#!/usr/bin/env perl
use strict;
use warnings;

use Inline C => Config => CCFLAGSEX => '-O2 -fomit-frame-pointer';
use Inline C => <<'END_OF_C_CODE';

#include <stdint.h>

void num_steps_c( SV* _n, SV* _s )
{
    uint64_t n, input;
    int steps = 0;

    n = input = SvUV(_n);

    while ( n != 1 && n >= input ) {
        n % 2 ? ( steps += 2, n = (3 * n + 1) >> 1 )
              : ( steps += 1, n = n >> 1 );
    }

    sv_setuv(_n, n);
    sv_setiv(_s, steps);

    return;
}

END_OF_C_CODE

my $size = shift || 1e6;

$size = 1e6 if $size < 1e6;  # minimum
$size = 1e9 if $size > 1e9;  # maximum

##
# iM71's demonstration + applied T(x) notation and compression
#   https://stackoverflow.com/a/55361008
#   https://www.youtube.com/watch?v=t1I9uHF9X5Y (1 min into video)
#
# Parallel solution
#   https://www.perlmonks.org/?node_id=11115780
##

my @cache = (0, 1, 2);
my @seqs;

sub collatz_seq {
    my $size = shift;
    my ($n, $steps);
    for my $input (2..$size) {
        num_steps_c($n = $input, $steps);
        $cache[$input] = $steps += $cache[$n];
        push @seqs, [ $input, $steps ] if $steps > 400;
    }
}

collatz_seq($size);
@seqs = ( sort { $b->[1] <=> $a->[1]} @seqs )[ 0..19 ];

printf "Collatz(%5d) has sequence length of %3d steps\n", @$_
    for @seqs;