semaphore.verona

class Semaphore {
  _count: U64 & imm;
  _signal: Promise[None] & iso;

  create(count: U64 & imm): cown[Semaphore] & imm {
    var result = new Semaphore;
    result._count = count;
    result._signal = Promise.create();
    cown.create(result)
  }

  up(sema: cown[Semaphore] & imm) {
    when(sema) {
      sema._count = sema._count + 1;
      (sema._signal = Promise.create()).fulfill(new None);
    }
  }

  _down(sema: cown[Semaphore] & imm, promise: Promise[None] & iso) {
    when(var s = sema) {
      if (s._count > 0) {
        s._count = s._count - 1;
        promise.fulfill(new None);
      } else {
        when (var _ = (mut-view (s._signal)).wait_handle()) {
          Semaphore._down(sema, promise);
        };
      }
    };
  }

  down(sema: cown[Semaphore] & imm): cown[None] & imm {
    var p = Promise.create();
    var f = (mut-view p).wait_handle();
    Semaphore._down(sema, p);
    f
  }
}