Hi Laurent,

Thank you, for your caching algorithm. It has kept me busy after hours. I added Step 4 for less caching. Basically, commenting 2 lines.

I updated my posts here and here.

sub collatz_seq {
    my $input = shift;
    my $n = $input;
    my $result = 0;
    my $new_n;

    while ($n != 1) {
        $result += $cache[$n], last
            if defined $cache[$n];

        $n % 2 ? ( $result += 2, $new_n = (3 * $n + 1) >> 1 )
               : ( $result += 1, $new_n = $n >> 1 );

      # $cache[$n] = $cache[$new_n] + ($n % 2 ? 2 : 1)
      #     if defined $cache[$new_n] and $n < $max;

        $n = $new_n;
    }

    $cache[$input] = $result if $input < $max;
    return $result;
}
[download]

A new member collatz3_e was added to the list.

# Intel i7 laptop, Docker Container, Ubuntu + Perlbrew Perl 5.30.1

collatz3_a.pl 1e7  32.291s  (a) original, accepts argument
collatz3_b.pl 1e7  30.134s  (b) a + replaced division with >> 1
collatz3_c.pl 1e7  28.503s  (c) b + removed 1 level of branching
collatz3_d.pl 1e7  21.464s  (d) c + reduced loop iterations
collatz3_e.pl 1e7  19.357s  (e) d + caching less

# AMD 3970x, Docker Container, Ubuntu + Perlbrew Perl 5.30.1

collatz3_a.pl 1e7  13.130s  (a) original, accepts argument
collatz3_b.pl 1e7  12.394s  (b) a + replaced division with >> 1
collatz3_c.pl 1e7  12.261s  (c) b + removed 1 level of branching
collatz3_d.pl 1e7   9.170s  (d) c + reduced loop iterations
collatz3_e.pl 1e7   7.681s  (e) d + caching less
[download]

The 32-core machine reaches below 0.5 seconds for size 1e7. That includes the time to launch Perl, load modules, spin up and reap workers.

The Collatz Conjenture took over me. And finally am able to move on.

Kind regards, Mario