CLJS-977: implement IKVReduce in Subvec

Author: michalmarczyk

src/cljs/cljs/core.cljs

@@ -4949,6 +4949,16 @@ reduces them without incurring seq initialization"
   (-reduce [coll f start]
     (ci-reduce coll f start))
 
+  IKVReduce
+  (-kv-reduce [coll f init]
+    (loop [i start j 0 init init]
+      (if (< i end)
+        (let [init (f init j (-nth v i))]
+          (if (reduced? init)
+            @init
+            (recur (inc i) (inc j) init)))
+        init)))
+
   IFn
   (-invoke [coll k]
     (-nth coll k))

test/cljs/cljs/core_test.cljs

@@ -459,7 +459,10 @@
         ;; no layered subvecs
         (is (identical? v1 (.-v (subvec s 1 4))))
         (let [m {:x 1}]
-          (is (= m (meta (with-meta s m))))))
+          (is (= m (meta (with-meta s m)))))
+        ;; CLJS-997
+        (is (= (reduce-kv + 123 (vec (range 10 50)))
+               (reduce-kv + 123 (subvec (vec (range 100)) 10 50)))))
       ;; CLJS-513
       (let [sentinel (js-obj)
             s (subvec [0 1 2 3] 1 2)]