/*
 * Copyright (c) 1997, 2018, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */

package java.util;

import java.io.Serializable;
import java.util.function.Function;
import java.util.function.ToIntFunction;
import java.util.function.ToLongFunction;
import java.util.function.ToDoubleFunction;
import java.util.Comparators;

/**
 * A comparison function, which imposes a <i>total ordering</i> on some
 * collection of objects. Comparators can be passed to a sort method (such
 * as {@link Collections#sort(List,Comparator) Collections.sort} or {@link
 * Arrays#sort(Object[],Comparator) Arrays.sort}) to allow precise control
 * over the sort order. Comparators can also be used to control the order of
 * certain data structures (such as {@link SortedSet sorted sets} or {@link
 * SortedMap sorted maps}), or to provide an ordering for collections of
 * objects that don't have a {@link Comparable natural ordering}.<p>
 *
 * The ordering imposed by a comparator {@code c} on a set of elements
 * {@code S} is said to be <i>consistent with equals</i> if and only if
 * {@code c.compare(e1, e2)==0} has the same boolean value as
 * {@code e1.equals(e2)} for every {@code e1} and {@code e2} in
 * {@code S}.<p>
 *
 * Caution should be exercised when using a comparator capable of imposing an
 * ordering inconsistent with equals to order a sorted set (or sorted map).
 * Suppose a sorted set (or sorted map) with an explicit comparator {@code c}
 * is used with elements (or keys) drawn from a set {@code S}. If the
 * ordering imposed by {@code c} on {@code S} is inconsistent with equals,
 * the sorted set (or sorted map) will behave "strangely." In particular the
 * sorted set (or sorted map) will violate the general contract for set (or
 * map), which is defined in terms of {@code equals}.<p>
 *
 * For example, suppose one adds two elements {@code a} and {@code b} such that
 * {@code (a.equals(b) && c.compare(a, b) != 0)}
 * to an empty {@code TreeSet} with comparator {@code c}.
 * The second {@code add} operation will return
 * true (and the size of the tree set will increase) because {@code a} and
 * {@code b} are not equivalent from the tree set's perspective, even though
 * this is contrary to the specification of the
 * {@link Set#add Set.add} method.<p>
 *
 * Note: It is generally a good idea for comparators to also implement
 * {@code java.io.Serializable}, as they may be used as ordering methods in
 * serializable data structures (like {@link TreeSet}, {@link TreeMap}). In
 * order for the data structure to serialize successfully, the comparator (if
 * provided) must implement {@code Serializable}.<p>
 *
 * For the mathematically inclined, the <i>relation</i> that defines the
 * <i>imposed ordering</i> that a given comparator {@code c} imposes on a
 * given set of objects {@code S} is:<pre>
 * {(x, y) such that c.compare(x, y) &lt;= 0}.
 * </pre> The <i>quotient</i> for this total order is:<pre>
 * {(x, y) such that c.compare(x, y) == 0}.
 * </pre>
 *
 * It follows immediately from the contract for {@code compare} that the
 * quotient is an <i>equivalence relation</i> on {@code S}, and that the
 * imposed ordering is a <i>total order</i> on {@code S}. When we say that
 * the ordering imposed by {@code c} on {@code S} is <i>consistent with
 * equals</i>, we mean that the quotient for the ordering is the equivalence
 * relation defined by the objects' {@link Object#equals(Object)
 * equals(Object)} method(s):<pre>
 * {(x, y) such that x.equals(y)}. </pre>
 *
 * <p>Unlike {@code Comparable}, a comparator may optionally permit
 * comparison of null arguments, while maintaining the requirements for
 * an equivalence relation.
 *
 * <p>This interface is a member of the
 * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework">
 * Java Collections Framework</a>.
 *
 * @param <T> the type of objects that may be compared by this comparator
 *
 * @author Josh Bloch
 * @author Neal Gafter
 * @see Comparable
 * @see java.io.Serializable
 * @since 1.2
 */
@FunctionalInterface
public interface Comparator<T> {
 /**
 * Compares its two arguments for order. Returns a negative integer,
 * zero, or a positive integer as the first argument is less than, equal
 * to, or greater than the second.<p>
 *
 * The implementor must ensure that {@code sgn(compare(x, y)) ==
 * -sgn(compare(y, x))} for all {@code x} and {@code y}. (This
 * implies that {@code compare(x, y)} must throw an exception if and only
 * if {@code compare(y, x)} throws an exception.)<p>
 *
 * The implementor must also ensure that the relation is transitive:
 * {@code ((compare(x, y)>0) && (compare(y, z)>0))} implies
 * {@code compare(x, z)>0}.<p>
 *
 * Finally, the implementor must ensure that {@code compare(x, y)==0}
 * implies that {@code sgn(compare(x, z))==sgn(compare(y, z))} for all
 * {@code z}.<p>
 *
 * It is generally the case, but <i>not</i> strictly required that
 * {@code (compare(x, y)==0) == (x.equals(y))}. Generally speaking,
 * any comparator that violates this condition should clearly indicate
 * this fact. The recommended language is "Note: this comparator
 * imposes orderings that are inconsistent with equals."<p>
 *
 * In the foregoing description, the notation
 * {@code sgn(}<i>expression</i>{@code )} designates the mathematical
 * <i>signum</i> function, which is defined to return one of {@code -1},
 * {@code 0}, or {@code 1} according to whether the value of
 * <i>expression</i> is negative, zero, or positive, respectively.
 *
 * @param o1 the first object to be compared.
 * @param o2 the second object to be compared.
 * @return a negative integer, zero, or a positive integer as the
 * first argument is less than, equal to, or greater than the
 * second.
 * @throws NullPointerException if an argument is null and this
 * comparator does not permit null arguments
 * @throws ClassCastException if the arguments' types prevent them from
 * being compared by this comparator.
 */
 int compare(T o1, T o2);

