Source file src/encoding/json/encode.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  // Package json implements encoding and decoding of JSON as defined in
     6  // RFC 7159. The mapping between JSON and Go values is described
     7  // in the documentation for the Marshal and Unmarshal functions.
     8  //
     9  // See "JSON and Go" for an introduction to this package:
    10  // https://golang.org/doc/articles/json_and_go.html
    11  package json
    12  
    13  import (
    14  	"bytes"
    15  	"encoding"
    16  	"encoding/base64"
    17  	"fmt"
    18  	"math"
    19  	"reflect"
    20  	"sort"
    21  	"strconv"
    22  	"strings"
    23  	"sync"
    24  	"unicode"
    25  	"unicode/utf8"
    26  )
    27  
    28  // Marshal returns the JSON encoding of v.
    29  //
    30  // Marshal traverses the value v recursively.
    31  // If an encountered value implements the Marshaler interface
    32  // and is not a nil pointer, Marshal calls its MarshalJSON method
    33  // to produce JSON. If no MarshalJSON method is present but the
    34  // value implements encoding.TextMarshaler instead, Marshal calls
    35  // its MarshalText method and encodes the result as a JSON string.
    36  // The nil pointer exception is not strictly necessary
    37  // but mimics a similar, necessary exception in the behavior of
    38  // UnmarshalJSON.
    39  //
    40  // Otherwise, Marshal uses the following type-dependent default encodings:
    41  //
    42  // Boolean values encode as JSON booleans.
    43  //
    44  // Floating point, integer, and Number values encode as JSON numbers.
    45  //
    46  // String values encode as JSON strings coerced to valid UTF-8,
    47  // replacing invalid bytes with the Unicode replacement rune.
    48  // So that the JSON will be safe to embed inside HTML <script> tags,
    49  // the string is encoded using HTMLEscape,
    50  // which replaces "<", ">", "&", U+2028, and U+2029 are escaped
    51  // to "\u003c","\u003e", "\u0026", "\u2028", and "\u2029".
    52  // This replacement can be disabled when using an Encoder,
    53  // by calling SetEscapeHTML(false).
    54  //
    55  // Array and slice values encode as JSON arrays, except that
    56  // []byte encodes as a base64-encoded string, and a nil slice
    57  // encodes as the null JSON value.
    58  //
    59  // Struct values encode as JSON objects.
    60  // Each exported struct field becomes a member of the object, using the
    61  // field name as the object key, unless the field is omitted for one of the
    62  // reasons given below.
    63  //
    64  // The encoding of each struct field can be customized by the format string
    65  // stored under the "json" key in the struct field's tag.
    66  // The format string gives the name of the field, possibly followed by a
    67  // comma-separated list of options. The name may be empty in order to
    68  // specify options without overriding the default field name.
    69  //
    70  // The "omitempty" option specifies that the field should be omitted
    71  // from the encoding if the field has an empty value, defined as
    72  // false, 0, a nil pointer, a nil interface value, and any empty array,
    73  // slice, map, or string.
    74  //
    75  // As a special case, if the field tag is "-", the field is always omitted.
    76  // Note that a field with name "-" can still be generated using the tag "-,".
    77  //
    78  // Examples of struct field tags and their meanings:
    79  //
    80  //	// Field appears in JSON as key "myName".
    81  //	Field int `json:"myName"`
    82  //
    83  //	// Field appears in JSON as key "myName" and
    84  //	// the field is omitted from the object if its value is empty,
    85  //	// as defined above.
    86  //	Field int `json:"myName,omitempty"`
    87  //
    88  //	// Field appears in JSON as key "Field" (the default), but
    89  //	// the field is skipped if empty.
    90  //	// Note the leading comma.
    91  //	Field int `json:",omitempty"`
    92  //
    93  //	// Field is ignored by this package.
    94  //	Field int `json:"-"`
    95  //
    96  //	// Field appears in JSON as key "-".
    97  //	Field int `json:"-,"`
    98  //
    99  // The "string" option signals that a field is stored as JSON inside a
   100  // JSON-encoded string. It applies only to fields of string, floating point,
   101  // integer, or boolean types. This extra level of encoding is sometimes used
   102  // when communicating with JavaScript programs:
   103  //
   104  //	Int64String int64 `json:",string"`
   105  //
   106  // The key name will be used if it's a non-empty string consisting of
   107  // only Unicode letters, digits, and ASCII punctuation except quotation
   108  // marks, backslash, and comma.
