import { XMLParser } from 'fast-xml-parser';

/** One feed item: title always present (non-empty), link where the feed provides one. */
export interface FeedItem {
  title: string;
  link?: string;
}

const parser = new XMLParser({ ignoreAttributes: false });

function asText(value: unknown): string | undefined {
  if (typeof value === 'string') return value.trim() || undefined;
  // element with attributes/CDATA parses to an object holding its text under #text
  const t = (value as Record<string, unknown>)?.['#text'];
  if (typeof t === 'string' && t.trim()) return t.trim();
  return undefined;
}

function asLink(value: unknown): string | undefined {
  const fromText = asText(value);
  if (fromText) return fromText; // RSS 2.0 <link>url</link>
  // Atom <link rel="alternate" href="..."/> — the URL is an attribute
  const href = (value as Record<string, unknown>)?.['@_href'];
  if (typeof href === 'string' && href.trim()) return href.trim();
  return undefined;
}

function collect(value: unknown): Record<string, unknown>[] {
  // a single item/entry parses to an object, several to an array
  if (!value) return [];
  const arr = Array.isArray(value) ? value : [value];
  return arr.filter(
    (v): v is Record<string, unknown> => !!v && typeof v === 'object',
  );
}

/**
 * Parse RSS 2.0 (`<item><title>/<link>`) or Atom (`<entry><title>`) XML into items with
 * non-empty titles, in document order (feeds are newest-first). Returns [] for malformed
 * input or feeds without usable items — that empty result IS the "not a valid feed" signal.
 */
export function parseFeed(xml: string): FeedItem[] {
  let doc: unknown;
  try {
    doc = parser.parse(xml);
  } catch {
    return []; // fast-xml-parser is lenient, but a hard failure means no items either
  }
  const root = doc as Record<string, any>;
  // coalesce the RAW values first — collect() always returns an array, so `collect(a) ?? collect(b)` could never fall through to b
  const rawItems = collect(root?.rss?.channel?.item ?? root?.feed?.entry);
  const items: FeedItem[] = [];
  for (const item of rawItems) {
    const title = asText(item?.title);
    if (!title) continue; // skip entries without a usable title
    const link = asLink(item.link);
    items.push(link ? { title, link } : { title });
  }
  return items;
}