parameter <S> lets us avoid the extra <String> in statements like:
// Ordering<Iterable<String>> o =
// Ordering.<String>natural().lexicographical();
public <S extends T> Ordering<Iterable<S>> lexicographical() {
/*
* Note that technically the returned ordering should be capable of
* handling not just {@code Iterable<S>} instances, but also any {@code
* Iterable<? extends S>}. However, the need for this comes up so rarely
* that it doesn't justify making everyone else deal with the very ugly
* wildcard.
*/
return new LexicographicalOrdering<S>(this);
}
// Regular instance methods
// Override to add @Nullable
@Override public abstract int compare(@Nullable T left, @Nullable T right);
/**
* Returns the least of the specified values according to this ordering. If
* there are multiple least values, the first of those is returned. The
* iterator will be left exhausted: its {@code hasNext()} method will return
* {@code false}.
*
* @param iterator the iterator whose minimum element is to be determined
* @throws NoSuchElementException if {@code iterator} is empty
* @throws ClassCastException if the parameters are not <i>mutually
* comparable</i> under this ordering.
*
* @since 11.0
*/
public <E extends T> E min(Iterator<E> iterator) {
// let this throw NoSuchElementException as necessary
E minSoFar = iterator.next();
while (iterator.hasNext()) {
minSoFar = min(minSoFar, iterator.next());
}
return minSoFar;
}
/**
* Returns the least of the specified values according to this ordering. If
* there are multiple least values, the first of those is returned.
*
* @param iterable the iterable whose minimum element is to be determined
* @throws NoSuchElementException if {@code iterable} is empty
* @throws ClassCastException if the parameters are not <i>mutually
* comparable</i> under this ordering.
*/
public <E extends T> E min(Iterable<E> iterable) {
return min(iterable.iterator());
}
/**
* Returns the lesser of the two values according to this ordering. If the
* values compare as 0, the first is returned.
*
* <p><b>Implementation note:</b> this method is invoked by the default
* implementations of the other {@code min} overloads, so overriding it will
* affect their behavior.
*
* @param a value to compare, returned if less than or equal to b.
* @param b value to compare.
* @throws ClassCastException if the parameters are not <i>mutually
* comparable</i> under this ordering.
*/
public <E extends T> E min(@Nullable E a, @Nullable E b) {
return (compare(a, b) <= 0) ? a : b;
}
/**
* Returns the least of the specified values according to this ordering. If
* there are multiple least values, the first of those is returned.
*
* @param a value to compare, returned if less than or equal to the rest.
* @param b value to compare
* @param c value to compare
* @param rest values to compare
* @throws ClassCastException if the parameters are not <i>mutually
* comparable</i> under this ordering.
*/
public <E extends T> E min(
@Nullable E a, @Nullable E b, @Nullable E c, E... rest) {
E minSoFar = min(min(a, b), c);
for (E r : rest) {
minSoFar = min(minSoFar, r);
}
return minSoFar;
}
/**
* Returns the greatest of the specified values according to this ordering. If
* there are multiple greatest values, the first of those is returned. The
* iterator will be left exhausted: its {@code hasNext()} method will return
* {@code false}.
*
* @param iterator the iterator whose maximum element is to be determined
* @throws NoSuchElementException if {@code iterator} is empty
* @throws ClassCastException if the parameters are not <i>mutually
* comparable</i> under this ordering.
*
* @since 11.0
*/
public <E extends T> E max(Iterator<E> iterator) {
// let this throw NoSuchElementException as necessary
E maxSoFar = iterator.next();
while (iterator.hasNext()) {
maxSoFar = max(maxSoFar, iterator.next());
}
return maxSoFar;
}
/**
* Returns the greatest of the specified values according to this ordering. If
* there are multiple greatest values, the first of those is returned.
*
* @param iterable the iterable whose maximum element is to be determined
* @throws NoSuchElementException if {@code iterable} is empty
* @throws ClassCastException if the parameters are not <i>mutually
* comparable</i> under this ordering.
