Merge pull request #480 from stuarth/impl-cycle

add cycle

Author: heyLu

pixie/stdlib.pxi

@@ -1746,6 +1746,19 @@ The new value is thus `(apply f current-value-of-atom args)`."
                       s)))]
        (lazy-seq (step pred coll)))))
 
+(defn cycle
+  [coll]
+  (if (empty? coll)
+    ()
+    (let [cycle'
+          (fn cycle' [current]
+            (lazy-seq
+             (cons
+              (first current)
+              (let [rst (rest current)]
+                (cycle' (if (empty? rst) coll rst))))))]
+      (cycle' coll))))
+
 ;; TODO: use a transient map in the future
 (defn group-by
   {:doc "Groups the collection into a map keyed by the result of applying f on each element. The value at each key is a vector of elements in order of appearance."

tests/pixie/tests/test-stdlib.pxi

@@ -568,6 +568,12 @@
   (t/assert= (transduce (drop-while even?) conj [0 2] [1 4 6]) [0 2 1 4 6])
   (t/assert= (transduce (drop-while even?) conj [0 2] [2 4 6 7 8]) [0 2 7 8]))
 
+(t/deftest test-cycle
+  (t/assert= (cycle ()) ())
+  (t/assert= (cycle nil) ())
+  (t/assert= (take 5 (cycle '(1 2))) '(1 2 1 2 1))
+  (t/assert= (take 3 (cycle [nil])) '(nil nil nil)))
+
 (t/deftest test-trace
   (try
     (/ 0 0)