Adding String() method to Packet interface

Nearly all RTCP packet types already include String() methods, which
are quite valuable for diagnostic and logging purposes. This change
updates the base Packet interface to include String() so that it
can be used by applications without having to perform a complex set
of type assertions.

As a minor bit of housekeeping, it also moves assertions that the
Packet interface is satisfied from implementation files into test
files.

compound_packet.go

@@ -1,5 +1,9 @@
 package rtcp
 
+import (
+	"strings"
+)
+
 // A CompoundPacket is a collection of RTCP packets transmitted as a single packet with
 // the underlying protocol (for example UDP).
 //
@@ -14,8 +18,6 @@ package rtcp
 // Other RTCP packet types may follow in any order. Packet types may appear more than once.
 type CompoundPacket []Packet
 
-var _ Packet = (*CompoundPacket)(nil) // assert is a Packet
-
 // Validate returns an error if this is not an RFC-compliant CompoundPacket.
 func (c CompoundPacket) Validate() error {
 	if len(c) == 0 {
@@ -134,3 +136,12 @@ func (c CompoundPacket) DestinationSSRC() []uint32 {
 
 	return c[0].DestinationSSRC()
 }
+
+func (c CompoundPacket) String() string {
+	out := "CompoundPacket\n"
+	for _, p := range c {
+		out += p.String()
+	}
+	out = strings.TrimSuffix(strings.ReplaceAll(out, "\n", "\n\t"), "\t")
+	return out
+}

compound_packet_test.go

@@ -8,6 +8,8 @@ import (
 	"github.com/stretchr/testify/assert"
 )
 
+var _ Packet = (*CompoundPacket)(nil) // assert is a Packet
+
 func TestReadEOF(t *testing.T) {
 	shortHeader := []byte{
 		0x81, 0xc9, // missing type & len

goodbye.go

@@ -2,6 +2,7 @@ package rtcp
 
 import (
 	"encoding/binary"
+	"fmt"
 )
 
 // The Goodbye packet indicates that one or more sources are no longer active.
@@ -12,8 +13,6 @@ type Goodbye struct {
 	Reason string
 }
 
-var _ Packet = (*Goodbye)(nil) // assert is a Packet
-
 // Marshal encodes the Goodbye packet in binary
 func (g Goodbye) Marshal() ([]byte, error) {
 	/*
@@ -144,3 +143,13 @@ func (g *Goodbye) DestinationSSRC() []uint32 {
 	copy(out, g.Sources)
 	return out
 }
+
+func (g Goodbye) String() string {
+	out := "Goodbye\n"
+	for i, s := range g.Sources {
+		out += fmt.Sprintf("\tSource %d: %x\n", i, s)
+	}
+	out += fmt.Sprintf("\tReason: %s\n", g.Reason)
+
+	return out
+}

goodbye_test.go

@@ -6,6 +6,8 @@ import (
 	"testing"
 )
 
+var _ Packet = (*Goodbye)(nil) // assert is a Packet
+
 func TestGoodbyeUnmarshal(t *testing.T) {
 	for _, test := range []struct {
 		Name      string

packet.go

@@ -7,6 +7,7 @@ type Packet interface {
 
 	Marshal() ([]byte, error)
 	Unmarshal(rawPacket []byte) error
+	String() string
 }
 
 // Unmarshal takes an entire udp datagram (which may consist of multiple RTCP packets) and

picture_loss_indication.go

@@ -14,8 +14,6 @@ type PictureLossIndication struct {
 	MediaSSRC uint32
 }
 
-var _ Packet = (*PictureLossIndication)(nil) // assert is a Packet
-
 const (
 	pliLength = 2
 )

picture_loss_indication_test.go

@@ -6,6 +6,8 @@ import (
 	"testing"
 )
 
+var _ Packet = (*PictureLossIndication)(nil) // assert is a Packet
+
 func TestPictureLossIndicationUnmarshal(t *testing.T) {
 	for _, test := range []struct {
 		Name      string

rapid_resynchronization_request.go

@@ -14,8 +14,6 @@ type RapidResynchronizationRequest struct {
 	MediaSSRC uint32
 }
 
-var _ Packet = (*RapidResynchronizationRequest)(nil) // assert is a Packet
-
 const (
 	rrrLength       = 2
 	rrrHeaderLength = ssrcLength * 2

rapid_resynchronization_request_test.go

@@ -6,6 +6,8 @@ import (
 	"testing"
 )
 
+var _ Packet = (*RapidResynchronizationRequest)(nil) // assert is a Packet
+
 func TestRapidResynchronizationRequestUnmarshal(t *testing.T) {
 	for _, test := range []struct {
 		Name      string

raw_packet.go

@@ -6,8 +6,6 @@ import "fmt"
 // a packet with an unknown type is encountered.
 type RawPacket []byte
 
-var _ Packet = (*RawPacket)(nil) // assert is a Packet
-
 // Marshal encodes the packet in binary.
 func (r RawPacket) Marshal() ([]byte, error) {
 	return r, nil

raw_packet_test.go

@@ -6,6 +6,8 @@ import (
 	"testing"
 )
 
