diff --git a/audit/window.go b/audit/window.go
new file mode 100644
index 0000000..2709686
--- /dev/null
+++ b/audit/window.go
@@ -0,0 +1,23 @@
+package audit
+
+type Window struct {
+    Last uint16
+    HasLast bool
+    Summary Summary
+}
+
+func (w *Window) Observe(sequence uint16) string {
+    if !w.HasLast {
+        w.Last, w.HasLast = sequence, true
+        w.Summary.Accepted++
+        w.Summary.Latest = sequence
+        return "accepted"
+    }
+    delta := uint16(sequence - w.Last)
+    if delta == 0 { w.Summary.Duplicate++; return "duplicate" }
+    if delta > 32767 { w.Summary.Reordered++; return "reordered" }
+    if delta > 1 { w.Summary.Lost += uint64(delta - 1) }
+    w.Last, w.Summary.Latest = sequence, sequence
+    w.Summary.Accepted++
+    return "accepted"
+}