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<?php
/**
* @author Tassos.gr <info@tassos.gr>
* @link https://www.tassos.gr
* @copyright Copyright © 2024 Tassos All Rights Reserved
* @license GNU GPLv3 <http://www.gnu.org/licenses/gpl.html> or later
*/
namespace NRFramework\Parser;
defined('_JEXEC') or die;
use NRFramework\Parser\Lexer;
use NRFramework\Parser\RingBuffer;
/**
* Parser base class
* LL(k) recursive-decent parser with backtracking support
*/
abstract class Parser
{
/**
* Lexer instance (feeds the parser with tokens)
*
* @var NRFramework\Parser\Lexer
*/
protected $input = null;
/**
* Ring buffer of the next k tokens
* from the input stream
*
* @var RingBuffer
*/
protected $lookahead = null;
/**
* k: Number of lookahead tokens
*
* @var int
*/
protected $k;
/**
* Array(stack) containing the current
* contents of the lookahead buffer when
* marking the position of the stream
*
* @var array
*/
protected $lookahead_history = null;
/**
* Lexer constructor
*
* @param Lexer $input
* @param integer $k, number of lookahead tokens
*/
public function __construct(Lexer $input, $k = 1)
{
if (!is_integer($k) || ($k < 1))
{
throw new \InvalidArgumentException('Parser: $k must be greater than 0!');
}
$this->k = $k;
$this->input = $input;
$this->lookahead_history = [];
// initialize lookahead buffer
$this->resetBuffer();
}
/**
* Checks the type of the next token.
* Advances the position in the token stream.
*
* @param string $type
* @return void
*
* @throws Exception
*/
public function match($type)
{
if ($this->lookahead[0]->type === $type)
{
$this->consume();
return;
}
throw new Exceptions\SyntaxErrorException('Expecting token ' . $type . ', found ' . $this->lookahead[0]);
}
/**
* Retrieves the next token from the input stream
* and add it to the buffer.
*
* @return void
*/
public function consume()
{
$this->lookahead[] = $this->input->nextToken();
}
/**
* Marks the position in the token stream
*
* @return void
*/
public function mark()
{
array_push($this->lookahead_history, $this->lookahead);
$this->input->mark();
}
/**
* Reset to a previously marked position
* in the token stream
*
* @return void
*/
public function reset()
{
$this->input->reset();
// reset lookahead buffer if not marked
if (empty($this->lookahead_history))
{
$this->resetBuffer();
}
// normal reset
else
{
$this->lookahead = array_pop($this->lookahead_history);
}
}
/**
* Resets and refills the lookahead buffer starting
* from the current position in the token stream
*
* @return void
*/
protected function resetBuffer()
{
$this->lookahead = new RingBuffer($this->k);
for ($i=0; $i < $this->k; $i++)
{
$this->consume();
}
}
}