如何工作Bitap算法？

I'm trying to understand the algorithm Bitap for string matching The following code related to bitap algorithm that write with PHP language :

function SearchString($text, $pattern)
{
$m = strlen($pattern);
$textLen = strlen($text);
$patternMask = array();
$i;

if (empty($pattern)) return 0;
if ($m > 31) return -1; //Error: The pattern is too long!

$R = ~1;

for ($i = 0; $i <= 127; ++$i)
    $patternMask[$i] = ~0;

for ($i = 0; $i < $m; ++$i)
    $patternMask[ord($pattern[$i])] &= ~(1 << $i);

for ($i = 0; $i < $textLen; ++$i)
{
    $R |= $patternMask[ord($text[$i])];
    $R <<= 1;

    if (0 == ($R & (1 << $m)))
        return ($i - $m) + 1;
}

return -1;
}

Please explain how this code works?

i am not understand this part :

for ($i = 0; $i < $m; ++$i)
$patternMask[ord($pattern[$i])] &= ~(1 << $i);

for ($i = 0; $i < $textLen; ++$i)
{
$R |= $patternMask[ord($text[$i])];
$R <<= 1;

if (0 == ($R & (1 << $m)))
    return ($i - $m) + 1;
}

return -1;

thanks