Source file src/encoding/json/decode.go

     1  // Copyright 2010 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // Represents JSON data structure using native Go types: booleans, floats,
     6  // strings, arrays, and maps.
     7  
     8  package json
     9  
    10  import (
    11  	"encoding"
    12  	"encoding/base64"
    13  	"fmt"
    14  	"reflect"
    15  	"strconv"
    16  	"strings"
    17  	"unicode"
    18  	"unicode/utf16"
    19  	"unicode/utf8"
    20  )
    21  
    22  // Unmarshal parses the JSON-encoded data and stores the result
    23  // in the value pointed to by v. If v is nil or not a pointer,
    24  // Unmarshal returns an InvalidUnmarshalError.
    25  //
    26  // Unmarshal uses the inverse of the encodings that
    27  // Marshal uses, allocating maps, slices, and pointers as necessary,
    28  // with the following additional rules:
    29  //
    30  // To unmarshal JSON into a pointer, Unmarshal first handles the case of
    31  // the JSON being the JSON literal null. In that case, Unmarshal sets
    32  // the pointer to nil. Otherwise, Unmarshal unmarshals the JSON into
    33  // the value pointed at by the pointer. If the pointer is nil, Unmarshal
    34  // allocates a new value for it to point to.
    35  //
    36  // To unmarshal JSON into a value implementing the Unmarshaler interface,
    37  // Unmarshal calls that value's UnmarshalJSON method, including
    38  // when the input is a JSON null.
    39  // Otherwise, if the value implements encoding.TextUnmarshaler
    40  // and the input is a JSON quoted string, Unmarshal calls that value's
    41  // UnmarshalText method with the unquoted form of the string.
    42  //
    43  // To unmarshal JSON into a struct, Unmarshal matches incoming object
    44  // keys to the keys used by Marshal (either the struct field name or its tag),
    45  // preferring an exact match but also accepting a case-insensitive match. By
    46  // default, object keys which don't have a corresponding struct field are
    47  // ignored (see Decoder.DisallowUnknownFields for an alternative).
    48  //
    49  // To unmarshal JSON into an interface value,
    50  // Unmarshal stores one of these in the interface value:
    51  //
    52  //	bool, for JSON booleans
    53  //	float64, for JSON numbers
    54  //	string, for JSON strings
    55  //	[]interface{}, for JSON arrays
    56  //	map[string]interface{}, for JSON objects
    57  //	nil for JSON null
    58  //
    59  // To unmarshal a JSON array into a slice, Unmarshal resets the slice length
    60  // to zero and then appends each element to the slice.
    61  // As a special case, to unmarshal an empty JSON array into a slice,
    62  // Unmarshal replaces the slice with a new empty slice.
    63  //
    64  // To unmarshal a JSON array into a Go array, Unmarshal decodes
    65  // JSON array elements into corresponding Go array elements.
    66  // If the Go array is smaller than the JSON array,
    67  // the additional JSON array elements are discarded.
    68  // If the JSON array is smaller than the Go array,
    69  // the additional Go array elements are set to zero values.
    70  //
    71  // To unmarshal a JSON object into a map, Unmarshal first establishes a map to
    72  // use. If the map is nil, Unmarshal allocates a new map. Otherwise Unmarshal
    73  // reuses the existing map, keeping existing entries. Unmarshal then stores
    74  // key-value pairs from the JSON object into the map. The map's key type must
    75  // either be any string type, an integer, implement json.Unmarshaler, or
    76  // implement encoding.TextUnmarshaler.
    77  //
    78  // If a JSON value is not appropriate for a given target type,
    79  // or if a JSON number overflows the target type, Unmarshal
    80  // skips that field and completes the unmarshaling as best it can.
    81  // If no more serious errors are encountered, Unmarshal returns
    82  // an UnmarshalTypeError describing the earliest such error. In any
    83  // case, it's not guaranteed that all the remaining fields following
    84  // the problematic one will be unmarshaled into the target object.
    85  //
    86  // The JSON null value unmarshals into an interface, map, pointer, or slice
    87  // by setting that Go value to nil. Because null is often used in JSON to mean
    88  // ``not present,'' unmarshaling a JSON null into any other Go type has no effect
    89  // on the value and produces no error.
    90  //
    91  // When unmarshaling quoted strings, invalid UTF-8 or
    92  // invalid UTF-16 surrogate pairs are not treated as an error.
    93  // Instead, they are replaced by the Unicode replacement
    94  // character U+FFFD.
    95  //
    96  func Unmarshal(data []byte, v interface{}) error {
    97  	// Check for well-formedness.
    98  	// Avoids filling out half a data structure
    99  	// before discovering a JSON syntax error.
   100  	var d decodeState
   101  	err := checkValid(data, &d.scan)
   102  	if err != nil {
   103  		return err
   104  	}
   105  
   106  	d.init(data)
   107  	return d.unmarshal(v)
   108  }
   109  
   110  // Unmarshaler is the interface implemented by types
   111  // that can unmarshal a JSON description of themselves.
   112  // The input can be assumed to be a valid encoding of
   113  // a JSON value. UnmarshalJSON must copy the JSON data
   114  // if it wishes to retain the data after returning.
   115  //
   116  // By convention, to approximate the behavior of Unmarshal itself,
   117  // Unmarshalers implement UnmarshalJSON([]byte("null")) as a no-op.
   118  type Unmarshaler interface {
   119  	UnmarshalJSON([]byte) error
   120  }
   121  
   122  // An UnmarshalTypeError describes a JSON value that was
   123  // not appropriate for a value of a specific Go type.
   124  type UnmarshalTypeError struct {
   125  	Value  string       // description of JSON value - "bool", "array", "number -5"
   126  	Type   reflect.Type // type of Go value it could not be assigned to
   127  	Offset int64        // error occurred after reading Offset bytes
   128  	Struct string       // name of the struct type containing the field
   129  	Field  string       // the full path from root node to the field
   130  }
   131  
   132  func (e *UnmarshalTypeError) Error() string {
   133  	if e.Struct != "" || e.Field != "" {
   134  		return "json: cannot unmarshal " + e.Value + " into Go struct field " + e.Struct + "." + e.Field + " of type " + e.Type.String()
   135  	}
   136  	return "json: cannot unmarshal " + e.Value + " into Go value of type " + e.Type.String()
   137  }
   138  
   139  // An UnmarshalFieldError describes a JSON object key that
   140  // led to an unexported (and therefore unwritable) struct field.
