Unexport unused `Writer` API until it is re-implemented

Unexport the CARv2 writer API until it is reimplemented using WriterAt
or WriteSeeker to be more efficient. This API is not used anyway and we
can postpone releasing it until SelectiveCar writer API is figured out.
For now we unexport it to carry on with interface finalization and alpha
release.

v2/writer.go

@@ -21,8 +21,8 @@ var bulkPadding = make([]byte, bulkPaddingBytesSize)
 type (
 	// padding represents the number of padding bytes.
 	padding uint64
-	// Writer writes CAR v2 into a give io.Writer.
-	Writer struct {
+	// writer writes CAR v2 into a given io.Writer.
+	writer struct {
 		NodeGetter   format.NodeGetter
 		CarV1Padding uint64
 		IndexPadding uint64
@@ -31,7 +31,6 @@ type (
 		roots        []cid.Cid
 		encodedCarV1 *bytes.Buffer
 	}
-	WriteOption func(*Writer)
 )
 
 // WriteTo writes this padding to the given writer as default value bytes.
@@ -56,9 +55,9 @@ func (p padding) WriteTo(w io.Writer) (n int64, err error) {
 	return
 }
 
-// NewWriter instantiates a new CAR v2 writer.
-func NewWriter(ctx context.Context, ng format.NodeGetter, roots []cid.Cid) *Writer {
-	return &Writer{
+// newWriter instantiates a new CAR v2 writer.
+func newWriter(ctx context.Context, ng format.NodeGetter, roots []cid.Cid) *writer {
+	return &writer{
 		NodeGetter:   ng,
 		ctx:          ctx,
 		roots:        roots,
@@ -67,7 +66,7 @@ func NewWriter(ctx context.Context, ng format.NodeGetter, roots []cid.Cid) *Writ
 }
 
 // WriteTo writes the given root CIDs according to CAR v2 specification.
-func (w *Writer) WriteTo(writer io.Writer) (n int64, err error) {
+func (w *writer) WriteTo(writer io.Writer) (n int64, err error) {
 	_, err = writer.Write(Pragma)
 	if err != nil {
 		return
@@ -113,14 +112,14 @@ func (w *Writer) WriteTo(writer io.Writer) (n int64, err error) {
 	return
 }
 
-func (w *Writer) writeHeader(writer io.Writer, carV1Len int) (int64, error) {
+func (w *writer) writeHeader(writer io.Writer, carV1Len int) (int64, error) {
 	header := NewHeader(uint64(carV1Len)).
 		WithCarV1Padding(w.CarV1Padding).
 		WithIndexPadding(w.IndexPadding)
 	return header.WriteTo(writer)
 }
 
-func (w *Writer) writeIndex(writer io.Writer, carV1 []byte) (int64, error) {
+func (w *writer) writeIndex(writer io.Writer, carV1 []byte) (int64, error) {
 	// TODO avoid recopying the bytes by refactoring index once it is integrated here.
 	// Right now we copy the bytes since index takes a writer.
 	// Consider refactoring index to make this process more efficient.
@@ -185,7 +184,7 @@ func WrapV1(src io.ReadSeeker, dst io.Writer) error {
 		return err
 	}
 
-	// Similar to the Writer API, write all components of a CARv2 to the
+	// Similar to the writer API, write all components of a CARv2 to the
 	// destination file: Pragma, Header, CARv1, Index.
 	v2Header := NewHeader(uint64(v1Size))
 	if _, err := dst.Write(Pragma); err != nil {

v2/writer_test.go

@@ -60,7 +60,7 @@ func TestPadding_WriteTo(t *testing.T) {
 func TestNewWriter(t *testing.T) {
 	dagService := dstest.Mock()
 	wantRoots := generateRootCid(t, dagService)
-	writer := NewWriter(context.Background(), dagService, wantRoots)
+	writer := newWriter(context.Background(), dagService, wantRoots)
 	assert.Equal(t, wantRoots, writer.roots)
 }