Fast cardinality of C# BitArray from Clojure

fast-cardinality.cljr

;; Fast cardinality of C# BitArray from Clojure because for some reason BitArray lacks this (see java.util.BitSet)

(import '[System.Collections BitArray] '[System.Reflection BindingFlags FieldInfo])

(def field-info (-> BitArray (.GetField "m_array" (enum-or BindingFlags/NonPublic BindingFlags/Instance))))

(defn get-cardinality
  [^BitArray ba]
  (let [array (.GetValue ^FieldInfo field ba)
        n (count array)]
    (loop [i 0 acc 0]
      (if (< i n)
        (recur (unchecked-inc i) (unchecked-add-int acc (Int32/PopCount (aget ^ints array i))))
	acc))))

(comment
(def t (doto (new BitArray 1000) (.Set 500 true) (.Set 600 true)))
(count (filter true? t)) ;; 2
(get-cardinality t) ;; 2
)