gistfile1.m

- (NSArray *)mapConcurrent;
{
  NSArray *originals = self.listOfObjects;

  NSUInteger count = originals.count;
  NSMutableArray *collected = [[NSMutableArray alloc] initWithCapacity:count];
  for (NSUInteger i = 0; i < count; i++) {
    [collected addObject:[NSNull null]];
  }

  dispatch_semaphore_t itemLock = dispatch_semaphore_create(1);
  dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
  dispatch_apply(count, queue, ^(size_t i) {
    id original = originals[i];
    id result = [original performWork];
    dispatch_semaphore_wait(itemLock, DISPATCH_TIME_FOREVER);
    collected[i] = result;
    dispatch_semaphore_signal(itemLock);
  });
  dispatch_release(itemLock);

  return collected;
}