transducer

(ns scratch)

(defn map [f]
  (fn [xf]
    (fn
      ([] (xf))
      ([acc] (xf acc))
      ([acc itm]
       (xf acc (f itm))))))

(defn filter [f]
  (fn [xf]
    (fn
      ([] (xf))
      ([acc] (xf acc))
      ([acc itm]
       (if (f itm)
         (xf acc itm)
         acc)))))

(defn compose [a b]
  (fn [x]
    (a (b x))))

(defn transduce [xform rf coll]
  (let [rf (xform rf)
        start (rf)]
    (reduce rf start coll)0))



(defprotocol ITransducer
  (start [this])
  (on-next! [this itm])
  (on-end! [this])
  (set-next! [this nxt]))

(defrecord Map [f next]
  ITransducer
  (start [this]
    (start @next))
  (on-next! [this itm]
    (on-next! @next (f itm)))
  (on-end! [this]
    (on-end! @next))
  (set-next! [this nxt]
    (vreset! next nxt)))

(defrecord Filter [f next]
  ITransducer
  (start [this]
    (start @next))
  (on-next! [this itm]
    (if (f itm)
      (on-next! @next itm)))
  (on-end! [this]
    (on-end! @next))
  (set-next! [this nxt]
    (vreset! next nxt)))

(defn compose [a b]
  (set-next! a b))

(defn transduce [xform rf coll]
  (let [acc (volatile! (rf))]
    (set-next! xform (reify ITransducer
                       (start [this] 
                         (vreset! acc (rf)))
                       (on-next! [this itm]
                         (vswap! acc rf itm))
                       (on-end! [this]
                         nil)))
    (doseq [itm coll]
      (on-next! xform itm))))