 Solution

A counting semaphore holds a nonnegative number of permits. down waits until a permit is available and consumes one; up returns a permit and can release a waiting caller. A binary semaphore restricts the number available to zero or one. Unlike a mutex, a semaphore need not be released by the same software thread that acquired it; this permits producer-to-consumer signalling.

Interpret maxCount as the initial number of permits and, for this bounded implementation, the maximum permitted number of unmatched releases. An up beyond that capacity is a programming error, rather than a blocking operation. Clients obey this release discipline. If an unbounded counting semaphore is desired instead, remove the upper-bound requirement and use a sufficiently wide counter; a capacity-one instance is binary only under the corresponding balanced-use discipline.

class CountingSemaphore(maxCount: Int) extends Semaphore {
  require(maxCount > 0)
  private var count = maxCount
  def down: Unit = synchronized {
    while (count == 0) wait()
    count -= 1
  }
  def up: Unit = synchronized {
    require(count < maxCount, "unmatched semaphore release")
    count += 1
    notify()
  }
}

All accesses to count occur under the same monitor. wait atomically releases that monitor while sleeping, then reacquires it before returning. The while is needed because of spurious wakeups and because another caller can consume the returned permit before the notified caller reacquires the monitor. Only down callers wait here, and they all wait on the same predicate; hence waking one with notify is sufficient. A later up notifies another waiter if necessary. This use of notify will not be safe for the mixed predicates in the next part.^[ref]

A binary semaphore initially available is new CountingSemaphore(1). To initialize it unavailable, call its down once during construction, before publishing it to other software threads. Its state obeysThis implementation provides no first-come-first-served guarantee: a new caller can acquire the monitor before a notified waiter. The following examples assume ordinary successful execution without cancellation or interruption of a partially completed buffer operation.