*/
public <E extends T> E max(Iterable<E> iterable) {
return max(iterable.iterator());
}
/**
* Returns the greater of the two values according to this ordering. If the
* values compare as 0, the first is returned.
*
* <p><b>Implementation note:</b> this method is invoked by the default
* implementations of the other {@code max} overloads, so overriding it will
* affect their behavior.
*
* @param a value to compare, returned if greater than or equal to b.
* @param b value to compare.
* @throws ClassCastException if the parameters are not <i>mutually
* comparable</i> under this ordering.
*/
public <E extends T> E max(@Nullable E a, @Nullable E b) {
return (compare(a, b) >= 0) ? a : b;
}
/**
* Returns the greatest of the specified values according to this ordering. If
* there are multiple greatest values, the first of those is returned.
*
* @param a value to compare, returned if greater than or equal to the rest.
* @param b value to compare
* @param c value to compare
* @param rest values to compare
* @throws ClassCastException if the parameters are not <i>mutually
* comparable</i> under this ordering.
*/
public <E extends T> E max(
@Nullable E a, @Nullable E b, @Nullable E c, E... rest) {
E maxSoFar = max(max(a, b), c);
for (E r : rest) {
maxSoFar = max(maxSoFar, r);
}
return maxSoFar;
}
/**
* Returns the {@code k} least elements of the given iterable according to
* this ordering, in order from least to greatest. If there are fewer than
* {@code k} elements present, all will be included.
*
* <p>The implementation does not necessarily use a <i>stable</i> sorting
* algorithm; when multiple elements are equivalent, it is undefined which
* will come first.
*
* @return an immutable {@code RandomAccess} list of the {@code k} least
* elements in ascending order
* @throws IllegalArgumentException if {@code k} is negative
* @since 8.0
*/
public <E extends T> List<E> leastOf(Iterable<E> iterable, int k) {
if (iterable instanceof Collection) {
Collection<E> collection = (Collection<E>) iterable;
if (collection.size() <= 2L * k) {
// In this case, just dumping the collection to an array and sorting is
// faster than using the implementation for Iterator, which is
// specialized for k much smaller than n.
@SuppressWarnings("unchecked") // c only contains E's and doesn't escape
E[] array = (E[]) collection.toArray();
Arrays.sort(array, this);
if (array.length > k) {
array = ObjectArrays.arraysCopyOf(array, k);
}
return Collections.unmodifiableList(Arrays.asList(array));
}
}
return leastOf(iterable.iterator(), k);
}
/**
* Returns the {@code k} least elements from the given iterator according to
* this ordering, in order from least to greatest. If there are fewer than
* {@code k} elements present, all will be included.
*
* <p>The implementation does not necessarily use a <i>stable</i> sorting
* algorithm; when multiple elements are equivalent, it is undefined which
* will come first.
*
* @return an immutable {@code RandomAccess} list of the {@code k} least
* elements in ascending order
* @throws IllegalArgumentException if {@code k} is negative
* @since 14.0
*/
public <E extends T> List<E> leastOf(Iterator<E> elements, int k) {
checkNotNull(elements);
checkNonnegative(k, "k");
if (k == 0 || !elements.hasNext()) {
return ImmutableList.of();
} else if (k >= Integer.MAX_VALUE / 2) {
// k is really large; just do a straightforward sorted-copy-and-sublist
ArrayList<E> list = Lists.newArrayList(elements);
Collections.sort(list, this);
if (list.size() > k) {
list.subList(k, list.size()).clear();
}
list.trimToSize();
return Collections.unmodifiableList(list);
}
/*
* Our goal is an O(n) algorithm using only one pass and O(k) additional
* memory.
*
* We use the following algorithm: maintain a buffer of size 2*k. Every time
* the buffer gets full, find the median and partition around it, keeping
* only the lowest k elements. This requires n/k find-median-and-partition
* steps, each of which take O(k) time with a traditional quickselect.
*
* After sorting the output, the whole algorithm is O(n + k log k). It
* degrades gracefully for worst-case input (descending order), performs
* competitively or wins outright for randomly ordered input, and doesn't
* require the whole collection to fit into memory.
