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

use Time::HiRes 'time';
my $t = time;

use constant MAX => 1e6;
use constant TOP => MAX < 20 ? MAX : 20;

my $seqs = sequence( longlong, 1 + MAX );
$seqs-> setbadat( 0 );
$seqs-> setbadat( 1 );
$seqs-> badvalue( 2 );

my $lengths = ones( short, 1 + MAX );
$lengths-> inplace-> setvaltobad( 1 );
$lengths-> set( 1, 1 );
$lengths-> set( 2, 2 );
$lengths-> set( 4, 3 );

my $current = zeroes( short, 1 + MAX );

while ( any $seqs-> isgood ) {      # sic

    my ( $seqs_odd, $current_odd_masked )
        = where( $seqs, $current, $seqs & 1 );

    $current_odd_masked ++;
    $current ++;
    ( $seqs_odd *= 3 ) ++;
    $seqs >>= 1; 
    
    my ( $seqs_cap, $lengths_cap, $current_cap )
        = where( $seqs, $lengths, $current, $seqs <= MAX );

    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
}

$lengths-> badflag( 0 );

my $sorted_i = $lengths-> qsorti;
my $sorted   = $lengths-> index( $sorted_i );
my $value    = $sorted-> at( MAX + 1 - TOP );
my $pos      = vsearch_insert_leftmost( $value, $sorted );
my $top_i    = $sorted_i-> slice([ MAX, $pos ]);

( my $result = $lengths
    -> index( $top_i )
    -> longlong
    -> bitnot
    -> cat( $top_i )
    -> transpose
    -> qsortvec
    -> slice([], [ 0, TOP - 1 ])

)-> slice([ 0 ], [])
 -> inplace
 -> bitnot;

say $result;
say time - $t;

__END__

[
 [   525 837799]
 [   509 626331]
...
 [   445 886953]
 [   445 906175]
 [   445 922524]
 [   445 922525]
]

2.88385105133057