use warnings; use strict; use feature 'say';
use MCE::Util;
use MCE::Flow;

sub collatz {
    my ($start) = @_;
    my @seq = $start;
    push @seq, ($seq[-1] / 2, 3 * $seq[-1] + 1)[$seq[-1] % 2]
        while $seq[-1] != 1;
    return @seq;
}

my @sizes;

mce_flow_s {
    max_workers => MCE::Util::get_ncpu(),
    bounds_only => 1,
    gather      => \@sizes,
}, sub {
   my ($mce, $chunk, $chunk_id ) = @_;
   my ($start, $end) = @$chunk;
   my @chunk_sizes;

   push @chunk_sizes, [$_, scalar collatz($_)] for $start .. $end;

   MCE->gather( @chunk_sizes );
}, 1, 1e6;

say "@$_" for reverse +(sort { $b->[1] <=> $a->[1] } @sizes)[0..19];