/**
 * Keyset paging for 「过往的报告」 (2026-09-30, BUG-1124): newest first by
 * (created_at desc, id desc), `limit + 1` rows to know whether more exist.
 * Modelled on session-cursor.ts; reports page on created_at, not updated_at.
 */
const reportIdPattern = /^[0-9a-f]{8}-[0-9a-f]{4}-[1-8][0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}$/i;

/** One batch under 「加载更多」 (product: 10 at a time). Also the first page. */
export const REPORT_PAGE_SIZE = 10;
export const REPORT_PAGE_LIMIT_MAX = 50;

export type ReportCursor = {
  readonly createdAt: string;
  readonly id: string;
};

export function clampReportLimit(raw: string | null | undefined): number {
  if (raw == null || raw === "") return REPORT_PAGE_SIZE;
  const parsed = Number.parseInt(raw, 10);
  if (!Number.isFinite(parsed)) return REPORT_PAGE_SIZE;
  return Math.min(REPORT_PAGE_LIMIT_MAX, Math.max(1, Math.trunc(parsed)));
}

export function encodeReportCursor(createdAt: string, id: string): string {
  return `${createdAt},${id}`;
}

export function parseReportCursor(raw: string | null | undefined): ReportCursor | null {
  if (!raw) return null;
  const comma = raw.lastIndexOf(",");
  if (comma <= 0 || comma === raw.length - 1) return null;
  const createdAt = raw.slice(0, comma);
  const id = raw.slice(comma + 1);
  if (!Number.isFinite(Date.parse(createdAt))) return null;
  if (!reportIdPattern.test(id)) return null;
  return { createdAt, id };
}

/** PostgREST `or=` filter: strictly older than the cursor row. */
export function reportCursorFilter(cursor: ReportCursor): string {
  const createdAt = JSON.stringify(cursor.createdAt);
  const id = JSON.stringify(cursor.id);
  return `created_at.lt.${createdAt},and(created_at.eq.${createdAt},id.lt.${id})`;
}

function rawCreatedAt(value: unknown): string {
  if (typeof value === "string") return Number.isFinite(Date.parse(value)) ? value : "";
  if (value instanceof Date && Number.isFinite(value.getTime())) return value.toISOString();
  return "";
}

/** The cursor after the `limit`-th row when a `limit + 1`-th row came back, else null. */
export function nextReportCursor(
  rows: readonly unknown[],
  limit: number,
): string | null {
  if (rows.length <= limit) return null;
  const last = rows[limit - 1];
  if (!last || typeof last !== "object") return null;
  const row = last as { id?: unknown; created_at?: unknown };
  const createdAt = rawCreatedAt(row.created_at);
  if (!createdAt || typeof row.id !== "string") return null;
  return encodeReportCursor(createdAt, row.id);
}

type Keyed = { readonly id: string; readonly createdAt: string };

/** Newest first, as the endpoint orders them. */
function olderThan(left: Keyed, right: Keyed): boolean {
  const l = Date.parse(left.createdAt);
  const r = Date.parse(right.createdAt);
  if (l !== r) return l < r;
  return left.id < right.id;
}

/**
 * A fresh first page merged over the rows already on screen: the first page
 * wins for every row it carries; rows the user loaded with 「加载更多」 that are
 * older than the first page's last row stay. Used by the 3s poll and 刷新, so
 * a refresh never throws away pages the user opened.
 */
export function mergeFirstReportPage<T extends Keyed>(
  firstPage: readonly T[],
  loaded: readonly T[],
): T[] {
  const tail = firstPage[firstPage.length - 1];
  if (!tail) return [];
  const ids = new Set(firstPage.map((row) => row.id));
  return [...firstPage, ...loaded.filter((row) => !ids.has(row.id) && olderThan(row, tail))];
}

/** One more page appended after what is on screen; duplicates are dropped. */
export function appendReportPage<T extends Keyed>(loaded: readonly T[], page: readonly T[]): T[] {
  const ids = new Set(loaded.map((row) => row.id));
  return [...loaded, ...page.filter((row) => !ids.has(row.id))];
}