map-alt transducer

map-alt.clj

;; map-alt is like the map xform but calls the funtions in an alternating order
;; (f i0) (g i1) (h i2) (f i3) ...
;;
;; In other words, map-alt spreads fn calls across elements, whereas (mapcat (juxt ...)) calls all
;; fns on each element.

(defn map-alt
  ([] (map identity))

  ([f] (map f))

  ([f g]
   (fn [rf]
     (let [toggle (volatile! false)]
       (fn
         ([] (rf))
         ([result] (rf result))
         ([result input]
          (rf result (if (vswap! toggle not) (f input) (g input))))))))

  ([f g & more]
   (fn [rf]
     (let [fv (into [f g] more)
           cnt (count fv)
           inc0 (fn [n] (let [n (unchecked-inc n)] (if (>= n cnt) 0 n)))
           i (volatile! -1)]
       (fn
         ([] (rf))
         ([result] (rf result))
         ([result input]
          (rf result ((nth fv (vswap! i inc0)) input))))))))