Re^4: Equivalent of unpack 'q' with 32-bit Perl (a8)

Anonymous Monk,

I really appreciate your help and I am not trying to intentionally be obtuse but I am still not following. The code needs to work on the output of Digest::MD5's md5() which according to the docs is This function will concatenate all arguments, calculate the MD5 digest of this "message", and return it in binary form. The returned string will be 16 bytes long.

The following code works

#!/usr/bin/perl
use strict;
use warnings;
use Digest::MD5 'md5';
use Math::BigInt;

my $thing = 'This is a thing';
my $md5 = md5($thing);

print unpack('q', $md5), "\n";
print md5_32bit_sol($md5), "\n";

sub md5_32bit_sol {
    my ($md5) = @_;

    return md5_32bit_positive($md5) if vec($md5, 63, 1) == 0;
    return md5_32bit_negative($md5);
}

sub md5_32bit_negative {
    my ($md5) = @_;
    my $base_2 = '';

    for (reverse 0 .. 63) {
        $base_2 .= abs(vec($md5, $_, 1) - 1);
    }
    $base_2 =~ s/^0+//;

    my $decimal = Math::BigInt->new();

    my $power = Math::BigInt->new(2);
    $power->bpow(length($base_2) - 1);

    for my $pos (0 .. length($base_2) - 1) {
        $decimal->badd($power * substr($base_2, $pos, 1));
        $power->bdiv(2);
    }
    return ($decimal + 1) * -1;

}

sub md5_32bit_positive {
    my ($md5) = @_;
    my $base_2 = '';

    for (reverse 0 .. 63) {
        $base_2 .= vec($md5, $_, 1);
    }
    $base_2 =~ s/^0+//;

    my $decimal = Math::BigInt->new();

    my $power = Math::BigInt->new(2);
    $power->bpow(length($base_2) - 1);

    for my $pos (0 .. length($base_2) - 1) {
        $decimal->badd($power * substr($base_2, $pos, 1));
        $power->bdiv(2);
    }
    return $decimal;
}
[download]

I am not sure why the code for handling negative numbers is so weird but it works for everything I tested.

Cheers - L~R

Comment on Re^4: Equivalent of unpack 'q' with 32-bit Perl (a8) Download Code

Replies are listed 'Best First'.
Re^5: Equivalent of unpack 'q' with 32-bit Perl (a8) by Anonymous Monk on Sep 06, 2016 at 23:41 UTC
well, shouldn't need to involve vec at all, just use md5_hex to get hex version of digest ... you could unpack 'H*' ... and give that to bigint hex ... also negatives, yeah I've no idea :)	[reply]
Re^6: Equivalent of unpack 'q' with 32-bit Perl (a8) by Limbic~Region (Chancellor) on Sep 06, 2016 at 23:51 UTC
Anonymous Monk, just use md5_hex to get hex version of digest That's a brilliant (obvious) idea. I wish I had thought of it before I wrote the code above (which now handles negative numbers as well). I assume your approach will work but I am not inclined to verify (the guy I am helping should be able to do it himself). Thanks! Edit: After thinking about it for a minute, I don't think that would work. Basically, you have an 128 bit integer in binary form and you are then converting the first 64 bits of it into decimal form. I'm pretty sure a hex digest isn't going to help here because you can't simply just cut it in half. Cheers - L~R	[reply]
Re^7: Equivalent of unpack 'q' with 32-bit Perl (a8) by RonW (Parson) on Sep 07, 2016 at 20:41 UTC
After thinking about it for a minute, I don't think that would work. Basically, you have an 128 bit integer in binary form and you are then converting the first 64 bits of it into decimal form. I'm pretty sure a hex digest isn't going to help here because you can't simply just cut it in half. That seems to be what `unpack('q', md5($foo))` is doing - cutting the digest in half. `use Digest::MD5 'md5'; my $foo = 'hello, world!'; my @v = unpack('q', md5($foo)); printf "%x %x\n", @v;` [download] `$ perl unpackq.pl e3ba1f79d1badb3a 0` [download] If you really need to convert the whole digest to an integer, try `Math::BigInt->from_hex(md5_hex($foo))`	[reply] [d/l] [select]
Re^8: Equivalent of unpack 'q' with 32-bit Perl (a8) by Limbic~Region (Chancellor) on Sep 07, 2016 at 20:54 UTC
Re^9: Equivalent of unpack 'q' with 32-bit Perl (a8) by RonW (Parson) on Sep 08, 2016 at 16:34 UTC
Re^7: Equivalent of unpack 'q' with 32-bit Perl (a8) by Anonymous Monk on Sep 07, 2016 at 22:14 UTC
Edit: After thinking about it for a minute, I don't think that would work. Basically, you have an 128 bit integer in binary form and you are then converting the first 64 bits of it into decimal form. I'm pretty sure a hex digest isn't going to help here because you can't simply just cut it in half. What are you talking about?	[reply]