Pre: Add util/group-by-kv.

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)))

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]]))