SPUG: Primes

[Fred Morris]

Somebody told me this wasn't math, just accounting. But I can't seem to find 
it exactly described in the literature. Have fun. Let me know.

#!/usr/bin/perl 
# Expects -p:max-to-test where max-to-test is an integer.

use Getopt::Std;
use vars '$opt_p';
use strict;

# Annoying hidden voodoo.
getopts('p:');

($opt_p =~ m/^\d+$/) or die 'p must be an unsigned decimal integer';

my $i = 2; # First possible value.
my %numbers;

sub next_number($$) {

    my $i = shift;
    my $k = shift;

    my $next_i = $i + $k;
    if (exists $numbers{$next_i}) {
        push @{$numbers{$next_i}}, $k;
    }
    else {
        $numbers{$next_i} = [$k];
    }

    return;
}

while ($i < $opt_p) {

    if (exists $numbers{$i}) {
        map next_number($i,$_), @{$numbers{$i}};
        delete $numbers{$i};
    }
    else {
        printf "%8d\n", $i;
        next_number($i,$i);
    }

    $i += 1;
}

exit(0);