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<?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; use RuntimeException; /** * A queue is a collection in which the entities in the collection are kept in * order. * * The principal operations on the queue are the addition of entities to the end * (tail), also known as *enqueue*, and removal of entities from the front * (head), also known as *dequeue*. This makes the queue a first-in-first-out * (FIFO) data structure. * * Besides basic array operations, queues provide additional insertion, * extraction, and inspection operations. 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 `QueueInterface` implementations; in most * implementations, insert operations cannot fail. * * <table> * <caption>Summary of QueueInterface methods</caption> * <thead> * <tr> * <td></td> * <td><em>Throws exception</em></td> * <td><em>Returns special value</em></td> * </tr> * </thead> * <tbody> * <tr> * <th>Insert</th> * <td><code>add()</code></td> * <td><code>offer()</code></td> * </tr> * <tr> * <th>Remove</th> * <td><code>remove()</code></td> * <td><code>poll()</code></td> * </tr> * <tr> * <th>Examine</th> * <td><code>element()</code></td> * <td><code>peek()</code></td> * </tr> * </tbody> * </table> * * Queues typically, but do not necessarily, order elements in a FIFO * (first-in-first-out) manner. Among the exceptions are priority queues, which * order elements according to a supplied comparator, or the elements' natural * ordering, and LIFO queues (or stacks) which order the elements LIFO * (last-in-first-out). Whatever the ordering used, the head of the queue is * that element which would be removed by a call to remove() or poll(). In a * FIFO queue, all new elements are inserted at the tail of the queue. Other * kinds of queues may use different placement rules. Every `QueueInterface` * implementation must specify its ordering properties. * * The `offer()` method inserts an element if possible, otherwise returning * `false`. This differs from the `add()` method, which can fail to add an * element only by throwing an unchecked exception. The `offer()` method is * designed for use when failure is a normal, rather than exceptional * occurrence, for example, in fixed-capacity (or "bounded") queues. * * The `remove()` and `poll()` methods remove and return the head of the queue. * Exactly which element is removed from the queue is a function of the queue's * ordering policy, which differs from implementation to implementation. The * `remove()` and `poll()` methods differ only in their behavior when the queue * is empty: the `remove()` method throws an exception, while the `poll()` * method returns `null`. * * The `element()` and `peek()` methods return, but do not remove, the head of * the queue. * * `QueueInterface` implementations generally do not allow insertion of `null` * elements, although some implementations do not prohibit insertion of `null`. * Even in the implementations that permit it, `null` should not be inserted * into a queue, as `null` is also used as a special return value by the * `poll()` method to indicate that the queue contains no elements. * * @template T * @extends ArrayInterface<T> */ interface QueueInterface extends ArrayInterface { /** * Ensures that this queue contains the specified element (optional * operation). * * Returns `true` if this queue changed as a result of the call. (Returns * `false` if this queue does not permit duplicates and already contains the * specified element.) * * Queues that support this operation may place limitations on what elements * may be added to this queue. In particular, some queues will refuse to add * `null` elements, and others will impose restrictions on the type of * elements that may be added. Queue classes should clearly specify in their * documentation any restrictions on what elements may be added. * * If a queue refuses to add a particular element for any reason other than * that it already contains the element, it must throw an exception (rather * than returning `false`). This preserves the invariant that a queue always * contains the specified element after this call returns. * * @see self::offer() * * @param T $element The element to add to 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 add(mixed $element): bool; /** * Retrieves, but does not remove, the head of this queue. * * This method differs from `peek()` only in that it throws an exception if * this queue is empty. * * @see self::peek() * * @return T the head of this queue. * * @throws NoSuchElementException if this queue is empty. */ public function element(): mixed; /** * Inserts the specified element into 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 `add()`, which can fail to insert an element only by * throwing an exception. * * @see self::add() * * @param T $element The element to add to this queue. * * @return bool `true` if the element was added to this queue, else `false`. */ public function offer(mixed $element): bool; /** * Retrieves, but does not remove, the head of this queue, or returns `null` * if this queue is empty. * * @see self::element() * * @return T | null the head of this queue, or `null` if this queue is empty. */ public function peek(): mixed; /** * Retrieves and removes the head of this queue, or returns `null` * if this queue is empty. * * @see self::remove() * * @return T | null the head of this queue, or `null` if this queue is empty. */ public function poll(): mixed; /** * Retrieves and removes the head of this queue. * * This method differs from `poll()` only in that it throws an exception if * this queue is empty. * * @see self::poll() * * @return T the head of this queue. * * @throws NoSuchElementException if this queue is empty. */ public function remove(): mixed; /** * Returns the type associated with this queue. */ public function getType(): string; }