Compute permutation that sorts the input

order2.clj

;; Written to answer https://stackoverflow.com/questions/47254742/sort-primitive-array-with-custom-comparator-on-clojure

(defn order2 [xs]
  (let [rnd (java.util.Random.)
        a1 (double-array xs)
        a2 (long-array (alength a1))]
    (dotimes [i (alength a2)]
      (aset a2 i i))
    (letfn [(quicksort [^long l ^long h]
              (if (< l h)
                (let [p (.invokePrim ^clojure.lang.IFn$LLL partition l h)]
                  (quicksort l (dec p))
                  (quicksort (inc p) h))))
            (partition ^long [^long l ^long h]
              (let [pidx (+ l (.nextInt rnd (- h l)))
                    pivot (aget a1 pidx)]
                (swap1 a1 pidx h)
                (swap2 a2 pidx h)
                (loop [i (dec l)
                       j l]
                  (if (< j h)
                    (if (< (aget a1 j) pivot)
                      (let [i (inc i)]
                        (swap1 a1 i j)
                        (swap2 a2 i j)
                        (recur i (inc j)))
                      (recur i (inc j)))
                    (let [i (inc i)]
                      (when (< (aget a1 h) (aget a1 i))
                        (swap1 a1 i h)
                        (swap2 a2 i h))
                      i)))))
            (swap1 [^doubles a ^long i ^long j]
              (let [tmp (aget a i)]
                (aset a i (aget a j))
                (aset a j tmp)))
            (swap2 [^longs a ^long i ^long j]
              (let [tmp (aget a i)]
                (aset a i (aget a j))
                (aset a j tmp)))]
      (quicksort 0 (dec (alength a1)))
      (vec a2))))