class ctools_math_expr_stack

class ctools_math_expr_stack {
    
    /**
     * The stack.
     *
     * @var array
     */
    private $stack;
    
    /**
     * The stack pointer, points at the first empty space.
     *
     * @var int
     */
    private $count;
    
    /**
     * Ctools_math_expr_stack constructor.
     */
    public function __construct() {
        $this->stack = array();
        $this->count = 0;
    }
    
    /**
     * Push the value onto the stack.
     *
     * @param mixed $val
     */
    public function push($val) {
        $this->stack[$this->count] = $val;
        $this->count++;
    }
    
    /**
     * Remove the most recently pushed value and return it.
     *
     * @return mixed|null
     *   The most recently pushed value, or NULL if the stack was empty.
     */
    public function pop() {
        if ($this->count > 0) {
            $this->count--;
            return $this->stack[$this->count];
        }
        return NULL;
    }
    
    /**
     * "Peek" the stack, or Return a value from the stack without removing it.
     *
     * @param int $n
     *   Integer indicating which value to return. 1 is the topmost (i.e. the
     *   value that pop() would return), 2 indicates the next, 3 the third, etc.
     *
     * @return mixed|null
     *   A value pushed onto the stack at the nth position, or NULL if the stack
     *   was empty.
     */
    public function last($n = 1) {
        return !empty($this->stack[$this->count - $n]) ? $this->stack[$this->count - $n] : NULL;
    }
    
    /**
     * Return the number of items on the stack.
     *
     * @return int
     *   The number of items.
     */
    public function count() {
        return $this->count;
    }

}