-module(rps3).
-export([get_rules/0, winning_hand/1]).
get_rules() -> [rock, paper, scissors].
beats(H, [H|Tail]) ->
    lists:last(Tail);
beats(H, [B,H|_]) ->
    B;
beats(H, [_|T]) ->
    beats(H, T).
loses(H) ->
    beats(H, lists:reverse(get_rules())).
% winner for 3 hands is:
% rock, rock, rock      -> rock
% rock, paper, scissors -> undefined
% rock, paper, paper    -> paper
% rock, rock, paper     -> paper
winning_hand(Hands) ->
    HSet = sets:from_list(Hands),
    sets:fold(
        fun(Hand, Acc) ->
            case sets:is_element(loses(Hand), HSet) of
                true -> Acc;
                false -> Hand
            end
        end, undefined, HSet).