From bc011bbf43d865d2003ebed26eaf512c7ef2224e Mon Sep 17 00:00:00 2001
From: Nick Alexander <nalexander@mozilla.com>
Date: Tue, 4 Oct 2016 11:34:28 -0700
Subject: [PATCH] Pre: Add util/group-by-kv.

---
 src/common/datomish/util.cljc | 14 ++++++++++++++
 test/datomish/util_test.cljc  |  5 +++++
 2 files changed, 19 insertions(+)

diff --git a/src/common/datomish/util.cljc b/src/common/datomish/util.cljc
index 13a9ee09..dde3955b 100644
--- a/src/common/datomish/util.cljc
+++ b/src/common/datomish/util.cljc
@@ -154,3 +154,17 @@
 
 (defn unlimited-buffer []
   (UnlimitedBuffer. #?(:cljs (array) :clj (java.util.LinkedList.))))
+
+(defn group-by-kv
+  "Returns a map of the elements of coll keyed by the first element of
+  the result of f on each element. The value at each key will be a
+  vector of the second element of the result of f on the corresponding
+  elements, in the order they appeared in coll."
+  {:static true}
+  [f coll]
+  (persistent!
+    (reduce
+      (fn [ret x]
+        (let [[k v] (f x)]
+          (assoc! ret k (conj (get ret k []) v))))
+      (transient {}) coll)))
diff --git a/test/datomish/util_test.cljc b/test/datomish/util_test.cljc
index 87f25ce1..f1e215b2 100644
--- a/test/datomish/util_test.cljc
+++ b/test/datomish/util_test.cljc
@@ -46,3 +46,8 @@
   (is (util/unblocking-chan? (a/chan (a/sliding-buffer 10))))
   (is (util/unblocking-chan? (a/chan (util/unlimited-buffer))))
   (is (not (util/unblocking-chan? (a/chan (a/buffer 10))))))
+
+(deftest test-group-by-kvs
+  (are [m xs] (= m (util/group-by-kv identity xs))
+    {:a [1 2] :b [3]}
+    [[:a 1] [:a 2] [:b 3]]))