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

start()->
    register(semaphore, spawn(?MODULE, init, [])).

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

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

init() ->
    unlocked(Lock).

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

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