   141  //
   142  // Deprecated: No longer used; kept for compatibility.
   143  type UnmarshalFieldError struct {
   144  	Key   string
   145  	Type  reflect.Type
   146  	Field reflect.StructField
   147  }
   148  
   149  func (e *UnmarshalFieldError) Error() string {
   150  	return "json: cannot unmarshal object key " + strconv.Quote(e.Key) + " into unexported field " + e.Field.Name + " of type " + e.Type.String()
   151  }
   152  
   153  // An InvalidUnmarshalError describes an invalid argument passed to Unmarshal.
   154  // (The argument to Unmarshal must be a non-nil pointer.)
   155  type InvalidUnmarshalError struct {
   156  	Type reflect.Type
   157  }
   158  
   159  func (e *InvalidUnmarshalError) Error() string {
   160  	if e.Type == nil {
   161  		return "json: Unmarshal(nil)"
   162  	}
   163  
   164  	if e.Type.Kind() != reflect.Ptr {
   165  		return "json: Unmarshal(non-pointer " + e.Type.String() + ")"
   166  	}
   167  	return "json: Unmarshal(nil " + e.Type.String() + ")"
   168  }
   169  
   170  func (d *decodeState) unmarshal(v interface{}) error {
   171  	rv := reflect.ValueOf(v)
   172  	if rv.Kind() != reflect.Ptr || rv.IsNil() {
   173  		return &InvalidUnmarshalError{reflect.TypeOf(v)}
   174  	}
   175  
   176  	d.scan.reset()
   177  	d.scanWhile(scanSkipSpace)
   178  	// We decode rv not rv.Elem because the Unmarshaler interface
   179  	// test must be applied at the top level of the value.
   180  	err := d.value(rv)
   181  	if err != nil {
   182  		return d.addErrorContext(err)
   183  	}
   184  	return d.savedError
   185  }
   186  
   187  // A Number represents a JSON number literal.
   188  type Number string
   189  
   190  // String returns the literal text of the number.
   191  func (n Number) String() string { return string(n) }
   192  
   193  // Float64 returns the number as a float64.
   194  func (n Number) Float64() (float64, error) {
   195  	return strconv.ParseFloat(string(n), 64)
   196  }
   197  
   198  // Int64 returns the number as an int64.
   199  func (n Number) Int64() (int64, error) {
   200  	return strconv.ParseInt(string(n), 10, 64)
   201  }
   202  
   203  // An errorContext provides context for type errors during decoding.
   204  type errorContext struct {
   205  	Struct     reflect.Type
   206  	FieldStack []string
   207  }
   208  
   209  // decodeState represents the state while decoding a JSON value.
   210  type decodeState struct {
   211  	data                  []byte
   212  	off                   int // next read offset in data
   213  	opcode                int // last read result
   214  	scan                  scanner
   215  	errorContext          *errorContext
   216  	savedError            error
   217  	useNumber             bool
   218  	disallowUnknownFields bool
   219  }
   220  
   221  // readIndex returns the position of the last byte read.
   222  func (d *decodeState) readIndex() int {
   223  	return d.off - 1
   224  }
   225  
   226  // phasePanicMsg is used as a panic message when we end up with something that
   227  // shouldn't happen. It can indicate a bug in the JSON decoder, or that
   228  // something is editing the data slice while the decoder executes.
   229  const phasePanicMsg = "JSON decoder out of sync - data changing underfoot?"
   230  
   231  func (d *decodeState) init(data []byte) *decodeState {
   232  	d.data = data
   233  	d.off = 0
   234  	d.savedError = nil
   235  	if d.errorContext != nil {
   236  		d.errorContext.Struct = nil
   237  		// Reuse the allocated space for the FieldStack slice.
   238  		d.errorContext.FieldStack = d.errorContext.FieldStack[:0]
   239  	}
   240  	return d
   241  }
   242  
   243  // saveError saves the first err it is called with,
   244  // for reporting at the end of the unmarshal.
   245  func (d *decodeState) saveError(err error) {
   246  	if d.savedError == nil {
   247  		d.savedError = d.addErrorContext(err)
   248  	}
   249  }
   250  
   251  // addErrorContext returns a new error enhanced with information from d.errorContext
   252  func (d *decodeState) addErrorContext(err error) error {
   253  	if d.errorContext != nil && (d.errorContext.Struct != nil || len(d.errorContext.FieldStack) > 0) {
   254  		switch err := err.(type) {
   255  		case *UnmarshalTypeError:
   256  			err.Struct = d.errorContext.Struct.Name()
   257  			err.Field = strings.Join(d.errorContext.FieldStack, ".")
   258  		}
   259  	}
   260  	return err
   261  }
   262  
   263  // skip scans to the end of what was started.
   264  func (d *decodeState) skip() {
   265  	s, data, i := &d.scan, d.data, d.off
   266  	depth := len(s.parseState)
   267  	for {
   268  		op := s.step(s, data[i])
   269  		i++
   270  		if len(s.parseState) < depth {
   271  			d.off = i
   272  			d.opcode = op
   273  			return
   274  		}
   275  	}
   276  }
   277  
   278  // scanNext processes the byte at d.data[d.off].
   279  func (d *decodeState) scanNext() {
   280  	if d.off < len(d.data) {
   281  		d.opcode = d.scan.step(&d.scan, d.data[d.off])
   282  		d.off++
   283  	} else {
   284  		d.opcode = d.scan.eof()
   285  		d.off = len(d.data) + 1 // mark processed EOF with len+1
   286  	}
   287  }
   288  
   289  // scanWhile processes bytes in d.data[d.off:] until it
   290  // receives a scan code not equal to op.
   291  func (d *decodeState) scanWhile(op int) {
   292  	s, data, i := &d.scan, d.data, d.off
   293  	for i < len(data) {
   294  		newOp := s.step(s, data[i])
   295  		i++
   296  		if newOp != op {
   297  			d.opcode = newOp
   298  			d.off = i
   299  			return
   300  		}
   301  	}
   302  
   303  	d.off = len(data) + 1 // mark processed EOF with len+1
   304  	d.opcode = d.scan.eof()
   305  }
   306  
   307  // rescanLiteral is similar to scanWhile(scanContinue), but it specialises the
   308  // common case where we're decoding a literal. The decoder scans the input
   309  // twice, once for syntax errors and to check the length of the value, and the
   310  // second to perform the decoding.
