public function getStream($stream)
    {
        if ($stream === null) {
            return null;
        }

        $streamData = '';

        if ($this->props[$stream]['size'] < self::SMALL_BLOCK_THRESHOLD) {
            $rootdata = $this->_readData($this->props[$this->rootentry]['startBlock']);

            $block = $this->props[$stream]['startBlock'];

            while ($block != -2) {
                $pos = $block * self::SMALL_BLOCK_SIZE;
                $streamData .= substr($rootdata, $pos, self::SMALL_BLOCK_SIZE);

                $block = self::getInt4d($this->smallBlockChain, $block * 4);
            }

            return $streamData;
        }
        $numBlocks = $this->props[$stream]['size'] / self::BIG_BLOCK_SIZE;
        if ($this->props[$stream]['size'] % self::BIG_BLOCK_SIZE != 0) {
            ++$numBlocks;
        }

        if ($numBlocks == 0) {
            return '';
        }

        $block = $this->props[$stream]['startBlock'];

        while ($block != -2) {
            $pos = ($block + 1) * self::BIG_BLOCK_SIZE;
            $streamData .= substr($this->data, $pos, self::BIG_BLOCK_SIZE);
            $block = self::getInt4d($this->bigBlockChain, $block * 4);
        }

        return $streamData;
    }