public function offsetGet($offset)
    {
        if (($offset < 0) || ($offset >= $this->charLen)) {
            return null;
        }

        $delta = $offset - $this->charIdx;

        if ($delta > 0) {
            // Fast forwarding.
            while ($delta-- > 0) {
                $this->byteIdx += static::getCharLength($this->str[$this->byteIdx]);
                ++$this->charIdx;
            }
        } elseif ($delta < 0) {
            // Rewinding.
            while ($delta++ < 0) {
                do {
                    $byte = ord($this->str[--$this->byteIdx]);
                } while (($byte >= 128) && ($byte < 192));
                --$this->charIdx;
            }
        }

        $bytesCount = static::getCharLength($this->str[$this->byteIdx]);

        $ret = '';
        for ($i = 0; $bytesCount-- > 0; ++$i) {
            $ret .= $this->str[$this->byteIdx + $i];
        }

        return $ret;
    }