import type { ObjectResolution, UnionSubject } from "./port";

/**
 * Why an empty read is `unresolved` and never `present`.
 *
 * `readUnion` cannot tell an absent object from one it failed to read, and does
 * not claim to: its per-document read is wrapped in `try {…} catch { return [] }`
 * (`readDoc`, in `@ng-eventually/polyfill`'s `src/surface/read-model.ts`), and
 * documents whose cap this user does not hold are dropped from the batch before
 * it reads at all. An empty result means absent OR unreadable OR failed, with
 * nothing to separate them.
 *
 * That is not a gap this package has to close, because nothing it does depends on
 * the answer: an index only ever grows, so every reading leads to the same act —
 * do not add, and say so.
 *
 * The rule lives here, apart from the I/O, for one reason: in the adapter it was
 * unreachable by any test, and dropping it there left the suite green while a
 * FAILED read got reported as `skipped: "no-field"` — a failure filed as a
 * property of the object. That is exactly the misclassification this layer exists
 * to avoid, so the rule is now a pure function with its own tests.
 */
export const EMPTY_READ =
  "the read came back empty — the object is absent, unreadable, or the read failed; " +
  "the polyfill does not distinguish them, and this layer does not need it to";

/** Turns what a read returned into the two-state answer the curator acts on. */
export function resolutionFromRead(subjects: readonly UnionSubject[]): ObjectResolution {
  if (subjects.length === 0) return { state: "unresolved", reason: EMPTY_READ };
  return { state: "present", subjects };
}

/** Turns a read that threw into the same two-state answer. Never `present`. */
export function resolutionFromFailure(error: unknown): ObjectResolution {
  return { state: "unresolved", reason: String(error) };
}