   311  //
   312  // Only in the second step do we use decodeState to tokenize literals, so we
   313  // know there aren't any syntax errors. We can take advantage of that knowledge,
   314  // and scan a literal's bytes much more quickly.
   315  func (d *decodeState) rescanLiteral() {
   316  	data, i := d.data, d.off
   317  Switch:
   318  	switch data[i-1] {
   319  	case '"': // string
   320  		for ; i < len(data); i++ {
   321  			switch data[i] {
   322  			case '\\':
   323  				i++ // escaped char
   324  			case '"':
   325  				i++ // tokenize the closing quote too
   326  				break Switch
   327  			}
   328  		}
   329  	case '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '-': // number
   330  		for ; i < len(data); i++ {
   331  			switch data[i] {
   332  			case '0', '1', '2', '3', '4', '5', '6', '7', '8', '9',
   333  				'.', 'e', 'E', '+', '-':
   334  			default:
   335  				break Switch
   336  			}
   337  		}
   338  	case 't': // true
   339  		i += len("rue")
   340  	case 'f': // false
   341  		i += len("alse")
   342  	case 'n': // null
   343  		i += len("ull")
   344  	}
   345  	if i < len(data) {
   346  		d.opcode = stateEndValue(&d.scan, data[i])
   347  	} else {
   348  		d.opcode = scanEnd
   349  	}
   350  	d.off = i + 1
   351  }
   352  
   353  // value consumes a JSON value from d.data[d.off-1:], decoding into v, and
   354  // reads the following byte ahead. If v is invalid, the value is discarded.
   355  // The first byte of the value has been read already.
   356  func (d *decodeState) value(v reflect.Value) error {
   357  	switch d.opcode {
   358  	default:
   359  		panic(phasePanicMsg)
   360  
   361  	case scanBeginArray:
   362  		if v.IsValid() {
   363  			if err := d.array(v); err != nil {
   364  				return err
   365  			}
   366  		} else {
   367  			d.skip()
   368  		}
   369  		d.scanNext()
   370  
   371  	case scanBeginObject:
   372  		if v.IsValid() {
   373  			if err := d.object(v); err != nil {
   374  				return err
   375  			}
   376  		} else {
   377  			d.skip()
   378  		}
   379  		d.scanNext()
   380  
   381  	case scanBeginLiteral:
   382  		// All bytes inside literal return scanContinue op code.
   383  		start := d.readIndex()
   384  		d.rescanLiteral()
   385  
   386  		if v.IsValid() {
   387  			if err := d.literalStore(d.data[start:d.readIndex()], v, false); err != nil {
   388  				return err
   389  			}
   390  		}
   391  	}
   392  	return nil
   393  }
   394  
   395  type unquotedValue struct{}
   396  
   397  // valueQuoted is like value but decodes a
   398  // quoted string literal or literal null into an interface value.
   399  // If it finds anything other than a quoted string literal or null,
   400  // valueQuoted returns unquotedValue{}.
   401  func (d *decodeState) valueQuoted() interface{} {
   402  	switch d.opcode {
   403  	default:
   404  		panic(phasePanicMsg)
   405  
   406  	case scanBeginArray, scanBeginObject:
   407  		d.skip()
   408  		d.scanNext()
   409  
   410  	case scanBeginLiteral:
   411  		v := d.literalInterface()
   412  		switch v.(type) {
   413  		case nil, string:
   414  			return v
   415  		}
   416  	}
   417  	return unquotedValue{}
   418  }
   419  
   420  // indirect walks down v allocating pointers as needed,
   421  // until it gets to a non-pointer.
   422  // If it encounters an Unmarshaler, indirect stops and returns that.
   423  // If decodingNull is true, indirect stops at the first settable pointer so it
   424  // can be set to nil.
   425  func indirect(v reflect.Value, decodingNull bool) (Unmarshaler, encoding.TextUnmarshaler, reflect.Value) {
   426  	// Issue #24153 indicates that it is generally not a guaranteed property
   427  	// that you may round-trip a reflect.Value by calling Value.Addr().Elem()
   428  	// and expect the value to still be settable for values derived from
   429  	// unexported embedded struct fields.
   430  	//
   431  	// The logic below effectively does this when it first addresses the value
   432  	// (to satisfy possible pointer methods) and continues to dereference
   433  	// subsequent pointers as necessary.
   434  	//
   435  	// After the first round-trip, we set v back to the original value to
   436  	// preserve the original RW flags contained in reflect.Value.
   437  	v0 := v
   438  	haveAddr := false
   439  
   440  	// If v is a named type and is addressable,
   441  	// start with its address, so that if the type has pointer methods,
   442  	// we find them.
   443  	if v.Kind() != reflect.Ptr && v.Type().Name() != "" && v.CanAddr() {
   444  		haveAddr = true
   445  		v = v.Addr()
   446  	}
   447  	for {
   448  		// Load value from interface, but only if the result will be
   449  		// usefully addressable.
   450  		if v.Kind() == reflect.Interface && !v.IsNil() {
   451  			e := v.Elem()
   452  			if e.Kind() == reflect.Ptr && !e.IsNil() && (!decodingNull || e.Elem().Kind() == reflect.Ptr) {
   453  				haveAddr = false
   454  				v = e
   455  				continue
   456  			}
   457  		}
   458  
   459  		if v.Kind() != reflect.Ptr {
   460  			break
   461  		}
   462  
   463  		if decodingNull && v.CanSet() {
   464  			break
   465  		}
   466  
   467  		// Prevent infinite loop if v is an interface pointing to its own address:
   468  		//     var v interface{}
   469  		//     v = &v
   470  		if v.Elem().Kind() == reflect.Interface && v.Elem().Elem() == v {
   471  			v = v.Elem()
   472  			break
   473  		}
   474  		if v.IsNil() {
   475  			v.Set(reflect.New(v.Type().Elem()))
   476  		}
   477  		if v.Type().NumMethod() > 0 && v.CanInterface() {
   478  			if u, ok := v.Interface().(Unmarshaler); ok {
   479  				return u, nil, reflect.Value{}
   480  			}
   481  			if !decodingNull {
   482  				if u, ok := v.Interface().(encoding.TextUnmarshaler); ok {
   483  					return nil, u, reflect.Value{}
   484  				}
   485  			}
   486  		}
   487  
   488  		if haveAddr {
   489  			v = v0 // restore original value after round-trip Value.Addr().Elem()
   490  			haveAddr = false
   491  		} else {
   492  			v = v.Elem()
   493  		}
   494  	}
   495  	return nil, nil, v
   496  }
   497  
   498  // array consumes an array from d.data[d.off-1:], decoding into v.
