Add repeated-keys utility.

src/common/datomish/util.cljc

@@ -174,3 +174,22 @@
         (let [[k v] (f x)]
           (assoc! ret k (conj (get ret k []) v))))
       (transient {}) coll)))
+
+(defn repeated-keys
+  "Takes a seq of maps.
+   Returns the set of keys that appear in more than one map."
+  [maps]
+  (if (not (seq (rest maps)))
+    #{}
+    ;; This is a perfect use case for transients, except that
+    ;; you can't use them for intersection due to CLJ-700.
+    ;; http://dev.clojure.org/jira/browse/CLJ-700
+    (loop [overlapping #{}
+           seen #{}
+           key-sets (map (comp set keys) maps)]
+      (if-let [ks (first key-sets)]
+        (let [overlap (clojure.set/intersection seen ks)]
+          (recur (clojure.set/union overlapping overlap)
+                 (clojure.set/union seen ks)
+                 (rest key-sets)))
+        overlapping))))

test/datomish/util_test.cljc

@@ -51,3 +51,18 @@
   (are [m xs] (= m (util/group-by-kv identity xs))
     {:a [1 2] :b [3]}
     [[:a 1] [:a 2] [:b 3]]))
+
+(deftest test-repeated-keys
+  (let [abc {:a 1 :b 2 :c 3}
+        def {:d 1 :e 2 :f 3}
+        bcd {:b 1 :c 2 :d 3}
+        efg {:e 1 :f 2 :g 3}
+        empty {}]
+    (is (= #{} (util/repeated-keys [])))
+    (is (= #{} (util/repeated-keys [empty])))
+    (is (= #{} (util/repeated-keys [empty empty])))
+    (is (= #{} (util/repeated-keys [abc empty empty])))
+    (is (= #{} (util/repeated-keys [abc def empty])))
+    (is (= #{:b :c} (util/repeated-keys [bcd abc])))
+    (is (= #{:b :c :d} (util/repeated-keys [abc def bcd])))
+    (is (= #{:b :c :d :e :f :g} (util/repeated-keys [abc efg def efg bcd])))))