use strict;
use warnings;
use Benchmark qw(timethese);

sub popcount1 {
   my $x = shift;
   my $count;
   for ($count = 0; $x; ++$count) { $x &= $x - 1 }
   return $count;
}

sub popcount2 {
   return sprintf('%b', shift) =~ tr/1//;
}

# Update: unpack('%b*', pack('J', shift)) is better (see Rosetta ref. below)
# This works but is slower: unpack('b*', pack('J', shift)) =~ tr/1//;
sub popcount3 {
   return unpack('%32b*', pack('Q', shift));
}

my $start = 2**32 - 42;
my $end   = $start + 1000000;

print "sanity test for correctness\n";
for my $i (0 .. 256, $start .. $end) {
   my $one = popcount1($i);
   my $two = popcount2($i);
   my $three = popcount3($i);
   # print "$i: $one $two $three\n";
   $one == $two or die;
   $one == $three or die;
}

timethese 50, {
   One   => sub { popcount1($_) for $start .. $end },
   Two   => sub { popcount2($_) for $start .. $end },
   Three => sub { popcount3($_) for $start .. $end },
};