// THE TIMELINE — a report's dated entries (report.json `entries`): small
// cited blocks appended over time, read newest first, each its own anchor on
// the page and its own item in the report's feeds (./feeds.ts).
//
// The one order every reader shares — the page, the slides, the citation
// numbering (./uses.ts), the exports, the feeds, MCP: newest first by date;
// entries of the same instant keep their order in the document.
//
// Pure, no imports: the browser and the validator read the one copy.

const DAY_RE = /^\d{4}-\d{2}-\d{2}$/;

// A report date (`YYYY-MM-DD`, midnight UTC, or an ISO 8601 date-time with a
// zone) as epoch milliseconds; NaN for anything else.
export function reportDateMs(v: string | undefined): number {
  if (!v) return Number.NaN;
  return Date.parse(DAY_RE.test(v) ? `${v}T00:00:00Z` : v);
}

// The source indexes of `entries`, newest first. A date that does not parse
// sorts last (a published report has none: the validator refuses it).
export function entryOrder(entries: readonly { date: string }[]): number[] {
  const at = entries.map((e) => {
    const ms = reportDateMs(e.date);
    return Number.isNaN(ms) ? Number.NEGATIVE_INFINITY : ms;
  });
  return entries.map((_e, i) => i).sort((a, b) => at[b] - at[a] || a - b);
}

export function entriesNewestFirst<T extends { date: string }>(entries: readonly T[]): T[] {
  return entryOrder(entries).map((i) => entries[i]);
}

// When the report last changed, its timeline counted: the latest of its own
// `updated` and every entry's `date` and `updated` — when that is after
// `published`. Else `updated` as given (an entry dated before the report was
// published does not make it "updated").
export function effectiveUpdated(report: {
  published?: string;
  updated?: string;
  entries?: readonly { date: string; updated?: string }[];
}): string | undefined {
  let best = report.updated;
  let bestMs = reportDateMs(best);
  for (const e of report.entries ?? []) {
    for (const d of [e.date, e.updated]) {
      const ms = reportDateMs(d);
      if (!Number.isNaN(ms) && (Number.isNaN(bestMs) || ms > bestMs)) {
        best = d;
        bestMs = ms;
      }
    }
  }
  if (best === report.updated) return report.updated;
  const published = reportDateMs(report.published);
  return !Number.isNaN(published) && bestMs <= published ? report.updated : best;
}