-module(eventdetector).
-export([start/2,edetector/3]).

start(InitialState, TransitionFun) ->
    spawn(?MODULE, edetector, [InitialState, TransitionFun, []]).

edetector(State, TransitionFun, Obs) ->
    receive
    {Pid, add_me} ->
        % send {added} to Pid, then add Pid to head of
        % observers list
        Pid ! {added},
        New = [Pid] ++ Obs,
        edetector(State, TransitionFun, New);
    {Pid, add_yourself_to, EDPid} ->
        EDPid ! {self(), add_me},
        receive
        {added} ->
            Pid ! {added}
        end,
        edetector(State, TransitionFun, Obs);
    {Pid, state_value} ->
        Pid ! {value_is, State},
        edetector(State, TransitionFun, Obs);
    atom ->
        {NewState, Event} = TransitionFun(State, atom),
        if Event == none ->
            edetector(State, TransitionFun, Obs);
        true ->
            sendTo(Obs, Event),
            edetector(NewState, TransitionFun, Obs)
        end
    end.

sendTo([], Event) ->
    _ = Event,
    [];
sendTo([H|T], Event) ->
    H ! Event,
    sendTo(T, Event).