diff --git a/src/test/grub/test/unit/sync.clj b/src/test/grub/test/unit/sync.clj
index c198d80..1aa8dd4 100644
--- a/src/test/grub/test/unit/sync.clj
+++ b/src/test/grub/test/unit/sync.clj
@@ -25,3 +25,21 @@
     (s/get-history-state states "hash1") => {:a :b}
     (s/get-history-state states "hash2") => {:c :d}
     (s/get-history-state states "hash3") => {:e :f}))
+
+(fact "Add history state appends state to the end"
+  (let [states [{:hash "hash1" :state {:a :b}}
+                {:hash "hash2" :state {:c :d}}]]
+    (:state (last (s/add-history-state states {:e :f}))) => {:e :f}))
+
+(fact "Add history state appends state to the end and drops first state if full"
+  (let [states (into [] (for [i (range 20)] {:hash (str "hash" i) :state {:i i}}))
+        new-states (s/add-history-state states {:i 21})]
+    (count new-states) => 20
+    (:state (last new-states)) => {:i 21}
+    (:state (first new-states)) => {:i 1}))
+
+(fact "Add history state does not add consecutive duplicate states"
+  (let [hash (hasch/uuid {:c :d})
+        states [{:hash "hash1" :state {:a :b}}
+                {:hash hash :state {:c :d}}]]
+    (s/add-history-state states {:c :d}) => states))