Add prefix-lookup

src/useful/experimental.clj

@@ -1,9 +1,9 @@
 (ns useful.experimental
   (:use [useful.utils :only [split-vec]]
-        [useful.seq :only [alternates]]
+        [useful.seq :only [alternates find-first]]
         [useful.map :only [keyed]]
         [clojure.tools.macro :only [name-with-attributes]]
-        [useful.fn :only [any as-fn]]))
+        [useful.fn :only [any as-fn knit]]))
 
 (defn comp-partial
   "A version of comp that \"rescues\" the first N args, passing them to every composed function
@@ -234,3 +234,18 @@
   [m & ks]
   (-> (apply dissoc m ks)
       (vary-meta merge (select-keys m ks))))
+
+(defn prefix-lookup
+  "Takes a map whose keys are names, and returns a function that does fast prefix
+   matching on its input against the names in the original map, returning the
+   first value whose key is a prefix.
+
+   If order is important (eg because your prefixes overlap, or you want to test
+   common prefixes first for performance), you can pass a sequence of pairs
+   instead of a map."
+  [prefix-map]
+  (let [name-pairs (map (knit name identity) prefix-map)]
+    (fn [^String s]
+      (when-let [pair (find-first #(.startsWith s (first %))
+                                  name-pairs)]
+        (second pair)))))

test/useful/experimental_test.clj

@@ -160,3 +160,15 @@
   (let [m (lift-meta {:a 1 :b 2} :a)]
     (is (= {:b 2} m))
     (is (= {:a 1} (meta m)))))
+
+(deftest prefix-lookup-test
+  (let [lookup (prefix-lookup [["a" :apple]
+                               ["person" :person]
+                               [:p :pineapple]
+                               ["abbrev" :abbreviation]])]
+    (are [in out] (= out (lookup in))
+         "apropos" :apple
+         "persona" :person
+         "pursues" :pineapple ;; keywords should work
+         "abbrev." :apple ;; should test in order, and short-circuit
+         )))