Add extmap support

Extmaps are configured via the SettingEngine. This allows a user
to set arbitrary values, and when answering ids and entries are
properly excluded.

Co-authored-by: Gabor Pongracz <gabor.pongracz@proemergotech.com>

settingengine.go

@@ -8,6 +8,7 @@ import (
 
 	"github.com/pion/ice/v2"
 	"github.com/pion/logging"
+	"github.com/pion/sdp/v2"
 	"github.com/pion/transport/vnet"
 )
 
@@ -49,6 +50,7 @@ type SettingEngine struct {
 		SRTCP *uint
 	}
 	sdpMediaLevelFingerprints                 bool
+	sdpExtensions                             map[SDPSectionType][]sdp.ExtMap
 	answeringDTLSRole                         DTLSRole
 	disableCertificateFingerprintVerification bool
 	disableSRTPReplayProtection               bool
@@ -245,3 +247,65 @@ func (e *SettingEngine) DisableSRTCPReplayProtection(isDisabled bool) {
 func (e *SettingEngine) SetSDPMediaLevelFingerprints(sdpMediaLevelFingerprints bool) {
 	e.sdpMediaLevelFingerprints = sdpMediaLevelFingerprints
 }
+
+// AddSDPExtensions adds available and offered extensions for media type.
+//
+// Ext IDs are optional and generated if you do not provide them
+// SDP answers will only include extensions supported by both sides
+func (e *SettingEngine) AddSDPExtensions(mediaType SDPSectionType, exts []sdp.ExtMap) {
+	if e.sdpExtensions == nil {
+		e.sdpExtensions = make(map[SDPSectionType][]sdp.ExtMap)
+	}
+	if _, ok := e.sdpExtensions[mediaType]; !ok {
+		e.sdpExtensions[mediaType] = []sdp.ExtMap{}
+	}
+	e.sdpExtensions[mediaType] = append(e.sdpExtensions[mediaType], exts...)
+}
+
+func (e *SettingEngine) getSDPExtensions() map[SDPSectionType][]sdp.ExtMap {
+	var lastID int
+	idMap := map[string]int{}
+
+	// Build provided ext id map
+	for _, extList := range e.sdpExtensions {
+		for _, ext := range extList {
+			if ext.Value != 0 {
+				idMap[ext.URI.String()] = ext.Value
+			}
+		}
+	}
+
+	// Find next available ID
+	nextID := func() {
+		var done bool
+		for !done {
+			lastID++
+			var found bool
+			for _, v := range idMap {
+				if lastID == v {
+					found = true
+					break
+				}
+			}
+			if !found {
+				done = true
+			}
+		}
+	}
+
+	// Assign missing IDs across all media types based on URI
+	for mType, extList := range e.sdpExtensions {
+		for i, ext := range extList {
+			if ext.Value == 0 {
+				if id, ok := idMap[ext.URI.String()]; ok {
+					e.sdpExtensions[mType][i].Value = id
+				} else {
+					nextID()
+					e.sdpExtensions[mType][i].Value = lastID
+					idMap[ext.URI.String()] = lastID
+				}
+			}
+		}
+	}
+	return e.sdpExtensions
+}