 /**
 * Indicates whether some other object is &quot;equal to&quot; this
 * comparator. This method must obey the general contract of
 * {@link Object#equals(Object)}. Additionally, this method can return
 * {@code true} <i>only</i> if the specified object is also a comparator
 * and it imposes the same ordering as this comparator. Thus,
 * {@code comp1.equals(comp2)} implies that {@code sgn(comp1.compare(o1,
 * o2))==sgn(comp2.compare(o1, o2))} for every object reference
 * {@code o1} and {@code o2}.<p>
 *
 * Note that it is <i>always</i> safe <i>not</i> to override
 * {@code Object.equals(Object)}. However, overriding this method may,
 * in some cases, improve performance by allowing programs to determine
 * that two distinct comparators impose the same order.
 *
 * @param obj the reference object with which to compare.
 * @return {@code true} only if the specified object is also
 * a comparator and it imposes the same ordering as this
 * comparator.
 * @see Object#equals(Object)
 * @see Object#hashCode()
 */
 boolean equals(Object obj);

 /**
 * Returns a comparator that imposes the reverse ordering of this
 * comparator.
 *
 * @return a comparator that imposes the reverse ordering of this
 * comparator.
 * @since 1.8
 */
 default Comparator<T> reversed() {
 return Collections.reverseOrder(this);
 }

 /**
 * Returns a lexicographic-order comparator with another comparator.
 * If this {@code Comparator} considers two elements equal, i.e.
 * {@code compare(a, b) == 0}, {@code other} is used to determine the order.
 *
 * <p>The returned comparator is serializable if the specified comparator
 * is also serializable.
 *
 * @apiNote
 * For example, to sort a collection of {@code String} based on the length
 * and then case-insensitive natural ordering, the comparator can be
 * composed using following code,
 *
 * <pre>{@code
 * Comparator<String> cmp = Comparator.comparingInt(String::length)
 * .thenComparing(String.CASE_INSENSITIVE_ORDER);
 * }</pre>
 *
 * @param other the other comparator to be used when this comparator
 * compares two objects that are equal.
 * @return a lexicographic-order comparator composed of this and then the
 * other comparator
 * @throws NullPointerException if the argument is null.
 * @since 1.8
 */
 default Comparator<T> thenComparing(Comparator<? super T> other) {
 Objects.requireNonNull(other);
 return (Comparator<T> & Serializable) (c1, c2) -> {
 int res = compare(c1, c2);
 return (res != 0) ? res : other.compare(c1, c2);
 };
 }

