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/*
* Copyright (C) 2003-2024 Paolo Boldi and Sebastiano Vigna
*
* 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 it.unimi.dsi.fastutil;
import java.util.Comparator;
import java.util.NoSuchElementException;
/** A priority queue.
*
* <p>A priority queue provides a way to {@linkplain #enqueue(Object) enqueue}
* elements, and to {@linkplain #dequeue() dequeue} them in some specified
* order. Elements that are <em>smaller</em> in the specified order are
* dequeued first. It is also possible to get the {@linkplain #first() first
* element}, that is, the element that would be dequeued next.
*
* <p>Additionally, the queue may provide a method to peek at
* element that would be dequeued {@linkplain #last() last}.
*
* <p>The relative order of the elements enqueued should not change during
* queue operations. Nonetheless, some implementations may give the caller a
* way to notify the queue that the {@linkplain #changed() first element has
* changed its relative position in the order}.
*/
public interface PriorityQueue<K> {
/** Enqueues a new element.
*
* @param x the element to enqueue.
*/
void enqueue(K x);
/** Dequeues the {@linkplain #first() first} element from the queue.
*
* @return the dequeued element.
* @throws NoSuchElementException if the queue is empty.
*/
K dequeue();
/** Checks whether this queue is empty.
*
* <P>This default implementation checks whether {@link #size()} is zero.
* @return true if this queue is empty.
*/
default boolean isEmpty() {
return size() == 0;
}
/** Returns the number of elements in this queue.
*
* @return the number of elements in this queue.
*/
int size();
/** Removes all elements from this queue.
*/
void clear();
/** Returns the first element of the queue.
*
* @return the first element.
* @throws NoSuchElementException if the queue is empty.
*/
K first();
/** Returns the last element of the queue, that is, the element the would be dequeued last (optional operation).
*
* <p>This default implementation just throws an {@link UnsupportedOperationException}.
* @return the last element.
* @throws NoSuchElementException if the queue is empty.
*/
default K last() { throw new UnsupportedOperationException(); }
/** Notifies the queue that the {@linkplain #first() first} element has changed (optional operation).
* <p>This default implementation just throws an {@link UnsupportedOperationException}.
*/
default void changed() { throw new UnsupportedOperationException(); }
/** Returns the comparator associated with this queue, or {@code null} if it uses its elements' natural ordering.
*
* @return the comparator associated with this sorted set, or {@code null} if it uses its elements' natural ordering.
*/
Comparator<? super K> comparator();
}
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