decode: Switch over to Parser

1 files changed, 215 insertions, 289 deletions

diff --git a/decode.go b/decode.go
index 32912ec..0769bc7 100644
--- a/decode.go
+++ b/decode.go
@@ -27,41 +27,54 @@ type runeBuffer interface {
 }
 
 type Decoder struct {
-	r io.RuneScanner
+	io  runeTypeScanner
+	buf *bufio.Reader
 
 	// config
 	disallowUnknownFields bool
 	useNumber             bool
 
 	// state
-	err    error
-	curPos int64
-	nxtPos int64
-	stack  []any
+	err   error
+	stack []any
 }
 
-var forceBufio bool
+var forceBufio bool // used by the tests to implement .Buffered()
 
 func NewDecoder(r io.Reader) *Decoder {
-	rs, ok := r.(io.RuneScanner)
+	rr, ok := r.(io.RuneReader)
+	var ret Decoder
 	if forceBufio || !ok {
-		rs = bufio.NewReader(r)
+		rr = bufio.NewReader(r)
+		if forceBufio {
+			ret.buf = rr.(*bufio.Reader)
+		}
 	}
-	return &Decoder{
-		r: rs,
+	ret.io = &noWSRuneTypeScanner{
+		inner: &runeTypeScannerImpl{
+			inner: rr,
+		},
 	}
+	return &ret
 }
 
 func (dec *Decoder) DisallowUnknownFields() { dec.disallowUnknownFields = true }
 func (dec *Decoder) UseNumber()             { dec.useNumber = true }
-func (dec *Decoder) InputOffset() int64     { return dec.curPos }
+func (dec *Decoder) InputOffset() int64     { return dec.io.InputOffset() }
 
 func (dec *Decoder) More() bool {
-	dec.decodeWS()
-	_, ok := dec.peekRuneOrEOF()
-	return ok
+	dec.io.Reset()
+	_, _, t, e := dec.io.ReadRuneType()
+	dec.io.UnreadRune()
+	return e == nil && t != RuneTypeEOF
 }
 
+func (dec *Decoder) stackPush(idx any) {
+	dec.stack = append(dec.stack, idx)
+}
+func (dec *Decoder) stackPop() {
+	dec.stack = dec.stack[:len(dec.stack)-1]
+}
 func (dec *Decoder) stackStr() string {
 	var buf strings.Builder
 	buf.WriteString("v")
@@ -71,30 +84,6 @@ func (dec *Decoder) stackStr() string {
 	return buf.String()
 }
 
-func (dec *Decoder) stackPush(idx any) {
-	dec.stack = append(dec.stack, idx)
-}
-func (dec *Decoder) stackPop() {
-	dec.stack = dec.stack[:len(dec.stack)-1]
-}
-
-type decodeError struct {
-	Err error
-}
-
-func (dec *Decoder) panicIO(err error) {
-	panic(decodeError{fmt.Errorf("json: I/O error at input byte %v: %s: %w",
-		dec.nxtPos, dec.stackStr(), err)})
-}
-func (dec *Decoder) panicSyntax(err error) {
-	panic(decodeError{fmt.Errorf("json: syntax error at input byte %v: %s: %w",
-		dec.curPos, dec.stackStr(), err)})
-}
-func (dec *Decoder) panicType(typ reflect.Type, err error) {
-	panic(decodeError{fmt.Errorf("json: type mismatch error at input byte %v: %s: type %v: %w",
-		dec.curPos, dec.stackStr(), typ, err)})
-}
-
 func Decode(r io.Reader, ptr any) error {
 	return NewDecoder(r).Decode(ptr)
 }
@@ -112,6 +101,7 @@ func (dec *Decoder) Decode(ptr any) (err error) {
 		return dec.err
 	}
 
+	dec.io.Reset()
 	defer func() {
 		if r := recover(); r != nil {
 			if de, ok := r.(decodeError); ok {
@@ -122,90 +112,98 @@ func (dec *Decoder) Decode(ptr any) (err error) {
 			}
 		}
 	}()
-	dec.decodeWS()
 	dec.decode(ptrVal.Elem(), false)
 	return nil
 }
 
