sliding.clj

(defn sliding
  ([^long n] (sliding n 1))
  ([^long n ^long step]
   (fn [rf]
     (let [a (java.util.ArrayDeque. n)]
       (fn
         ([] (rf))
         ([result]
            (let [result (if (.isEmpty a)
                           result
                           (let [v (vec (.toArray a))]
                             ;;clear first!
                             (.clear a)
                             (unreduced (rf result v))))]
              (rf result)))
         ([result input]
            (.add a input)
            (if (= n (.size a))
              (let [v (vec (.toArray a))]
                (dorun (take step (repeatedly #(.removeFirst a))))
                (rf result v))
              result)))))))

This isn't getting us quite the same thing as partition-all, in terms of the sequences returned (and getting a transducing version of partition-all is what we want here, right?):

user=> (eduction (sliding 3 1) [1 2 3 4])
([1 2 3] [2 3 4] [3 4])
user=> (partition-all 3 1 [1 2 3 4])
((1 2 3) (2 3 4) (3 4) (4))