   499  // The first byte of the array ('[') has been read already.
   500  func (d *decodeState) array(v reflect.Value) error {
   501  	// Check for unmarshaler.
   502  	u, ut, pv := indirect(v, false)
   503  	if u != nil {
   504  		start := d.readIndex()
   505  		d.skip()
   506  		return u.UnmarshalJSON(d.data[start:d.off])
   507  	}
   508  	if ut != nil {
   509  		d.saveError(&UnmarshalTypeError{Value: "array", Type: v.Type(), Offset: int64(d.off)})
   510  		d.skip()
   511  		return nil
   512  	}
   513  	v = pv
   514  
   515  	// Check type of target.
   516  	switch v.Kind() {
   517  	case reflect.Interface:
   518  		if v.NumMethod() == 0 {
   519  			// Decoding into nil interface? Switch to non-reflect code.
   520  			ai := d.arrayInterface()
   521  			v.Set(reflect.ValueOf(ai))
   522  			return nil
   523  		}
   524  		// Otherwise it's invalid.
   525  		fallthrough
   526  	default:
   527  		d.saveError(&UnmarshalTypeError{Value: "array", Type: v.Type(), Offset: int64(d.off)})
   528  		d.skip()
   529  		return nil
   530  	case reflect.Array, reflect.Slice:
   531  		break
   532  	}
   533  
   534  	i := 0
   535  	for {
   536  		// Look ahead for ] - can only happen on first iteration.
   537  		d.scanWhile(scanSkipSpace)
   538  		if d.opcode == scanEndArray {
   539  			break
   540  		}
   541  
   542  		// Get element of array, growing if necessary.
   543  		if v.Kind() == reflect.Slice {
   544  			// Grow slice if necessary
   545  			if i >= v.Cap() {
   546  				newcap := v.Cap() + v.Cap()/2
   547  				if newcap < 4 {
   548  					newcap = 4
   549  				}
   550  				newv := reflect.MakeSlice(v.Type(), v.Len(), newcap)
   551  				reflect.Copy(newv, v)
   552  				v.Set(newv)
   553  			}
   554  			if i >= v.Len() {
   555  				v.SetLen(i + 1)
   556  			}
   557  		}
   558  
   559  		if i < v.Len() {
   560  			// Decode into element.
   561  			if err := d.value(v.Index(i)); err != nil {
   562  				return err
   563  			}
   564  		} else {
   565  			// Ran out of fixed array: skip.
   566  			if err := d.value(reflect.Value{}); err != nil {
   567  				return err
   568  			}
   569  		}
   570  		i++
   571  
   572  		// Next token must be , or ].
   573  		if d.opcode == scanSkipSpace {
   574  			d.scanWhile(scanSkipSpace)
   575  		}
   576  		if d.opcode == scanEndArray {
   577  			break
   578  		}
   579  		if d.opcode != scanArrayValue {
   580  			panic(phasePanicMsg)
   581  		}
   582  	}
   583  
   584  	if i < v.Len() {
   585  		if v.Kind() == reflect.Array {
   586  			// Array. Zero the rest.
   587  			z := reflect.Zero(v.Type().Elem())
   588  			for ; i < v.Len(); i++ {
   589  				v.Index(i).Set(z)
   590  			}
   591  		} else {
   592  			v.SetLen(i)
   593  		}
   594  	}
   595  	if i == 0 && v.Kind() == reflect.Slice {
   596  		v.Set(reflect.MakeSlice(v.Type(), 0, 0))
   597  	}
   598  	return nil
   599  }
   600  
   601  var nullLiteral = []byte("null")
   602  var textUnmarshalerType = reflect.TypeOf((*encoding.TextUnmarshaler)(nil)).Elem()
   603  
   604  // object consumes an object from d.data[d.off-1:], decoding into v.
   605  // The first byte ('{') of the object has been read already.
   606  func (d *decodeState) object(v reflect.Value) error {
   607  	// Check for unmarshaler.
   608  	u, ut, pv := indirect(v, false)
   609  	if u != nil {
   610  		start := d.readIndex()
   611  		d.skip()
   612  		return u.UnmarshalJSON(d.data[start:d.off])
   613  	}
   614  	if ut != nil {
   615  		d.saveError(&UnmarshalTypeError{Value: "object", Type: v.Type(), Offset: int64(d.off)})
   616  		d.skip()
   617  		return nil
   618  	}
   619  	v = pv
   620  	t := v.Type()
   621  
   622  	// Decoding into nil interface? Switch to non-reflect code.
   623  	if v.Kind() == reflect.Interface && v.NumMethod() == 0 {
   624  		oi := d.objectInterface()
   625  		v.Set(reflect.ValueOf(oi))
   626  		return nil
   627  	}
   628  
   629  	var fields structFields
   630  
   631  	// Check type of target:
   632  	//   struct or
   633  	//   map[T1]T2 where T1 is string, an integer type,
   634  	//             or an encoding.TextUnmarshaler
   635  	switch v.Kind() {
   636  	case reflect.Map:
   637  		// Map key must either have string kind, have an integer kind,
   638  		// or be an encoding.TextUnmarshaler.
   639  		switch t.Key().Kind() {
   640  		case reflect.String,
   641  			reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64,
   642  			reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
   643  		default:
   644  			if !reflect.PtrTo(t.Key()).Implements(textUnmarshalerType) {
   645  				d.saveError(&UnmarshalTypeError{Value: "object", Type: t, Offset: int64(d.off)})
   646  				d.skip()
   647  				return nil
   648  			}
   649  		}
   650  		if v.IsNil() {
   651  			v.Set(reflect.MakeMap(t))
   652  		}
   653  	case reflect.Struct:
   654  		fields = cachedTypeFields(t)
   655  		// ok
   656  	default:
   657  		d.saveError(&UnmarshalTypeError{Value: "object", Type: t, Offset: int64(d.off)})
   658  		d.skip()
   659  		return nil
   660  	}
   661  
   662  	var mapElem reflect.Value
   663  	var origErrorContext errorContext
   664  	if d.errorContext != nil {
   665  		origErrorContext = *d.errorContext
   666  	}
   667  
   668  	for {
   669  		// Read opening " of string key or closing }.