-func (dec *Decoder) readRune() rune {
-	c, size, err := dec.r.ReadRune()
-	if err != nil {
-		if err == io.EOF {
-			dec.panicSyntax(io.ErrUnexpectedEOF)
-		}
-		dec.panicIO(err)
-	}
-	dec.curPos = dec.nxtPos
-	dec.nxtPos = dec.curPos + int64(size)
-	return c
+// io helpers //////////////////////////////////////////////////////////////////////////////////////
+
+type decodeError struct {
+	Err error
 }
 
-func (dec *Decoder) readRuneOrEOF() (c rune, ok bool) {
-	c, size, err := dec.r.ReadRune()
-	if err != nil {
-		if err == io.EOF {
-			return 0, false
-		}
-		dec.panicIO(err)
+func (dec *Decoder) panicType(typ reflect.Type, err error) {
+	panic(decodeError{fmt.Errorf("json: type mismatch error at input byte %v: %s: type %v: %w",
+		dec.InputOffset(), dec.stackStr(), typ, err)})
+}
+
+func (dec *Decoder) readRune() (rune, RuneType) {
+	c, _, t, e := dec.io.ReadRuneType()
+	if e != nil {
+		panic(decodeError{e})
 	}
-	dec.curPos = dec.nxtPos
-	dec.nxtPos = dec.curPos + int64(size)
-	return c, true
+	return c, t
 }
 
 func (dec *Decoder) unreadRune() {
-	if err := dec.r.UnreadRune(); err != nil {
+	if err := dec.io.UnreadRune(); err != nil {
 		// .UnreadRune() must succeed if the previous call was
 		// .ReadRune(), which it always is for this code.
-		panic(err)
+		panic("should not happen")
 	}
-	dec.nxtPos = dec.curPos
 }
 
-func (dec *Decoder) peekRune() rune {
-	c, _, err := dec.r.ReadRune()
-	if err != nil {
-		if err == io.EOF {
-			dec.panicSyntax(io.ErrUnexpectedEOF)
-		}
-		dec.panicIO(err)
+func (dec *Decoder) peekRuneType() RuneType {
+	_, t := dec.readRune()
+	dec.unreadRune()
+	return t
+}
+
+func (dec *Decoder) expectRune(ec rune, et RuneType) {
+	ac, at := dec.readRune()
+	if ac != ec || at != et {
+		panic("should not happen")
 	}
-	if err := dec.r.UnreadRune(); err != nil {
-		// .UnreadRune() must succeed if the previous call was
-		// .ReadRune(), which it always is for this code.
-		panic(err)
+}
+
+func (dec *Decoder) expectRuneType(ec rune, et RuneType) {
+	ac, at := dec.readRune()
+	if ac != ec || at != et {
+		dec.panicType(nil, fmt.Errorf("TODO error message"))
 	}
-	return c
 }
 
-func (dec *Decoder) peekRuneOrEOF() (rune, bool) {
-	c, _, err := dec.r.ReadRune()
-	if err != nil {
-		if err == io.EOF {
-			return 0, false
-		}
-		dec.panicIO(err)
+type decRuneTypeScanner struct {
+	dec *Decoder
+}
+
+func (sc *decRuneTypeScanner) ReadRuneType() (rune, int, RuneType, error) {
+	c, s, t, e := sc.dec.io.ReadRuneType()
+	if e != nil {
+		panic(decodeError{e})
 	}
-	if err := dec.r.UnreadRune(); err != nil {
-		// .UnreadRune() must succeed if the previous call was
-		// .ReadRune(), which it always is for this code.
-		panic(err)
+	return c, s, t, nil
+}
+func (sc *decRuneTypeScanner) ReadRune() (rune, int, error) {
+	r, s, t, _ := sc.ReadRuneType()
+	switch t {
+	case RuneTypeEOF:
+		return 0, 0, io.EOF
+	default:
+		return r, s, nil
 	}
-	return c, true
 }
 
