diff --git a/src/state.mjs b/src/state.mjs
new file mode 100644
index 0000000..6c29dda
--- /dev/null
+++ b/src/state.mjs
@@ -0,0 +1,15 @@
+export function advance(state, event) {
+  const next = { ...state, anomalies: [...state.anomalies] };
+  if (event.type === 'ALARM') {
+    if (next.open) next.anomalies.push('duplicate alarm');
+    else { next.open = true; next.firstAlarm = event.time; next.acknowledged = false; }
+  } else if (event.type === 'ACK') {
+    if (!next.open) next.anomalies.push('ack without alarm');
+    else if (next.acknowledged) next.anomalies.push('duplicate ack');
+    else next.acknowledged = true;
+  } else if (event.type === 'CLEAR') {
+    if (!next.open) next.anomalies.push('clear without alarm');
+    else { next.open = false; next.closedAt = event.time; }
+  }
+  return next;
+}