Reduce a lazy sequence like a loop with a condition in Clojure

Question

I have an infinite lazy sequence that produce a "preamble" and a "pattern repetition", a simple example of this kind of sequence could be implemented in Clojure with:

(def infinite-lazy-sequence
  (lazy-cat [4] (cycle [1 2 3])))

=> (take 20 infinite-lazy-sequence)
(4 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1)

I would like to get a set of the elements forming this infinite-lazy-sequence, here is a way to implement it using loop:

(loop
  [[head & tail] infinite-lazy-sequence
   result #{}]
  (if (contains? result head)
    result
    (recur tail (conj result head))))

=> #{1 2 3 4}

Here is the question: is it possible to achieve the same result using reduce? And using take-while?

Edit:

Benchmark results using clojure-1.5.1 and perforate-0.2.4

proposed loop-recur implementation:

Case:  :loop-recur
Evaluation count : 60 in 60 samples of 1 calls.
             Execution time mean : 1.054975 sec
    Execution time std-deviation : 26.316442 ms
   Execution time lower quantile : 1.026187 sec ( 2.5%)
   Execution time upper quantile : 1.125854 sec (97.5%)

@sw1nn reduce-reduced implementation:

Case:  :reduce-reduced
Evaluation count : 120 in 60 samples of 2 calls.
             Execution time mean : 875.091831 ms
    Execution time std-deviation : 19.745142 ms
   Execution time lower quantile : 850.871606 ms ( 2.5%)
   Execution time upper quantile : 921.947759 ms (97.5%)

So no element in your infinite sequence may occur twice before the repeated pattern starts repeating, is that correct? — svk
– svk, Commented Mar 25, 2013 at 21:59
Yes, it is correct. Unfortunately I don't know how long the preamble could be... Good point. — Filippo Vitale
– Filippo Vitale, Commented Mar 25, 2013 at 22:01

sw1nn · Accepted Answer · 2013-03-25 22:13:51Z

4

Clojure 1.5 added reduced to allow shortcircuiting reduces...

 (reduce (fn [a v] (if-not (a v) (conj a v) (reduced a))) 
         #{} 
         infinite-lazy-sequence)
 => #{1 2 3 4}

edited Mar 25, 2013 at 22:13

answered Mar 25, 2013 at 22:05

sw1nn

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1 Comment

Filippo Vitale Over a year ago

thanks @sw1nn, I have just benchmarked it with my target infinite-lazy-sequence and your reduce-reduced version seems also faster with Clojure 1.5.1

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