-func (dec *Decoder) expectRune(exp rune) {
-	act := dec.readRune()
-	if act != exp {
-		dec.panicSyntax(fmt.Errorf("expected %q but got %q", exp, act))
+func (sc *decRuneTypeScanner) UnreadRune() error  { return sc.dec.io.UnreadRune() }
+func (sc *decRuneTypeScanner) InputOffset() int64 { return sc.dec.InputOffset() }
+func (sc *decRuneTypeScanner) Reset()             { sc.dec.io.Reset() }
+
+func (dec *Decoder) limitingScanner() runeTypeScanner {
+	return &elemRuneTypeScanner{
+		inner: &decRuneTypeScanner{
+			dec: dec,
+		},
 	}
 }
 
+// decoder main ////////////////////////////////////////////////////////////////////////////////////
+
 var (
 	rawMessagePtrType   = reflect.TypeOf((*json.RawMessage)(nil))
 	decodableType       = reflect.TypeOf((*Decodable)(nil)).Elem()
 	jsonUnmarshalerType = reflect.TypeOf((*json.Unmarshaler)(nil)).Elem()
 	textUnmarshalerType = reflect.TypeOf((*encoding.TextUnmarshaler)(nil)).Elem()
+	boolType            = reflect.TypeOf(true)
 )
 
