Add ability to reuse Decoder, Scanner, and byteReader

bench_test.go

@@ -106,6 +106,19 @@ func BenchmarkDecoderDecodeInterfaceAny(b *testing.B) {
 				check(b, err)
 			}
 		})
+		b.Run("pkgjson/reuse/"+tc.path, func(b *testing.B) {
+			b.ReportAllocs()
+			b.SetBytes(r.Size())
+			b.ResetTimer()
+			dec := NewDecoderBuffer(r, buf[:])
+			for i := 0; i < b.N; i++ {
+				r.Seek(0, 0)
+				dec.Reset(r)
+				var i interface{}
+				err := dec.Decode(&i)
+				check(b, err)
+			}
+		})
 		b.Run("encodingjson/"+tc.path, func(b *testing.B) {
 			b.ReportAllocs()
 			b.SetBytes(r.Size())

decoder.go

@@ -111,6 +111,13 @@ func (d *Decoder) NextToken() ([]byte, error) {
 	return d.state(d)
 }
 
+// Reset resets the decoder to read from a new io.Reader to avoid reallocation.
+func (d *Decoder) Reset(r io.Reader) {
+	d.scanner.Reset(r)
+	d.state = (*Decoder).stateValue
+	d.stack = d.stack[:0]
+}
+
 func (d *Decoder) stateObjectString() ([]byte, error) {
 	tok := d.scanner.Next()
 	if len(tok) < 1 {

reader.go

@@ -68,3 +68,11 @@ func (b *byteReader) compact() {
 	copy(b.data, b.data[b.offset:])
 	b.offset = 0
 }
+
+// reset resets the byteReader to read from a new r without reallocating.
+func (b *byteReader) reset(r io.Reader) {
+	b.data = b.data[:0]
+	b.offset = 0
+	b.r = r
+	b.err = nil
+}

scanner.go

@@ -107,6 +107,12 @@ func (s *Scanner) Next() []byte {
 	}
 }
 
+// Reset resets the scanner to read from a new io.Reader to avoid reallocation.
+func (s *Scanner) Reset(r io.Reader) {
+	s.br.reset(r)
+	s.offset = 0
+}
+
 func (s *Scanner) validateToken(expected string) int {
 	for {
 		w := s.br.window()