/**
 * @factory/shared — realtime presence palette (PRODUCT-SPEC realtime row,
 * phase-07 TASK-0092).
 *
 * `presenceColor(userId)` is the single deterministic source of a peer's
 * cursor/avatar color, shared so the server (presence broadcasts) and
 * clients agree without negotiating. 20 distinct colors; the a11y rule is
 * that color is never the only identifier (the peer's name label is
 * mandatory — ux-requirements §12), so a color collision is harmless.
 *
 * Deterministic FNV-1a 32-bit hash → palette index. No allocation, no
 * Math.random (rule 25: deterministic behavior).
 */

/** The 20-color presence palette (closed set; names, not raw hex, on the wire). */
export const PRESENCE_PALETTE: readonly string[] = [
  "red",
  "orange",
  "amber",
  "yellow",
  "lime",
  "green",
  "teal",
  "cyan",
  "sky",
  "blue",
  "indigo",
  "violet",
  "purple",
  "fuchsia",
  "pink",
  "rose",
  "slate",
  "gray",
  "zinc",
  "stone",
];

/**
 * FNV-1a 32-bit hash of a string. Deterministic across platforms (32-bit
 * math is exact in JS doubles).
 */
export function fnv1a32(input: string): number {
  let hash = 0x811c9dc5;
  for (let i = 0; i < input.length; i += 1) {
    hash ^= input.charCodeAt(i);
    // hash *= 16777619 (FNV prime) — via multiply + >>> 0 (32-bit wrap)
    hash = Math.imul(hash, 0x01000193) >>> 0;
  }
  return hash >>> 0;
}

/**
 * The deterministic presence color name for a user.
 *
 * Distribution guarantee (unit-tested): over any 1,000-user fixture no
 * color is assigned to more than 10 % of users (palette of 20 ⇒ expected
 * 5 %; the test asserts the FNV-1a mix is uniform enough).
 */
export function presenceColor(userId: string): string {
  const index = fnv1a32(userId) % PRESENCE_PALETTE.length;
  return PRESENCE_PALETTE[index] ?? "gray";
}