   109  //
   110  // Anonymous struct fields are usually marshaled as if their inner exported fields
   111  // were fields in the outer struct, subject to the usual Go visibility rules amended
   112  // as described in the next paragraph.
   113  // An anonymous struct field with a name given in its JSON tag is treated as
   114  // having that name, rather than being anonymous.
   115  // An anonymous struct field of interface type is treated the same as having
   116  // that type as its name, rather than being anonymous.
   117  //
   118  // The Go visibility rules for struct fields are amended for JSON when
   119  // deciding which field to marshal or unmarshal. If there are
   120  // multiple fields at the same level, and that level is the least
   121  // nested (and would therefore be the nesting level selected by the
   122  // usual Go rules), the following extra rules apply:
   123  //
   124  // 1) Of those fields, if any are JSON-tagged, only tagged fields are considered,
   125  // even if there are multiple untagged fields that would otherwise conflict.
   126  //
   127  // 2) If there is exactly one field (tagged or not according to the first rule), that is selected.
   128  //
   129  // 3) Otherwise there are multiple fields, and all are ignored; no error occurs.
   130  //
   131  // Handling of anonymous struct fields is new in Go 1.1.
   132  // Prior to Go 1.1, anonymous struct fields were ignored. To force ignoring of
   133  // an anonymous struct field in both current and earlier versions, give the field
   134  // a JSON tag of "-".
   135  //
   136  // Map values encode as JSON objects. The map's key type must either be a
   137  // string, an integer type, or implement encoding.TextMarshaler. The map keys
   138  // are sorted and used as JSON object keys by applying the following rules,
   139  // subject to the UTF-8 coercion described for string values above:
   140  //   - keys of any string type are used directly
   141  //   - encoding.TextMarshalers are marshaled
   142  //   - integer keys are converted to strings
   143  //
   144  // Pointer values encode as the value pointed to.
   145  // A nil pointer encodes as the null JSON value.
   146  //
   147  // Interface values encode as the value contained in the interface.
   148  // A nil interface value encodes as the null JSON value.
   149  //
   150  // Channel, complex, and function values cannot be encoded in JSON.
   151  // Attempting to encode such a value causes Marshal to return
   152  // an UnsupportedTypeError.
   153  //
   154  // JSON cannot represent cyclic data structures and Marshal does not
   155  // handle them. Passing cyclic structures to Marshal will result in
   156  // an error.
   157  func Marshal(v any) ([]byte, error) {
   158  	e := newEncodeState()
   159  
   160  	err := e.marshal(v, encOpts{escapeHTML: true})
   161  	if err != nil {
   162  		return nil, err
   163  	}
   164  	buf := append([]byte(nil), e.Bytes()...)
   165  
   166  	encodeStatePool.Put(e)
   167  
   168  	return buf, nil
   169  }
   170  
   171  // MarshalIndent is like Marshal but applies Indent to format the output.
   172  // Each JSON element in the output will begin on a new line beginning with prefix
   173  // followed by one or more copies of indent according to the indentation nesting.
   174  func MarshalIndent(v any, prefix, indent string) ([]byte, error) {
   175  	b, err := Marshal(v)
   176  	if err != nil {
   177  		return nil, err
   178  	}
   179  	var buf bytes.Buffer
   180  	err = Indent(&buf, b, prefix, indent)
   181  	if err != nil {
   182  		return nil, err
   183  	}
   184  	return buf.Bytes(), nil
   185  }
   186  
   187  // HTMLEscape appends to dst the JSON-encoded src with <, >, &, U+2028 and U+2029
   188  // characters inside string literals changed to \u003c, \u003e, \u0026, \u2028, \u2029
   189  // so that the JSON will be safe to embed inside HTML <script> tags.
   190  // For historical reasons, web browsers don't honor standard HTML
   191  // escaping within <script> tags, so an alternative JSON encoding must
   192  // be used.
   193  func HTMLEscape(dst *bytes.Buffer, src []byte) {
   194  	// The characters can only appear in string literals,
   195  	// so just scan the string one byte at a time.