*/
int bufferCap = k * 2;
@SuppressWarnings("unchecked") // we'll only put E's in
E[] buffer = (E[]) new Object[bufferCap];
E threshold = elements.next();
buffer[0] = threshold;
int bufferSize = 1;
// threshold is the kth smallest element seen so far. Once bufferSize >= k,
// anything larger than threshold can be ignored immediately.
while (bufferSize < k && elements.hasNext()) {
E e = elements.next();
buffer[bufferSize++] = e;
threshold = max(threshold, e);
}
while (elements.hasNext()) {
E e = elements.next();
if (compare(e, threshold) >= 0) {
continue;
}
buffer[bufferSize++] = e;
if (bufferSize == bufferCap) {
// We apply the quickselect algorithm to partition about the median,
// and then ignore the last k elements.
int left = 0;
int right = bufferCap - 1;
int minThresholdPosition = 0;
// The leftmost position at which the greatest of the k lower elements
// -- the new value of threshold -- might be found.
while (left < right) {
int pivotIndex = (left + right + 1) >>> 1;
int pivotNewIndex = partition(buffer, left, right, pivotIndex);
if (pivotNewIndex > k) {
right = pivotNewIndex - 1;
} else if (pivotNewIndex < k) {
left = Math.max(pivotNewIndex, left + 1);
minThresholdPosition = pivotNewIndex;
} else {
break;
}
}
bufferSize = k;
threshold = buffer[minThresholdPosition];
for (int i = minThresholdPosition + 1; i < bufferSize; i++) {
threshold = max(threshold, buffer[i]);
}
}
}
Arrays.sort(buffer, 0, bufferSize, this);
bufferSize = Math.min(bufferSize, k);
return Collections.unmodifiableList(
Arrays.asList(ObjectArrays.arraysCopyOf(buffer, bufferSize)));
// We can't use ImmutableList; we have to be null-friendly!
}
private <E extends T> int partition(
E[] values, int left, int right, int pivotIndex) {
E pivotValue = values[pivotIndex];
values[pivotIndex] = values[right];
values[right] = pivotValue;
int storeIndex = left;
for (int i = left; i < right; i++) {
if (compare(values[i], pivotValue) < 0) {
ObjectArrays.swap(values, storeIndex, i);
storeIndex++;
}
}
ObjectArrays.swap(values, right, storeIndex);
return storeIndex;
}
/**
* Returns the {@code k} greatest elements of the given iterable according to
* this ordering, in order from greatest to least. If there are fewer than
* {@code k} elements present, all will be included.
*
* <p>The implementation does not necessarily use a <i>stable</i> sorting
* algorithm; when multiple elements are equivalent, it is undefined which
* will come first.
*
* @return an immutable {@code RandomAccess} list of the {@code k} greatest
* elements in <i>descending order</i>
* @throws IllegalArgumentException if {@code k} is negative
* @since 8.0
*/
public <E extends T> List<E> greatestOf(Iterable<E> iterable, int k) {
// TODO(kevinb): see if delegation is hurting performance noticeably
// TODO(kevinb): if we change this implementation, add full unit tests.
return reverse().leastOf(iterable, k);
}
/**
* Returns the {@code k} greatest elements from the given iterator according to
* this ordering, in order from greatest to least. If there are fewer than
* {@code k} elements present, all will be included.
*
* <p>The implementation does not necessarily use a <i>stable</i> sorting
* algorithm; when multiple elements are equivalent, it is undefined which
* will come first.
*
* @return an immutable {@code RandomAccess} list of the {@code k} greatest
* elements in <i>descending order</i>
* @throws IllegalArgumentException if {@code k} is negative
* @since 14.0
*/
public <E extends T> List<E> greatestOf(Iterator<E> iterator, int k) {
return reverse().leastOf(iterator, k);
}
/**
* Returns a <b>mutable</b> list containing {@code elements} sorted by this
* ordering; use this only when the resulting list may need further
* modification, or may contain {@code null}. The input is not modified. The
* returned list is serializable and has random access.
