Source File
	header_extension_interceptor.go

Belonging Package
	github.com/pion/interceptor/pkg/twcc

// SPDX-FileCopyrightText: 2023 The Pion community <https://pion.ly>
// SPDX-License-Identifier: MIT

package twcc

import (
	"errors"
	"sync/atomic"

	"github.com/pion/interceptor"
	"github.com/pion/rtp"
)

var errHeaderIsNil = errors.New("header is nil")

// HeaderExtensionInterceptorFactory is a interceptor.Factory for a HeaderExtensionInterceptor.
type HeaderExtensionInterceptorFactory struct{}

// NewInterceptor constructs a new HeaderExtensionInterceptor.
func (h *HeaderExtensionInterceptorFactory) NewInterceptor(_ string) (interceptor.Interceptor, error) {
	return &HeaderExtensionInterceptor{}, nil
}

// NewHeaderExtensionInterceptor returns a HeaderExtensionInterceptorFactory.
func NewHeaderExtensionInterceptor() (*HeaderExtensionInterceptorFactory, error) {
	return &HeaderExtensionInterceptorFactory{}, nil
}

// HeaderExtensionInterceptor adds transport wide sequence numbers as header extension to each RTP packet.
type HeaderExtensionInterceptor struct {
	interceptor.NoOp
	nextSequenceNr uint32
}

const transportCCURI = "http://www.ietf.org/id/draft-holmer-rmcat-transport-wide-cc-extensions-01"

// BindLocalStream returns a writer that adds a rtp.TransportCCExtension
// header with increasing sequence numbers to each outgoing packet.
func (h *HeaderExtensionInterceptor) BindLocalStream(
	info *interceptor.StreamInfo,
	writer interceptor.RTPWriter,
) interceptor.RTPWriter {
	var hdrExtID uint8
	for _, e := range info.RTPHeaderExtensions {
		if e.URI == transportCCURI {
			hdrExtID = uint8(e.ID) //nolint:gosec // G115

			break
		}
	}
	if hdrExtID == 0 { // Don't add header extension if ID is 0, because 0 is an invalid extension ID
		return writer
	}

	return interceptor.RTPWriterFunc(
		func(header *rtp.Header, payload []byte, attributes interceptor.Attributes) (int, error) {
			sequenceNumber := atomic.AddUint32(&h.nextSequenceNr, 1) - 1
			//nolint:gosec // G115
			tcc, err := (&rtp.TransportCCExtension{TransportSequence: uint16(sequenceNumber)}).Marshal()
			if err != nil {
				return 0, err
			}
			if header == nil {
				return 0, errHeaderIsNil
			}
			err = header.SetExtension(hdrExtID, tcc)
			if err != nil {
				return 0, err
			}

			return writer.Write(header, payload, attributes)
		},
	)
}

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