CODEWITHSUNDEEP
phppackunpack
Syhm
Syhm

Reputation: 13

Implement unpack b from Perl in PHP

Given a string of bytes I want their binary representation such that the bits of each byte are ordered from least to most significant (as produced by Perl's unpack "b*").

For example,

"\x28\x9b"

should return

"0001010011011001"

In this post Ilmari Karonen described how to achieve pack "b*" in PHP. So I thought all I have to do is split the hex string into bytes and run them through base_convert.

function unpack_B($data) {
    $unpacked = unpack("H*", $data)[1];
    $nibbles = str_split($unpacked, 2);
    foreach ($nibbles as $i => $nibble) {
        $nibbles[$i] = base_convert($nibble, 16, 2);
    }
    return implode("", $nibbles);
}

However, It's returning something different.

What am I missing here?

Upvotes: 1

Views: 109

Answers (1)

Sammitch
Sammitch

Reputation: 32252

Looking at the docs for perl's pack() it seems like B is the usual "big endian" [I know I'm abusing this term] "descending" bit order, and b is "little endian"/"ascending".

I honestly cannot parse what on earth that the code/answer you've linked is supposed to do, so I've written it all from scratch based on what the perl docs say the pack arguments do.

function bin_to_litbin($input, $be=true) {
    return implode(
        '',
        array_map(
            function($a)use($be){
                $ret = str_pad(decbin(ord($a)), 8, '0', STR_PAD_LEFT);
                if(!$be) {
                    $ret = strrev($ret);
                }
                return $ret;
            },
            str_split($input, 1)
        )
    );
}

function litbin_to_bin($input, $be=true) {
    return implode(
        '',
        array_map(
            function($a)use($be){
                if(!$be) {
                    $a=strrev($a);
                }
                return chr(bindec($a));
            },
            str_split($input, 8)
        )
    );
}

$hex = '00289b150065b302a06c560094cd0a80';
$bin = hex2bin($hex);

var_dump(
    $hex,
    $cur =  bin_to_litbin($bin, false),
    bin2hex(litbin_to_bin($cur, false))
);

where $be=true is B/"big endian" and $be=false is b/"little endian".

Output:

string(32) "00289b150065b302a06c560094cd0a80"
string(128) "00000000000101001101100110101000000000001010011011001101010000000000010100110110011010100000000000101001101100110101000000000001"
string(32) "00289b150065b302a06c560094cd0a80"

Though truth be told I cannot think of any practical reason to ever encode data as literal zero and one characters. It is wildly unnecessary and wasteful compared to literally any other encoding. I would wager that that is why B and b were never implemented in PHP.

Base64 is 1.33x the length its input, hex is 2x, and literal binary is 8x.

Upvotes: 1

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