+var _ Packet = (*RawPacket)(nil) // assert is a Packet
+
 func TestRawPacketRoundTrip(t *testing.T) {
 	for _, test := range []struct {
 		Name               string

receiver_estimated_maximum_bitrate.go

@@ -20,8 +20,6 @@ type ReceiverEstimatedMaximumBitrate struct {
 	SSRCs []uint32
 }
 
-var _ Packet = (*ReceiverEstimatedMaximumBitrate)(nil) // assert is a Packet
-
 // Marshal serializes the packet and returns a byte slice.
 func (p ReceiverEstimatedMaximumBitrate) Marshal() (buf []byte, err error) {
 	// Allocate a buffer of the exact output size.

receiver_estimated_maximum_bitrate_test.go

@@ -7,6 +7,8 @@ import (
 	"github.com/stretchr/testify/assert"
 )
 
+var _ Packet = (*ReceiverEstimatedMaximumBitrate)(nil) // assert is a Packet
+
 func TestReceiverEstimatedMaximumBitrateMarshal(t *testing.T) {
 	assert := assert.New(t)
 

receiver_report.go

@@ -19,8 +19,6 @@ type ReceiverReport struct {
 	ProfileExtensions []byte
 }
 
-var _ Packet = (*ReceiverReport)(nil) // assert is a Packet
-
 const (
 	ssrcLength     = 4
 	rrSSRCOffset   = headerLength

receiver_report_test.go

@@ -6,6 +6,8 @@ import (
 	"testing"
 )
 
+var _ Packet = (*ReceiverReport)(nil) // assert is a Packet
+
 func TestReceiverReportUnmarshal(t *testing.T) {
 	for _, test := range []struct {
 		Name      string

sender_report.go

@@ -39,8 +39,6 @@ type SenderReport struct {
 	ProfileExtensions []byte
 }
 
-var _ Packet = (*SenderReport)(nil) // assert is a Packet
-
 const (
 	srHeaderLength      = 24
 	srSSRCOffset        = 0

sender_report_test.go

@@ -6,6 +6,8 @@ import (
 	"testing"
 )
 
+var _ Packet = (*SenderReport)(nil) // assert is a Packet
+
 func TestSenderReportUnmarshal(t *testing.T) {
 	for _, test := range []struct {
 		Name      string

slice_loss_indication.go

@@ -30,8 +30,6 @@ type SliceLossIndication struct {
 	SLI []SLIEntry
 }
 
-var _ Packet = (*SliceLossIndication)(nil) // assert is a Packet
-
 const (
 	sliLength = 2
 	sliOffset = 8

slice_loss_indication_test.go

@@ -6,6 +6,8 @@ import (
 	"testing"
 )
 
+var _ Packet = (*SliceLossIndication)(nil) // assert is a Packet
+
 func TestSliceLossIndicationUnmarshal(t *testing.T) {
 	for _, test := range []struct {
 		Name      string

source_description.go

@@ -61,8 +61,6 @@ type SourceDescription struct {
 	Chunks []SourceDescriptionChunk
 }
 
-var _ Packet = (*SourceDescription)(nil) // assert is a Packet
-
 // Marshal encodes the SourceDescription in binary
 func (s SourceDescription) Marshal() ([]byte, error) {
 	/*

source_description_test.go

@@ -6,6 +6,8 @@ import (
 	"testing"
 )
 
+var _ Packet = (*SourceDescription)(nil) // assert is a Packet
+
 func TestSourceDescriptionUnmarshal(t *testing.T) {
 	for _, test := range []struct {
 		Name      string

transport_layer_cc.go

@@ -75,8 +75,6 @@ func numOfBitsOfSymbolSize() map[uint16]uint16 {
 	}
 }
 
-var _ Packet = (*TransportLayerCC)(nil) // assert is a Packet
-
 var (
 	errPacketStatusChunkLength = errors.New("packet status chunk must be 2 bytes")
 	errDeltaExceedLimit        = errors.New("delta exceed limit")

transport_layer_cc_test.go

@@ -5,6 +5,8 @@ import (
 	"testing"
 )
 
+var _ Packet = (*TransportLayerCC)(nil) // assert is a Packet
+
 func TestTransportLayerCC_RunLengthChunkUnmarshal(t *testing.T) {
 	for _, test := range []struct {
 		Name      string

transport_layer_nack.go

@@ -33,8 +33,6 @@ type TransportLayerNack struct {
 	Nacks []NackPair
 }
 
-var _ Packet = (*TransportLayerNack)(nil) // assert is a Packet
-
 // NackPairsFromSequenceNumbers generates a slice of NackPair from a list of SequenceNumbers
 // This handles generating the proper values for PacketID/LostPackets
 func NackPairsFromSequenceNumbers(sequenceNumbers []uint16) (pairs []NackPair) {

transport_layer_nack_test.go

@@ -6,6 +6,8 @@ import (
 	"testing"
 )
 
+var _ Packet = (*TransportLayerNack)(nil) // assert is a Packet
+
 func TestTransportLayerNackUnmarshal(t *testing.T) {
 	for _, test := range []struct {
 		Name      string