Bitwise ops inconsistent

smrtdrmmr has asked for the wisdom of the Perl Monks concerning the following question:

After having searched for a while and trying to figure it out, I am at the mercy of Perl Monks. Here is the code that should effectively right rotate $p, bitwise, by 1 spot each time through the loop:

for($a = 0; $a < ($n % $SIZE); $a++)
{
    $carry = 0x1 & $p; #extract least significant bit
    $p >>= 1; #shift $p by one spot to the right
    if($carry == 1) #to rotate in a leading 1
    {
        my $or = 2 ** ($SIZE - 1); #$SIZE is # of bits in $p
        print "OR: $or\n";
        print "pre|: $p. carry: $carry\n";
        $p |= $or; #bitwise or in $or to add the leading 1 to $p
    }
    print "$a-$p\n";
}
[download]

I am using Perl 5.6.1. Here are some sample outputs for a given $p.

#this case works when $SIZE = 32 and $n = 3
p: 204229738. #ok
0-102114869 #ok, carry was 0 before 1st shift
OR: 2147483648 #carry is 1 before 2nd shift
pre|: 51057434. carry: 1 #p after second shift
1-2198541082 #p after | with $or
2-1099270541 #p after third shift

#this case fails when $SIZE = 40 and $n = 1
p: 418262508595. #ok
OR: 549755813888 #ok
pre|: 2147483647. carry: 1 #not ok
0-4294967295 #not ok

It seems that anything of size 5 bytes or greater fails consistently. Any thoughts why and how to combat this?

Comment on Bitwise ops inconsistent Download Code

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Re: Bitwise ops inconsistent by Zaxo (Archbishop) on Aug 24, 2005 at 00:21 UTC
`use Math::BigInt;` You can stretch to eight bytes if your perl is compiled to handle 64-bit integers. After Compline, Zaxo	[reply] [d/l]
Re^2: Bitwise ops inconsistent by ysth (Canon) on Aug 24, 2005 at 05:00 UTC
`use Math::BigInt;` But not the one shipped with 5.6.1; upgrade to the latest version on CPAN.	[reply] [d/l]
Re: Bitwise ops inconsistent by pg (Canon) on Aug 24, 2005 at 01:26 UTC
Your numbers have been sliently converted to floating (when they are too big for integer), and those integer operators no longer produce meaningful results (or as those good documents put: result is undefined)	[reply]
Re: Bitwise ops inconsistent by GrandFather (Saint) on Aug 24, 2005 at 00:22 UTC
First, you should provide a complete example including the data that generates the result shown. Second, your results would be much clearer of you printed hex rather than decimal numbers (use `printf "OR: %08x\n", $or;` for example). Third, always `use strict; use warnings;` Perl is Huffman encoded by design.	[reply] [d/l] [select]
Re: Bitwise ops inconsistent by QM (Parson) on Aug 24, 2005 at 15:31 UTC
Excellent advice above. I'd add/enhance it by saying: `use integer;` [download] might help, or use bit vectors; -QM -- Quantum Mechanics: The dreams stuff is made of	[reply] [d/l]