the-mews/app/lib/application/controllers/browser_controller.dart

import 'package:actors/actors.dart';
import 'package:drift/drift.dart' show Value;
import 'package:signals/signals.dart';

import '../../database/database.dart';
import '../../mews/document.dart';
import '../actors/fetch_actor.dart';
import '../internal_actions.dart';
import '../internal_pages.dart';
import '../pages/mews_urls.dart';

/// What went wrong while browsing. The presentation layer maps these to
/// user-facing messages; no raw codes cross into the UI.
sealed class BrowseError {
  const BrowseError();
}

class LoadFailed extends BrowseError {
  const LoadFailed();
}

class BrokenPage extends BrowseError {
  const BrokenPage(this.source);

  /// The markup that failed to parse; "show page text" recovers its content.
  final String source;
}

/// Coordinates navigation: owns the signals the reader UI observes and
/// dispatches fetch commands to the fetch actor. Local-first: a cached page is
/// revalidated with the server but survives any network failure.
class BrowserController {
  BrowserController({
    required MewsDatabase db,
    required Actor<FetchCommand, FetchResult> fetch,
    InternalPages? internalPages,
    InternalActions? internalActions,
  })  : _db = db,
        _fetch = fetch,
        _internalPages = internalPages,
        _internalActions = internalActions;

  final MewsDatabase _db;
  final Actor<FetchCommand, FetchResult> _fetch;
  final InternalPages? _internalPages;
  final InternalActions? _internalActions;

  /// The stack of visited pages; the reader is at [_stackIndex].
  final List<String> _stack = [];
  int _stackIndex = -1;

  final currentPage = signal<MewsPage?>(null);
  final loading = signal(false);
  final error = signal<BrowseError?>(null);

  /// Pages opened this session, so links can show the visited color.
  final visitedUrls = signal(<String>{});

  /// The parsed pages next to the current one in the stack, ready to show
  /// while swiping between them.
  final backPage = signal<MewsPage?>(null);
  final forwardPage = signal<MewsPage?>(null);

  bool get canGoBack => _stackIndex > 0;
  bool get canGoForward => _stackIndex >= 0 && _stack.length - 1 > _stackIndex;

  /// Normalizes what the reader typed into an address: bare domains become
  /// https URLs; anything already carrying a scheme (mews:, https:, ...) is
  /// left alone.
  static String normalizeAddress(String input) {
    final trimmed = input.trim();
    if (trimmed.isEmpty) return '';
    if (_schemePattern.hasMatch(trimmed)) return trimmed;
    return 'https://$trimmed';
  }

  static final _schemePattern = RegExp(r'^[a-zA-Z][a-zA-Z0-9+.-]*:');

  /// The wap. subdomain equivalent of [url], or null when the host already
  /// starts with it or the address is not a plain web URL.
  static String? wapUrlOf(String url) {
    final uri = Uri.tryParse(url);
    if (uri == null || !uri.hasScheme || uri.host.isEmpty) return null;
    if (uri.scheme != 'http' && uri.scheme != 'https') return null;
    if (uri.host.startsWith('wap.')) return null;
    return uri.replace(host: 'wap.${uri.host}').toString();
  }

  Future<void> open(String input) async {
    final url = MewsUrls.canonical(normalizeAddress(input));
    if (url.isEmpty) return;

    // Action links change something and stay on the current page instead of
    // joining the walk history.
    if (MewsUrls.isAction(url)) {
      // Only the client's own pages may change the reader's data. A fetched
      // page linking to mews:do/... must not act on their behalf (spec 8.1).
      if (!_onInternalPage) return;
      if (await (_internalActions?.apply(url) ?? Future.value(false))) {
        await _loadCurrent();
      }
      return;
    }

    _stack.removeRange(_stackIndex + 1, _stack.length);
    _stack.add(url);
    _stackIndex++;
    _updateNeighbors();
    await _loadCurrent();

    // Mews sites commonly live on a wap. subdomain; try that before showing
    // an error when the address as typed does not work out.
    if (error.value != null && _stack[_stackIndex] == url) {
      final wap = wapUrlOf(url);
      if (wap != null) {
        final firstError = error.value;
        _stack[_stackIndex] = wap;
        _updateNeighbors();
        await _loadCurrent();
        // The address as typed is the closer match; keep its error when the
        // wap. attempt does not work either.
        if (error.value != null && firstError != null) {
          error.value = firstError;
        }
      }
    }
  }

  Future<void> back() async {
    if (!canGoBack) return;
    _stackIndex--;
    // The neighbor was parsed for the swipe already; show it right away so
    // the transition lands on content instead of a blank page while
    // _loadCurrent revalidates it against the server.
    _showParsedNeighbor(backPage.value);
    _updateNeighbors();
    await _loadCurrent();
  }

  Future<void> forward() async {
    if (!canGoForward) return;
    _stackIndex++;
    _showParsedNeighbor(forwardPage.value);
    _updateNeighbors();
    await _loadCurrent();
  }

  void _showParsedNeighbor(MewsPage? page) {
    if (page == null) return;
    currentPage.value = page;
    error.value = null;
    visitedUrls.value = {...visitedUrls.value, page.url};
  }

