sub collatz-length(Int $n) {
state %seq = { 1 => 1 };
if $n +& 1 {
%seq{$n} //= 2 + collatz-length((3 * $n + 1) +> 1);
} else {
collatz-length($n +> 1) + 1;
}
}

sub collatz-length(Int $n) {
state %seq = { 1 => 1 };
%seq{$n} //= $n +& 1 ??
2 + collatz-length((3 * $n + 1) +> 1) !!
1 + collatz-length($n +> 1);
}

subset Odd of Int where {$_ !%% 2};
subset Even of Int where {$_ %% 2};

multi collatz-length (1 ) { 1 }
multi collatz-length (Even $n) { 1 + collatz-length($n div 2) }
multi collatz-length (Odd $n) { 1 + collatz-length(3 * $n + 1) }

sub MAIN(*@numbers) {
for @numbers -> $n {
say “$n: ” ~ collatz-length($n.Int);
}
}

Conclusion: I need a better benchmarking framework for this. Must ponder.

sub MAIN() {
my $start = now;
for @numbers -> $number {
collatz-length($number);
}
my $time = now – $start;

say “$time”;
}

I only wrote it to support testing a single function at a time, but it measures the runtime of the actual function execution, so I’ve found it useful for comparing implementations with better granularity and less dependence on system load.

%seq{$n} //= 1 + collatz-length($n +& 1 ?? 3 * $n + 1 !! $n +> 1);