Slowdown example

euler-35.pl6

# How many circular primes are there below one million?
# real	0m46.458s
# user	0m46.080s
# sys	  0m0.068s
# This is Rakudo version 2016.03-110-g5bfc8fb built on MoarVM version 2016.03-104-g10d3971
use v6;
my $max = 1_000_000;
# find all primes up to $max using The Sieve of Erathostenes
my int @a = (0..$max);

for (2..($max div 2)) -> $i {
  my $j = 2;
  @a[$i * $j++] = 1 while $i * $j <= $max;
}
# remove discarded numbers and store the rest in a set for ease of lookup
my @primes = @a.grep(-> $num {$num > 1});
my $set = SetHash.new(@primes);

say "The number of circular primes is ", @primes.grep( -> $prime { &checkPrime($prime) } ).elems;

sub checkPrime($p) {
  my $tmp = $p;
  my $numDigits = (~$p).chars;
  return True if $numDigits == 1;
  loop (my $i = 1; $i <= $numDigits - 1; $i++) {
    $tmp = $tmp % 10 * 10 ** ($numDigits - 1) + $tmp div 10;
    return False if !$set{$tmp};
  }
  return True;
}