 var kind2bits = map[reflect.Kind]int{
@@ -234,22 +232,22 @@ func (dec *Decoder) decode(val reflect.Value, nullOK bool) {
 		var buf bytes.Buffer
 		dec.scan(&buf)
 		if err := val.Addr().Interface().(*json.RawMessage).UnmarshalJSON(buf.Bytes()); err != nil {
-			dec.panicSyntax(err)
+			dec.panicType(typ, err)
 		}
 	case val.CanAddr() && reflect.PointerTo(typ).Implements(decodableType):
 		obj := val.Addr().Interface().(Decodable)
-		if err := obj.DecodeJSON(dec.r); err != nil {
-			dec.panicSyntax(err)
+		if err := obj.DecodeJSON(dec.limitingScanner()); err != nil {
+			dec.panicType(typ, err)
 		}
 	case val.CanAddr() && reflect.PointerTo(typ).Implements(jsonUnmarshalerType):
 		var buf bytes.Buffer
 		dec.scan(&buf)
 		obj := val.Addr().Interface().(json.Unmarshaler)
 		if err := obj.UnmarshalJSON(buf.Bytes()); err != nil {
-			dec.panicSyntax(err)
+			dec.panicType(typ, err)
 		}
 	case val.CanAddr() && reflect.PointerTo(typ).Implements(textUnmarshalerType):
-		if nullOK && dec.peekRune() == 'n' {
+		if nullOK && dec.peekRuneType() == RuneTypeNullN {
 			dec.decodeNull()
 			return
 		}
@@ -257,19 +255,19 @@ func (dec *Decoder) decode(val reflect.Value, nullOK bool) {
 		dec.decodeString(&buf)
 		obj := val.Addr().Interface().(encoding.TextUnmarshaler)
 		if err := obj.UnmarshalText(buf.Bytes()); err != nil {
-			dec.panicSyntax(err)
+			dec.panicType(typ, err)
 		}
 	default:
 		kind := typ.Kind()
 		switch kind {
 		case reflect.Bool:
-			if nullOK && dec.peekRune() == 'n' {
+			if nullOK && dec.peekRuneType() == RuneTypeNullN {
 				dec.decodeNull()
 				return
 			}
 			val.SetBool(dec.decodeBool())
 		case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
-			if nullOK && dec.peekRune() == 'n' {
+			if nullOK && dec.peekRuneType() == RuneTypeNullN {
 				dec.decodeNull()
 				return
 			}
@@ -277,11 +275,11 @@ func (dec *Decoder) decode(val reflect.Value, nullOK bool) {
 			dec.scanNumber(&buf)
 			n, err := strconv.ParseInt(buf.String(), 10, kind2bits[kind])
 			if err != nil {
-				dec.panicSyntax(err)
+				dec.panicType(typ, err)
 			}
 			val.SetInt(n)
 		case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
-			if nullOK && dec.peekRune() == 'n' {
+			if nullOK && dec.peekRuneType() == RuneTypeNullN {
 				dec.decodeNull()
 				return
 			}
@@ -289,11 +287,11 @@ func (dec *Decoder) decode(val reflect.Value, nullOK bool) {
 			dec.scanNumber(&buf)
 			n, err := strconv.ParseUint(buf.String(), 10, kind2bits[kind])
 			if err != nil {
-				dec.panicSyntax(err)
+				dec.panicType(typ, err)
 			}
 			val.SetUint(n)
 		case reflect.Float32, reflect.Float64:
-			if nullOK && dec.peekRune() == 'n' {
+			if nullOK && dec.peekRuneType() == RuneTypeNullN {
 				dec.decodeNull()
 				return
 			}
@@ -301,11 +299,11 @@ func (dec *Decoder) decode(val reflect.Value, nullOK bool) {
 			dec.scanNumber(&buf)
 			n, err := strconv.ParseFloat(buf.String(), kind2bits[kind])
 			if err != nil {
-				dec.panicSyntax(err)
+				dec.panicType(typ, err)
 			}
 			val.SetFloat(n)
 		case reflect.String:
-			if nullOK && dec.peekRune() == 'n' {
+			if nullOK && dec.peekRuneType() == RuneTypeNullN {
 				dec.decodeNull()
 				return
 			}
@@ -321,8 +319,8 @@ func (dec *Decoder) decode(val reflect.Value, nullOK bool) {
 			if typ.NumMethod() > 0 {
 				dec.panicType(typ, fmt.Errorf("cannot decode in to non-empty interface"))
 			}
-			switch dec.peekRune() {
-			case 'n':
+			switch dec.peekRuneType() {
+			case RuneTypeNullN:
 				if !val.IsNil() && val.Elem().Kind() == reflect.Pointer && val.Elem().Elem().Kind() == reflect.Pointer {
 					// XXX: I can't justify this case, other than "it's what encoding/json does, but
 					// I don't understand their rationale".
@@ -339,7 +337,7 @@ func (dec *Decoder) decode(val reflect.Value, nullOK bool) {
 				}
 			}
 		case reflect.Struct:
-			if nullOK && dec.peekRune() == 'n' {
+			if nullOK && dec.peekRuneType() == RuneTypeNullN {
 				dec.decodeNull()
 				return
 			}
@@ -372,36 +370,36 @@ func (dec *Decoder) decode(val reflect.Value, nullOK bool) {
 					fVal = fVal.Field(idx)
 				}
 				if field.Quote {
-					switch dec.peekRune() {
-					case 'n':
+					switch dec.peekRuneType() {
+					case RuneTypeNullN:
 						dec.decodeNull()
 						switch fVal.Kind() {
 						// XXX: I can't justify this list, other than "it's what encoding/json
 						// does, but I don't understand their rationale".
 						case reflect.Interface, reflect.Pointer, reflect.Map, reflect.Slice:
 							fVal.Set(reflect.Zero(fVal.Type()))
+						default:
+							// do nothing???
 						}
-					case '"':
-						// TODO: Figure out how to do this without buffering.
+					case RuneTypeStringBeg:
+						// TODO: Figure out how to do this without buffering, have correct offsets.
 						var buf bytes.Buffer
-						subD := *dec // capture the .curPos *before* calling .decodeString
 						dec.decodeString(&buf)
-						subD.r = &buf
+						subD := NewDecoder(&buf)
 						subD.decode(fVal, false)
 					default:
-						dec.panicSyntax(fmt.Errorf(",string field: expected %q or %q but got %q",
-							'n', '"', dec.peekRune()))
+						dec.panicType(typ, fmt.Errorf(",string field TODO ERROR MESSAGE"))
 					}
 				} else {
 					dec.decode(fVal, true)
 				}
 			})
 		case reflect.Map:
-			switch dec.peekRune() {
-			case 'n':
+			switch dec.peekRuneType() {
+			case RuneTypeNullN:
 				dec.decodeNull()
 				val.Set(reflect.Zero(typ))
-			case '{':
+			case RuneTypeObjectBeg:
 				if val.IsNil() {
 					val.Set(reflect.MakeMap(typ))
 				}
@@ -413,7 +411,7 @@ func (dec *Decoder) decode(val reflect.Value, nullOK bool) {
 					case reflect.PointerTo(nameValTyp).Implements(textUnmarshalerType):
 						obj := nameValPtr.Interface().(encoding.TextUnmarshaler)
 						if err := obj.UnmarshalText(nameBuf.Bytes()); err != nil {
-							dec.panicSyntax(err)
+							dec.panicType(nameValTyp, err)
 						}
 					default:
 						switch nameValTyp.Kind() {
@@ -422,13 +420,13 @@ func (dec *Decoder) decode(val reflect.Value, nullOK bool) {
 						case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
 							n, err := strconv.ParseInt(nameBuf.String(), 10, kind2bits[nameValTyp.Kind()])
 							if err != nil {
-								dec.panicSyntax(err)
+								dec.panicType(nameValTyp, err)
 							}
 							nameValPtr.Elem().SetInt(n)
 						case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
 							n, err := strconv.ParseUint(nameBuf.String(), 10, kind2bits[nameValTyp.Kind()])
 							if err != nil {
-								dec.panicSyntax(err)
+								dec.panicType(nameValTyp, err)
 							}
 							nameValPtr.Elem().SetUint(n)
 						default:
@@ -444,16 +442,16 @@ func (dec *Decoder) decode(val reflect.Value, nullOK bool) {
 					val.SetMapIndex(nameValPtr.Elem(), fValPtr.Elem())
 				})
 			default:
-				dec.panicSyntax(fmt.Errorf("map: expected %q or %q bug got %q", 'n', '{', dec.peekRune()))
+				dec.panicType(typ, fmt.Errorf("map: TODO"))
 			}
 		case reflect.Slice:
 			switch {
 			case typ.Elem().Kind() == reflect.Uint8:
-				switch dec.peekRune() {
-				case 'n':
+				switch dec.peekRuneType() {
+				case RuneTypeNullN:
 					dec.decodeNull()
 					val.Set(reflect.Zero(typ))
-				case '"':
+				case RuneTypeStringBeg:
 					var buf bytes.Buffer
 					dec.decodeString(newBase64Decoder(&buf))
 					if typ.Elem() == byteType {
@@ -467,14 +465,14 @@ func (dec *Decoder) decode(val reflect.Value, nullOK bool) {
 						}
 					}
 				default:
-					dec.panicSyntax(fmt.Errorf("byte slice: expected %q or %q but got %q", 'n', '"', dec.peekRune()))
+					dec.panicType(typ, fmt.Errorf("byte slice: TODO"))
 				}
 			default:
-				switch dec.peekRune() {
-				case 'n':
+				switch dec.peekRuneType() {
+				case RuneTypeNullN:
 					dec.decodeNull()
 					val.Set(reflect.Zero(typ))
-				case '[':
+				case RuneTypeArrayBeg:
 					if val.IsNil() {
 						val.Set(reflect.MakeSlice(typ, 0, 0))
 					}
@@ -491,11 +489,11 @@ func (dec *Decoder) decode(val reflect.Value, nullOK bool) {
 						i++
 					})
 				default:
-					dec.panicSyntax(fmt.Errorf("slice: expected %q or %q but got %q", 'n', '[', dec.peekRune()))
+					dec.panicType(typ, fmt.Errorf("slice: TODO"))
 				}
 			}
 		case reflect.Array:
-			if nullOK && dec.peekRune() == 'n' {
+			if nullOK && dec.peekRuneType() == RuneTypeNullN {
 				dec.decodeNull()
 				return
 			}
@@ -517,8 +515,8 @@ func (dec *Decoder) decode(val reflect.Value, nullOK bool) {
 				val.Index(i).Set(reflect.Zero(typ.Elem()))
 			}
 		case reflect.Pointer:
-			switch dec.peekRune() {
-			case 'n':
+			switch dec.peekRuneType() {
+			case RuneTypeNullN:
 				dec.decodeNull()
 				/*
 					for typ.Elem().Kind() == reflect.Pointer {
@@ -542,67 +540,27 @@ func (dec *Decoder) decode(val reflect.Value, nullOK bool) {
 	}
 }
 
