// What the library detail page's error slot shows after a load.
//
// Extracted from `/library/[id]/+page.svelte` so it can be unit-tested.

export type LoadOutcome = { ok: true } | { ok: false; error: unknown };

/**
 * The text of whatever a load threw. Backend commands reject with a plain
 * string, not an `Error`, so reading only `Error.message` replaced every
 * backend error with the generic fallback and hid what actually failed.
 */
export function loadErrorMessage(error: unknown): string {
  if (error instanceof Error && error.message) return error.message;
  if (typeof error === "string" && error) return error;
  return "Failed to load item";
}

/**
 * The error to show once a load has ended.
 *
 * A success always clears it — a same-item reload used to leave a stale error
 * up for good. A failed *refresh* of a page already on screen shows none: the
 * page is still showing what the cache answered, and a background refresh
 * (reconnect, resume, filter change) must not blank it. Only failing to open an
 * item leaves the viewer with nothing to see.
 *
 * TRACES: UR-062 | DR-297 | UT-267
 */
export function errorAfterLoad(
  outcome: LoadOutcome,
  options: { refreshing: boolean },
): string | null {
  if (outcome.ok || options.refreshing) return null;
  return loadErrorMessage(outcome.error);
}