v1.2.0: Adding index-on

README.md

@@ -30,6 +30,9 @@ Built with Leiningen 1.x
 ### v1.1.0
 * Add `with-temp-ns`, a sandboxing macro (may move to a different ns in future)
 
+### v1.2.0
+* Add `index-on`, a generalization of group-by
+
 ## License
 
 Copyright (C) 2012 Tim McCormack

project.clj

@@ -1,4 +1,4 @@
-(defproject org.timmc/handy "1.1.0"
+(defproject org.timmc/handy "1.2.0"
   :description "Common utilities to fill in the gaps"
   :url "https://github.com/timmc/handy"
   :license {:name "Eclipse Public License - v1.0"

src/org/timmc/handy.clj

@@ -67,3 +67,16 @@ logical true/false."
         (finally
          (in-ns '~old-ns)
          (remove-ns '~tmp-ns))))))
+
+;;;; Structural manipulation
+
+(defn ^{:since "1.2.0"} index-on
+  "From a table (coll of record maps) produce a map of index key values
+to projections on the other keys. r->k is a function of a record to some
+key value, e.g. #(get % 5) or (juxt :a :b) or just :c.
+
+Example: (index-on [{:a 0, :b 1, :c 2}, {:a 3, :b 4, :c 5}] :a [:b])
+         => {0 {:b 1}, 3 {:b 4}}"
+  [table r->k keep-keys]
+  (into {} (for [record table]
+             [(r->k record) (select-keys record keep-keys)])))

test/org/timmc/test/handy.clj

@@ -103,3 +103,9 @@
                         (def a (org.timmc.test.handy/resolver Shape))]
            (+ 2 3))
          5)))
+
+;;;; Structural
+
+(deftest indexing
+  (is (= (index-on [{:a 0, :b 1, :c 2}, {:a 3, :b 4, :c 5}] (juxt :a :b) [:c])
+         {[0 1] {:c 2}, [3 4] {:c 5}})))