-module(semaphore).
-compile(export_all).

start()->
    Lock = spawn(?MODULE, init, []),
        spawn(?MODULE, peer, ["this", Lock]),
        spawn(?MODULE, peer, ["andthat", Lock]).

p(Lock) ->
    semaphore ! {p, Lock},
    receive
        go ->
            go
    end.

v(Lock) ->
    semaphore ! {v, Lock},
    ok.

init() ->
    unlocked().

unlocked() ->
    receive {p, Lock} ->
            Lock ! go,
            locked(Lock)
    end.

locked(Lock) ->
    receive
        {v, Lock} ->
            unlocked()
    end.

peer(Text, Lock) ->
    p(Lock),
    io:format("~w~n", [Text]),
    io:format("~w~n", [Text]),
    io:format("~w~n", [Text]),
    v(Lock),
    peer(Text, Lock).