-module(rps).
-export([lobby/0, room/1, beats/1, winner/1, go/0, fool/0]).
-define(RULES, [rock, paper, scissors]).
beats(C) ->
    beats(C, ?RULES).
beats(C, [C|Tail]) ->
    lists:last(Tail);
beats(C, [B,C|_]) ->
    B;
beats(C, [_|T]) ->
    beats(C, T).
% winner for 3 players is:
% rock, rock, rock      -> undefined
% rock, paper, scissors -> undefined
% rock, paper, paper    -> undefined
% rock, rock, paper     -> {paper, Who}
winner(Choices) when length(Choices) > 1 ->
    CSet = sets:from_list([C || {C, _} <- Choices]),
    RSet = sets:from_list(?RULES),
    Absents = sets:subtract(RSet, CSet),
    case sets:size(Absents) of
        1 ->
            Absent = hd(sets:to_list(Absents)),
            Unbeaten = beats(Absent),
            Winners = [{C, Who} || {C, Who} <- Choices, C =:= Unbeaten],
            case length(Winners) of
                1 -> hd(Winners);
                _ -> undefined
            end;
        _ -> undefined
    end.
go() ->
    register(lobby, spawn(rps, lobby, [])),
    [spawn(rps, fool, []) || _ <- lists:seq(1, 3)].
    
lobby() ->
    lobby([]).
lobby(Players) when length(Players) =:= 3 ->
    Room = spawn(rps, room, [Players]),
    lists:foreach(fun(Player) -> Player ! {room, Room} end, Players),
    lobby([]);
lobby(Players) ->
    receive
        {join, Who} ->
            lobby([Who|Players])
    end.
room(Players) ->
    lists:foreach(fun(Player) -> Player ! start end, Players),
    room(Players, []).
room(Players, Choices) when length(Players) =:= length(Choices) ->
    Best = winner(Choices),
    Msg = if
              Best =:= undefined ->
                  io:format("draw: ~w.~n", [Choices]),
                  draw;
              true ->
                  io:format("the winner is ~w.~n", [Best]),
                  {winner, Best}
          end,
    lists:foreach(fun(Player) -> Player ! Msg end, Players),
    room(Players);
room(Players, Choices) ->
    receive
        {Object, Who} ->
            ObjectIsOk = lists:any(fun(X) -> X =:= Object end, ?RULES),
            ChoiceMade = lists:any(fun({_, Player}) -> Player =:= Who end, Choices),
            case ObjectIsOk andalso not ChoiceMade of
                true ->
                    room(Players, [{Object, Who}|Choices]);
                false -> 
                    io:format("~w picks ~w, not allowed.~n", [Who, Object]),
                    room(Players, Choices)
            end
    end.
fool() ->
    lobby ! {join, self()},
    fool(joined).
fool(joined) ->
    receive
        {room, Room} ->
            random:seed(now()),
            fool(Room)
    end;
fool(Room) ->
    receive
        start ->
            Choice = lists:nth(random:uniform(length(?RULES)), ?RULES),
            Room ! {Choice, self()},
            fool(Room);
        {winner, _} ->
            fool(Room)
    end.