* @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:
*
*
* Summary of DoubleEndedQueueInterface methods
*
*
* |
* First Element (Head) |
* Last Element (Tail) |
*
*
* |
* Throws exception |
* Special value |
* Throws exception |
* Special value |
*
*
*
*
* | Insert |
* addFirst() |
* offerFirst() |
* addLast() |
* offerLast() |
*
*
* | Remove |
* removeFirst() |
* pollFirst() |
* removeLast() |
* pollLast() |
*
*
* | Examine |
* firstElement() |
* peekFirst() |
* lastElement() |
* peekLast() |
*
*
*
*
* 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:
*
*
* Comparison of QueueInterface and DoubleEndedQueueInterface methods
*
*
* | QueueInterface Method |
* DoubleEndedQueueInterface Method |
*
*
*
*
* add() |
* addLast() |
*
*
* offer() |
* offerLast() |
*
*
* remove() |
* removeFirst() |
*
*
* poll() |
* pollFirst() |
*
*
* element() |
* firstElement() |
*
*
* peek() |
* peekFirst() |
*
*
*
*
* 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:
*
*
* Comparison of stack concepts and DoubleEndedQueueInterface methods
*
*
* | Stack concept |
* DoubleEndedQueueInterface Method |
*
*
*
*
* | push |
* addFirst() |
*
*
* | pop |
* removeFirst() |
*
*
* | peek |
* peekFirst() |
*
*
*
*
* 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();
}