sbool parallelMap2_debug;

// Note: an executor passed in is not shut down 

static <A, B> L<B> parallelMap2(ThreadPoolExecutor executor default null, Collection<A> l, F1<A, B> f) {
  if (l(l) <= 1)
    ret map(f, l);
  ret parallelMap2(executor, iterator(l), l(l), f);
}

static <A, B> L<B> parallelMap2(ThreadPoolExecutor executor default null, Iterator<A> it, final int count, final F1<A, B> f) ctex {
  if (executor == null && coresToUse_fixed() == 1)
    ret map(f, iterable(it));
    
  ThreadPoolExecutor e = executor == null ? defaultThreadPoolExecutor() : executor;
    
  L<Future<B>> futures = emptyList(count);
  new Var<Throwable> error;
  if (parallelMap2_debug)
    print("parallelMap2: " + count + " elements, " + e.getMaximumPoolSize() + " threads");
    
  long time = sysNow();
  try {
    int i = 0;
    for (final A o : iterable(it)) {
      ++i;
      final int _i = i;
      futures.add(e.submit(() -> {
        try {
          ret callF(f, o);
        } catch e2 {
          error.set(e2);
          null;
        }
      }));
    }
    /*if (parallelMap2_debug)
      print("parallelMap2: scheduling done after " + (sysNow()-time));*/
    L<B> out = getFutures(futures);
    if (error.has())
      throw rethrow(error!);
    if (parallelMap2_debug)
      print("parallelMap2: main done after " + (sysNow()-time));
    ret out;
  } finally {
    if (executor == null) e.shutdown();
  }
}

static <A, B> L<B> lambdaMapLike parallelMap2(IF1<A, B> f, Cl<A> l) {
  ret parallelMap2(l, if1ToF1(f));
}

Began life as a copy of #1011925