[ILUG] puzzle of the day

[John P. Looney]

On Thu, Feb 14, 2002 at 05:13:26PM +0000, Colm Buckley mentioned:
> On Thu 14 Feb 2002 17:06, Kate wrote:
> 
> > Anyone know how to turn a signed number into an unsigned one.
> 
> Er.  *If* you have a type which can hold the unsigned result, then
> it's:
> 
>       2^(size) + (signed number)
> 
> eg : 4294967296 + (-1407713022) = 2887254274 (not 2392992828; don't
> know where you got that from).
 
 That was a guess :) 

 Ah. Right. Found out what the problem was. Horrid bug. Basically, I was
trying to do a PHP "sort by IP" type function. Problems;

 printf("%u", signed_int);

 printed nothing (despite being told by the docs it should do an integer
conversion on it). So, I'd to do it with:

function ip_compare($arg1,$arg2) {
    $ip1=ip2long($arg1);
    $ip2=ip2long($arg2);
    if($ip1 < 0 ) {
        $ip1 += pow(2,32);
    }
    if($ip2 < 0 ) {
        $ip2 += pow(2,32);
    }
    return($ip1-$ip2);
}

 The idea being that if it return negative, then $ip2 was bigger than
$ip1. Simple. Only, some times it didn't work. Why ?

 The calling function (in this case uasort()) expected an integer. So cast
back the number from a float to an int. Which worked, most of the time,
except if $ip1 was way bigger (more than 2 billion) than $ip2. In which
cast it would return a number bigger than 2 billion...which would get
turned into a negative signed int.

 Bah.

Kate

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John Looney             Chief Scientist
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