-func (dec *Decoder) decodeWS() {
-	for {
-		c, ok := dec.readRuneOrEOF()
-		if !ok {
-			return
-		}
-		switch c {
-		// NB: The JSON definition of whitespace is more
-		// narrow than unicode.IsSpace
-		case 0x0020, 0x000A, 0x000D, 0x0009:
-			// do nothing
-		default:
-			dec.unreadRune()
-			return
-		}
-	}
-}
-
 func (dec *Decoder) scan(out io.Writer) {
-	var scanner Parser
+	limiter := dec.limitingScanner()
 	for {
-		c, ok := dec.readRuneOrEOF()
-
-		var t RuneType
-		var err error
-		if ok {
-			t, err = scanner.HandleRune(c)
-		} else {
-			t, err = scanner.HandleEOF()
-		}
-
-		switch t {
-		case RuneTypeError:
-			dec.panicSyntax(err)
-		case RuneTypeEOF:
-			if ok {
-				dec.unreadRune()
-			}
+		c, _, err := limiter.ReadRune()
+		if err == io.EOF {
 			return
-		case RuneTypeSpace:
-			// ignore
-		default:
-			if _, err := writeRune(out, c); err != nil {
-				dec.panicIO(err)
-			}
 		}
+		_, _ = writeRune(out, c)
 	}
 }
 