 /**
 * Returns a lexicographic-order comparator with a function that
 * extracts a key to be compared with the given {@code Comparator}.
 *
 * @implSpec This default implementation behaves as if {@code
 * thenComparing(comparing(keyExtractor, cmp))}.
 *
 * @param <U> the type of the sort key
 * @param keyExtractor the function used to extract the sort key
 * @param keyComparator the {@code Comparator} used to compare the sort key
 * @return a lexicographic-order comparator composed of this comparator
 * and then comparing on the key extracted by the keyExtractor function
 * @throws NullPointerException if either argument is null.
 * @see #comparing(Function, Comparator)
 * @see #thenComparing(Comparator)
 * @since 1.8
 */
 default <U> Comparator<T> thenComparing(
 Function<? super T, ? extends U> keyExtractor,
 Comparator<? super U> keyComparator)
 {
 return thenComparing(comparing(keyExtractor, keyComparator));
 }

 /**
 * Returns a lexicographic-order comparator with a function that
 * extracts a {@code Comparable} sort key.
 *
 * @implSpec This default implementation behaves as if {@code
 * thenComparing(comparing(keyExtractor))}.
 *
 * @param <U> the type of the {@link Comparable} sort key
 * @param keyExtractor the function used to extract the {@link
 * Comparable} sort key
 * @return a lexicographic-order comparator composed of this and then the
 * {@link Comparable} sort key.
 * @throws NullPointerException if the argument is null.
 * @see #comparing(Function)
 * @see #thenComparing(Comparator)
 * @since 1.8
 */
 default <U extends Comparable<? super U>> Comparator<T> thenComparing(
 Function<? super T, ? extends U> keyExtractor)
 {
 return thenComparing(comparing(keyExtractor));
 }

 /**
 * Returns a lexicographic-order comparator with a function that
 * extracts an {@code int} sort key.
 *
 * @implSpec This default implementation behaves as if {@code
 * thenComparing(comparingInt(keyExtractor))}.
 *
 * @param keyExtractor the function used to extract the integer sort key
 * @return a lexicographic-order comparator composed of this and then the
 * {@code int} sort key
 * @throws NullPointerException if the argument is null.
 * @see #comparingInt(ToIntFunction)
 * @see #thenComparing(Comparator)
 * @since 1.8
 */
 default Comparator<T> thenComparingInt(ToIntFunction<? super T> keyExtractor) {
 return thenComparing(comparingInt(keyExtractor));
 }

 /**
 * Returns a lexicographic-order comparator with a function that
 * extracts a {@code long} sort key.
 *
 * @implSpec This default implementation behaves as if {@code
 * thenComparing(comparingLong(keyExtractor))}.
 *
 * @param keyExtractor the function used to extract the long sort key
 * @return a lexicographic-order comparator composed of this and then the
 * {@code long} sort key
 * @throws NullPointerException if the argument is null.
 * @see #comparingLong(ToLongFunction)
 * @see #thenComparing(Comparator)
 * @since 1.8
 */
 default Comparator<T> thenComparingLong(ToLongFunction<? super T> keyExtractor) {
 return thenComparing(comparingLong(keyExtractor));
 }

 /**
 * Returns a lexicographic-order comparator with a function that
 * extracts a {@code double} sort key.
 *
 * @implSpec This default implementation behaves as if {@code
 * thenComparing(comparingDouble(keyExtractor))}.
 *
 * @param keyExtractor the function used to extract the double sort key
 * @return a lexicographic-order comparator composed of this and then the
 * {@code double} sort key
 * @throws NullPointerException if the argument is null.
 * @see #comparingDouble(ToDoubleFunction)
 * @see #thenComparing(Comparator)
 * @since 1.8
 */
 default Comparator<T> thenComparingDouble(ToDoubleFunction<? super T> keyExtractor) {
 return thenComparing(comparingDouble(keyExtractor));
 }

