use strict;
use warnings;
use feature 'say';

use PDL;
use Time::HiRes 'time';

{
    no warnings 'once'; $PDL::BIGPDL = 1;
    eval q{ PDL::set_autopthread_targ(1) };
}

use constant MAX    => shift || 500000;
use constant MAXLEN => MAX * 1;

my $t = time;
my $lengths = ones( short, 3 + MAXLEN );

$lengths-> inplace-> setvaltobad( 1 );
$lengths-> set( 1, 1 );
$lengths-> set( 2, 2 );
$lengths-> set( 4, 3 );

my ($from, $to) = (0, MAX);
my $seqs_c = $from + sequence( longlong, $to - $from + 1 );

$seqs_c-> setbadat( 0 );
$seqs_c-> setbadat( 1 );
$seqs_c-> badvalue( 2 );

my $lengths_c = $lengths-> slice([ $from, $to ]);
my $current   = zeroes( short, nelem( $seqs_c ));

while ( any $seqs_c-> isgood ) {
    my ( $seqs_c_odd, $current_odd_masked )
        = where( $seqs_c, $current, $seqs_c & 1 );

    $current_odd_masked ++;
    $current ++;
    ( $seqs_c_odd *= 3 ) ++;
    $seqs_c >>= 1;

    my ( $seqs_cap, $lengths_cap, $current_cap )
        = where( $seqs_c, $lengths_c, $current,
            $seqs_c <= MAXLEN );

    my $lut = $lengths-> index( $seqs_cap );

    # "_f" is for "finished"
    my ( $seqs_f, $lengths_f, $lut_f, $current_f )
        = where( $seqs_cap, $lengths_cap, $lut, $current_cap,
            $lut-> isgood );

    $lengths_f .= $lut_f + $current_f;
    $seqs_f    .= 2;                    # i.e. BAD
}

say {*STDERR} time - $t;