import { createHmac } from "node:crypto";

/**
 * Derives a daily-rotating, non-reversible pseudonymous visitor identifier
 * for session grouping - the same approach used by privacy-friendly
 * analytics tools (Plausible/Fathom-style): hash(ip + userAgent + host)
 * salted with a value that changes every calendar day. Raw IP/UA are never
 * persisted anywhere, only this hash - and since the salt rotates daily,
 * the same visitor hashes differently tomorrow, so there is no cross-day
 * linkage and no long-term individual profile.
 *
 * ANALYTICS_SALT_SECRET is a dedicated secret (not PAYLOAD_SECRET) - this
 * codebase keeps one secret per purpose on purpose (see
 * MEETING_SESSION_SECRET's comment in .env.example).
 */
function secret(): string {
  const value = process.env.ANALYTICS_SALT_SECRET;
  if (!value) throw new Error("ANALYTICS_SALT_SECRET is not set");
  return value;
}

function dateKey(date: Date): string {
  return date.toISOString().slice(0, 10);
}

function dailySalt(date: Date): string {
  return createHmac("sha256", secret()).update(dateKey(date)).digest("hex");
}

export function visitorHash(args: { ip: string; userAgent: string; host: string; date?: Date }): string {
  const salt = dailySalt(args.date ?? new Date());
  return createHmac("sha256", salt).update(`${args.ip}|${args.userAgent}|${args.host}`).digest("hex");
}