 /**
 * Returns a comparator that imposes the reverse of the <em>natural
 * ordering</em>.
 *
 * <p>The returned comparator is serializable and throws {@link
 * NullPointerException} when comparing {@code null}.
 *
 * @param <T> the {@link Comparable} type of element to be compared
 * @return a comparator that imposes the reverse of the <i>natural
 * ordering</i> on {@code Comparable} objects.
 * @see Comparable
 * @since 1.8
 */
 public static <T extends Comparable<? super T>> Comparator<T> reverseOrder() {
 return Collections.reverseOrder();
 }

 /**
 * Returns a comparator that compares {@link Comparable} objects in natural
 * order.
 *
 * <p>The returned comparator is serializable and throws {@link
 * NullPointerException} when comparing {@code null}.
 *
 * @param <T> the {@link Comparable} type of element to be compared
 * @return a comparator that imposes the <i>natural ordering</i> on {@code
 * Comparable} objects.
 * @see Comparable
 * @since 1.8
 */
 @SuppressWarnings("unchecked")
 public static <T extends Comparable<? super T>> Comparator<T> naturalOrder() {
 return (Comparator<T>) Comparators.NaturalOrderComparator.INSTANCE;
 }

 /**
 * Returns a null-friendly comparator that considers {@code null} to be
 * less than non-null. When both are {@code null}, they are considered
 * equal. If both are non-null, the specified {@code Comparator} is used
 * to determine the order. If the specified comparator is {@code null},
 * then the returned comparator considers all non-null values to be equal.
 *
 * <p>The returned comparator is serializable if the specified comparator
 * is serializable.
 *
 * @param <T> the type of the elements to be compared
 * @param comparator a {@code Comparator} for comparing non-null values
 * @return a comparator that considers {@code null} to be less than
 * non-null, and compares non-null objects with the supplied
 * {@code Comparator}.
 * @since 1.8
 */
 public static <T> Comparator<T> nullsFirst(Comparator<? super T> comparator) {
 return new Comparators.NullComparator<>(true, comparator);
 }

 /**
 * Returns a null-friendly comparator that considers {@code null} to be
 * greater than non-null. When both are {@code null}, they are considered
 * equal. If both are non-null, the specified {@code Comparator} is used
 * to determine the order. If the specified comparator is {@code null},
 * then the returned comparator considers all non-null values to be equal.
 *
 * <p>The returned comparator is serializable if the specified comparator
 * is serializable.
 *
 * @param <T> the type of the elements to be compared
 * @param comparator a {@code Comparator} for comparing non-null values
 * @return a comparator that considers {@code null} to be greater than
 * non-null, and compares non-null objects with the supplied
 * {@code Comparator}.
 * @since 1.8
 */
 public static <T> Comparator<T> nullsLast(Comparator<? super T> comparator) {
 return new Comparators.NullComparator<>(false, comparator);
 }

 /**
 * Accepts a function that extracts a sort key from a type {@code T}, and
 * returns a {@code Comparator<T>} that compares by that sort key using
 * the specified {@link Comparator}.
 *
 * <p>The returned comparator is serializable if the specified function
 * and comparator are both serializable.
 *
 * @apiNote
 * For example, to obtain a {@code Comparator} that compares {@code
 * Person} objects by their last name ignoring case differences,
 *
 * <pre>{@code
 * Comparator<Person> cmp = Comparator.comparing(
 * Person::getLastName,
 * String.CASE_INSENSITIVE_ORDER);
 * }</pre>
 *
 * @param <T> the type of element to be compared
 * @param <U> the type of the sort key
 * @param keyExtractor the function used to extract the sort key
 * @param keyComparator the {@code Comparator} used to compare the sort key
 * @return a comparator that compares by an extracted key using the
 * specified {@code Comparator}
 * @throws NullPointerException if either argument is null
 * @since 1.8
 */
 public static <T, U> Comparator<T> comparing(
 Function<? super T, ? extends U> keyExtractor,
 Comparator<? super U> keyComparator)
 {
 Objects.requireNonNull(keyExtractor);
 Objects.requireNonNull(keyComparator);
 return (Comparator<T> & Serializable)
 (c1, c2) -> keyComparator.compare(keyExtractor.apply(c1),
 keyExtractor.apply(c2));
 }