*
* <p>Unlike {@link Sets#newTreeSet(Iterable)}, this method does not discard
* elements that are duplicates according to the comparator. The sort
* performed is <i>stable</i>, meaning that such elements will appear in the
* returned list in the same order they appeared in {@code elements}.
*
* <p><b>Performance note:</b> According to our
* benchmarking
* on Open JDK 7, {@link #immutableSortedCopy} generally performs better (in
* both time and space) than this method, and this method in turn generally
* performs better than copying the list and calling {@link
* Collections#sort(List)}.
*/
public <E extends T> List<E> sortedCopy(Iterable<E> elements) {
@SuppressWarnings("unchecked") // does not escape, and contains only E's
E[] array = (E[]) Iterables.toArray(elements);
Arrays.sort(array, this);
return Lists.newArrayList(Arrays.asList(array));
}
/**
* Returns an <b>immutable</b> list containing {@code elements} sorted by this
* ordering. The input is not modified.
*
* <p>Unlike {@link Sets#newTreeSet(Iterable)}, this method does not discard
* elements that are duplicates according to the comparator. The sort
* performed is <i>stable</i>, meaning that such elements will appear in the
* returned list in the same order they appeared in {@code elements}.
*
* <p><b>Performance note:</b> According to our
* benchmarking
* on Open JDK 7, this method is the most efficient way to make a sorted copy
* of a collection.
*
* @throws NullPointerException if any of {@code elements} (or {@code
* elements} itself) is null
* @since 3.0
*/
public <E extends T> ImmutableList<E> immutableSortedCopy(
Iterable<E> elements) {
@SuppressWarnings("unchecked") // we'll only ever have E's in here
E[] array = (E[]) Iterables.toArray(elements);
for (E e : array) {
checkNotNull(e);
}
Arrays.sort(array, this);
return ImmutableList.asImmutableList(array);
}
/**
* Returns {@code true} if each element in {@code iterable} after the first is
* greater than or equal to the element that preceded it, according to this
* ordering. Note that this is always true when the iterable has fewer than
* two elements.
*/
public boolean isOrdered(Iterable<? extends T> iterable) {
Iterator<? extends T> it = iterable.iterator();
if (it.hasNext()) {
T prev = it.next();
while (it.hasNext()) {
T next = it.next();
if (compare(prev, next) > 0) {
return false;
}
prev = next;
}
}
return true;
}
/**
* Returns {@code true} if each element in {@code iterable} after the first is
* <i>strictly</i> greater than the element that preceded it, according to
* this ordering. Note that this is always true when the iterable has fewer
* than two elements.
*/
public boolean isStrictlyOrdered(Iterable<? extends T> iterable) {
Iterator<? extends T> it = iterable.iterator();
if (it.hasNext()) {
T prev = it.next();
while (it.hasNext()) {
T next = it.next();
if (compare(prev, next) >= 0) {
return false;
}
prev = next;
}
}
return true;
}
/**
* {@link Collections#binarySearch(List, Object, Comparator) Searches}
* {@code sortedList} for {@code key} using the binary search algorithm. The
* list must be sorted using this ordering.
*
* @param sortedList the list to be searched
* @param key the key to be searched for
*/
public int binarySearch(List<? extends T> sortedList, @Nullable T key) {
return Collections.binarySearch(sortedList, key, this);
}
/**
* Exception thrown by a {@link Ordering#explicit(List)} or {@link
* Ordering#explicit(Object, Object[])} comparator when comparing a value
* outside the set of values it can compare. Extending {@link
* ClassCastException} may seem odd, but it is required.
*/
// TODO(kevinb): make this public, document it right
@VisibleForTesting
static class IncomparableValueException extends ClassCastException {
final Object value;
IncomparableValueException(Object value) {
super("Cannot compare value: " + value);
this.value = value;
}
private static final long serialVersionUID = 0;
}
// Never make these public
static final int LEFT_IS_GREATER = 1;
static final int RIGHT_IS_GREATER = -1;
}