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>

test/utils/beacon/duration-test.ts

@@ -0,0 +1,83 @@
+/*
+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";
+
+import { msUntilExpiry, sortBeaconsByLatestExpiry } from "../../../src/utils/beacon";
+import { makeBeaconInfoEvent } from "../../test-utils";
+
+describe('beacon utils', () => {
+    // 14.03.2022 16:15
+    const now = 1647270879403;
+    const HOUR_MS = 3600000;
+
+    beforeEach(() => {
+        jest.spyOn(global.Date, 'now').mockReturnValue(now);
+    });
+
+    afterAll(() => {
+        jest.spyOn(global.Date, 'now').mockRestore();
+    });
+
+    describe('msUntilExpiry', () => {
+        it('returns remaining duration', () => {
+            const start = now - HOUR_MS;
+            const durationMs = HOUR_MS * 3;
+
+            expect(msUntilExpiry(start, durationMs)).toEqual(HOUR_MS * 2);
+        });
+
+        it('returns 0 when expiry has already passed', () => {
+            // created 3h ago
+            const start = now - HOUR_MS * 3;
+            // 1h durations
+            const durationMs = HOUR_MS;
+
+            expect(msUntilExpiry(start, durationMs)).toEqual(0);
+        });
+    });
+
+    describe('sortBeaconsByLatestExpiry()', () => {
+        const roomId = '!room:server';
+        const aliceId = '@alive:server';
+
+        // 12h old, 12h left
+        const beacon1 = new Beacon(makeBeaconInfoEvent(aliceId,
+            roomId,
+            { timeout: HOUR_MS * 24, timestamp: now - 12 * HOUR_MS },
+            '$1',
+        ));
+        // 10h left
+        const beacon2 = new Beacon(makeBeaconInfoEvent(aliceId,
+            roomId,
+            { timeout: HOUR_MS * 10, timestamp: now },
+            '$2',
+        ));
+
+        // 1ms left
+        const beacon3 = new Beacon(makeBeaconInfoEvent(aliceId,
+            roomId,
+            { timeout: HOUR_MS + 1, timestamp: now - HOUR_MS },
+            '$3',
+        ));
+
+        it('sorts beacons by descending expiry time', () => {
+            expect([beacon2, beacon3, beacon1].sort(sortBeaconsByLatestExpiry)).toEqual([
+                beacon1, beacon2, beacon3,
+            ]);
+        });
+    });
+});