Support staged rollout of migration to Rust Crypto (#12184)

* Rust migration staged rollout

* Phased rollout unit tests

test/utils/PhasedRolloutFeature-test.ts

@@ -0,0 +1,89 @@
+/*
+Copyright 2024 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 { PhasedRolloutFeature } from "../../src/utils/PhasedRolloutFeature";
+
+describe("Test PhasedRolloutFeature", () => {
+    function randomUserId() {
+        const characters = "abcdefghijklmnopqrstuvwxyz0123456789.=_-/+";
+        let result = "";
+        const charactersLength = characters.length;
+        const idLength = Math.floor(Math.random() * 15) + 6; // Random number between 6 and 20
+        for (let i = 0; i < idLength; i++) {
+            result += characters.charAt(Math.floor(Math.random() * charactersLength));
+        }
+        return "@" + result + ":matrix.org";
+    }
+
+    function randomDeviceId() {
+        const characters = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
+        let result = "";
+        const charactersLength = characters.length;
+        for (let i = 0; i < 10; i++) {
+            result += characters.charAt(Math.floor(Math.random() * charactersLength));
+        }
+        return result;
+    }
+
+    it("should only accept valid percentage", () => {
+        expect(() => new PhasedRolloutFeature("test", 0.8)).toThrow();
+        expect(() => new PhasedRolloutFeature("test", -1)).toThrow();
+        expect(() => new PhasedRolloutFeature("test", 123)).toThrow();
+    });
+
+    it("should enable for all if percentage is 100", () => {
+        const phasedRolloutFeature = new PhasedRolloutFeature("test", 100);
+
+        for (let i = 0; i < 1000; i++) {
+            expect(phasedRolloutFeature.isFeatureEnabled(randomUserId())).toBeTruthy();
+        }
+    });
+
+    it("should not enable for anyone if percentage is 0", () => {
+        const phasedRolloutFeature = new PhasedRolloutFeature("test", 0);
+
+        for (let i = 0; i < 1000; i++) {
+            expect(phasedRolloutFeature.isFeatureEnabled(randomUserId())).toBeFalsy();
+        }
+    });
+
+    it("should enable for more users if percentage grows", () => {
+        let rolloutPercentage = 0;
+        let previousBatch: string[] = [];
+        const allUsers = new Array(1000).fill(0).map(() => randomDeviceId());
+
+        while (rolloutPercentage <= 90) {
+            rolloutPercentage += 10;
+            const nextRollout = new PhasedRolloutFeature("test", rolloutPercentage);
+            const nextBatch = allUsers.filter((userId) => nextRollout.isFeatureEnabled(userId));
+            expect(previousBatch.length).toBeLessThan(nextBatch.length);
+            expect(previousBatch.every((user) => nextBatch.includes(user))).toBeTruthy();
+            previousBatch = nextBatch;
+        }
+    });
+
+    it("should distribute differently depending on the feature name", () => {
+        const allUsers = new Array(1000).fill(0).map(() => randomUserId());
+
+        const featureARollout = new PhasedRolloutFeature("FeatureA", 50);
+        const featureBRollout = new PhasedRolloutFeature("FeatureB", 50);
+
+        const featureAUsers = allUsers.filter((userId) => featureARollout.isFeatureEnabled(userId));
+        const featureBUsers = allUsers.filter((userId) => featureBRollout.isFeatureEnabled(userId));
+
+        expect(featureAUsers).not.toEqual(featureBUsers);
+    });
+});