bounded-worker-pool-pmap

(import '[java.util.concurrent Callable Executors ExecutionException Future])

(defn pmap-bounded
  "Runs f over coll with at most n workers, returning results in input order."
  [n f coll]
  (let [pool  (Executors/newFixedThreadPool n)
        tasks (mapv (fn [x]
                      (.submit pool ^Callable (reify Callable (call [_] (f x)))))
                    coll)]
    (try
      (mapv (fn [^Future t]
              (try (.get t) (catch ExecutionException e (throw (.getCause e)))))
            tasks)
      (finally (.shutdownNow pool)))))