Make partitionv faster

Question

If we use the cat transducer instead of calling (concat p pad), the code is much faster:

(defn partitionv [n step pad coll]
  (lazy-seq
    (when-let [s (seq coll)]
      (let [p (into [] (take n) s)]
        (if (= n (count p))
          (cons p (partitionv n step pad (nthrest s step)))
          (list (into [] (take n) (concat p pad))))))))

can be changed to

(defn partitionv2 [n step pad coll]
  (lazy-seq
    (when-let [s (seq coll)]
      (let [p (into [] (take n) s)]
        (if (= n (count p))
          (cons p (partitionv n step pad (nthrest s step)))
          (list (into [] (comp cat (take n)) [p pad])))))))

Comments

To answer the question in the edit - it's actually the standard markdown syntax. :) https://www.markdownguide.org/basic-syntax#code-blocks-1 It's easier to use the editor buttons than to manually indent stuff.

Answer 1

You're only changing the construction of the last possibly padded value, right?

It seems hard to believe that is "much faster". What are you testing?

Comments

Well, that's interesting, it seemed much faster on a few benchmarks in a row... But sometimes it's the other way...