make get-in faster #256

Signed-off-by: Stuart Halloway <stu@thinkrelevance.com>

Author: stuarthalloway

src/clj/clojure/core.clj

@@ -4910,12 +4910,19 @@
   "Returns the value in a nested associative structure,
   where ks is a sequence of ke(ys. Returns nil if the key is not present,
   or the not-found value if supplied."
+  {:added "1.2"}
   ([m ks]
-   (reduce get m ks))
+     (reduce get m ks))
   ([m ks not-found]
-   (if (seq ks)
-     (get (reduce get m (butlast ks)) (last ks) not-found)
-     m)))
+     (loop [sentinel (Object.)
+            m m
+            ks (seq ks)]
+       (if ks
+         (let [m (get m (first ks) sentinel)]
+           (if (identical? sentinel m)
+             not-found
+             (recur sentinel m (next ks))))
+         m))))
 
 (defn assoc-in
   "Associates a value in a nested associative structure, where ks is a