/**
 * Pure, deterministic dedup key for maintenance threads.
 *
 * Derives a stable key from the merge-base + head SHAs so that repeated
 * scheduled sync runs against the same upstream state collapse onto a single
 * maintenance thread instead of creating a new one each run. NEVER involves
 * Date.now()/random — the key must be identical for identical inputs.
 *
 * Returns `null` when the head SHA is missing/empty (unresolvable head → the
 * caller cannot dedup and should SKIP thread creation rather than mint a
 * unique key).
 */
export const maintenanceDedupKey = (input: {
  mergeBaseSha?: string;
  headSha?: string;
}): string | null => {
  const head = input.headSha?.trim();
  if (!head) return null;
  // Intentional `||`: an empty/whitespace merge base must collapse to the
  // 'nobase' sentinel, which `??` would not do (it keeps the empty string).
  // eslint-disable-next-line @typescript-eslint/prefer-nullish-coalescing
  const base = input.mergeBaseSha?.trim() || 'nobase';
  return `${base}:${head}`;
};