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

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) {
        $n = $input, $steps = 0;
        $n % 2 ? ( $steps += 2, $n = (3 * $n + 1) >> 1 )
               : ( $steps += 1, $n = $n >> 1 )
            while $n != 1 && $n >= $input;

        $cache[$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;