   670  		d.scanWhile(scanSkipSpace)
   671  		if d.opcode == scanEndObject {
   672  			// closing } - can only happen on first iteration.
   673  			break
   674  		}
   675  		if d.opcode != scanBeginLiteral {
   676  			panic(phasePanicMsg)
   677  		}
   678  
   679  		// Read key.
   680  		start := d.readIndex()
   681  		d.rescanLiteral()
   682  		item := d.data[start:d.readIndex()]
   683  		key, ok := unquoteBytes(item)
   684  		if !ok {
   685  			panic(phasePanicMsg)
   686  		}
   687  
   688  		// Figure out field corresponding to key.
   689  		var subv reflect.Value
   690  		destring := false // whether the value is wrapped in a string to be decoded first
   691  
   692  		if v.Kind() == reflect.Map {
   693  			elemType := t.Elem()
   694  			if !mapElem.IsValid() {
   695  				mapElem = reflect.New(elemType).Elem()
   696  			} else {
   697  				mapElem.Set(reflect.Zero(elemType))
   698  			}
   699  			subv = mapElem
   700  		} else {
   701  			var f *field
   702  			if i, ok := fields.nameIndex[string(key)]; ok {
   703  				// Found an exact name match.
   704  				f = &fields.list[i]
   705  			} else {
   706  				// Fall back to the expensive case-insensitive
   707  				// linear search.
   708  				for i := range fields.list {
   709  					ff := &fields.list[i]
   710  					if ff.equalFold(ff.nameBytes, key) {
   711  						f = ff
   712  						break
   713  					}
   714  				}
   715  			}
   716  			if f != nil {
   717  				subv = v
   718  				destring = f.quoted
   719  				for _, i := range f.index {
   720  					if subv.Kind() == reflect.Ptr {
   721  						if subv.IsNil() {
   722  							// If a struct embeds a pointer to an unexported type,
   723  							// it is not possible to set a newly allocated value
   724  							// since the field is unexported.
   725  							//
   726  							// See https://golang.org/issue/21357
   727  							if !subv.CanSet() {
   728  								d.saveError(fmt.Errorf("json: cannot set embedded pointer to unexported struct: %v", subv.Type().Elem()))
   729  								// Invalidate subv to ensure d.value(subv) skips over
   730  								// the JSON value without assigning it to subv.
   731  								subv = reflect.Value{}
   732  								destring = false
   733  								break
   734  							}
   735  							subv.Set(reflect.New(subv.Type().Elem()))
   736  						}
   737  						subv = subv.Elem()
   738  					}
   739  					subv = subv.Field(i)
   740  				}
   741  				if d.errorContext == nil {
   742  					d.errorContext = new(errorContext)
   743  				}
   744  				d.errorContext.FieldStack = append(d.errorContext.FieldStack, f.name)
   745  				d.errorContext.Struct = t
   746  			} else if d.disallowUnknownFields {
   747  				d.saveError(fmt.Errorf("json: unknown field %q", key))
   748  			}
   749  		}
   750  
   751  		// Read : before value.
   752  		if d.opcode == scanSkipSpace {
   753  			d.scanWhile(scanSkipSpace)
   754  		}
   755  		if d.opcode != scanObjectKey {
   756  			panic(phasePanicMsg)
   757  		}
   758  		d.scanWhile(scanSkipSpace)
   759  
   760  		if destring {
   761  			switch qv := d.valueQuoted().(type) {
   762  			case nil:
   763  				if err := d.literalStore(nullLiteral, subv, false); err != nil {
   764  					return err
   765  				}
   766  			case string:
   767  				if err := d.literalStore([]byte(qv), subv, true); err != nil {
   768  					return err
   769  				}
   770  			default:
   771  				d.saveError(fmt.Errorf("json: invalid use of ,string struct tag, trying to unmarshal unquoted value into %v", subv.Type()))
   772  			}
   773  		} else {
   774  			if err := d.value(subv); err != nil {
   775  				return err
   776  			}
   777  		}
   778  
   779  		// Write value back to map;
   780  		// if using struct, subv points into struct already.
   781  		if v.Kind() == reflect.Map {
   782  			kt := t.Key()
   783  			var kv reflect.Value
   784  			switch {
   785  			case reflect.PtrTo(kt).Implements(textUnmarshalerType):
   786  				kv = reflect.New(kt)
   787  				if err := d.literalStore(item, kv, true); err != nil {
   788  					return err
   789  				}
   790  				kv = kv.Elem()
   791  			case kt.Kind() == reflect.String:
   792  				kv = reflect.ValueOf(key).Convert(kt)
   793  			default:
   794  				switch kt.Kind() {
   795  				case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
   796  					s := string(key)
   797  					n, err := strconv.ParseInt(s, 10, 64)
   798  					if err != nil || reflect.Zero(kt).OverflowInt(n) {
   799  						d.saveError(&UnmarshalTypeError{Value: "number " + s, Type: kt, Offset: int64(start + 1)})
   800  						break
   801  					}
   802  					kv = reflect.ValueOf(n).Convert(kt)
   803  				case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
   804  					s := string(key)
   805  					n, err := strconv.ParseUint(s, 10, 64)
   806  					if err != nil || reflect.Zero(kt).OverflowUint(n) {
   807  						d.saveError(&UnmarshalTypeError{Value: "number " + s, Type: kt, Offset: int64(start + 1)})
   808  						break
   809  					}
   810  					kv = reflect.ValueOf(n).Convert(kt)
   811  				default:
   812  					panic("json: Unexpected key type") // should never occur
   813  				}
   814  			}
   815  			if kv.IsValid() {
   816  				v.SetMapIndex(kv, subv)
   817  			}
   818  		}
   819  
   820  		// Next token must be , or }.
   821  		if d.opcode == scanSkipSpace {
   822  			d.scanWhile(scanSkipSpace)
   823  		}
   824  		if d.errorContext != nil {
   825  			// Reset errorContext to its original state.
