-module(rps_rules).
-export([parse/1, winning_hand/2]).
-record(state, {hands=[], beats=dict:new(), loses=dict:new(), why=dict:new()}).
parse(Text) ->
    Tokens = lists:map(fun list_to_atom/1, string:tokens(Text, " ")),
    parse(Tokens, #state{}).
parse([V1,Label,V2|T], #state{hands=Hands, beats=Beats, loses=Loses, why=Why}) ->
    Acc = #state{
        hands=[V1|Hands],
        beats=dict:append(V1, V2, Beats),
        loses=dict:append(V2, V1, Loses),
        why=dict:store({V1, V2}, Label, Why)},
    parse([V2|T], Acc);
parse([_V2], Acc) ->
    Acc.
% winner for 3 hands is:
% rock, rock, rock      -> rock
% rock, paper, scissors -> undefined
% rock, paper, paper    -> paper
% rock, rock, paper     -> paper
winning_hand(Hands, State=#state{}) ->
    HSet = sets:from_list(Hands),
    sets:fold(
        fun(Hand, Acc) ->
            Loses = dict:fetch(Hand, State#state.loses),
            case sets:is_disjoint(sets:from_list(Loses), HSet) of
                false -> Acc;
                true -> Hand
            end
        end, undefined, HSet).