Question

I’m trying to implement a Overhand Shuffle in Clojure as a bit of a learning exercise

So I’ve got this code…

(defn overhand [cards]
    (let [ card_count (count cards)
          _new_cards '()
         _rand_ceiling (if (> card_count 4) (int (* 0.2 card_count)) 1)]
      (take card_count
            (reduce into (mapcat
                           (fn [c]
                             (-> (inc (rand-int _rand_ceiling))
                                 (take cards)
                                 (cons _new_cards)))
                           cards)))))

It is very close to doing what I want, but it is repeatedly taking the first (random) N number of cards off the front, but I want it to progress through the list…

calling as

(overhand [1 2 3 4 5 6 7 8 9])

instead of ending up with

(1 2 3 1 2 1 2 3 4)

I want to end up with

(7 8 9 5 6 1 2 3 4)

Also, as a side note this feels like a really ugly way to indent/organize this function, is there a more obvious way?

Answer 1

this function is creating a list of lists, transforming each of them, and cating them back together. the problem it that it is pulling from the same thing every time and appending to a fixed value. essentially it is running the same operation every time and so it is repeating the output over with out progressing thgough the list. If you break the problem down differently and split the creation of random sized chunks from the stringing them together it gets a bit easier to see how to make it work correctly.

some ways to split the sequence:

(defn random-partitions [cards]
  (let [card_count (count cards)
        rand_ceiling (if (> card_count 4) (inc (int (* 0.2 card_count))) 1)]
   (partition-by (ƒ [_](= 0 (rand-int rand_ceiling))) cards)))

to keep the partitions less than length four

(defn random-partitions [cards]
  (let [[h t] (split-at (inc (rand-int 4)) cards)]
    (when (not-empty h) (lazy-seq (cons h (random-partition t))))))

or to keep the partitions at the sizes in your original question

(defn random-partitions [cards]
  (let [card_count (count cards)
        rand_ceiling (if (> card_count 4) (inc (int (* 0.2 card_count))) 1)
        [h t] (split-at (inc (rand-int rand_ceiling)) cards)]
    (when (not-empty h) (lazy-seq (cons h (random-partition t))))))

(random-partitions [1 2 3 4 5 6 7 8 9 10])
((1 2 3 4) (5) (6 7 8 9) (10))

this can also be written without directly using lazy-seq:

(defn random-partitions [cards]
  (->> [[] cards]
       (iterate
        (ƒ [[h t]]
          (split-at (inc (rand-int 4)) t)))
       rest ;iterate returns its input as the first argument, drop it.
       (map first)
       (take-while not-empty)))

which can then be reduced back into a single sequence:

(reduce  into (random-partitions [1 2 3 4 5 6 7 8 9 10]))
(10 9 8 7 6 5 4 3 1 2)

if you reverse the arguments to into it looks like a much better shuffle

 (reduce #(into %2 %1) (random-partitions [1 2 3 4 5 6 7 8 9 10]))
(8 7 1 2 3 4 5 6 9 10)