1: <?php
 2: /**
 3:  * BN-PHP (https://bitbucket.org/zdenekdrahos/bn-php)
 4:  * @license New BSD License
 5:  * @author Zdenek Drahos
 6:  */
 7: 
 8: namespace BN\Collections;
 9: 
10: /**
11:  * Class represeting LIFO data structure. The last element added
12:  * to the stack will be the first one to be removed.
13:  */
14: class Stack extends LinearDataStructure
15: {
16:     /**
17:      * Inserts element to the beggining of the stack
18:      * @param  mixed                                 $element
19:      * @throws \BN\Collections\NullArgumentException
20:      */
21:     public function push($element)
22:     {
23:         parent::checkIfElementIsNotNull($element);
24:         array_unshift($this->data, $element);
25:     }
26: 
27:     /**
28:      * Removes and returns first element of the stack.
29:      * Returns null if the stack is empty
30:      * @return mixed
31:      */
32:     public function pop()
33:     {
34:         if (!$this->isEmpty()) {
35:             return array_shift($this->data);
36:         }
37:         return null;
38:     }
39: 
40:     /**
41:      * Returns first element of the stack (element is not removed from the stack).
42:      * Returns null if the stack is empty
43:      * @return mixed
44:      */
45:     public function peek()
46:     {
47:         if (!$this->isEmpty()) {
48:             return $this->data[0];
49:         }
50:         return null;
51:     }
52: }
53: