Add repeatedly that is reducable foldable

src/gen/reducers.clj

@@ -0,0 +1,99 @@
+(ns gen.reducers
+  (:refer-clojure :exclude [repeatedly])
+  (:require [clojure.core :as clojure]
+            [clojure.core.reducers :as reducers]))
+
+(declare fold-repeatedly)
+
+(deftype Repeatedly [inv f]
+  clojure.core.reducers/CollFold
+  (coll-fold [_ n combinef reducef]
+    (fold-repeatedly inv f n combinef reducef))
+
+  clojure.lang.IReduce
+  (reduce [_ rf]
+    (if (pos? inv)
+      (.reduce (Repeatedly. (dec inv) f)
+               rf
+               (f))
+      (rf)))
+  (reduce [_ rf x]
+    (loop [ret x
+           inv inv]
+      (if (pos? inv)
+        (let [ret (rf ret (f))]
+          (if (reduced? ret)
+            @ret
+            (recur ret (dec inv))))
+        ret))))
+
+(defn repeatedly
+  "Like `clojure.core/repeatedly`, but is reducible and foldable."
+  [n f]
+  (Repeatedly. n f))
+
+(def fjfork @#'reducers/fjfork)
+(def fjjoin @#'reducers/fjjoin)
+(def fjtask @#'reducers/fjtask)
+(def fjinvoke @#'reducers/fjinvoke)
+
+(defn- fold-repeatedly
+  [inv f n combinef reducef]
+  (cond (< inv 1) (combinef)
+
+        (<= inv n)
+        (reduce reducef (combinef) (Repeatedly. inv f))
+
+        :else
+        (let [inv1 (quot inv 2)
+              inv2 (+ inv1 (mod inv 2))
+              fc (fn [inv]
+                   #(fold-repeatedly inv f n combinef reducef))]
+          (fjinvoke
+           #(let [f1 (fc inv1)
+                  t2 (fjtask (fc inv2))]
+              (fjfork t2)
+              (combinef (f1) (fjjoin t2)))))))
+
+(comment
+
+  (time (into [] (clojure/repeatedly 1000000 rand)))
+  (time (into [] (repeatedly 1000000 rand)))
+
+  (time (reduce + (clojure/repeatedly 1000000 rand)))
+  (time (reduce + (repeatedly 1000000 rand)))
+
+  (time (reducers/fold + (clojure/repeatedly 1000000 rand)))
+  (time (reducers/fold + (repeatedly 1000000 rand)))
+
+  (time (reducers/fold 10 + + (clojure/repeatedly 1000000 rand)))
+  (time (reducers/fold 10 + + (repeatedly 1000000 rand)))
+
+  (time (reducers/fold 2 + + (clojure/repeatedly 1000000 rand)))
+  (time (reducers/fold 2 + + (repeatedly 1000000 rand)))
+
+  (time (reducers/fold 1 + + (clojure/repeatedly 1000000 rand)))
+  (time (reducers/fold 1 + + (repeatedly 1000000 rand)))
+
+  ;;
+
+  (def slow-rand #(do (Thread/sleep 0 1) (rand)))
+
+  (time (reduce + (clojure/repeatedly 1000 slow-rand)))
+  (time (reduce + (repeatedly 1000 slow-rand)))
+
+  (time (reducers/fold + (clojure/repeatedly 1000 slow-rand)))
+  (time (reducers/fold + (repeatedly 1000 slow-rand)))
+
+  (time (reducers/fold 10 + + (clojure/repeatedly 1000 slow-rand)))
+  (time (reducers/fold 10 + + (repeatedly 1000 slow-rand)))
+
+  (time (reducers/fold 2 + + (clojure/repeatedly 1000 slow-rand)))
+  (time (reducers/fold 2 + + (repeatedly 1000 slow-rand)))
+
+  (time (reducers/fold 1 + + (clojure/repeatedly 1000 slow-rand)))
+  (time (reducers/fold 1 + + (repeatedly 1000 slow-rand)))
+
+  (set! *print-length* 10)
+
+  ,)