#!/usr/local/bin/perl -w

use strict;

################################################################################

my $resultFile = "results.txt";

my $lowrange = 0;
my $hirange = 0;

################################################################################

print "Welcome to the prime number program.\n";
print "This program will tell you every prime number within a certain range.\n";

print "Please enter the lower range now: ";
chomp($lowrange = <STDIN>);

die "\nThe low must be a positive integer\n"
        if ($lowrange && $lowrange =~ /\D/);

print "\nHow high should the program search for primes? ";
chomp($hirange = <STDIN>);

die "\nThe high must be a positive integer\n"
        if ($hirange && $hirange =~ /\D/);
die "\nThe high must be greater than or equal to the low\n"
        if ($hirange < $lowrange);

print "Now listing the range of all primes between $lowrange and $hirange. \n";
print "Please wait.\n";

$lowrange++ if ($lowrange % 2 == 0);

open RESULTS, ">$resultFile"
        || die "Cannot open $resultFile for writing\n";

my $counter = 0;

if ($lowrange <= 3) {
        if ($lowrange <= 2) {
                print RESULTS "2\n";
                $counter++;
        }

        print RESULTS "3\n";
        $lowrange = 5;
        $counter++;
}

CANDIDATE:
for (my $candidate = $lowrange; $candidate <= $hirange; $candidate += 2) {

        for my $divisor (3 .. int(sqrt($candidate))) {
                next CANDIDATE
                        unless $candidate % $divisor;
        }


        $counter++;
        print RESULTS "$candidate\n";
}

close RESULTS;

print "\nTotal amount of primes: $counter \n";
print "Your results have been saved to $resultFile\n";

__END__