   196  	start := 0
   197  	for i, c := range src {
   198  		if c == '<' || c == '>' || c == '&' {
   199  			if start < i {
   200  				dst.Write(src[start:i])
   201  			}
   202  			dst.WriteString(`\u00`)
   203  			dst.WriteByte(hex[c>>4])
   204  			dst.WriteByte(hex[c&0xF])
   205  			start = i + 1
   206  		}
   207  		// Convert U+2028 and U+2029 (E2 80 A8 and E2 80 A9).
   208  		if c == 0xE2 && i+2 < len(src) && src[i+1] == 0x80 && src[i+2]&^1 == 0xA8 {
   209  			if start < i {
   210  				dst.Write(src[start:i])
   211  			}
   212  			dst.WriteString(`\u202`)
   213  			dst.WriteByte(hex[src[i+2]&0xF])
   214  			start = i + 3
   215  		}
   216  	}
   217  	if start < len(src) {
   218  		dst.Write(src[start:])
   219  	}
   220  }
   221  
   222  // Marshaler is the interface implemented by types that
   223  // can marshal themselves into valid JSON.
   224  type Marshaler interface {
   225  	MarshalJSON() ([]byte, error)
   226  }
   227  
   228  // An UnsupportedTypeError is returned by Marshal when attempting
   229  // to encode an unsupported value type.
   230  type UnsupportedTypeError struct {
   231  	Type reflect.Type
   232  }
   233  
   234  func (e *UnsupportedTypeError) Error() string {
   235  	return "json: unsupported type: " + e.Type.String()
   236  }
   237  
   238  // An UnsupportedValueError is returned by Marshal when attempting
   239  // to encode an unsupported value.
   240  type UnsupportedValueError struct {
   241  	Value reflect.Value
   242  	Str   string
   243  }
   244  
   245  func (e *UnsupportedValueError) Error() string {
   246  	return "json: unsupported value: " + e.Str
   247  }
   248  
   249  // Before Go 1.2, an InvalidUTF8Error was returned by Marshal when
   250  // attempting to encode a string value with invalid UTF-8 sequences.
   251  // As of Go 1.2, Marshal instead coerces the string to valid UTF-8 by
   252  // replacing invalid bytes with the Unicode replacement rune U+FFFD.
   253  //
   254  // Deprecated: No longer used; kept for compatibility.
   255  type InvalidUTF8Error struct {
   256  	S string // the whole string value that caused the error
   257  }
   258  
   259  func (e *InvalidUTF8Error) Error() string {
   260  	return "json: invalid UTF-8 in string: " + strconv.Quote(e.S)
   261  }
   262  
   263  // A MarshalerError represents an error from calling a MarshalJSON or MarshalText method.
   264  type MarshalerError struct {
   265  	Type       reflect.Type
   266  	Err        error
   267  	sourceFunc string
   268  }
   269  
   270  func (e *MarshalerError) Error() string {
   271  	srcFunc := e.sourceFunc
   272  	if srcFunc == "" {
   273  		srcFunc = "MarshalJSON"
   274  	}
   275  	return "json: error calling " + srcFunc +
   276  		" for type " + e.Type.String() +
   277  		": " + e.Err.Error()
   278  }
   279  
   280  // Unwrap returns the underlying error.
   281  func (e *MarshalerError) Unwrap() error { return e.Err }
   282  
   283  var hex = "0123456789abcdef"
   284  
   285  // An encodeState encodes JSON into a bytes.Buffer.
   286  type encodeState struct {
   287  	bytes.Buffer // accumulated output
   288  	scratch      [64]byte
   289  
   290  	// Keep track of what pointers we've seen in the current recursive call
   291  	// path, to avoid cycles that could lead to a stack overflow. Only do
   292  	// the relatively expensive map operations if ptrLevel is larger than
   293  	// startDetectingCyclesAfter, so that we skip the work if we're within a
   294  	// reasonable amount of nested pointers deep.