 /**
 * Accepts a function that extracts a {@link java.lang.Comparable
 * Comparable} sort key from a type {@code T}, and returns a {@code
 * Comparator<T>} that compares by that sort key.
 *
 * <p>The returned comparator is serializable if the specified function
 * is also serializable.
 *
 * @apiNote
 * For example, to obtain a {@code Comparator} that compares {@code
 * Person} objects by their last name,
 *
 * <pre>{@code
 * Comparator<Person> byLastName = Comparator.comparing(Person::getLastName);
 * }</pre>
 *
 * @param <T> the type of element to be compared
 * @param <U> the type of the {@code Comparable} sort key
 * @param keyExtractor the function used to extract the {@link
 * Comparable} sort key
 * @return a comparator that compares by an extracted key
 * @throws NullPointerException if the argument is null
 * @since 1.8
 */
 public static <T, U extends Comparable<? super U>> Comparator<T> comparing(
 Function<? super T, ? extends U> keyExtractor)
 {
 Objects.requireNonNull(keyExtractor);
 return (Comparator<T> & Serializable)
 (c1, c2) -> keyExtractor.apply(c1).compareTo(keyExtractor.apply(c2));
 }

 /**
 * Accepts a function that extracts an {@code int} sort key from a type
 * {@code T}, and returns a {@code Comparator<T>} that compares by that
 * sort key.
 *
 * <p>The returned comparator is serializable if the specified function
 * is also serializable.
 *
 * @param <T> the type of element to be compared
 * @param keyExtractor the function used to extract the integer sort key
 * @return a comparator that compares by an extracted key
 * @see #comparing(Function)
 * @throws NullPointerException if the argument is null
 * @since 1.8
 */
 public static <T> Comparator<T> comparingInt(ToIntFunction<? super T> keyExtractor) {
 Objects.requireNonNull(keyExtractor);
 return (Comparator<T> & Serializable)
 (c1, c2) -> Integer.compare(keyExtractor.applyAsInt(c1), keyExtractor.applyAsInt(c2));
 }

 /**
 * Accepts a function that extracts a {@code long} sort key from a type
 * {@code T}, and returns a {@code Comparator<T>} that compares by that
 * sort key.
 *
 * <p>The returned comparator is serializable if the specified function is
 * also serializable.
 *
 * @param <T> the type of element to be compared
 * @param keyExtractor the function used to extract the long sort key
 * @return a comparator that compares by an extracted key
 * @see #comparing(Function)
 * @throws NullPointerException if the argument is null
 * @since 1.8
 */
 public static <T> Comparator<T> comparingLong(ToLongFunction<? super T> keyExtractor) {
 Objects.requireNonNull(keyExtractor);
 return (Comparator<T> & Serializable)
 (c1, c2) -> Long.compare(keyExtractor.applyAsLong(c1), keyExtractor.applyAsLong(c2));
 }

 /**
 * Accepts a function that extracts a {@code double} sort key from a type
 * {@code T}, and returns a {@code Comparator<T>} that compares by that
 * sort key.
 *
 * <p>The returned comparator is serializable if the specified function
 * is also serializable.
 *
 * @param <T> the type of element to be compared
 * @param keyExtractor the function used to extract the double sort key
 * @return a comparator that compares by an extracted key
 * @see #comparing(Function)
 * @throws NullPointerException if the argument is null
 * @since 1.8
 */
 public static<T> Comparator<T> comparingDouble(ToDoubleFunction<? super T> keyExtractor) {
 Objects.requireNonNull(keyExtractor);
 return (Comparator<T> & Serializable)
 (c1, c2) -> Double.compare(keyExtractor.applyAsDouble(c1), keyExtractor.applyAsDouble(c2));
 }
}