From: [email protected]
[mailto:[email protected]] On Behalf Of
[email protected]
Sent: 14 June 2011 14:39
To: [email protected]
Subject: reverse of modulo (inverse?)
> Greetings.
>
> I'm not having much luck finding this, and it's been a long time since
> college math, so I thought I'd throw it > at you folks and cross my fingers...
>
> I'm looking for an algorithm that reverses the modulo function. That is, if x
> = y % 256, given x, what is y?
That's not possible, because you have discarded necessary information.
>
> Broader picture is, I'm trying to back-figure a site number from an IP
> address. E.g., given number x = 12345, > the IP's middle octets ( x/256 and
> x%256 respectively) come out to 48 and 57 . Is there an algorithm I can put >
> the numbers 48 and 57 into and have 12345 come out?
Well, that's a different question (assuming that I have understood the last
paragraph). If you have both x/256 and x%256, and you know the modulus/divisor
was 256, then you have enough to reconstruct the original. For example ...
use strict;
use warnings;
my $mod = 256;
for my $x (@ARGV) {
my $o1 = int($x / $mod);
my $o2 = $x % $mod;
my $orig_x = $o1 * $mod + $o2;
print "Starting with x=$x and mod=$mod, o1=$o1 o2=$o2, reconstructed
x=$orig_x\n";
}
HTH
--
Brian Raven
Please consider the environment before printing this e-mail.
This e-mail may contain confidential and/or privileged information. If you are
not the intended recipient or have received this e-mail in error, please advise
the sender immediately by reply e-mail and delete this message and any
attachments without retaining a copy.
Any unauthorised copying, disclosure or distribution of the material in this
e-mail is strictly forbidden.
_______________________________________________
ActivePerl mailing list
[email protected]
To unsubscribe: http://listserv.ActiveState.com/mailman/mysubs