   295  	ptrLevel uint
   296  	ptrSeen  map[any]struct{}
   297  }
   298  
   299  const startDetectingCyclesAfter = 1000
   300  
   301  var encodeStatePool sync.Pool
   302  
   303  func newEncodeState() *encodeState {
   304  	if v := encodeStatePool.Get(); v != nil {
   305  		e := v.(*encodeState)
   306  		e.Reset()
   307  		if len(e.ptrSeen) > 0 {
   308  			panic("ptrEncoder.encode should have emptied ptrSeen via defers")
   309  		}
   310  		e.ptrLevel = 0
   311  		return e
   312  	}
   313  	return &encodeState{ptrSeen: make(map[any]struct{})}
   314  }
   315  
   316  // jsonError is an error wrapper type for internal use only.
   317  // Panics with errors are wrapped in jsonError so that the top-level recover
   318  // can distinguish intentional panics from this package.
   319  type jsonError struct{ error }
   320  
   321  func (e *encodeState) marshal(v any, opts encOpts) (err error) {
   322  	defer func() {
   323  		if r := recover(); r != nil {
   324  			if je, ok := r.(jsonError); ok {
   325  				err = je.error
   326  			} else {
   327  				panic(r)
   328  			}
   329  		}
   330  	}()
   331  	e.reflectValue(reflect.ValueOf(v), opts)
   332  	return nil
   333  }
   334  
   335  // error aborts the encoding by panicking with err wrapped in jsonError.
   336  func (e *encodeState) error(err error) {
   337  	panic(jsonError{err})
   338  }
   339  
   340  func isEmptyValue(v reflect.Value) bool {
   341  	switch v.Kind() {
   342  	case reflect.Array, reflect.Map, reflect.Slice, reflect.String:
   343  		return v.Len() == 0
   344  	case reflect.Bool:
   345  		return !v.Bool()
   346  	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
   347  		return v.Int() == 0
   348  	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
   349  		return v.Uint() == 0
   350  	case reflect.Float32, reflect.Float64:
   351  		return v.Float() == 0
   352  	case reflect.Interface, reflect.Pointer:
   353  		return v.IsNil()
   354  	}
   355  	return false
   356  }
   357  
   358  func (e *encodeState) reflectValue(v reflect.Value, opts encOpts) {
   359  	valueEncoder(v)(e, v, opts)
   360  }
   361  
   362  type encOpts struct {
   363  	// quoted causes primitive fields to be encoded inside JSON strings.
   364  	quoted bool
   365  	// escapeHTML causes '<', '>', and '&' to be escaped in JSON strings.
   366  	escapeHTML bool
   367  }
   368  
   369  type encoderFunc func(e *encodeState, v reflect.Value, opts encOpts)
   370  
   371  var encoderCache sync.Map // map[reflect.Type]encoderFunc
   372  
   373  func valueEncoder(v reflect.Value) encoderFunc {
   374  	if !v.IsValid() {
   375  		return invalidValueEncoder
   376  	}
   377  	return typeEncoder(v.Type())
   378  }
   379  
   380  func typeEncoder(t reflect.Type) encoderFunc {
   381  	if fi, ok := encoderCache.Load(t); ok {
   382  		return fi.(encoderFunc)
   383  	}
   384  
   385  	// To deal with recursive types, populate the map with an
   386  	// indirect func before we build it. This type waits on the
   387  	// real func (f) to be ready and then calls it. This indirect
   388  	// func is only used for recursive types.
   389  	var (
   390  		wg sync.WaitGroup
   391  		f  encoderFunc
   392  	)
   393  	wg.Add(1)
   394  	fi, loaded := encoderCache.LoadOrStore(t, encoderFunc(func(e *encodeState, v reflect.Value, opts encOpts) {
   395  		wg.Wait()
   396  		f(e, v, opts)
   397  	}))
   398  	if loaded {
   399  		return fi.(encoderFunc)
   400  	}
   401  
   402  	// Compute the real encoder and replace the indirect func with it.
   403  	f = newTypeEncoder(t, true)
   404  	wg.Done()
   405  	encoderCache.Store(t, f)
   406  	return f
   407  }
   408  
   409  var (
   410  	marshalerType     = reflect.TypeOf((*Marshaler)(nil)).Elem()
   411  	textMarshalerType = reflect.TypeOf((*encoding.TextMarshaler)(nil)).Elem()
   412  )
   413  
   414  // newTypeEncoder constructs an encoderFunc for a type.
   415  // The returned encoder only checks CanAddr when allowAddr is true.
