For background see Re^9: Compress positive integers and the thread above which suggests an algorithm and the thread above which details the task.

Here is an implementation of the MSB serialisation strategy using fixed width 4 byte ints for simplicity.This task really asks to be implemented in C but can of course be done in Perl. << is a bitshift operator | is binary OR and & is binary AND. 

use strict;
use Data::Dumper;

my $data = { 1 => [ 1..10 ],
             2 => [ 11..20 ],
             3 => [ 21..30 ],
};

my $MSB       = 1<<31;
print "$MSB is MSB in decimal\n";

my $SET_MSB   = pack "I",$MSB;
printf "%s SET MASK in binary\n", (unpack "b32", $SET_MSB);

my $UNSET_MSB = pack "I",$MSB-1;
printf "%s UNSET MASK in binary\n", (unpack "b32", $UNSET_MSB);

my $res = serialise($data);
my $dat = unserialise($res);
print Data::Dumper::Dumper($dat);

sub serialise {
    my $data = shift;
    my $str = '';
    for my $doc_id( keys %$data ) {
        $str .= (pack "I", $doc_id) | $SET_MSB;
        for my $pos ( @{$data->{$doc_id}} ) {
            $str .= (pack "I", $pos) & $UNSET_MSB;
        }
    }
  return $str;
}
sub unserialise {
    my $data = shift;
    my $dat = {};
    my $doc_id = undef;
    for( my $i=0;$i<length $data; $i+=4) {
        my $int = unpack "I", (substr $data,$i);
        if ( $int > $MSB ) {
            $doc_id = unpack "I", ((pack "I", $int) & $UNSET_MSB);
        }
        else {
            push @{$dat->{$doc_id}}, $int;
        }
    }
  return $dat;
}

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In reply to Re: Byte allign compression in Perl.. by tachyon-II
in thread Byte allign compression in Perl.. by MimisIVI

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