add helpers for beacon duration calculations (#8101)

* add helpers for beacon duration calculations

Signed-off-by: Kerry Archibald <kerrya@element.io>

* makeBeaconInfoEvent timestsamp prop

Signed-off-by: Kerry Archibald <kerrya@element.io>

* ad copyright

Signed-off-by: Kerry Archibald <kerrya@element.io>

src/utils/beacon/duration.ts

@@ -0,0 +1,36 @@
+/*
+Copyright 2022 The Matrix.org Foundation C.I.C
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import { Beacon } from "matrix-js-sdk/src/matrix";
+
+/**
+ * Get ms until expiry
+ * Returns 0 when expiry is already passed
+ * @param startTimestamp
+ * @param durationMs
+ * @returns remainingMs
+ */
+export const msUntilExpiry = (startTimestamp: number, durationMs: number): number =>
+    Math.max(0, (startTimestamp + durationMs) - Date.now());
+
+export const getBeaconMsUntilExpiry = (beacon: Beacon): number =>
+    msUntilExpiry(beacon.beaconInfo.timestamp, beacon.beaconInfo.timeout);
+
+export const getBeaconExpiryTimestamp = (beacon: Beacon): number =>
+    beacon.beaconInfo.timestamp + beacon.beaconInfo.timeout;
+
+export const sortBeaconsByLatestExpiry = (left: Beacon, right: Beacon): number =>
+    getBeaconExpiryTimestamp(right) - getBeaconExpiryTimestamp(left);