exception Timeout

type t = {mutex : Mutex.t;
          condition_lock : Condition.t;
          mutable value : int}

let create initial_value =
  assert (0 <= initial_value);
  {mutex = Mutex.create ();
   condition_lock = Condition.create ();
   value = initial_value}

let up semaphore =
  Mutex.lock semaphore.mutex;
  semaphore.value <- succ semaphore.value;
  Mutex.unlock semaphore.mutex;
  Condition.signal semaphore.condition_lock

let down semaphore =
  Mutex.lock semaphore.mutex;
  assert (0 <= semaphore.value);
  while semaphore.value = 0 do
    Condition.wait semaphore.condition_lock semaphore.mutex
  done;
  semaphore.value <- pred semaphore.value;
  Mutex.unlock semaphore.mutex

(* If it is possible to decrement the semaphore, do it and return "true".
   Otherwise return "false". *)
let try_down semaphore =
  Mutex.lock semaphore.mutex;
  if 0 < semaphore.value then
    begin
      semaphore.value <- pred semaphore.value;
      Mutex.unlock semaphore.mutex;
      true
    end
  else
    begin
      Mutex.unlock semaphore.mutex;
      false
    end