Neda/call/lib/services/secure_channel.dart

import 'dart:async';
import 'dart:typed_data';
import '../core/codec/lpc_codec.dart';
import '../core/crypto/key_manager.dart';
import '../core/framing/deframer.dart';
import '../core/framing/framer.dart';
import '../utils/audio_math.dart';
import '../utils/constants.dart';
import 'audio_service.dart';

/// Channel state machine.
enum ChannelState {
  idle,       // not started
  txReady,    // key loaded, ready to transmit
  transmitting,
  receiving,
  error,
}

/// High-level secure voice channel.
///
/// Owns the complete TX and RX pipelines:
///   TX: microphone PCM → LPC encode → Framer → FSK audio → speaker
///   RX: microphone PCM → Deframer → LPC decode → speaker (earpiece)
///
/// Operation is half-duplex (push-to-talk).
/// Call [startTransmit] / [stopTransmit] to switch directions.
class SecureChannel {
  final AudioService _audio;
  final KeyManager   _keys;

  late final Framer   _framer;
  late final Deframer _deframer;
  late final LpcCodec _encoder;
  late final LpcCodec _decoder;

  ChannelState _state = ChannelState.idle;
  ChannelState get state => _state;

  // Accumulate microphone sub-frames into a full LPC super-frame.
  final _txBuffer = <Uint8List>[];   // list of 20-ms Int16 LE chunks

  StreamSubscription<dynamic>? _captureSub;

  // Stats exposed to UI.
  int  txFrames     = 0;
  int  rxFrames     = 0;
  int  rxErrors     = 0;
  double txRms      = 0.0;
  double rxSignalRms = 0.0;

  // Notifier for state / stats updates.
  final _stateController = StreamController<ChannelState>.broadcast();
  Stream<ChannelState> get onStateChange => _stateController.stream;

  // 4-byte session ID (timestamp-derived at session creation).
  late final Uint8List _sessionId;

  SecureChannel(this._audio, this._keys) {
    _sessionId = _buildSessionId();
    _framer    = Framer(_keys, _sessionId);
    _deframer  = Deframer(_keys, _sessionId);
    _encoder   = LpcCodec();
    _decoder   = LpcCodec();
  }

  // ── Lifecycle ──────────────────────────────────────────────────────

  /// Initialise audio hardware (speaker ON, playback started).
  Future<void> open() async {
    await _audio.setSpeakerMode(true);   // loudspeaker for acoustic coupling
    await _audio.startPlayback();
    _setState(ChannelState.txReady);
  }

  /// Stop all audio, clean up.
  Future<void> close() async {
    await _stopCapture();
    await _audio.stopPlayback();
    _framer.reset();
    _deframer.reset();
    _txBuffer.clear();
    _setState(ChannelState.idle);
  }

  // ── Transmit (PTT press) ───────────────────────────────────────────

  /// Begin transmitting: capture mic audio, encode LPC, modulate FSK, play.
  Future<void> startTransmit() async {
    if (_state == ChannelState.transmitting) return;
    _txBuffer.clear();
    _setState(ChannelState.transmitting);

    // Start mic capture using UNPROCESSED source (no AEC / noise suppression).
    await _audio.startCapture(source: 9 /* AudioSource.UNPROCESSED */);

    _captureSub = _audio.captureStream.listen(_onTxSamples);
  }

  /// Stop transmitting (PTT release).
  Future<void> stopTransmit() async {
    if (_state != ChannelState.transmitting) return;
    await _stopCapture();
    _txBuffer.clear();
    _setState(ChannelState.txReady);
  }

  // ── Receive ────────────────────────────────────────────────────────

  /// Begin receive mode: capture mic audio (which picks up the earpiece FSK).
  Future<void> startReceive() async {
    if (_state == ChannelState.receiving) return;
    _deframer.reset();
    _setState(ChannelState.receiving);

    // VOICE_COMMUNICATION (7) lets us capture audio while the call is active.
    // Falls back to UNPROCESSED (9) → MIC (1) inside AudioEngine.
    await _audio.startCapture(source: 7 /* AudioSource.VOICE_COMMUNICATION */);
    _captureSub = _audio.captureStream.listen(_onRxSamples);
  }

  /// Stop receive mode.
  Future<void> stopReceive() async {
    if (_state != ChannelState.receiving) return;
    await _stopCapture();
    _setState(ChannelState.txReady);
  }

  // ── TX pipeline ────────────────────────────────────────────────────

  /// Called for each 160-sample (20 ms) microphone chunk during TX.
  void _onTxSamples(Float64List samples) {
    txRms = AudioMath.rms(samples);

    // Convert Float64 → Int16 LE bytes for LPC encoder.
    final pcm = AudioMath.floatToInt16Bytes(samples);
    _txBuffer.add(pcm);

    // Once we have a full super-frame (10 sub-frames × 20 ms = 200 ms) encode.
    if (_txBuffer.length >= C.lpcSubframesPerSuper) {
      // Concatenate all sub-frame PCM into one 3200-byte buffer.
      final superPcm = Uint8List(C.lpcSubframeSamples * C.lpcSubframesPerSuper * 2);
      for (int i = 0; i < C.lpcSubframesPerSuper; i++) {
        superPcm.setRange(
          i * C.lpcSubframeSamples * 2,
          (i + 1) * C.lpcSubframeSamples * 2,
          _txBuffer[i],
        );
      }
      _txBuffer.clear();

      // LPC encode → 9 bytes.
      final lpcBits = _encoder.encode(superPcm);
      txFrames++;

      // Framer: encrypt + RS FEC + FSK modulate → audio samples.
      final fskAudio = _framer.frameAndModulate(lpcBits);

      // Play FSK audio through loudspeaker → goes into the cellular mic.
      _audio.writePlayback(fskAudio);
    }
  }

  // ── RX pipeline ────────────────────────────────────────────────────

  /// Called for each 160-sample (20 ms) microphone chunk during RX.
  void _onRxSamples(Float64List samples) {
    rxSignalRms = AudioMath.rms(samples);

    // Feed into FSK demodulator / deframer.
    _deframer.pushAudio(samples);

    // Drain any decoded LPC voice payloads.
    while (_deframer.voiceQueue.isNotEmpty) {
      final lpcBits = _deframer.voiceQueue.removeAt(0);
      final pcm     = _decoder.decode(lpcBits);
      rxFrames++;

      // Play decoded voice through earpiece.
      _audio.writePlaybackBytes(pcm);
    }

    // Mirror deframer error counts to our own stats.
    rxErrors = _deframer.rxErrors;
  }

  // ── Helpers ────────────────────────────────────────────────────────

  Future<void> _stopCapture() async {
    await _captureSub?.cancel();
    _captureSub = null;
    await _audio.stopCapture();
  }

  void _setState(ChannelState s) {
    _state = s;
    _stateController.add(s);
  }

  static Uint8List _buildSessionId() {
    final ts  = DateTime.now().microsecondsSinceEpoch;
    final sid = Uint8List(4);
    sid[0] = (ts >> 24) & 0xFF;
    sid[1] = (ts >> 16) & 0xFF;
    sid[2] = (ts >>  8) & 0xFF;
    sid[3] =  ts        & 0xFF;
    return sid;
  }

  Future<void> dispose() async {
    await close();
    await _stateController.close();
  }
}