clojure - 非线性减速在 Clojure 中创建惰性序列

Question

我实现了一个函数，该函数将给定输入集合的 n-gram 作为惰性序列返回。

(defn gen-ngrams
  [n coll]
  (if (>= (count coll) n)
    (lazy-seq (cons (take n coll) (gen-ngrams n (rest coll))))))

当我使用更大的输入集合调用此函数时，我希望看到执行时间线性增加。然而，我观察到的时间比那更糟:

user> (time (count (gen-ngrams 3 (take 1000 corpus))))
"Elapsed time: 59.426 msecs"
998
user> (time (count (gen-ngrams 3 (take 10000 corpus))))
"Elapsed time: 5863.971 msecs"
9998
user> (time (count (gen-ngrams 3 (take 20000 corpus))))
"Elapsed time: 23584.226 msecs"
19998
user> (time (count (gen-ngrams 3 (take 30000 corpus))))
"Elapsed time: 54905.999 msecs"
29998
user> (time (count (gen-ngrams 3 (take 40000 corpus))))
"Elapsed time: 100978.962 msecs"
39998

corpus 是字符串标记的 Cons。

是什么导致了这种行为，我该如何提高性能？

Answer 1

我认为您的问题与“(count coll)”有关，它会在每次调用 ngrams 时迭代 coll。

解决方案是使用内置分区函数:

user=> (time (count (gen-ngrams 3 (take 20000 corpus))))
"Elapsed time: 6212.894932 msecs"
19998
user=> (time (count (partition 3 1 (take 20000 corpus))))
"Elapsed time: 12.57996 msecs"
19998

如果对实现感到好奇，请查看分区源代码 http://clojuredocs.org/clojure_core/clojure.core/partition