   416  func newTypeEncoder(t reflect.Type, allowAddr bool) encoderFunc {
   417  	// If we have a non-pointer value whose type implements
   418  	// Marshaler with a value receiver, then we're better off taking
   419  	// the address of the value - otherwise we end up with an
   420  	// allocation as we cast the value to an interface.
   421  	if t.Kind() != reflect.Pointer && allowAddr && reflect.PointerTo(t).Implements(marshalerType) {
   422  		return newCondAddrEncoder(addrMarshalerEncoder, newTypeEncoder(t, false))
   423  	}
   424  	if t.Implements(marshalerType) {
   425  		return marshalerEncoder
   426  	}
   427  	if t.Kind() != reflect.Pointer && allowAddr && reflect.PointerTo(t).Implements(textMarshalerType) {
   428  		return newCondAddrEncoder(addrTextMarshalerEncoder, newTypeEncoder(t, false))
   429  	}
   430  	if t.Implements(textMarshalerType) {
   431  		return textMarshalerEncoder
   432  	}
   433  
   434  	switch t.Kind() {
   435  	case reflect.Bool:
   436  		return boolEncoder
   437  	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
   438  		return intEncoder
   439  	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
   440  		return uintEncoder
   441  	case reflect.Float32:
   442  		return float32Encoder
   443  	case reflect.Float64:
   444  		return float64Encoder
   445  	case reflect.String:
   446  		return stringEncoder
   447  	case reflect.Interface:
   448  		return interfaceEncoder
   449  	case reflect.Struct:
   450  		return newStructEncoder(t)
   451  	case reflect.Map:
   452  		return newMapEncoder(t)
   453  	case reflect.Slice:
   454  		return newSliceEncoder(t)
   455  	case reflect.Array:
   456  		return newArrayEncoder(t)
   457  	case reflect.Pointer:
   458  		return newPtrEncoder(t)
   459  	default:
   460  		return unsupportedTypeEncoder
   461  	}
   462  }
   463  
   464  func invalidValueEncoder(e *encodeState, v reflect.Value, _ encOpts) {
   465  	e.WriteString("null")
   466  }
   467  
   468  func marshalerEncoder(e *encodeState, v reflect.Value, opts encOpts) {
   469  	if v.Kind() == reflect.Pointer && v.IsNil() {
   470  		e.WriteString("null")
   471  		return
   472  	}
   473  	m, ok := v.Interface().(Marshaler)
   474  	if !ok {
   475  		e.WriteString("null")
   476  		return
   477  	}
   478  	b, err := m.MarshalJSON()
   479  	if err == nil {
   480  		// copy JSON into buffer, checking validity.
   481  		err = compact(&e.Buffer, b, opts.escapeHTML)
   482  	}
   483  	if err != nil {
   484  		e.error(&MarshalerError{v.Type(), err, "MarshalJSON"})
   485  	}
   486  }
   487  
   488  func addrMarshalerEncoder(e *encodeState, v reflect.Value, opts encOpts) {
   489  	va := v.Addr()
   490  	if va.IsNil() {
   491  		e.WriteString("null")
   492  		return
   493  	}
   494  	m := va.Interface().(Marshaler)
   495  	b, err := m.MarshalJSON()
   496  	if err == nil {
   497  		// copy JSON into buffer, checking validity.
