#!/usr/bin/perl
use strict;
use warnings;
use DBM::Deep;
use Inline C =>;

my $prime = DBM::Deep->new( 'primes.db' );
#$prime->{is}{$_} = 1 for (2, 3, 5, 7, 11, 13, 17, 19, 23); # 1st run 
+only
my $next = find_near_primes(10000 => 100);


while ( my $p = $next->() ) {
    print "$p\n";
}

sub find_near_primes {
    my ($num, $tot) = @_;
    my ($below, $above, $up, $down);
    my $by_2_4 = sub { my $n = shift; return sub { return $n = $n == 2
+ ? 4 : 2 } };
    my $dir = -1;

    return sub {
        return () if ! $tot--;
        $dir *= -1;
        if ( $num ) {
            $num = $num - $num % 3;
            $num += not ($num % 2) ? -1 : 2;
            ($below, $above, $num) = ($num, $num, undef); 
            ($up, $down) = ($by_2_4->( 4 ), $by_2_4->( 2 ));
            $below -= $down->();
        }
        if ( $dir == 1 || $below < 5 ) {
            if ( ! $prime->{is}{$above} ) {
                $above += $up->() while $prime->{not}{$above};
                while ( ! is_prime( $above ) ) {
                    $prime->{not}{$above} = 1;
                    $above += $up->() while $prime->{not}{$above};
                    last if $prime->{is}{$above};
                }
                $prime->{is}{$above} = 1;
            }
            my $p = $above;
            $above += $up->() and return $p; 
        }
        else {
            if ( ! $prime->{is}{$below} ) {
                $below -= $down->() while $prime->{not}{$below};
                while ( ! is_prime( $below ) ) {
                    $prime->{not}{$below} = 1;
                    $below -= $down->() while $prime->{not}{$below};
                    last if $prime->{is}{$below};
                }
                $prime->{is}{$below} = 1;
            }
            my $p = $below;
            $below -= $down->() and return $p; 
        }
    };
}
__END__
__C__
#include <math.h>

short is_prime (unsigned long num) {
    unsigned long i = 5;
    unsigned long j = sqrt( num );

    while ( i <= j ) {
        if ( num % i == 0 ) return 0;
        i += 2;
        if ( num % i == 0 ) return 0;
        i += 4;
    }
    return 1;
}
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