feat(helpers): add deterministic stringHash for stable daily selection

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)
+	})
+})

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
+}