 func (dec *Decoder) scanNumber(out io.Writer) {
-	c := dec.peekRune()
-	switch c {
-	case '-', '0', '1', '2', '3', '4', '5', '6', '7', '8', '9':
-		dec.scan(out)
-	default:
-		dec.panicSyntax(fmt.Errorf("number: expected %q or a digit, but got %q", '-', c))
+	if !dec.peekRuneType().IsNumber() {
+		dec.panicType(numberType, fmt.Errorf("number: not a number"))
 	}
+	dec.scan(out)
 }
 
 func (dec *Decoder) decodeAny() any {
-	c := dec.peekRune()
+	c, _ := dec.readRune()
+	dec.unreadRune()
 	switch c {
 	case '{':
 		ret := make(map[string]any)
@@ -628,14 +586,14 @@ func (dec *Decoder) decodeAny() any {
 		return buf.String()
 	case '-', '0', '1', '2', '3', '4', '5', '6', '7', '8', '9':
 		var buf strings.Builder
-		dec.scanNumber(&buf)
+		dec.scan(&buf)
 		num := json.Number(buf.String())
 		if dec.useNumber {
 			return num
 		}
 		f64, err := num.Float64()
 		if err != nil {
-			dec.panicSyntax(err)
+			panic("should not happen")
 		}
 		return f64
 	case 't', 'f':
@@ -644,168 +602,136 @@ func (dec *Decoder) decodeAny() any {
 		dec.decodeNull()
 		return nil
 	default:
-		dec.panicSyntax(fmt.Errorf("any: unexpected character: %c", c))
-		panic("not reached")
+		panic("should not happen")
 	}
 }
 