  Future<void> reload() => _loadCurrent();

  /// Whether the reader is standing on one of the client's own pages.
  bool get _onInternalPage =>
      _stackIndex >= 0 && MewsUrls.isInternal(_stack[_stackIndex]);

  /// Rebuilds the open page when it is one of ours, so a change made elsewhere
  /// shows up without the reader navigating.
  Future<void> refreshInternalPage() async {
    if (!_onInternalPage) return;
    await _loadCurrent();
  }

  Future<void> submitForm(String action, String method, Map<String, String> fields) async {
    // A form on one of our own pages is a navigation or an action, never a
    // network request; the fetch client cannot speak the mews scheme.
    if (MewsUrls.isInternal(action)) {
      final base = Uri.parse(action);
      final verb = MewsUrls.verbOf(action);
      return open(verb == null
          ? MewsUrls.page(base, fields)
          : MewsUrls.action(verb, {...base.queryParameters, ...fields}));
    }

    loading.value = true;
    error.value = null;
    final FetchResult result;
    try {
      result = await _fetch.send(SubmitForm(url: action, method: method, fields: fields));
    } catch (_) {
      // A submit that throws out of the actor is still just a failed load.
      loading.value = false;
      error.value = const LoadFailed();
      return;
    }
    loading.value = false;
    switch (result) {
      case FetchFormOk(:final page):
        _stack.removeRange(_stackIndex + 1, _stack.length);
        _stack.add(page.url);
        _stackIndex++;
        _updateNeighbors();
        _show(page);
      case _:
        error.value = const LoadFailed();
    }
  }

  /// Reopens the most recent page from the previous session, or the hub when
  /// there is nothing to return to.
  Future<void> restoreLastSession() async {
    final url = await _db.lastVisitedUrl() ?? MewsUrls.hub;
    _stack.add(url);
    _stackIndex = 0;
    _updateNeighbors();
    await _loadCurrent();
  }

  /// Parses the stack's neighboring pages in the fetch actor so swipes can
  /// show the real content. Results arriving after another navigation are
  /// dropped, so rapid navigation never shows a stale neighbor.
  void _updateNeighbors() {
    _loadNeighbor(_stackIndex - 1, backPage);
    _loadNeighbor(_stackIndex + 1, forwardPage);
  }

  Future<void> _loadNeighbor(int index, Signal<MewsPage?> target) async {
    final expectedIndex = _stackIndex;
    if (index < 0 || index >= _stack.length) {
      target.value = null;
      return;
    }
    final url = _stack[index];
    if (_internalPages?.handles(url) ?? false) {
      target.value = await _internalPages!.pageFor(url);
      return;
    }
    final cached = await _db.cachedPage(url);
    if (_stackIndex != expectedIndex) return;
    if (cached == null) {
      target.value = null;
      return;
    }
    final result = await _fetch.send(ParseSource(url: url, source: cached.source));
    if (_stackIndex != expectedIndex) return;
    target.value = result is FetchPageOk ? result.page : null;
  }

  Future<void> _loadCurrent() async {
    if (_stackIndex < 0) return;
    final url = _stack[_stackIndex];
    loading.value = true;
    error.value = null;

    // Internal pages are built from the local database, never fetched or
    // cached; they join the stack like any other page.
    if (_internalPages?.handles(url) ?? false) {
      final page = await _internalPages!.pageFor(url);
      loading.value = false;
      if (page != null) {
        currentPage.value = page;
        error.value = null;
        // Our own addresses stay out of visitedUrls: a sort or filter link
        // would otherwise take the visited color after one use.
      } else {
        currentPage.value = null;
        error.value = const LoadFailed();
      }
      return;
    }

    final cached = await _db.cachedPage(url);
    final result = await _fetch.send(FetchPage(
      url: url,
      etag: cached?.etag,
      lastModified: cached?.lastModified,
      cachedSource: cached?.source,
    ));
    loading.value = false;

    switch (result) {
      case FetchPageOk(:final page, :final source, :final etag, :final lastModified, :final notModified):
        currentPage.value = page;
        error.value = null;
        visitedUrls.value = {...visitedUrls.value, url};
        // Opening a post is reading it. Nothing happens when the address is
        // not one, so this needs no knowledge of the reader's subscriptions.
        await _db.markEntryRead(url);
        if (!notModified) {
          await _db.saveCachedPage(CachedPagesCompanion.insert(
            url: url,
            source: source,
            title: Value(page.title),
            etag: Value(etag),
            lastModified: Value(lastModified),
          ));
          await _db.recordVisit(url, page.title);
        }
      case FetchParseError(:final source):
        currentPage.value = null;
        error.value = BrokenPage(source);
      case _:
        currentPage.value = null;
        error.value = const LoadFailed();
    }
  }

  void _show(MewsPage page) {
    currentPage.value = page;
    _db.recordVisit(page.url, page.title);
  }
}