<?php
require_once 'CommandInterface.php';

class Queue
{
    private $_commands;
    private $_currentIndex;

    public function __construct()
    {
        $this->_commands     = array();
        $this->_currentIndex = 0;
    }

    public function addCommand(CommandInterface $command)
    {
        $this->_commands[] = $command;
    }

    public function run()
    {
        while (!is_null($command = $this->next())) {
            echo $command->execute();
        }
    }

    private function next()
    {
        if (count($this->_commands) === 0 || count($this->_commands) <= $this->_currentIndex) {
            return null;
        } else {
            return $this->_commands[$this->_currentIndex++];
        }
    }
}