 func (dec *Decoder) decodeObject(nameBuf runeBuffer, decodeKVal func()) {
-	dec.expectRune('{')
-	dec.decodeWS()
-	c := dec.readRune()
-	switch c {
-	case '"':
+	dec.expectRuneType('{', RuneTypeObjectBeg)
+	_, t := dec.readRune()
+	switch t {
+	case RuneTypeObjectEnd:
+		return
+	case RuneTypeStringBeg:
 	decodeMember:
 		dec.unreadRune()
 		nameBuf.Reset()
 		dec.decodeString(nameBuf)
-		dec.decodeWS()
-		dec.expectRune(':')
-		dec.decodeWS()
+		dec.expectRune(':', RuneTypeObjectColon)
 		decodeKVal()
-		dec.decodeWS()
-		c := dec.readRune()
-		switch c {
-		case ',':
-			dec.decodeWS()
-			dec.expectRune('"')
+		_, t := dec.readRune()
+		switch t {
+		case RuneTypeObjectComma:
+			dec.expectRune('"', RuneTypeStringBeg)
 			goto decodeMember
-		case '}':
+		case RuneTypeObjectEnd:
 			return
 		default:
-			dec.panicSyntax(fmt.Errorf("object: expected %q or %q but got %q", ',', '}', c))
+			panic("should not happen")
 		}
-	case '}':
-		return
 	default:
-		dec.panicSyntax(fmt.Errorf("object: expected %q or %q but got %q", '"', '}', c))
+		panic("should not happen")
 	}
 }
 
 func (dec *Decoder) decodeArray(decodeMember func()) {
-	dec.expectRune('[')
-	dec.decodeWS()
-	c := dec.readRune()
-	switch c {
-	case ']':
+	dec.expectRuneType('[', RuneTypeArrayBeg)
+	_, t := dec.readRune()
+	switch t {
+	case RuneTypeArrayEnd:
 		return
 	default:
 		dec.unreadRune()
 	decodeNextMember:
 		decodeMember()
-		dec.decodeWS()
-		c := dec.readRune()
-		switch c {
-		case ',':
-			dec.decodeWS()
+		_, t := dec.readRune()
+		switch t {
+		case RuneTypeArrayComma:
 			goto decodeNextMember
-		case ']':
+		case RuneTypeArrayEnd:
 			return
 		default:
-			dec.panicSyntax(fmt.Errorf("array: expected %c or %c but got %c", ',', ']', c))
+			panic("should not happen")
 		}
 	}
 }
 
