brainfuck interpreter

brainfuck.php

<?php

$b = new Brainfuck(file_get_contents('hello.b'));
$b->execute();

class BrainFuck {

	private $code;
	private $cells = array();
	private $cell = 0;
	private $loopCount = 0;

	public function __construct($code) {
		$code = $this->cleanCode($code);
		
		if (!$this->testLoops($code)) {
		echo $code;
			throw new Exception('Incorrectly formatted code, loops don\'t fit');
		}
		
		$this->code = $code;
	}
	
	public function execute() {
		for ($i = 0; $i < strlen($this->code); $i++) {
			switch ($this->code[$i]) {
			
				case '>':
					$this->cell++;
				break;
				
				case '<':
					$this->cell = max(0, $this->cell - 1);
				break;
				
				case '+':
					$this->incrementCell($this->cells[$this->cell]);
				break;
				
				case '-':
					$this->decrementCell($this->cells[$this->cell]);
				break;
				
				case '.':
					echo chr($this->cells[$this->cell]);
				break;
				
				case ',':
					$this->cells[$this->cell] = ord(fgetc(STDIN));
				break;
				
				case '[':
					if (empty($this->cells[$this->cell])) {
						$i = $this->getLoopEnd($i);
					}
				break;
				
				case ']':
					if (!empty($this->cells[$this->cell])) {
						$i = $this->getLoopStart($i);
					}
				break;
			
			}
		}
		
		echo "\nDone";
	}
	
	private function incrementCell(&$cell) {
		if (isset($cell)) {
			$cell = min($cell + 1, 255);
		} else {
			$cell = 1;
		}
	}
	
	private function decrementCell(&$cell) {
		if (isset($cell)) {
			$cell = max($cell - 1, 0);
		} else {
			$cell = -1;
		}
	}
	
	private function getLoopStart($pos) {
		$counter = 1;
		for ($i = $pos - 1; $i != 0; $i--) {
			if ($this->code[$i] == ']') {
				$counter++;
			}
			
			if ($this->code[$i] == '[') {
				 $counter--;
			}
			
			if ($counter == 0) {
				return $i;
			}
		}
	}
	
	private function getLoopEnd($pos) {
		$counter = 1;
		for ($i = $pos + 1; $i < strlen($this->code); $i++) {
			if ($this->code[$i] == ']') {
				$counter--;
			}
			
			if ($this->code[$i] == '[') {
				 $counter++;
			}
			
			if ($counter == 0) {
				return $i;
			}
		}
	}
	
	
	private function testLoops($code) {
		$counter = 0;
		
		for ($i = 0; $i < strlen($code); $i++) {
			if ($code[$i] != '[' && $code[$i] != ']') {
				continue;
			} else if ($code[$i] == '[') {
				$counter++;		
			} else {
				$counter--;
			}
			
			if ($counter < 0) {
				return false;
			}						
		}
		
		return $counter === 0;
	}
	
	private function cleanCode($code = '') {
		return preg_replace('/[^\[\],\.><+-]/', '', $code);
	}

}