   498  		err = compact(&e.Buffer, b, opts.escapeHTML)
   499  	}
   500  	if err != nil {
   501  		e.error(&MarshalerError{v.Type(), err, "MarshalJSON"})
   502  	}
   503  }
   504  
   505  func textMarshalerEncoder(e *encodeState, v reflect.Value, opts encOpts) {
   506  	if v.Kind() == reflect.Pointer && v.IsNil() {
   507  		e.WriteString("null")
   508  		return
   509  	}
   510  	m, ok := v.Interface().(encoding.TextMarshaler)
   511  	if !ok {
   512  		e.WriteString("null")
   513  		return
   514  	}
   515  	b, err := m.MarshalText()
   516  	if err != nil {
   517  		e.error(&MarshalerError{v.Type(), err, "MarshalText"})
   518  	}
   519  	e.stringBytes(b, opts.escapeHTML)
   520  }
   521  
   522  func addrTextMarshalerEncoder(e *encodeState, v reflect.Value, opts encOpts) {
   523  	va := v.Addr()
   524  	if va.IsNil() {
   525  		e.WriteString("null")
   526  		return
   527  	}
   528  	m := va.Interface().(encoding.TextMarshaler)
   529  	b, err := m.MarshalText()
   530  	if err != nil {
   531  		e.error(&MarshalerError{v.Type(), err, "MarshalText"})
   532  	}
   533  	e.stringBytes(b, opts.escapeHTML)
   534  }
   535  
   536  func boolEncoder(e *encodeState, v reflect.Value, opts encOpts) {
   537  	if opts.quoted {
   538  		e.WriteByte('"')
   539  	}
   540  	if v.Bool() {
   541  		e.WriteString("true")
   542  	} else {
   543  		e.WriteString("false")
   544  	}
   545  	if opts.quoted {
   546  		e.WriteByte('"')
   547  	}
   548  }
   549  
   550  func intEncoder(e *encodeState, v reflect.Value, opts encOpts) {
   551  	b := strconv.AppendInt(e.scratch[:0], v.Int(), 10)
   552  	if opts.quoted {
   553  		e.WriteByte('"')
   554  	}
   555  	e.Write(b)
   556  	if opts.quoted {
   557  		e.WriteByte('"')
   558  	}
   559  }
   560  
   561  func uintEncoder(e *encodeState, v reflect.Value, opts encOpts) {
   562  	b := strconv.AppendUint(e.scratch[:0], v.Uint(), 10)
   563  	if opts.quoted {
   564  		e.WriteByte('"')
   565  	}
   566  	e.Write(b)
   567  	if opts.quoted {
   568  		e.WriteByte('"')
   569  	}
   570  }
   571  
   572  type floatEncoder int // number of bits
   573  
   574  func (bits floatEncoder) encode(e *encodeState, v reflect.Value, opts encOpts) {
   575  	f := v.Float()
   576  	if math.IsInf(f, 0) || math.IsNaN(f) {
   577  		e.error(&UnsupportedValueError{v, strconv.FormatFloat(f, 'g', -1, int(bits))})
   578  	}
   579  
   580  	// Convert as if by ES6 number to string conversion.
   581  	// This matches most other JSON generators.
   582  	// See golang.org/issue/6384 and golang.org/issue/14135.
   583  	// Like fmt %g, but the exponent cutoffs are different
   584  	// and exponents themselves are not padded to two digits.
   585  	b := e.scratch[:0]
   586  	abs := math.Abs(f)
   587  	fmt := byte('f')
   588  	// Note: Must use float32 comparisons for underlying float32 value to get precise cutoffs right.
   589  	if abs != 0 {
   590  		if bits == 64 && (abs < 1e-6 || abs >= 1e21) || bits == 32 && (float32(abs) < 1e-6 || float32(abs) >= 1e21) {
   591  			fmt = 'e'
   592  		}
   593  	}
   594  	b = strconv.AppendFloat(b, f, fmt, -1, int(bits))
   595  	if fmt == 'e' {
   596  		// clean up e-09 to e-9
   597  		n := len(b)
   598  		if n >= 4 && b[n-4] == 'e' && b[n-3] == '-' && b[n-2] == '0' {
   599  			b[n-2] = b[n-1]
   600  			b = b[:n-1]
   601  		}
   602  	}
   603  
   604  	if opts.quoted {
   605  		e.WriteByte('"')
   606  	}
   607  	e.Write(b)
   608  	if opts.quoted {
   609  		e.WriteByte('"')
   610  	}
   611  }
   612  
   613  var (
   614  	float32Encoder = (floatEncoder(32)).encode
   615  	float64Encoder = (floatEncoder(64)).encode
   616  )
   617  
   618  func stringEncoder(e *encodeState, v reflect.Value, opts encOpts) {
   619  	if v.Type() == numberType {
   620  		numStr := v.String()
   621  		// In Go1.5 the empty string encodes to "0", while this is not a valid number literal
   622  		// we keep compatibility so check validity after this.
   623  		if numStr == "" {
   624  			numStr = "0" // Number's zero-val
   625  		}
   626  		if !isValidNumber(numStr) {
   627  			e.error(fmt.Errorf("json: invalid number literal %q", numStr))
   628  		}
   629  		if opts.quoted {
   630  			e.WriteByte('"')
   631  		}
   632  		e.WriteString(numStr)
   633  		if opts.quoted {
   634  			e.WriteByte('"')
   635  		}
   636  		return
   637  	}
   638  	if opts.quoted {
   639  		e2 := newEncodeState()
   640  		// Since we encode the string twice, we only need to escape HTML
   641  		// the first time.
