From b0bc41291efbab251fae14e3300293e3f66556e8 Mon Sep 17 00:00:00 2001
From: kolaente <k@knt.li>
Date: Sun, 5 Apr 2026 13:29:41 +0200
Subject: [PATCH] feat(helpers): add deterministic stringHash for stable daily
 selection

---
 frontend/src/helpers/stringHash.test.ts | 21 +++++++++++++++++++++
 frontend/src/helpers/stringHash.ts      | 11 +++++++++++
 2 files changed, 32 insertions(+)
 create mode 100644 frontend/src/helpers/stringHash.test.ts
 create mode 100644 frontend/src/helpers/stringHash.ts

diff --git a/frontend/src/helpers/stringHash.test.ts b/frontend/src/helpers/stringHash.test.ts
new file mode 100644
index 000000000..3626acf0c
--- /dev/null
+++ b/frontend/src/helpers/stringHash.test.ts
@@ -0,0 +1,21 @@
+import {describe, it, expect} from 'vitest'
+import {stringHash} from './stringHash'
+
+describe('stringHash', () => {
+	it('returns a non-negative integer', () => {
+		expect(stringHash('hello')).toBeGreaterThanOrEqual(0)
+		expect(Number.isInteger(stringHash('hello'))).toBe(true)
+	})
+
+	it('is deterministic for the same input', () => {
+		expect(stringHash('foo')).toBe(stringHash('foo'))
+	})
+
+	it('returns different values for different inputs', () => {
+		expect(stringHash('foo')).not.toBe(stringHash('bar'))
+	})
+
+	it('handles the empty string', () => {
+		expect(stringHash('')).toBeGreaterThanOrEqual(0)
+	})
+})
diff --git a/frontend/src/helpers/stringHash.ts b/frontend/src/helpers/stringHash.ts
new file mode 100644
index 000000000..b843b1fbe
--- /dev/null
+++ b/frontend/src/helpers/stringHash.ts
@@ -0,0 +1,11 @@
+// Deterministic non-cryptographic string hash (djb2 variant).
+// Used for stable pseudo-random selection keyed on date + user + bucket.
+export function stringHash(input: string): number {
+	let hash = 5381
+	for (let i = 0; i < input.length; i++) {
+		// hash * 33 + char, kept in 32-bit range via `| 0`.
+		hash = ((hash << 5) + hash + input.charCodeAt(i)) | 0
+	}
+	// Ensure non-negative.
+	return hash >>> 0
+}