CGIpack

[jeroenes]

Category:

CGI programming or Text

Description:

CGIpack is a module I wrote in this response.

I came up with this solution. The encoder takes a list of data, together with a list with the desired number of bits to be saved. The bits are divided into chunks of 6 bits, that are encoded with the alphanumeric characters *and* % and - (64 characters in total). The resulting string is short, contains as little bits as possible and/or desired by the user.

See POD for further info.

Update:Version 0.02 has working 32 bit values now. Still pondering 64 bits.

Update2:Thanx to ChOas, I now know that perl 5.6 has another implementation of pack, *and* that 64 bits can be used only if you have a 64bit machine and 64 bit support is compiled in. I've chosen to comment that line in decode, you can uncomment it if your environment is right, otherwise there is a risk of a fatal error. Please let me know if you succesfully use 64 bits.

Wishlist:

• Apply tye's quad for 57 bit integers
• Add wrapper, to accomodate: characters, strings, signed integers.

package CGIpack;
use strict;

=head1 NAME

CGIpack - Transforms parameters into a packed list of URL-compatible c
+haracters
and vice versa.

=head1 SYNOPSIS

 use CGIpack;

 # Transform a list of numbers and back. 
 #Results in:
 @data=(15,13,2**23,0,2**31);
 @bitsizes = (4,14,24,1,55 );
 $encoded=encode({ bitsizes=>[@bitsizes], data=>[@data]});
 $decoded=join ', ', decode( {str=>$encoded, bitsizes=>[@bitsizes]} );
 ## only converted 32 bits instead of 55
 #Data: 15, 13, 8388608, 0, 2147483648
 #Encoded: V30000W000004000
 #Decoded array: 15, 13, 8388608, 0, 2147483648
 
=head1 DESCRIPTION

CGIpack is a module I wrote in response to a question.  Question went 
+like: 
"I want my users to save a URL containing a CGI with parameters.  To a
+void 
problems with email-readers, the list should be as short as possible, 
+and 
to avoid users messing around with the values I would like to see them
+ 
encoded in a certain way."

I came up with this solution.  The encoder takes a list of data, toget
+her 
with a list with the desired number of bits to be saved.  The bits are
+ 
divided into chunks of 6 bits, that are encoded with the alphanumeric 
characters *and* % and - (64 characters in total). The resulting strin
+g
is short, contains as little bits as possible and/or desired by the us
+er. 

=over 4

=item encode( $hashref )

Takes a hashref, which should contain a member 'data', referring to a 
+list, 
and a member 'bitsizes', referring to a list containing the number of 
desired bits for every value in the list. 

=item decode( $hashref )

Takes a hashref, which should contain a member 'str', containing a str
+ing 
of characters produced by encode (preferably, you may wanna roll your 
+own 
;-) and a member 'bitsizes', referring to a list containing the number
+ of 
desired bits for every value encoded in the string.

=back

=head1 CAVEAT

If you want to encode characters, you will have to convert them manual
+ly 
using unpack/pack 'c'.  I just may add a wrapper for that in the futur
+e, 
though.

It does work on 32 bit integers, but not yet on 64 bit. I'm a little p
+uzzled
why not, it may have something to do with the fact I'm working on a 32
+-bit
pentium right now. Will try at home with my PPC.

=head1 AUTHOR

Jeroen Elassaiss-Schaap

=head1 LICENSE

Perl/ artisitic license

=head1 STATUS

Alpha

=cut

use Exporter;
use vars qw( @EXPORT @ISA @VERSION);
@VERSION = 0.021;
@ISA = qw( Exporter );
@EXPORT = qw( &encode &decode);

sub encode{
    my $hash = shift;
    my @data = @{$hash->{'data'}};
    my @bitsizes = @{$hash->{'bitsizes'}};
    my ($str, $bitstr);
    for my $bits (@bitsizes) {
        $bitstr .= unpack("b$bits", pack('VV', shift( @data ) ));
    }
    $bitstr .= '0' x ( length($bitstr) % 6 );
     for my $item (0..( length($bitstr) / 6 - 1 )){
         my $val=pack('b6', substr($bitstr, $item*6, 6) );
         for ($val) {
             tr    [\100\077\000-\010\011-\043\044-\075]
                 [\045\055\060-\071\101-\132\141-\172];
            $str.=$_;
        }
     }
    $str;
}

sub decode{
    my $hash = shift;
    my $str = $hash->{'str'};
    my @bitsizes= @{$hash->{'bitsizes'}};
     my ($bitstr, $val);
     my @data;
    for (split //, $str){
        tr    [\045\055\060-\071\101-\132\141-\172]
            [\100\077\000-\010\011-\043\044-\075];
        $bitstr .= unpack("b6",$_); 
    }
    my $pointer = 0;
    for my $bits (@bitsizes) {
        my $val;
        for ($bits) {
            $val = unpack('c',pack("b$_",substr( $bitstr, $pointer, $_
+ ))), last
                if $bits < 9;
            $val = unpack('v',pack("b$_",substr( $bitstr, $pointer, $_
+ ))), last
                if $bits < 17;
            $val = unpack('V',pack("b32",substr( $bitstr, $pointer, $_
+ ))), last
                if $bits < 33;
            if (! eval('{$val = unpack("Q",pack("b$_",substr( '.
                    '$bitstr, $pointer, $_ )));1;}') ) {
                warn "only converted 32 bits instead of $_\n";
                $val = unpack('V',pack("b$_",
                    substr( $bitstr, $pointer, $_ )));
            }
        }
        push( @data, $val);
        $pointer += $bits;
    }
    @data;
}

1;

[yakko]

Since I have access to a 64bit platform, I can at least offer a data point. Here's what I get when running the snippet in the POD on an UltraSPARC-II (a quad-way E450):

Encoded: V30000W000004000
Decoded array: 15, 13, 8388608, 0, 549755813888

Looks like I get to sort out {,un}pack sooner than I thought, since this now has my curiosity :o)

(64bit OS, perl 5.6 compiled manually with 64bit ints, btw)

--
Me spell chucker work grate. Need grandma chicken.

[marius]

Now that is the coolest use of {un}pack that I've seen yet. (Not that I personally have seen many uses for it, but this one is cool!) This is definitely going into my personal snippets.. ++jeroenes!

-marius