miner · March 19, 2018 20:47
diff --git a/apropos3.clj b/apropos3.clj
 (defn digits [n]
  {:pre [(int? n) (>= n 0)]}
  (loop [digs () remainder n]
    (if (< remainder 10)
      (conj digs remainder)
      (recur (conj digs (rem remainder 10)) (quot remainder 10)))))

 (defn digits+rev [n]
  (let [ds (digits n)]
    (concat ds (reverse ds))))


 ;; based on preserving-reduced from clojure/core.clj
 ;; the extra level of `reduced` preserves the early termination value
 (defn rf-reduce [rf result xs]
  (let [rrf (fn [r x] (let [ret (rf r x)] (if (reduced? ret) (reduced ret) ret)))]
    (reduce rrf result xs)))

 ;; They asked for a stateful transducer solution...

 (defn revcat
  "Returns a lazy sequence of items from coll followed by the items in reverse order.  When
  no collection is provided, returns a stateful transducer that buffers a reversed collection
  of inputs.  Each input passes through to the output.  On reaching the end of input, the
  buffered elements are passed in as extra inputs. Inspired by @apropos_cast episode #3."
  ([]
   (fn [rf]
     (let [rv (volatile! ())]
       (fn
         ([] (rf))
         ([result] (rf (unreduced (rf-reduce rf result @rv))))
         ([result input]
          (vswap! rv conj input)
          (rf result input))))))
  ([coll] (concat coll (reverse coll))))

 (defn apropos3 [n]
  (sequence (revcat) (digits n)))
	(defn digits [n]
	{:pre [(int? n) (>= n 0)]}
	(loop [digs () remainder n]
	(if (< remainder 10)
	(conj digs remainder)
	(recur (conj digs (rem remainder 10)) (quot remainder 10)))))

	(defn digits+rev [n]
	(let [ds (digits n)]
	(concat ds (reverse ds))))


	;; based on preserving-reduced from clojure/core.clj
	;; the extra level of `reduced` preserves the early termination value
	(defn rf-reduce [rf result xs]
	(let [rrf (fn [r x] (let [ret (rf r x)] (if (reduced? ret) (reduced ret) ret)))]
	(reduce rrf result xs)))

	;; They asked for a stateful transducer solution...

	(defn revcat
	"Returns a lazy sequence of items from coll followed by the items in reverse order. When
	no collection is provided, returns a stateful transducer that buffers a reversed collection
	of inputs. Each input passes through to the output. On reaching the end of input, the
	buffered elements are passed in as extra inputs. Inspired by @apropos_cast episode #3."
	([]
	(fn [rf]
	(let [rv (volatile! ())]
	(fn
	([] (rf))
	([result] (rf (unreduced (rf-reduce rf result @rv))))
	([result input]
	(vswap! rv conj input)
	(rf result input))))))
	([coll] (concat coll (reverse coll))))

	(defn apropos3 [n]
	(sequence (revcat) (digits n)))