   826  			// Keep the same underlying array for FieldStack, to reuse the
   827  			// space and avoid unnecessary allocs.
   828  			d.errorContext.FieldStack = d.errorContext.FieldStack[:len(origErrorContext.FieldStack)]
   829  			d.errorContext.Struct = origErrorContext.Struct
   830  		}
   831  		if d.opcode == scanEndObject {
   832  			break
   833  		}
   834  		if d.opcode != scanObjectValue {
   835  			panic(phasePanicMsg)
   836  		}
   837  	}
   838  	return nil
   839  }
   840  
   841  // convertNumber converts the number literal s to a float64 or a Number
   842  // depending on the setting of d.useNumber.
   843  func (d *decodeState) convertNumber(s string) (interface{}, error) {
   844  	if d.useNumber {
   845  		return Number(s), nil
   846  	}
   847  	f, err := strconv.ParseFloat(s, 64)
   848  	if err != nil {
   849  		return nil, &UnmarshalTypeError{Value: "number " + s, Type: reflect.TypeOf(0.0), Offset: int64(d.off)}
   850  	}
   851  	return f, nil
   852  }
   853  
   854  var numberType = reflect.TypeOf(Number(""))
   855  
   856  // literalStore decodes a literal stored in item into v.
   857  //
   858  // fromQuoted indicates whether this literal came from unwrapping a
   859  // string from the ",string" struct tag option. this is used only to
   860  // produce more helpful error messages.
   861  func (d *decodeState) literalStore(item []byte, v reflect.Value, fromQuoted bool) error {
   862  	// Check for unmarshaler.
   863  	if len(item) == 0 {
   864  		//Empty string given
   865  		d.saveError(fmt.Errorf("json: invalid use of ,string struct tag, trying to unmarshal %q into %v", item, v.Type()))
   866  		return nil
   867  	}
   868  	isNull := item[0] == 'n' // null
   869  	u, ut, pv := indirect(v, isNull)
   870  	if u != nil {
   871  		return u.UnmarshalJSON(item)
   872  	}
   873  	if ut != nil {
   874  		if item[0] != '"' {
   875  			if fromQuoted {
   876  				d.saveError(fmt.Errorf("json: invalid use of ,string struct tag, trying to unmarshal %q into %v", item, v.Type()))
   877  				return nil
   878  			}
   879  			val := "number"
   880  			switch item[0] {
   881  			case 'n':
   882  				val = "null"
   883  			case 't', 'f':
   884  				val = "bool"
   885  			}
   886  			d.saveError(&UnmarshalTypeError{Value: val, Type: v.Type(), Offset: int64(d.readIndex())})
   887  			return nil
   888  		}
   889  		s, ok := unquoteBytes(item)
   890  		if !ok {
   891  			if fromQuoted {
   892  				return fmt.Errorf("json: invalid use of ,string struct tag, trying to unmarshal %q into %v", item, v.Type())
   893  			}
   894  			panic(phasePanicMsg)
   895  		}
   896  		return ut.UnmarshalText(s)
   897  	}
   898  
   899  	v = pv
   900  
   901  	switch c := item[0]; c {
   902  	case 'n': // null
   903  		// The main parser checks that only true and false can reach here,
   904  		// but if this was a quoted string input, it could be anything.
   905  		if fromQuoted && string(item) != "null" {
   906  			d.saveError(fmt.Errorf("json: invalid use of ,string struct tag, trying to unmarshal %q into %v", item, v.Type()))
   907  			break
   908  		}
   909  		switch v.Kind() {
   910  		case reflect.Interface, reflect.Ptr, reflect.Map, reflect.Slice:
   911  			v.Set(reflect.Zero(v.Type()))
   912  			// otherwise, ignore null for primitives/string
   913  		}
   914  	case 't', 'f': // true, false
   915  		value := item[0] == 't'
   916  		// The main parser checks that only true and false can reach here,
   917  		// but if this was a quoted string input, it could be anything.
   918  		if fromQuoted && string(item) != "true" && string(item) != "false" {
   919  			d.saveError(fmt.Errorf("json: invalid use of ,string struct tag, trying to unmarshal %q into %v", item, v.Type()))
   920  			break
   921  		}
   922  		switch v.Kind() {
   923  		default:
   924  			if fromQuoted {
   925  				d.saveError(fmt.Errorf("json: invalid use of ,string struct tag, trying to unmarshal %q into %v", item, v.Type()))
   926  			} else {
   927  				d.saveError(&UnmarshalTypeError{Value: "bool", Type: v.Type(), Offset: int64(d.readIndex())})
   928  			}
   929  		case reflect.Bool:
   930  			v.SetBool(value)
   931  		case reflect.Interface:
   932  			if v.NumMethod() == 0 {
   933  				v.Set(reflect.ValueOf(value))
   934  			} else {
   935  				d.saveError(&UnmarshalTypeError{Value: "bool", Type: v.Type(), Offset: int64(d.readIndex())})
   936  			}
   937  		}
   938  
   939  	case '"': // string
   940  		s, ok := unquoteBytes(item)
   941  		if !ok {
   942  			if fromQuoted {
   943  				return fmt.Errorf("json: invalid use of ,string struct tag, trying to unmarshal %q into %v", item, v.Type())
   944  			}
   945  			panic(phasePanicMsg)
   946  		}
   947  		switch v.Kind() {
   948  		default:
   949  			d.saveError(&UnmarshalTypeError{Value: "string", Type: v.Type(), Offset: int64(d.readIndex())})
   950  		case reflect.Slice:
   951  			if v.Type().Elem().Kind() != reflect.Uint8 {
   952  				d.saveError(&UnmarshalTypeError{Value: "string", Type: v.Type(), Offset: int64(d.readIndex())})
   953  				break
   954  			}
   955  			b := make([]byte, base64.StdEncoding.DecodedLen(len(s)))
   956  			n, err := base64.StdEncoding.Decode(b, s)
   957  			if err != nil {
   958  				d.saveError(err)
   959  				break
   960  			}
   961  			v.SetBytes(b[:n])
   962  		case reflect.String:
   963  			if v.Type() == numberType && !isValidNumber(string(s)) {
   964  				return fmt.Errorf("json: invalid number literal, trying to unmarshal %q into Number", item)
   965  			}
   966  			v.SetString(string(s))
   967  		case reflect.Interface:
   968  			if v.NumMethod() == 0 {
   969  				v.Set(reflect.ValueOf(string(s)))
   970  			} else {
   971  				d.saveError(&UnmarshalTypeError{Value: "string", Type: v.Type(), Offset: int64(d.readIndex())})
   972  			}
   973  		}
   974  
   975  	default: // number
   976  		if c != '-' && (c < '0' || c > '9') {
   977  			if fromQuoted {
   978  				return fmt.Errorf("json: invalid use of ,string struct tag, trying to unmarshal %q into %v", item, v.Type())
   979  			}
   980  			panic(phasePanicMsg)
   981  		}
   982  		s := string(item)
   983  		switch v.Kind() {
   984  		default:
   985  			if v.Kind() == reflect.String && v.Type() == numberType {
   986  				// s must be a valid number, because it's
   987  				// already been tokenized.
