Pack/unpack question

[croykee]

I am busy translating a perl script into php. The perl script contains a pack/unpack with B* template. I would like to know how to translate these 2 lines into php:-

1) The first just needs to translate an ASCII string into binary and I used a custom function to accomplish this.
$padBits = unpack("B*", $pad);

2) The second poses a problem where 2 binary strings are XOR-ed and I need to get a string back, like below in PERL
$encoded = encode_base64(pack("B*", $authBits ^ $padBits), "");

How would I go about translating this? Or what is an alternative in PHP for pack/unpack("B*" ?

Thanks,
Croykee

[laserlight]

Have you checked out PHP's own [man]pack/man, [man]unpack/man and [man]base64_encode/man?

[croykee]

Yes I have checked these functions out! The point is that there is no equivalent for pack/unpack("B*" (B template) in PHP's pack/unpack functions. I would like to know how to go about accomplishing this in an alternative way in PHP.

[laserlight]

I dont know what does the B* template do in Perl, so you'll have to provide an algorithm.

[croykee]

B in pack and unpack (PERL only!) Bit string, high-to-low order (See PHP's decbin() function. This is also roughly similar to the Perl vec() function, which has no cognate in PHP.

[laserlight]

Yeah, I decided to do some research.

Based on my understanding on what Perl does, here's the unoptimised PHP equivalent:

function packB($str) {
	$len = strlen($str);
	$ret = 0;
	$n = $len - 1;
	for ($i = 0; $i < $len; $i++) {
		if ($str{$i} != '0')
			$ret += pow(2, $n);
		$n--;
	}
	return $ret;
}

My understanding is that B* takes a string composed of '0's and '1's and returns an integer.

EDIT:
nope, this doesnt do what is needed (i.e. packing into bit string).

[laserlight]

ok, here's a fixed version, but it is horribly slow:

function packB($str) {
	$len = strlen($str);
	$pad_len = $len % 4;
	for ($i = 0; $i < $pad_len; $i++)
		$str = '0' . $str;
	$multiplier = pow(16, strlen($str) / 4 - 1);
	$ret = 0;
	for ($i = 0; $i < $len; $i += 4) {
		$num = 0;
		if ($str{$i} != 0)
			$num += 8 * $multiplier;
		if ($str{$i+1} != 0)
			$num += 4 * $multiplier;
		if ($str{$i+2} != 0)
			$num += 2 * $multiplier;
		if ($str{$i+3} != 0)
			$num += $multiplier;
		$ret += $num;
		$multiplier /= 16;
	}
	return $ret;
}

there should be a few shortcuts here and there that I am not using.

[laserlight]

Ah well, since I spent time on this, if you ever come back to this thread:

function packB($str) {
	$len = strlen($str);
	$pad_len = $len % 4;
	$str = str_pad($str, $pad_len, '0', STR_PAD_LEFT);
	$len += $pad_len;
	$ret = 0;
	$count = $len >> 2;
	for ($i = 0; $i < $len; $i += 4) {
		$count--;
		$num = 0;
		if ($str{$i}   != 0) $num += 8;
		if ($str{$i+1} != 0) $num += 4;
		if ($str{$i+2} != 0) $num += 2;
		if ($str{$i+3} != 0) $num++;
		$ret += $num << ($count << 2);
	}
	return $ret;
}

[croykee]

Thanks for all the hard work! I will take a look this afternoon to see if you cracked the problem ;-)

Thanks again..

[croykee]

This is great, now for the reverse of packB.. What would be the unpack version? unpackB() ??

[laserlight]

hmm... havent you tried writing it yourself?

function unpackB($num) {
	$temp = 0;
	for ($i = $num; $i > 1; $temp++)
		$i = $i >> 1;

$ret = '';
for ($i = $temp; $i >= 0; $i--) {
	$temp = $num - ((2 << $i) / 2);
	if ($temp >= 0) {
		$ret .= '1';
		$num = $temp;
	} else {
		$ret .= '0';
	}
}
return '0' . $ret;
}

I would note that there isnt any check for negative values.