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  1. (ns grep-n)
  2. (use 'clojure.test)
  3.  
  4. (defn grep-n [n f coll]
  5.   (lazy-seq
  6.    (when-let [s (seq coll)]
  7.      (let [p (take-while (complement f) (take n s))
  8.            n-till-first-match (count p)
  9.            matches (take-while f (nthrest s n-till-first-match))]
  10.        (if (seq matches)
  11.          (let [n-till-last-match (+ (count p) (count matches))
  12.                a (take n (nthrest s n-till-last-match))]
  13.            (if (some f a)
  14.              (concat p matches (grep-n n f (nthrest s n-till-last-match)))
  15.              (concat p matches a (grep-n n f (nthrest s (+ n-till-last-match (count a)))))))
  16.          (grep-n n f (rest s)))))))
  17.  
  18. (deftest grep-n-test
  19.   (is (= '(0 1 2)
  20.          (grep-n 1 #{1} (range 10))))
  21.  
  22.   (is (= '(2 3 4)
  23.          (grep-n 1 #{3} (range 10))))
  24.  
  25.   (is (= '(4 5 6 7 8)
  26.          (grep-n 2 #{6} (range 10))))
  27.  
  28.   (is (= '(3 4 5 6 7 12 13 14 15 16)
  29.          (grep-n 2 #{5 14} (range 20))))
  30.  
  31.   (is (= '(0 1 2 3 4 5 6 7 8)
  32.          (grep-n 3 #{3 5} (range 10))))
  33.  
  34.   (is (= '(4 5 6)
  35.          (grep-n 2 #{4} (range 4 10))))
  36.  
  37.   (is (= '(3 4 5 6 7)
  38.          (grep-n 2 #(= 0 (mod % 5)) (range 1 10)))))
  39.  
  40. (run-tests)
  41.  
  42. (time (doall (take 10 (grep-n 2 #{1 1111 1111111} (range 100000000)))))
Success #stdin #stdout 2.22s 130548KB
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stdin
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Standard input is empty
stdout
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Testing grep-n

Ran 1 tests containing 7 assertions.
0 failures, 0 errors.
"Elapsed time: 680.085665 msecs"
https://ideone.com/nggjrk
language:
Clojure (clojure 1.10.0)
created:
6 years ago
visibility:
public

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