<?php

/**
 * This file is part of the ramsey/collection library
 *
 * For the full copyright and license information, please view the LICENSE
 * file that was distributed with this source code.
 *
 * @copyright Copyright (c) Ben Ramsey <ben@benramsey.com>
 * @license http://opensource.org/licenses/MIT MIT
 */

declare(strict_types=1);

namespace Ramsey\Collection;

use Ramsey\Collection\Exception\NoSuchElementException;

/**
 * A linear collection that supports element insertion and removal at both ends.
 *
 * Most `DoubleEndedQueueInterface` implementations place no fixed limits on the
 * number of elements they may contain, but this interface supports
 * capacity-restricted double-ended queues as well as those with no fixed size
 * limit.
 *
 * This interface defines methods to access the elements at both ends of the
 * double-ended queue. Methods are provided to insert, remove, and examine the
 * element. Each of these methods exists in two forms: one throws an exception
 * if the operation fails, the other returns a special value (either `null` or
 * `false`, depending on the operation). The latter form of the insert operation
 * is designed specifically for use with capacity-restricted implementations; in
 * most implementations, insert operations cannot fail.
 *
 * The twelve methods described above are summarized in the following table:
 *
 * <table>
 * <caption>Summary of DoubleEndedQueueInterface methods</caption>
 * <thead>
 * <tr>
 * <th></th>
 * <th colspan=2>First Element (Head)</th>
 * <th colspan=2>Last Element (Tail)</th>
 * </tr>
 * <tr>
 * <td></td>
 * <td><em>Throws exception</em></td>
 * <td><em>Special value</em></td>
 * <td><em>Throws exception</em></td>
 * <td><em>Special value</em></td>
 * </tr>
 * </thead>
 * <tbody>
 * <tr>
 * <th>Insert</th>
 * <td><code>addFirst()</code></td>
 * <td><code>offerFirst()</code></td>
 * <td><code>addLast()</code></td>
 * <td><code>offerLast()</code></td>
 * </tr>
 * <tr>
 * <th>Remove</th>
 * <td><code>removeFirst()</code></td>
 * <td><code>pollFirst()</code></td>
 * <td><code>removeLast()</code></td>
 * <td><code>pollLast()</code></td>
 * </tr>
 * <tr>
 * <th>Examine</th>
 * <td><code>firstElement()</code></td>
 * <td><code>peekFirst()</code></td>
 * <td><code>lastElement()</code></td>
 * <td><code>peekLast()</code></td>
 * </tr>
 * </tbody>
 * </table>
 *
 * This interface extends the `QueueInterface`. When a double-ended queue is
 * used as a queue, FIFO (first-in-first-out) behavior results. Elements are
 * added at the end of the double-ended queue and removed from the beginning.
 * The methods inherited from the `QueueInterface` are precisely equivalent to
 * `DoubleEndedQueueInterface` methods as indicated in the following table:
 *
 * <table>
 * <caption>Comparison of QueueInterface and DoubleEndedQueueInterface methods</caption>
 * <thead>
 * <tr>
 * <th>QueueInterface Method</th>
 * <th>DoubleEndedQueueInterface Method</th>
 * </tr>
 * </thead>
 * <tbody>
 * <tr>
 * <td><code>add()</code></td>
 * <td><code>addLast()</code></td>
 * </tr>
 * <tr>
 * <td><code>offer()</code></td>
 * <td><code>offerLast()</code></td>
 * </tr>
 * <tr>
 * <td><code>remove()</code></td>
 * <td><code>removeFirst()</code></td>
 * </tr>
 * <tr>
 * <td><code>poll()</code></td>
 * <td><code>pollFirst()</code></td>
 * </tr>
 * <tr>
 * <td><code>element()</code></td>
 * <td><code>firstElement()</code></td>
 * </tr>
 * <tr>
 * <td><code>peek()</code></td>
 * <td><code>peekFirst()</code></td>
 * </tr>
 * </tbody>
 * </table>
 *
 * Double-ended queues can also be used as LIFO (last-in-first-out) stacks. When
 * a double-ended queue is used as a stack, elements are pushed and popped from
 * the beginning of the double-ended queue. Stack concepts are precisely
 * equivalent to `DoubleEndedQueueInterface` methods as indicated in the table
 * below:
 *
 * <table>
 * <caption>Comparison of stack concepts and DoubleEndedQueueInterface methods</caption>
 * <thead>
 * <tr>
 * <th>Stack concept</th>
 * <th>DoubleEndedQueueInterface Method</th>
 * </tr>
 * </thead>
 * <tbody>
 * <tr>
 * <td><em>push</em></td>
 * <td><code>addFirst()</code></td>
 * </tr>
 * <tr>
 * <td><em>pop</em></td>
 * <td><code>removeFirst()</code></td>
 * </tr>
 * <tr>
 * <td><em>peek</em></td>
 * <td><code>peekFirst()</code></td>
 * </tr>
 * </tbody>
 * </table>
 *
 * Note that the `peek()` method works equally well when a double-ended queue is
 * used as a queue or a stack; in either case, elements are drawn from the
 * beginning of the double-ended queue.
 *
 * While `DoubleEndedQueueInterface` implementations are not strictly required
 * to prohibit the insertion of `null` elements, they are strongly encouraged to
 * do so. Users of any `DoubleEndedQueueInterface` implementations that do allow
 * `null` elements are strongly encouraged *not* to take advantage of the
 * ability to insert nulls. This is so because `null` is used as a special
 * return value by various methods to indicated that the double-ended queue is
 * empty.
 */
interface DoubleEndedQueueInterface extends QueueInterface
{
    /**
     * Inserts the specified element at the front of this queue if it is
     * possible to do so immediately without violating capacity restrictions.
     *
     * When using a capacity-restricted double-ended queue, it is generally
     * preferable to use the `offerFirst()` method.
     *
     * @param mixed $element The element to add to the front of this queue.
     *
     * @return bool `true` if this queue changed as a result of the call.
     *
     * @throws \RuntimeException if a queue refuses to add a particular element
     *     for any reason other than that it already contains the element.
     *     Implementations should use a more-specific exception that extends
     *     `\RuntimeException`.
     */
    public function addFirst($element): bool;

