parseSdpAudio function

SdpAudio? parseSdpAudio(String sdp)

Parse the remote audio endpoint from an SDP body. Returns null if the body has no audio media line we can use.

Implementation

SdpAudio? parseSdpAudio(String sdp) {
  if (sdp.trim().isEmpty) return null;
  String? sessionConn;
  String? mediaConn;
  String? mLine;
  int? rtcpPort;
  SdpDirection? direction;
  int? telephoneEventPt;
  // fmtp parameters keyed by payload type (e.g. {101: '0-16'}).
  final fmtpByPt = <int, String>{};
  // Track which payload types appear in m= and any rtpmap entries so we
  // can pick PCMU (0) or PCMA (8) deterministically.
  final ptInMedia = <int>[];
  final rtpmapNames = <int, String>{};

  var inAudio = false;
  for (final raw in sdp.split('\n')) {
    final line = raw.trim();
    if (line.isEmpty) continue;
    if (line.startsWith('c=') && !inAudio) {
      sessionConn = line;
    } else if (line.startsWith('m=')) {
      inAudio = line.startsWith('m=audio');
      if (inAudio) {
        mLine = line;
        // m=audio <port> RTP/AVP <pt> <pt> ...
        final parts = line.split(RegExp(r'\s+'));
        for (var i = 3; i < parts.length; i++) {
          final pt = int.tryParse(parts[i]);
          if (pt != null) ptInMedia.add(pt);
        }
      }
    } else if (inAudio && line.startsWith('c=')) {
      mediaConn = line;
    } else if (inAudio && line.startsWith('a=rtpmap:')) {
      // a=rtpmap:<pt> <name>/<rate>[/channels]
      final rest = line.substring('a=rtpmap:'.length);
      final sp = rest.indexOf(' ');
      if (sp > 0) {
        final pt = int.tryParse(rest.substring(0, sp));
        if (pt != null) {
          final name = rest.substring(sp + 1).split('/').first.toUpperCase();
          rtpmapNames[pt] = name;
          if (name == 'TELEPHONE-EVENT') telephoneEventPt = pt;
        }
      }
    } else if (inAudio && line.startsWith('a=rtcp:')) {
      // a=rtcp:<port> [<nettype> <addrtype> <addr>]
      final rest = line.substring('a=rtcp:'.length).trim();
      final tok = rest.split(RegExp(r'\s+')).first;
      rtcpPort = int.tryParse(tok);
    } else if (inAudio && line.startsWith('a=fmtp:')) {
      // a=fmtp:<pt> <params>
      final rest = line.substring('a=fmtp:'.length);
      final sp = rest.indexOf(' ');
      if (sp > 0) {
        final pt = int.tryParse(rest.substring(0, sp));
        if (pt != null) fmtpByPt[pt] = rest.substring(sp + 1).trim();
      }
    } else if (inAudio) {
      switch (line) {
        case 'a=sendrecv':
          direction = SdpDirection.sendrecv;
          break;
        case 'a=sendonly':
          direction = SdpDirection.sendonly;
          break;
        case 'a=recvonly':
          direction = SdpDirection.recvonly;
          break;
        case 'a=inactive':
          direction = SdpDirection.inactive;
          break;
      }
    }
  }

  if (mLine == null) return null;
  final mParts = mLine.split(RegExp(r'\s+'));
  if (mParts.length < 4) return null;
  final port = int.tryParse(mParts[1]);
  if (port == null || port == 0) return null;

  final connLine = mediaConn ?? sessionConn;
  if (connLine == null) return null;
  // c=IN IP4 <host>
  final cParts = connLine.split(RegExp(r'\s+'));
  if (cParts.length < 3) return null;
  final host = cParts[2];

  // Pick first PT we know about (prefer PCMU then PCMA).
  G711Variant? chosen;
  for (final pt in ptInMedia) {
    final variant = G711Variant.fromPayloadType(pt);
    if (variant != null) {
      chosen = variant;
      break;
    }
  }
  // Fall back to rtpmap-by-name if the static PTs weren't 0/8.
  if (chosen == null) {
    for (final entry in rtpmapNames.entries) {
      if (entry.value == 'PCMU') {
        chosen = G711Variant.pcmu;
        break;
      }
      if (entry.value == 'PCMA') {
        chosen = G711Variant.pcma;
        break;
      }
    }
  }
  if (chosen == null) return null;

  return SdpAudio(
    host: host,
    port: port,
    codec: chosen,
    rtcpPort: rtcpPort,
    direction: direction ?? SdpDirection.sendrecv,
    telephoneEventPt: telephoneEventPt,
    telephoneEventRange: telephoneEventPt == null
        ? null
        : fmtpByPt[telephoneEventPt],
  );
}