// The stream half of the revision discipline; the READ half is `freshest()` in `queries.ts`.
//
// The split is deliberate. A frame has nothing to compare itself against, so its high-water mark
// has to be kept here. A read does — the query cache, which the stream patches — and keeping a
// second copy of it here was how progress rolled backwards: the copy went stale the moment a frame
// landed, and a read was compared against the copy instead of against what was on screen.

// One entry per book being watched, holding a number. Dropped when the subscription closes.
const applied = new Map<string, number>();

/**
 * Whether a stream frame carries anything new. Equal revisions pass: one transaction is one
 * revision but SEVERAL frames, and rejecting equality would drop the siblings of the frame already
 * applied — the same reason catch-up reads `>=` and not `>`.
 */
export function acceptFrame(scope: string, revision: number): boolean {
  const last = applied.get(scope);
  if (last !== undefined && revision < last) return false;
  applied.set(scope, revision);
  return true;
}

/** Drops the mark for a scope — after a resync there is nothing to compare against. */
export function forget(scope: string): void {
  applied.delete(scope);
}

/** Test seam: the registry is module state, and a leftover revision would leak between tests. */
export function forgetAll(): void {
  applied.clear();
}