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

* Rust migration staged rollout

* Phased rollout unit tests

src/utils/PhasedRolloutFeature.ts

@@ -0,0 +1,63 @@
+/*
+Copyright 2024 New Vector Ltd
+
+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 { xxHash32 } from "js-xxhash";
+
+/**
+ * The PhasedRolloutFeature class is used to manage the phased rollout of a new feature.
+ *
+ * It uses a hash of the user's identifier and the feature name to determine if a feature is enabled for a specific user.
+ * The rollout percentage determines the probability that a user will be enabled for the feature.
+ * The feature will be enabled for all users if the rollout percentage is 100, and for no users if the percentage is 0.
+ * If a user is enabled for a feature at x% rollout, it will also be for any greater than x percent.
+ *
+ * The process ensures a uniform distribution of enabled features across users.
+ *
+ * @property featureName - The name of the feature to be rolled out.
+ * @property rolloutPercentage - The int percentage (0..100) of users for whom the feature should be enabled.
+ */
+export class PhasedRolloutFeature {
+    public readonly featureName: string;
+    private readonly rolloutPercentage: number;
+    private readonly seed: number;
+
+    public constructor(featureName: string, rolloutPercentage: number) {
+        this.featureName = featureName;
+        if (!Number.isInteger(rolloutPercentage) || rolloutPercentage < 0 || rolloutPercentage > 100) {
+            throw new Error("Rollout percentage must be an integer between 0 and 100");
+        }
+        this.rolloutPercentage = rolloutPercentage;
+        // We add the feature name for the seed to ensure that the hash is different for each feature
+        this.seed = Array.from(featureName).reduce((sum, char) => sum + char.charCodeAt(0), 0);
+    }
+
+    /**
+     * Returns true if the feature should be enabled for the given user.
+     * @param userIdentifier - Some unique identifier for the user, e.g. their user ID or device ID.
+     */
+    public isFeatureEnabled(userIdentifier: string): boolean {
+        /*
+         * We use a hash function to convert the unique user ID string into an integer.
+         * This integer can then be used as a basis for deciding whether the user should have access to the new feature.
+         * We need some hash with good uniform distribution properties, security is not a concern here.
+         * We use xxHash32, which is fast and has good distribution properties.
+         */
+        const hash = xxHash32(userIdentifier, this.seed);
+        // We use the hash modulo 100 to get a number between 0 and 99.
+        // Modulo is simple and effective and the distribution should be uniform enough for our purposes.
+        return hash % 100 < this.rolloutPercentage;
+    }
+}