   988  				v.SetString(s)
   989  				break
   990  			}
   991  			if fromQuoted {
   992  				return fmt.Errorf("json: invalid use of ,string struct tag, trying to unmarshal %q into %v", item, v.Type())
   993  			}
   994  			d.saveError(&UnmarshalTypeError{Value: "number", Type: v.Type(), Offset: int64(d.readIndex())})
   995  		case reflect.Interface:
   996  			n, err := d.convertNumber(s)
   997  			if err != nil {
   998  				d.saveError(err)
   999  				break
  1000  			}
  1001  			if v.NumMethod() != 0 {
  1002  				d.saveError(&UnmarshalTypeError{Value: "number", Type: v.Type(), Offset: int64(d.readIndex())})
  1003  				break
  1004  			}
  1005  			v.Set(reflect.ValueOf(n))
  1006  
  1007  		case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
  1008  			n, err := strconv.ParseInt(s, 10, 64)
  1009  			if err != nil || v.OverflowInt(n) {
  1010  				d.saveError(&UnmarshalTypeError{Value: "number " + s, Type: v.Type(), Offset: int64(d.readIndex())})
  1011  				break
  1012  			}
  1013  			v.SetInt(n)
  1014  
  1015  		case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
  1016  			n, err := strconv.ParseUint(s, 10, 64)
  1017  			if err != nil || v.OverflowUint(n) {
  1018  				d.saveError(&UnmarshalTypeError{Value: "number " + s, Type: v.Type(), Offset: int64(d.readIndex())})
  1019  				break
  1020  			}
  1021  			v.SetUint(n)
  1022  
  1023  		case reflect.Float32, reflect.Float64:
  1024  			n, err := strconv.ParseFloat(s, v.Type().Bits())
  1025  			if err != nil || v.OverflowFloat(n) {
  1026  				d.saveError(&UnmarshalTypeError{Value: "number " + s, Type: v.Type(), Offset: int64(d.readIndex())})
  1027  				break
  1028  			}
  1029  			v.SetFloat(n)
  1030  		}
  1031  	}
  1032  	return nil
  1033  }
  1034  
  1035  // The xxxInterface routines build up a value to be stored
  1036  // in an empty interface. They are not strictly necessary,
  1037  // but they avoid the weight of reflection in this common case.
  1038  
  1039  // valueInterface is like value but returns interface{}
  1040  func (d *decodeState) valueInterface() (val interface{}) {
  1041  	switch d.opcode {
  1042  	default:
  1043  		panic(phasePanicMsg)
  1044  	case scanBeginArray:
  1045  		val = d.arrayInterface()
  1046  		d.scanNext()
  1047  	case scanBeginObject:
  1048  		val = d.objectInterface()
  1049  		d.scanNext()
  1050  	case scanBeginLiteral:
  1051  		val = d.literalInterface()
  1052  	}
  1053  	return
  1054  }
  1055  
  1056  // arrayInterface is like array but returns []interface{}.
  1057  func (d *decodeState) arrayInterface() []interface{} {
  1058  	var v = make([]interface{}, 0)
  1059  	for {
  1060  		// Look ahead for ] - can only happen on first iteration.
  1061  		d.scanWhile(scanSkipSpace)
  1062  		if d.opcode == scanEndArray {
  1063  			break
  1064  		}
  1065  
  1066  		v = append(v, d.valueInterface())
  1067  
  1068  		// Next token must be , or ].
  1069  		if d.opcode == scanSkipSpace {
  1070  			d.scanWhile(scanSkipSpace)
  1071  		}
  1072  		if d.opcode == scanEndArray {
  1073  			break
  1074  		}
  1075  		if d.opcode != scanArrayValue {
  1076  			panic(phasePanicMsg)
  1077  		}
  1078  	}
  1079  	return v
  1080  }
  1081  
  1082  // objectInterface is like object but returns map[string]interface{}.
  1083  func (d *decodeState) objectInterface() map[string]interface{} {
  1084  	m := make(map[string]interface{})
  1085  	for {
  1086  		// Read opening " of string key or closing }.
  1087  		d.scanWhile(scanSkipSpace)
  1088  		if d.opcode == scanEndObject {
  1089  			// closing } - can only happen on first iteration.
  1090  			break
  1091  		}
  1092  		if d.opcode != scanBeginLiteral {
  1093  			panic(phasePanicMsg)
  1094  		}
  1095  
  1096  		// Read string key.
  1097  		start := d.readIndex()
  1098  		d.rescanLiteral()
  1099  		item := d.data[start:d.readIndex()]
  1100  		key, ok := unquote(item)
  1101  		if !ok {
  1102  			panic(phasePanicMsg)
  1103  		}
  1104  
  1105  		// Read : before value.
  1106  		if d.opcode == scanSkipSpace {
  1107  			d.scanWhile(scanSkipSpace)
  1108  		}
  1109  		if d.opcode != scanObjectKey {
  1110  			panic(phasePanicMsg)
  1111  		}
  1112  		d.scanWhile(scanSkipSpace)
  1113  
  1114  		// Read value.
  1115  		m[key] = d.valueInterface()
  1116  
  1117  		// Next token must be , or }.
  1118  		if d.opcode == scanSkipSpace {
  1119  			d.scanWhile(scanSkipSpace)
  1120  		}
  1121  		if d.opcode == scanEndObject {
  1122  			break
  1123  		}
  1124  		if d.opcode != scanObjectValue {
  1125  			panic(phasePanicMsg)
  1126  		}
  1127  	}
  1128  	return m
  1129  }
  1130  
  1131  // literalInterface consumes and returns a literal from d.data[d.off-1:] and
  1132  // it reads the following byte ahead. The first byte of the literal has been
  1133  // read already (that's how the caller knows it's a literal).
