(defrecord Tree [value children])
(defrecord File [name size dir?])

(defn- parallel-map [f coll]
  (if (> (count coll) 3)
    (pmap f coll)
    (map f coll)))

(defn build-file-tree [file]
  (let [file (if (string? file) (java.io.File. file) file)
        name (.getName file)
        dir? (.isDirectory file)
        children (.listFiles file)
        sub-trees (parallel-map build-file-tree children)]
    (Tree.
      (File.
        name
        (if dir?
          (reduce + (parallel-map #(:size (:value %)) sub-trees))
          (.length file))
        dir?)
      sub-trees)))