-module(rps3).
-export([
    get_rules/0,
    beats/1, test_beats/0,
    loses/1, test_loses/0,
    winning_hand/1, test_winning_hand/0]).
get_rules() -> [rock, paper, scissors].
beats(H) ->
    beats(H, get_rules()).
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())).
test_beats() ->
    lists:foreach(
        fun(H) ->
            io:format("~w beats ~w.~n", [H, beats(H)])
        end,
        get_rules()).
test_loses() ->
    lists:foreach(
        fun(H) ->
            io:format("~w loses to ~w.~n", [H, loses(H)])
        end,
        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).
test_winning_hand() ->
    Cases = [
             [rock, rock, rock],
             [rock, paper, scissors],
             [rock, paper, paper],
             [rock, rock, paper],
             [rock, paper],
             [paper, scissors],
             [scissors, rock]
            ],
    lists:foreach(
        fun(Hands) ->
            Winner = winning_hand(Hands),
            io:format("~w wins in ~w.~n", [Winner, Hands])
        end,
        Cases).