  1134  func (d *decodeState) literalInterface() interface{} {
  1135  	// All bytes inside literal return scanContinue op code.
  1136  	start := d.readIndex()
  1137  	d.rescanLiteral()
  1138  
  1139  	item := d.data[start:d.readIndex()]
  1140  
  1141  	switch c := item[0]; c {
  1142  	case 'n': // null
  1143  		return nil
  1144  
  1145  	case 't', 'f': // true, false
  1146  		return c == 't'
  1147  
  1148  	case '"': // string
  1149  		s, ok := unquote(item)
  1150  		if !ok {
  1151  			panic(phasePanicMsg)
  1152  		}
  1153  		return s
  1154  
  1155  	default: // number
  1156  		if c != '-' && (c < '0' || c > '9') {
  1157  			panic(phasePanicMsg)
  1158  		}
  1159  		n, err := d.convertNumber(string(item))
  1160  		if err != nil {
  1161  			d.saveError(err)
  1162  		}
  1163  		return n
  1164  	}
  1165  }
  1166  
  1167  // getu4 decodes \uXXXX from the beginning of s, returning the hex value,
  1168  // or it returns -1.
  1169  func getu4(s []byte) rune {
  1170  	if len(s) < 6 || s[0] != '\\' || s[1] != 'u' {
  1171  		return -1
  1172  	}
  1173  	var r rune
  1174  	for _, c := range s[2:6] {
  1175  		switch {
  1176  		case '0' <= c && c <= '9':
  1177  			c = c - '0'
  1178  		case 'a' <= c && c <= 'f':
  1179  			c = c - 'a' + 10
  1180  		case 'A' <= c && c <= 'F':
  1181  			c = c - 'A' + 10
  1182  		default:
  1183  			return -1
  1184  		}
  1185  		r = r*16 + rune(c)
  1186  	}
  1187  	return r
  1188  }
  1189  
  1190  // unquote converts a quoted JSON string literal s into an actual string t.
  1191  // The rules are different than for Go, so cannot use strconv.Unquote.
  1192  func unquote(s []byte) (t string, ok bool) {
  1193  	s, ok = unquoteBytes(s)
  1194  	t = string(s)
  1195  	return
  1196  }
  1197  
  1198  func unquoteBytes(s []byte) (t []byte, ok bool) {
  1199  	if len(s) < 2 || s[0] != '"' || s[len(s)-1] != '"' {
  1200  		return
  1201  	}
  1202  	s = s[1 : len(s)-1]
  1203  
  1204  	// Check for unusual characters. If there are none,
  1205  	// then no unquoting is needed, so return a slice of the
  1206  	// original bytes.
  1207  	r := 0
  1208  	for r < len(s) {
  1209  		c := s[r]
  1210  		if c == '\\' || c == '"' || c < ' ' {
  1211  			break
  1212  		}
  1213  		if c < utf8.RuneSelf {
  1214  			r++
  1215  			continue
  1216  		}
  1217  		rr, size := utf8.DecodeRune(s[r:])
  1218  		if rr == utf8.RuneError && size == 1 {
  1219  			break
  1220  		}
  1221  		r += size
  1222  	}
  1223  	if r == len(s) {
  1224  		return s, true
  1225  	}
  1226  
  1227  	b := make([]byte, len(s)+2*utf8.UTFMax)
  1228  	w := copy(b, s[0:r])
  1229  	for r < len(s) {
  1230  		// Out of room? Can only happen if s is full of
  1231  		// malformed UTF-8 and we're replacing each
  1232  		// byte with RuneError.
  1233  		if w >= len(b)-2*utf8.UTFMax {
  1234  			nb := make([]byte, (len(b)+utf8.UTFMax)*2)
  1235  			copy(nb, b[0:w])
  1236  			b = nb
  1237  		}
  1238  		switch c := s[r]; {
  1239  		case c == '\\':
  1240  			r++
  1241  			if r >= len(s) {
  1242  				return
  1243  			}
  1244  			switch s[r] {
  1245  			default:
  1246  				return
  1247  			case '"', '\\', '/', '\'':
  1248  				b[w] = s[r]
  1249  				r++
  1250  				w++
  1251  			case 'b':
  1252  				b[w] = '\b'
  1253  				r++
  1254  				w++
  1255  			case 'f':
  1256  				b[w] = '\f'
  1257  				r++
  1258  				w++
  1259  			case 'n':
  1260  				b[w] = '\n'
  1261  				r++
  1262  				w++
  1263  			case 'r':
  1264  				b[w] = '\r'
  1265  				r++
  1266  				w++
  1267  			case 't':
  1268  				b[w] = '\t'
  1269  				r++
  1270  				w++
  1271  			case 'u':
  1272  				r--
  1273  				rr := getu4(s[r:])
  1274  				if rr < 0 {
  1275  					return
  1276  				}
  1277  				r += 6
  1278  				if utf16.IsSurrogate(rr) {
  1279  					rr1 := getu4(s[r:])
  1280  					if dec := utf16.DecodeRune(rr, rr1); dec != unicode.ReplacementChar {
  1281  						// A valid pair; consume.
  1282  						r += 6
  1283  						w += utf8.EncodeRune(b[w:], dec)
  1284  						break
  1285  					}
  1286  					// Invalid surrogate; fall back to replacement rune.
  1287  					rr = unicode.ReplacementChar
  1288  				}
  1289  				w += utf8.EncodeRune(b[w:], rr)
  1290  			}
  1291  
  1292  		// Quote, control characters are invalid.
  1293  		case c == '"', c < ' ':
  1294  			return
  1295  
  1296  		// ASCII
  1297  		case c < utf8.RuneSelf:
  1298  			b[w] = c
  1299  			r++
  1300  			w++
  1301  
  1302  		// Coerce to well-formed UTF-8.
  1303  		default:
  1304  			rr, size := utf8.DecodeRune(s[r:])
  1305  			r += size
  1306  			w += utf8.EncodeRune(b[w:], rr)
  1307  		}
  1308  	}
  1309  	return b[0:w], true
  1310  }
  1311  

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