For the new year, a somewhat more efficient implementation using Euclid's algorithm. The recursively defined iterator is somewhat cool as well. Memory usage is reasonable...

It would be faster were it not for the absurd function call overhead in Perl. :-( 

use strict;
sub build_sieve {
  my $n = 0;
  my @upcoming_factors = ();
  my ($next_small_p, $sub_iter);
  my $gives_next_square = 5;
  
  return sub {
    LOOP: {
      if (not $n++) {
        return 2; # Special case
      }
      if (not defined $upcoming_factors[0]) {
        if ($n == $gives_next_square) {
          if (not defined ($sub_iter)) {
            # Be lazy to avoid an infinite loop...
            $sub_iter = build_sieve();
            $sub_iter->(); # Throw away 2
            $next_small_p = $sub_iter->();
          }
  
          push @{$upcoming_factors[$next_small_p]}, $next_small_p;
          $next_small_p = $sub_iter->();
          my $next_p2 = $next_small_p * $next_small_p;
          $gives_next_square = ($next_p2 + 1)/2;
          shift @upcoming_factors;
          redo LOOP;
        }
        else {
          shift @upcoming_factors;
          return 2*$n-1;
        }
      }
      else {
        foreach my $i (@{$upcoming_factors[0]}) {
          push @{$upcoming_factors[$i]}, $i;
        }
        shift @upcoming_factors;
        redo LOOP;
      }
    }
  }
}

my $sieve = build_sieve();
print $sieve->(), "\n" for 1..(shift || 100);

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In reply to Re: Prime Iterator by Anonymous Monk
in thread Prime Iterator by Ovid

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