Lazy fishy

AoC day 6 is asking us to simulate a swarm of fish that happily reproduces every 6 days after getting mature at 8 days. My first attempt was to keep track of every single fish. For the requested 80 days, that is no problem at all. Calculating the swarm size after 256 days consumes several GB of RAM and takes halve an hour. According to Larry, laziness is a programmers virtue. All fish of the same age behave the same way. Instead of herding all the cats — I mean fish — we only need to keep track of 8 age-groups.

sub day6 {
    my @state = 3,4,3,1,2;

    my %duegroup := @state.BagHash;
    say %duegroup;

    my @swarmsize = lazy gather {
        take +@state;

        for 1..∞ -> $day {
            my $spawners = %duegroup{0};
            %duegroup{0..5} = %duegroup{1..6};
            %duegroup{6} = %duegroup{7} + $spawners;
            %duegroup{7..8} = |%duegroup{8}, |$spawners;

            take %duegroup.values.sum;
        }
    }

    say „After 80 days the Lanternfish-swarm is @swarmsize[80] fish strong.“;
    say „After 1 year the Lanternfish-swarm is @swarmsize[365] fish strong and fills the entire ocean.“;
}

I keep the age groups in a BagHash because it does the grouping by days-until-due-for-reproduction automatically. Since I want to use string interpolation at the end of the job, I keep the lazy list in a @-sigiled container.

say %duegroup; # BagHash(1 2 3(2) 4)
               #         ^left side, due in 1 day

To avoid filling it (and all RAM) up with fish-counts, I have to modify the Seq returned by gather with lazy. The result for day 0 is the number of fish in the input list. It holds the remainder of the days until reproduction. Each day we shift the not-due fish one group to the “left”. Those which would fall of the table are added to the new number of fish due in 6 days. I also add newly spawned fish at the “right” side of %duegroup.

This is quite fast. Calculating the 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fish strong swarm after 1000 years takes 40.345 seconds.

UPDATE:

The following version is 12.5x faster and inspired by a Haskellist.

sub day6_2 {
    my @state = 3,4,3,1,2;

    my ($a1, $a2, $a3, $a4, $a5, $a6, $a7, $a8, $a9) = @state.Bag{0..8};

    for 1..(365*1000) -> $day {
        ($a1, $a2, $a3, $a4, $a5, $a6, $a7, $a8, $a9) = ($a2, $a3, $a4, $a5, $a6, $a7, $a8 + $a1, $a9, $a1)
    }

    say [+] ($a1, $a2, $a3, $a4, $a5, $a6, $a7, $a8, $a9);
}

No wonder one can travel faster without baggage!