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PriorityQueue.php
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PriorityQueue.php
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<?php
declare(strict_types=1);
namespace Psl\DataStructure;
use Psl;
use Psl\Dict;
use Psl\Iter;
use Psl\Math;
use Psl\Vec;
/**
* @template T
*
* @implements PriorityQueueInterface<T>
*/
final class PriorityQueue implements PriorityQueueInterface
{
/**
* @var array<int, list<T>>
*/
private array $queue = [];
/**
* Adds a node to the queue.
*
* @param T $node
*/
public function enqueue($node, int $priority = 0): void
{
$nodes = $this->queue[$priority] ?? [];
$nodes[] = $node;
$this->queue[$priority] = $nodes;
}
/**
* Retrieves, but does not remove, the node at the head of this queue,
* or returns null if this queue is empty.
*
* @return null|T
*/
public function peek()
{
if (0 === $this->count()) {
return null;
}
$keys = Vec\keys($this->queue);
// Retrieve the highest priority.
$priority = Math\max($keys) ?? 0;
// Retrieve the list of nodes with the priority `$priority`.
$nodes = $this->queue[$priority] ?? [];
// Retrieve the first node of the list.
return Iter\first($nodes);
}
/**
* Retrieves and removes the node at the head of this queue,
* or returns null if this queue is empty.
*
* @return null|T
*/
public function pull()
{
if (0 === $this->count()) {
return null;
}
/** @psalm-suppress MissingThrowsDocblock - the queue is not empty */
return $this->dequeue();
}
/**
* Dequeues a node from the queue.
*
* @throws Psl\Exception\InvariantViolationException If the Queue is invalid.
*
* @return T
*/
public function dequeue()
{
Psl\invariant(0 !== $this->count(), 'Cannot dequeue a node from an empty Queue.');
/**
* Peeking into a non-empty queue always results in a value.
*
* @var T $node
*/
$node = $this->peek();
$this->drop();
return $node;
}
private function drop(): void
{
/**
* Retrieve the highest priority.
*
* @var int $priority
*/
$priority = Math\max(Vec\keys($this->queue));
/**
* Retrieve the list of nodes with the priority `$priority`.
*/
$nodes = $this->queue[$priority];
// If the list contained only this node,
// remove the list of nodes with priority `$priority`.
if (1 === Iter\count($nodes)) {
unset($this->queue[$priority]);
} else {
/**
* otherwise, drop the first node.
*
* @psalm-suppress MissingThrowsDocblock
*/
$this->queue[$priority] = Vec\values(Dict\drop($nodes, 1));
}
}
/**
* Count the nodes in the queue.
*/
public function count(): int
{
$count = 0;
foreach ($this->queue as $_priority => $list) {
$count += Iter\count($list);
}
return $count;
}
}