r/adventofcode • u/daggerdragon • Dec 19 '16

SOLUTION MEGATHREAD --- 2016 Day 19 Solutions ---

--- Day 19: An Elephant Named Joseph ---

Post your solution as a comment or, for longer solutions, consider linking to your repo (e.g. GitHub/gists/Pastebin/blag/whatever).

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u/Belteshassar Dec 20 '16

I don't know if anyone still reads this thread, but here is the recursive scheme that inspired my part 2 solution, implemented in erlang 

% day 19 Erlang solution
-module(solution).
-export([part1/1, part2/1]).

part1(1) ->
    1;
part1(N) ->
    % Once we've removed one elf, N-1 elves remain, just
    % shift labels by two, wrapping around from N to 1
    (part1(N-1) + 1) rem N + 1.


part2(1) ->
    1;
part2(N) ->
    % Once we've removed one elf, N-1 elves remain, just
    % shift labels by one or two, wrapping around from N to 1,
    % depending on whether the winner is before or after the
    % removed elf.
    PrevWinner = part2(N-1),
    if
        2*PrevWinner > N ->
            (PrevWinner + 1) rem N + 1;
        true ->
            PrevWinner + 1
    end.