/// Reads an Atom feed (spec 6.2) into the same shape dated links produce, so a
/// site's posts come from one store however they were published. Pure data: no
/// Flutter imports, so it can run inside the fetch actor's isolate.
library;

import 'package:xml/xml.dart';

import 'document.dart';
import 'parser.dart' show MewsParseException;

/// The entries of an Atom feed, newest first. Entries without a usable address
/// or date are skipped rather than guessed at.
///
/// Element names are matched without their namespace, so a feed using the
/// default Atom namespace and one using an `atom:` prefix both read the same.
List<MewsEntry> parseAtomEntries({
  required String source,
  required String feedUrl,
}) {
  final XmlDocument doc;
  try {
    doc = XmlDocument.parse(source);
  } on XmlException catch (e) {
    throw MewsParseException(e.toString());
  }

  final base = Uri.tryParse(feedUrl);
  final entries = <MewsEntry>[];
  for (final element in doc.rootElement.children.whereType<XmlElement>()) {
    if (element.name.local != 'entry') continue;
    final url = _entryUrl(element, base);
    final date = _entryDate(element);
    if (url == null || date == null) continue;
    entries.add(MewsEntry(
      url: url,
      date: date,
      title: _childText(element, 'title') ?? url,
    ));
  }

  entries.sort((a, b) => b.date.compareTo(a.date));
  return entries;
}

/// An entry's address: its alternate link, or its id when that is a web
/// address. The id is a fallback because many feeds use the post's URL for it.
String? _entryUrl(XmlElement entry, Uri? base) {
  for (final child in entry.children.whereType<XmlElement>()) {
    if (child.name.local != 'link') continue;
    final rel = child.getAttribute('rel');
    if (rel != null && rel != 'alternate') continue;
    final href = child.getAttribute('href');
    if (href == null || href.isEmpty) continue;
    final resolved = base?.resolve(href) ?? Uri.tryParse(href);
    if (resolved != null) return resolved.toString();
  }

  final id = _childText(entry, 'id');
  if (id == null) return null;
  final asUri = Uri.tryParse(id);
  return asUri != null && (asUri.scheme == 'http' || asUri.scheme == 'https')
      ? asUri.toString()
      : null;
}

/// When an entry was published. `updated` is required by Atom, so it comes
/// first, with `published` as the fallback.
DateTime? _entryDate(XmlElement entry) {
  for (final name in const ['updated', 'published']) {
    final text = _childText(entry, name);
    if (text == null) continue;
    final date = DateTime.tryParse(text);
    if (date != null) return date;
  }
  return null;
}

String? _childText(XmlElement parent, String name) {
  for (final child in parent.children.whereType<XmlElement>()) {
    if (child.name.local != name) continue;
    final text = child.innerText.trim();
    if (text.isNotEmpty) return text;
  }
  return null;
}