feat(study): use seeded PRNG for deterministic card shuffle order

Shuffle order is now fixed within a study day by seeding mulberry32
with the study day boundary timestamp (3:00 AM rollover).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

1 files changed, 32 insertions, 0 deletions

diff --git a/src/client/utils/random.ts b/src/client/utils/random.ts
new file mode 100644
index 0000000..53de45e
--- /dev/null
+++ b/src/client/utils/random.ts
@@ -0,0 +1,32 @@
+/**
+ * Mulberry32 seeded PRNG.
+ * Returns a function that produces deterministic values in [0, 1).
+ */
+export function createSeededRandom(seed: number): () => number {
+	let s = seed | 0;
+	return () => {
+		s = (s + 0x6d2b79f5) | 0;
+		let t = Math.imul(s ^ (s >>> 15), 1 | s);
+		t = (t + Math.imul(t ^ (t >>> 7), 61 | t)) ^ t;
+		return ((t ^ (t >>> 14)) >>> 0) / 4294967296;
+	};
+}
+
+/**
+ * Fisher-Yates shuffle algorithm.
+ * Returns a new shuffled array (does not mutate the original).
+ * Accepts an optional `random` function for deterministic shuffling.
+ */
+export function shuffle<T>(
+	array: T[],
+	random: () => number = Math.random,
+): T[] {
+	const result = [...array];
+	for (let i = result.length - 1; i > 0; i--) {
+		const j = Math.floor(random() * (i + 1));
+		const temp = result[i] as T;
+		result[i] = result[j] as T;
+		result[j] = temp;
+	}
+	return result;
+}