为什么我的Clojure代码运行这么慢？

Question

Below is my answer for 4clojure Problem 108

I'm able to pass the first three tests but the last test times out. The code runs really, really slowly on this last test. What exactly is causing this?

((fn [& coll] (loop [coll coll m {}]
    (do
        (let [ct (count coll)
                ns (mapv first coll)
                m' (reduce #(update-in %1 [%2] (fnil inc 0)) m ns)]
            (println m')
            (if (some #(<= ct %) (mapv m' ns))
                (apply min (map first (filter #(>= (val %) ct) m')))
                (recur (mapv rest coll) m'))))))
 (map #(* % % %) (range)) ;; perfect cubes
 (filter #(zero? (bit-and % (dec %))) (range)) ;; powers of 2
 (iterate inc 20))

Answer 1

You are gathering the next value from every input on every iteration (recur (mapv rest coll) m')

One of your inputs generates values extremely slowly, and accellarates to very high values very quickly: (filter #(zero? (bit-and % (dec %))) (range)) .

Your code is spending most of its time discovering powers of two by incrementing by one and testing the bits.

You don't need a map of all inputs with counts of occurrences. You don't need to find the next value for items that are not the lowest found so far. I won't post a solution since it is an exercise, but eliminating the lowest non matched value on each iteration should be a start.

Answer 2

In addition to the other good answers here, you're doing a bunch of math, but all numbers are boxed as objects rather than being used as primitives. Many tips for doing this better here .

Answer 3

This is a really inefficient way of counting powers of 2:

(filter #(zero? (bit-and % (dec %))) (range))

This is essentially counting from 0 to infinity, testing each number along the way to see if it's a power of two. The further you get into the sequence, the more expensive each call to rest is.

Given that it's the test input, and you can't change it, I think you need to re-think your approach. Rather than calling (mapv rest coll) , you probably only want to call rest on the sequence with the smallest first value.