Re^2: How do I make a random shuffle deterministic?

[LanX]

Most implementations of "srand" take an integer and will silently truncate decimal numbers. This means "srand(42)" will usually produce the same results as "srand(42.1)". To be safe, always pass "srand" an integer.

most strings will evaluate to 0 !

  DB<116> srand 0
 => 1

  DB<117> rand
 => "0.17082803610629"

  DB<118> rand
 => "0.749901980484964"

  DB<119> srand hostname
 => 1

  DB<120> rand
 => "0.17082803610629"

  DB<121> rand
 => "0.749901980484964"

  DB<122> 0+ hostname
 => 0

You need a checksum to produce a (pseudo) unique integer. You could try

$seed += ord($_) for split //, hostname

But whats wrong with the IP?

[Anonymous Monk]

The script has be as portable as possible. I'm not certain that the modules required to get an IP address will be available on an old version of perl that might be found on a Solaris or AIX host.

[LanX]

I just realized you had the same idea. But

1. The sum of two strings can be identical, try "rolf" and "rofl" and "flor"

2. for ( < @array > ) is dangerous nonsense, always do for (  @array  ) without glob

3. if you wanna play save, try a system cmd to get a mac adress.

4. You could determine a unique seed at installation time and store it in a module MyProject::Seed you use later. Like this you have full control (just edit the module) and you only need to fiddle once with the OS for a unique key.

5. you could even request a unique key over web at installation time.

[Anonymous Monk]

Thanks for the tips. It is acceptable if some hosts have the same seed. Still, lessening those occurrences is a good thing. How about this:

sub seed {
    my $string = shift;
    my $seed;

    my @ascii = map ord, split //, $string;
    my $product = pop @ascii;
    for ( @ascii ) { $seed += $_ };

    $seed = $seed * $product;

    srand $seed;
}

[Anonymous Monk]

New try after some research.

use Sys::Hostname;
use Digest::MD5 qw( md5 );

sub seed {
    my $string = shift;
    my $seed = 0;

    my $digest = md5( $string );
    
    for ( split //, $digest ) {
        $seed = ( $seed * 256 + ord ) % 1e12;
    }

    srand $seed;
}

# main matter
my $hostname = hostname;
seed ( $hostname );

[DrHyde]

If the modules aren't installed, then install them. If the most recent version of a module doesn't work on your ancient system, use cpXXXan.