Add Decoder.Buffered() to return buffered data

Buffered returns an io.Reader for data remaining in Decoder's buffer.
The returned reader is valid until the next call to Decode or Skip.

This basically matches the Decoder.Buffered() in Go's encoding/json.

stream.go

@@ -4,6 +4,7 @@
 package cbor
 
 import (
+	"bytes"
 	"errors"
 	"io"
 	"reflect"
@@ -65,6 +66,12 @@ func (dec *Decoder) NumBytesRead() int {
 	return dec.bytesRead
 }
 
+// Buffered returns a reader for data remaining in Decoder's buffer.
+// Returned reader is valid until the next call to Decode or Skip.
+func (dec *Decoder) Buffered() io.Reader {
+	return bytes.NewReader(dec.buf[dec.off:])
+}
+
 // readNext() reads next CBOR data item from Reader to buffer.
 // It returns the size of next CBOR data item.
 // It also returns validation error or read error if any.

stream_test.go

@@ -577,6 +577,92 @@ func TestDecoderStructTag(t *testing.T) {
 	}
 }
 
+func TestDecoderBuffered(t *testing.T) {
+	testCases := []struct {
+		name      string
+		cborData  []byte
+		buffered  []byte
+		decodeErr error
+	}{
+		{
+			name:      "empty",
+			cborData:  []byte{},
+			buffered:  []byte{},
+			decodeErr: io.EOF,
+		},
+		{
+			name:      "malformed CBOR data item",
+			cborData:  []byte{0xc0},
+			buffered:  []byte{0xc0},
+			decodeErr: io.ErrUnexpectedEOF,
+		},
+		{
+			name:     "1 CBOR data item",
+			cborData: []byte{0xc2, 0x49, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00},
+			buffered: []byte{},
+		},
+		{
+			name: "2 CBOR data items",
+			cborData: []byte{
+				// First CBOR data item
+				0xc2, 0x49, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+				// Second CBOR data item
+				0xc3, 0x49, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+			},
+			buffered: []byte{
+				// Second CBOR data item
+				0xc3, 0x49, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+			},
+		},
+		{
+			name: "1 CBOR data item followed by non-CBOR data",
+			cborData: []byte{
+				// CBOR data item
+				0xc2, 0x49, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+				// Extraneous non-CBOR data ("abc")
+				0x61, 0x62, 0x63,
+			},
+			buffered: []byte{
+				// non-CBOR data ("abc")
+				0x61, 0x62, 0x63,
+			},
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			r := bytes.NewReader(tc.cborData)
+
+			decoder := NewDecoder(r)
+
+			// Decoder's buffer has no data yet.
+			br := decoder.Buffered()
+			buffered, err := io.ReadAll(br)
+			if err != nil {
+				t.Errorf("failed to read from reader returned by Buffered(): %v", err)
+			}
+			if len(buffered) > 0 {
+				t.Errorf("Buffered() = 0x%x (%d bytes), want 0 bytes", buffered, len(buffered))
+			}
+
+			var v interface{}
+			err = decoder.Decode(&v)
+			if err != tc.decodeErr {
+				t.Errorf("Decode() returned error %v, want %v", err, tc.decodeErr)
+			}
+
+			br = decoder.Buffered()
+			buffered, err = io.ReadAll(br)
+			if err != nil {
+				t.Errorf("failed to read from reader returned by Buffered(): %v", err)
+			}
+			if !bytes.Equal(tc.buffered, buffered) {
+				t.Errorf("Buffered() = 0x%x (%d bytes), want 0x%x (%d bytes)", buffered, len(buffered), tc.buffered, len(tc.buffered))
+			}
+		})
+	}
+}
+
 func TestEncoder(t *testing.T) {
 	var want bytes.Buffer
 	var w bytes.Buffer