!/usr/local/bin/perl -w
use strict;

#These variables are user-configurable.
my $countermax = 10000;          #The max number of primes per file. (To regulate file size)
my $primefile = "Primes.seed";    #The location of the seed file. This is needed.
my $path = "./";     #The first part of the path to dump the files in. This must be pre-existing.
my $last = ".txt";                #The extension to append to the filenames. Your choice.
my $beginloc = 0;           #The number to start counting at when making filenames.

#End users should not edit below this line
my $outputloc = $beginloc;

#First this looks for a pre-existing primes file
#which has all the primes less than 10000 in it. 
#It needs this file. Assume it is present or
#suggest a better way or something...
open PRIMES, $primefile || die $!;
my @parray = <PRIMES>;
close PRIMES;

open RESULTS, ">".$path.$outputloc.$last || die $!;

print <<TXT;
Welcome to the prime number program.
This program will tell you every prime number within a certain range.
Please enter the lower range now:
TXT

#get from arg or stdin
my $lowrange = int($ARGV[0]||<STDIN>);
chomp $lowrange;

#Simple idiot-proofing. I didn't bother to check to make
#sure it's actually an integer. What's a good way to 
#do this?
if($lowrange < 0) { 
   $lowrange = 0; 
   print "\nYour number must be positive. I have set it to 0 for you.";
}

print "\nHow high should the program search for primes? ";

#get from arg or stdin
my $hirange = int($ARGV[1]||<STDIN>);
chomp $hirange;

if($hirange <= $lowrange) { 
  $hirange = $lowrange + 100;
  print <<TXT;

Your second number must be at least equal to the first.
I have arbitratily set it to be 100 greater.
TXT
}


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

#No need to check even numbers, right?
$lowrange++ unless $lowrange % 2;

my $counter=0;
#Of course, since 1 isn't a prime, we have to account
#for that.
if($lowrange == 1) {
   print RESULTS "2\n";
   $lowrange = 3;
   $counter++;
}

for(my $current = $lowrange; $current <= $hirange; $current += 2) {
  for(my $i = 0; $parray[$i] <= sqrt($current); $i++) {
    if($current % $parray[$i] == 0) { 
      $counter++;
      print RESULTS "$current\n";
      if($counter%$countermax == 0) {
        close RESULTS;
        $outputloc++;
        print "\nFile full. Currently ".($current *100/$hirange)."\% done.";

        open RESULTS, ">".$path.$outputloc.$last || die $!;
      }
      last;
     }
  }
}

print <<TXT;

Total amount of primes: $counter
Your results have been saved, starting with $path$beginloc$last

TXT