// The byte pipe to a smolmaild server: raw TCP (dart:io), framed elsewhere.
// Mobile is this app's only target platform, so dart:io is fine here.

import "dart:async";
import "dart:io";
import "dart:typed_data";

import "package:smol_mail/smol/errors.dart";
import "package:smol_mail/smol/proto.dart";

/// Buffers the socket's stream and serves exact-length reads, so protocol
/// code never sees a partial frame.
class TcpWire implements Wire {
  final Socket _socket;
  final _chunks = <Uint8List>[];
  final _waiters = <_ReadRequest>[];
  int _buffered = 0;
  Object? _closed;
  late final StreamSubscription<Uint8List> _subscription;

  TcpWire(this._socket) {
    _subscription = _socket.listen(_onData,
        onError: (Object error) => _fail(error),
        onDone: () => _fail(const SmolError("server closed the connection")));
  }

  static Future<TcpWire> connect(String host, int port) async {
    try {
      // Mobile networks routinely need longer than a LAN handshake; 30s keeps
      // flaky handovers from surfacing as user-facing timeouts.
      return TcpWire(await Socket.connect(host, port,
          timeout: const Duration(seconds: 30)));
    } on SocketException catch (error) {
      throw SmolError("cannot reach $host:$port (${error.message})");
    }
  }

  void _onData(Uint8List data) {
    _chunks.add(data);
    _buffered += data.length;
    _wake();
  }

  void _wake() {
    _waiters.removeWhere((w) {
      if (_closed != null) {
        w.completer.completeError(_closed!);
        return true;
      }
      if (_buffered >= w.need) {
        w.completer.complete();
        return true;
      }
      return false;
    });
  }

  void _fail(Object error) {
    _closed = error;
    for (final w in _waiters) {
      w.completer.completeError(error);
    }
    _waiters.clear();
  }

  @override
  void send(Uint8List bytes) {
    if (_closed != null) throw _closed!;
    _socket.add(bytes);
  }

  @override
  void close() {
    _subscription.cancel();
    _socket.destroy();
    _fail(const SmolError("connection closed"));
  }

  @override
  Future<Uint8List> readExact(int n) async {
    if (_closed != null) throw _closed!;
    if (_buffered < n) {
      final request = _ReadRequest(n);
      _waiters.add(request);
      try {
        await request.completer.future;
      } finally {
        _waiters.remove(request);
      }
      if (_closed != null) throw _closed!;
    }
    final out = Uint8List(n);
    var off = 0;
    while (off < n) {
      final chunk = _chunks.first;
      final take = chunk.length < n - off ? chunk.length : n - off;
      out.setRange(off, off + take, chunk);
      if (take == chunk.length) {
        _chunks.removeAt(0);
      } else {
        _chunks[0] = Uint8List.sublistView(chunk, take);
      }
      off += take;
      _buffered -= take;
    }
    return out;
  }
}

class _ReadRequest {
  final int need;
  final completer = Completer<void>();

  _ReadRequest(this.need);
}