port over latest merge-with, kv-reduce aware, can merge vectors

Co-authored-by: Rich Hickey <richhickey@gmail.com>

Author: swannodette

src/main/cljs/cljs/core.cljs

@@ -9301,16 +9301,33 @@ reduces them without incurring seq initialization"
   the first.  If a key occurs in more than one map, the mapping(s)
   from the latter (left-to-right) will be combined with the mapping in
   the result by calling (f val-in-result val-in-latter)."
-  [f & maps]
-  (when (some identity maps)
-    (let [merge-entry (fn [m e]
-                        (let [k (key e) v (val e)]
-                          (if (contains? m k)
-                            (assoc m k (f (get m k) v))
-                            (assoc m k v))))
-          merge2 (fn [m1 m2]
-                   (reduce merge-entry (or m1 {}) (seq m2)))]
-      (reduce merge2 maps))))
+  {:added "1.0"
+   :static true}
+  ([f] nil)
+  ([f x] x)
+  ([f x y]
+     (if (and x y)
+       (let [merge-kv
+             (fn this [m k v]
+               (let [mv (get m k this)]
+                 (if (identical? mv this)
+                   (assoc m k v)
+                   (assoc m k (f mv v)))))
+             merge-kv!
+             (fn this [m k v]
+               (let [mv (-lookup m k this)]
+                 (if (identical? mv this)
+                   (assoc! m k v)
+                   (assoc! m k (f mv v)))))]
+         (if (implements? IKVReduce y)
+           (if (implements? IEditableCollection x)
+             (persistent! (-kv-reduce y merge-kv! (transient x)))
+             (-kv-reduce y merge-kv x))
+           (reduce (fn [m e] (merge-kv m (key e) (val e))) x y)))
+       (or x y)))
+  ([f x y & maps]
+     (let [merge2 (fn [x y] (merge-with f x y))]
+       (reduce merge2 (merge2 x y) maps))))
 
 (defn select-keys
   "Returns a map containing only those entries in map whose key is in keys"