telegabber/calls/audio/fmtp.go

package audio

import (
	"strconv"
	"strings"
)

// opus fmtp params from peer SDP.
// ApplyTo honors UseInBandFEC, UseDTX, MaxAverageBitrate, MaxPlaybackRate.
// CBR/Stereo/Ptime are parsed but not applied (pipeline is fixed shape and
// hraban/opus.v2 doesn't wrap OPUS_SET_VBR).
type Fmtp struct {
	UseInBandFEC      bool // useinbandfec=1
	UseDTX            bool // usedtx=1
	CBR               bool // cbr=1 (not applied; see type doc)
	MaxAverageBitrate int  // maxaveragebitrate=<bps>, 0 = unspecified
	MaxPlaybackRate   int  // maxplaybackrate=<Hz>, 0 = unspecified
	SpropStereo       bool // sprop-stereo=1 (not applied)
	Stereo            bool // stereo=1 (not applied)
	MinPtime          int  // ms (not applied)
	MaxPtime          int  // ms (not applied)
}

// params half of the opus a=fmtp line; empty if no opus fmtp.
// Two-pass because pion may emit rtpmap and fmtp in any order.
func ExtractOpusFmtp(sdp string) string {
	opusPTs := make(map[string]bool)
	type fmtpEntry struct {
		pt     string
		params string
	}
	var fmtps []fmtpEntry
	for _, raw := range strings.Split(sdp, "\n") {
		line := strings.TrimRight(raw, "\r")
		if !strings.HasPrefix(line, "a=") {
			continue
		}
		body := line[2:]
		if rest, ok := strings.CutPrefix(body, "rtpmap:"); ok {
			// "<pt> opus/48000/2"
			sp := strings.IndexByte(rest, ' ')
			if sp < 0 {
				continue
			}
			pt, codec := rest[:sp], rest[sp+1:]
			if strings.HasPrefix(strings.ToLower(codec), "opus/") {
				opusPTs[pt] = true
			}
			continue
		}
		if rest, ok := strings.CutPrefix(body, "fmtp:"); ok {
			sp := strings.IndexByte(rest, ' ')
			if sp < 0 {
				continue
			}
			fmtps = append(fmtps, fmtpEntry{pt: rest[:sp], params: rest[sp+1:]})
		}
	}
	for _, f := range fmtps {
		if opusPTs[f.pt] {
			return f.params
		}
	}
	return ""
}

// unknown keys and malformed numerics are silently ignored; weird SDP
// shouldn't kill the call
func ParseFmtp(s string) Fmtp {
	var f Fmtp
	for _, kv := range strings.Split(s, ";") {
		kv = strings.TrimSpace(kv)
		if kv == "" {
			continue
		}
		eq := strings.IndexByte(kv, '=')
		if eq < 0 {
			continue
		}
		key := strings.ToLower(strings.TrimSpace(kv[:eq]))
		val := strings.TrimSpace(kv[eq+1:])
		switch key {
		case "useinbandfec":
			f.UseInBandFEC = val == "1"
		case "usedtx":
			f.UseDTX = val == "1"
		case "cbr":
			f.CBR = val == "1"
		case "maxaveragebitrate":
			if n, err := strconv.Atoi(val); err == nil {
				f.MaxAverageBitrate = n
			}
		case "maxplaybackrate":
			if n, err := strconv.Atoi(val); err == nil {
				f.MaxPlaybackRate = n
			}
		case "sprop-stereo":
			f.SpropStereo = val == "1"
		case "stereo":
			f.Stereo = val == "1"
		case "minptime":
			if n, err := strconv.Atoi(val); err == nil {
				f.MinPtime = n
			}
		case "maxptime":
			if n, err := strconv.Atoi(val); err == nil {
				f.MaxPtime = n
			}
		}
	}
	return f
}

func (f Fmtp) ApplyTo(e *Encoder) error {
	bitrate := f.MaxAverageBitrate
	if bitrate <= 0 {
		bitrate = DefaultBitrate
	}
	if err := e.SetBitrate(bitrate); err != nil {
		return err
	}
	if f.MaxPlaybackRate > 0 {
		if err := e.SetMaxBandwidth(BandwidthForPlaybackRate(f.MaxPlaybackRate)); err != nil {
			return err
		}
	}
	if f.UseInBandFEC {
		if err := e.SetInBandFEC(true); err != nil {
			return err
		}
		// hint to opus, not a guarantee
		if err := e.SetPacketLossPerc(5); err != nil {
			return err
		}
	}
	if f.UseDTX {
		if err := e.SetDTX(true); err != nil {
			return err
		}
	}
	return nil
}