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

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 :)

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