fix random seed logic

Author: sebasptsch

apps/client/src/utils/shuffleArray.ts

@@ -1,28 +1,46 @@
-function randomInRange(min: number, max: number, seed: Uint8Array = crypto.getRandomValues(new Uint8Array(4))): number {
-	const range = max - min + 1;
-	const randomValue = (seed[0] << 24) | (seed[1] << 16) | (seed[2] << 8) | seed[3];
-	return min + (randomValue % range);
+function makePRNG(seed: number) {
+	let state = seed >>> 0;
+	return () => {
+		// xorshift32
+		state ^= state << 13;
+		state ^= state >>> 17;
+		state ^= state << 5;
+		return state >>> 0;
+	};
 }
 
 export function makeRandomSeed(): Uint8Array {
 	return crypto.getRandomValues(new Uint8Array(4));
 }
 
-export function secureShuffle<T>(array: T[], seed: Uint8Array = crypto.getRandomValues(new Uint8Array(4))): T[] {
-	const numbers = [];
+function randomInRange(min: number, max: number, nextRand: () => number): number {
+	const range = max - min + 1;
+	const maxRand = 0xFFFFFFFF;
+	const limit = Math.floor(maxRand / range) * range;
 
-	// asynchronously generate an array of random numbers using a CSPRNG
-	for (let i = array.length - 1; i > 0; i--) {
-		numbers.push(randomInRange(0, i, seed));
-	}
+	let randomValue: number;
+	do {
+		randomValue = nextRand();
+	} while (randomValue >= limit);
+
+	return min + (randomValue % range);
+}
+
+export function secureShuffle<T>(array: T[], seedArray?: Uint8Array): T[] {
+	// Convert Uint8Array seed (4 bytes) into 32-bit integer
+	const seed =
+		seedArray
+			? ((seedArray[0] << 24) | (seedArray[1] << 16) | (seedArray[2] << 8) | seedArray[3]) >>> 0
+			: crypto.getRandomValues(new Uint8Array(4)).reduce((acc, v, i) => acc | (v << ((3 - i) * 8)), 0);
 
-	// apply durstenfeld shuffle with previously generated random numbers
-	for (let i = array.length - 1; i > 0; i--) {
-		const j = numbers[array.length - i - 1];
-		const temp = array[i];
-		array[i] = array[j];
-		array[j] = temp;
+	const nextRand = makePRNG(seed);
+	const result = array.slice();
+
+	// Fisher–Yates shuffle using seeded randoms
+	for (let i = result.length - 1; i > 0; i--) {
+		const j = randomInRange(0, i, nextRand);
+		[result[i], result[j]] = [result[j], result[i]];
 	}
 
-	return array;
-}
+	return result;
+}