    /**
     * Inserts the specified element at the end of this queue if it is possible
     * to do so immediately without violating capacity restrictions.
     *
     * When using a capacity-restricted double-ended queue, it is generally
     * preferable to use the `offerLast()` method.
     *
     * This method is equivalent to `add()`.
     *
     * @param mixed $element The element to add to the end of this queue.
     *
     * @return bool `true` if this queue changed as a result of the call.
     *
     * @throws \RuntimeException if a queue refuses to add a particular element
     *     for any reason other than that it already contains the element.
     *     Implementations should use a more-specific exception that extends
     *     `\RuntimeException`.
     */
    public function addLast($element): bool;

    /**
     * Inserts the specified element at the front of this queue if it is
     * possible to do so immediately without violating capacity restrictions.
     *
     * When using a capacity-restricted queue, this method is generally
     * preferable to `addFirst()`, which can fail to insert an element only by
     * throwing an exception.
     *
     * @param mixed $element The element to add to the front of this queue.
     *
     * @return bool `true` if the element was added to this queue, else `false`.
     */
    public function offerFirst($element): bool;

    /**
     * Inserts the specified element at the end of this queue if it is possible
     * to do so immediately without violating capacity restrictions.
     *
     * When using a capacity-restricted queue, this method is generally
     * preferable to `addLast()` which can fail to insert an element only by
     * throwing an exception.
     *
     * @param mixed $element The element to add to the end of this queue.
     *
     * @return bool `true` if the element was added to this queue, else `false`.
     */
    public function offerLast($element): bool;

    /**
     * Retrieves and removes the head of this queue.
     *
     * This method differs from `pollFirst()` only in that it throws an
     * exception if this queue is empty.
     *
     * @return mixed the first element in this queue.
     *
     * @throws NoSuchElementException if this queue is empty.
     */
    public function removeFirst();

    /**
     * Retrieves and removes the tail of this queue.
     *
     * This method differs from `pollLast()` only in that it throws an exception
     * if this queue is empty.
     *
     * @return mixed the last element in this queue.
     *
     * @throws NoSuchElementException if this queue is empty.
     */
    public function removeLast();

    /**
     * Retrieves and removes the head of this queue, or returns `null` if this
     * queue is empty.
     *
     * @return mixed|null the head of this queue, or `null` if this queue is empty.
     */
    public function pollFirst();

    /**
     * Retrieves and removes the tail of this queue, or returns `null` if this
     * queue is empty.
     *
     * @return mixed|null the tail of this queue, or `null` if this queue is empty.
     */
    public function pollLast();

    /**
     * Retrieves, but does not remove, the head of this queue.
     *
     * This method differs from `peekFirst()` only in that it throws an
     * exception if this queue is empty.
     *
     * @return mixed the head of this queue.
     *
     * @throws NoSuchElementException if this queue is empty.
     */
    public function firstElement();

    /**
     * Retrieves, but does not remove, the tail of this queue.
     *
     * This method differs from `peekLast()` only in that it throws an exception
     * if this queue is empty.
     *
     * @return mixed the tail of this queue.
     *
     * @throws NoSuchElementException if this queue is empty.
     */
    public function lastElement();

    /**
     * Retrieves, but does not remove, the head of this queue, or returns `null`
     * if this queue is empty.
     *
     * @return mixed|null the head of this queue, or `null` if this queue is empty.
     */
    public function peekFirst();

    /**
     * Retrieves, but does not remove, the tail of this queue, or returns `null`
     * if this queue is empty.
     *
     * @return mixed|null the tail of this queue, or `null` if this queue is empty.
     */
    public function peekLast();
}