-func (dec *Decoder) decodeHex() rune {
-	c := dec.readRune()
-	switch {
-	case '0' <= c && c <= '9':
-		return c - '0'
-	case 'a' <= c && c <= 'f':
-		return c - 'a' + 10
-	case 'A' <= c && c <= 'F':
-		return c - 'A' + 10
-	default:
-		dec.panicSyntax(fmt.Errorf("string: expected a hex digit but got %q", c))
-		panic("not reached")
-	}
-}
-
 func (dec *Decoder) decodeString(out io.Writer) {
-	dec.expectRune('"')
+	dec.expectRuneType('"', RuneTypeStringBeg)
+	var uhex [4]byte
 	for {
-		c := dec.readRune()
-		switch {
-		case 0x0020 <= c && c <= 0x10FFFF && c != '"' && c != '\\':
-			if _, err := writeRune(out, c); err != nil {
-				dec.panicSyntax(err)
-			}
-		case c == '\\':
-			c = dec.readRune()
+		c, t := dec.readRune()
+		switch t {
+		case RuneTypeStringChar:
+			_, _ = writeRune(out, c)
+		case RuneTypeStringEsc, RuneTypeStringEscU:
+			// do nothing
+		case RuneTypeStringEsc1:
 			switch c {
 			case '"':
-				if _, err := writeRune(out, '"'); err != nil {
-					dec.panicSyntax(err)
-				}
+				_, _ = writeRune(out, '"')
 			case '\\':
-				if _, err := writeRune(out, '\\'); err != nil {
-					dec.panicSyntax(err)
-				}
+				_, _ = writeRune(out, '\\')
 			case '/':
-				if _, err := writeRune(out, '/'); err != nil {
-					dec.panicSyntax(err)
-				}
+				_, _ = writeRune(out, '/')
 			case 'b':
-				if _, err := writeRune(out, '\b'); err != nil {
-					dec.panicSyntax(err)
-				}
+				_, _ = writeRune(out, '\b')
 			case 'f':
-				if _, err := writeRune(out, '\f'); err != nil {
-					dec.panicSyntax(err)
-				}
+				_, _ = writeRune(out, '\f')
 			case 'n':
-				if _, err := writeRune(out, '\n'); err != nil {
-					dec.panicSyntax(err)
-				}
+				_, _ = writeRune(out, '\n')
 			case 'r':
-				if _, err := writeRune(out, '\r'); err != nil {
-					dec.panicSyntax(err)
-				}
+				_, _ = writeRune(out, '\r')
 			case 't':
-				if _, err := writeRune(out, '\t'); err != nil {
-					dec.panicSyntax(err)
-				}
-			case 'u':
-				c = dec.decodeHex()
-				c = (c << 4) | dec.decodeHex()
-				c = (c << 4) | dec.decodeHex()
-				c = (c << 4) | dec.decodeHex()
-				if _, err := writeRune(out, c); err != nil {
-					dec.panicSyntax(err)
-				}
+				_, _ = writeRune(out, '\t')
+			default:
+				panic("should not happen")
 			}
-		case c == '"':
+		case RuneTypeStringEscUA:
+			uhex[0], _ = hex2int(c)
+		case RuneTypeStringEscUB:
+			uhex[1], _ = hex2int(c)
+		case RuneTypeStringEscUC:
+			uhex[2], _ = hex2int(c)
+		case RuneTypeStringEscUD:
+			uhex[3], _ = hex2int(c)
+			c = 0 |
+				rune(uhex[0])<<12 |
+				rune(uhex[1])<<8 |
+				rune(uhex[2])<<4 |
+				rune(uhex[3])<<0
+			_, _ = writeRune(out, c)
+		case RuneTypeStringEnd:
 			return
 		default:
-			dec.panicSyntax(fmt.Errorf("string: unexpected %c", c))
+			panic("should not happen")
 		}
 	}
 }
 
 func (dec *Decoder) decodeBool() bool {
-	c := dec.readRune()
+	c, _ := dec.readRune()
 	switch c {
 	case 't':
-		dec.expectRune('r')
-		dec.expectRune('u')
-		dec.expectRune('e')
+		dec.expectRune('r', RuneTypeTrueR)
+		dec.expectRune('u', RuneTypeTrueU)
+		dec.expectRune('e', RuneTypeTrueE)
 		return true
 	case 'f':
-		dec.expectRune('a')
-		dec.expectRune('l')
-		dec.expectRune('s')
-		dec.expectRune('e')
+		dec.expectRune('a', RuneTypeFalseA)
+		dec.expectRune('l', RuneTypeFalseL)
+		dec.expectRune('s', RuneTypeFalseS)
+		dec.expectRune('e', RuneTypeFalseE)
 		return false
 	default:
-		dec.panicSyntax(fmt.Errorf("bool: expected %q or %q but got %q", 't', 'f', c))
+		dec.panicType(boolType, fmt.Errorf("bool: expected %q or %q but got %q", 't', 'f', c))
 		panic("not reached")
 	}
 }
 
 func (dec *Decoder) decodeNull() {
-	dec.expectRune('n')
-	dec.expectRune('u')
-	dec.expectRune('l')
-	dec.expectRune('l')
+	dec.expectRuneType('n', RuneTypeNullN)
+	dec.expectRune('u', RuneTypeNullU)
+	dec.expectRune('l', RuneTypeNullL1)
+	dec.expectRune('l', RuneTypeNullL2)
 }