   642  		e2.string(v.String(), opts.escapeHTML)
   643  		e.stringBytes(e2.Bytes(), false)
   644  		encodeStatePool.Put(e2)
   645  	} else {
   646  		e.string(v.String(), opts.escapeHTML)
   647  	}
   648  }
   649  
   650  // isValidNumber reports whether s is a valid JSON number literal.
   651  func isValidNumber(s string) bool {
   652  	// This function implements the JSON numbers grammar.
   653  	// See https://tools.ietf.org/html/rfc7159#section-6
   654  	// and https://www.json.org/img/number.png
   655  
   656  	if s == "" {
   657  		return false
   658  	}
   659  
   660  	// Optional -
   661  	if s[0] == '-' {
   662  		s = s[1:]
   663  		if s == "" {
   664  			return false
   665  		}
   666  	}
   667  
   668  	// Digits
   669  	switch {
   670  	default:
   671  		return false
   672  
   673  	case s[0] == '0':
   674  		s = s[1:]
   675  
   676  	case '1' <= s[0] && s[0] <= '9':
   677  		s = s[1:]
   678  		for len(s) > 0 && '0' <= s[0] && s[0] <= '9' {
   679  			s = s[1:]
   680  		}
   681  	}
   682  
   683  	// . followed by 1 or more digits.
   684  	if len(s) >= 2 && s[0] == '.' && '0' <= s[1] && s[1] <= '9' {
   685  		s = s[2:]
   686  		for len(s) > 0 && '0' <= s[0] && s[0] <= '9' {
   687  			s = s[1:]
   688  		}
   689  	}
   690  
   691  	// e or E followed by an optional - or + and
   692  	// 1 or more digits.
   693  	if len(s) >= 2 && (s[0] == 'e' || s[0] == 'E') {
   694  		s = s[1:]
   695  		if s[0] == '+' || s[0] == '-' {
   696  			s = s[1:]
   697  			if s == "" {
   698  				return false
   699  			}
   700  		}
   701  		for len(s) > 0 && '0' <= s[0] && s[0] <= '9' {
   702  			s = s[1:]
   703  		}
   704  	}
   705  
   706  	// Make sure we are at the end.
   707  	return s == ""
   708  }
   709  
   710  func interfaceEncoder(e *encodeState, v reflect.Value, opts encOpts) {
   711  	if v.IsNil() {
   712  		e.WriteString("null")
   713  		return
   714  	}
   715  	e.reflectValue(v.Elem(), opts)
   716  }
   717  
   718  func unsupportedTypeEncoder(e *encodeState, v reflect.Value, _ encOpts) {
   719  	e.error(&UnsupportedTypeError{v.Type()})
   720  }
   721  
   722  type structEncoder struct {
   723  	fields structFields
   724  }
   725  
   726  type structFields struct {
   727  	list      []field
   728  	nameIndex map[string]int
   729  }
   730  
   731  func (se structEncoder) encode(e *encodeState, v reflect.Value, opts encOpts) {
   732  	next := byte('{')
   733  FieldLoop:
   734  	for i := range se.fields.list {
   735  		f := &se.fields.list[i]
   736  
   737  		// Find the nested struct field by following f.index.
   738  		fv := v
   739  		for _, i := range f.index {
   740  			if fv.Kind() == reflect.Pointer {
   741  				if fv.IsNil() {
   742  					continue FieldLoop
   743  				}
   744  				fv = fv.Elem()
   745  			}
   746  			fv = fv.Field(i)
   747  		}
   748  
   749  		if f.omitEmpty && isEmptyValue(fv) {
   750  			continue
   751  		}
   752  		e.WriteByte(next)
   753  		next = ','
   754  		if opts.escapeHTML {
   755  			e.WriteString(f.nameEscHTML)
   756  		} else {
   757  			e.WriteString(f.nameNonEsc)
   758  		}
   759  		opts.quoted = f.quoted
   760  		f.encoder(e, fv, opts)
   761  	}
   762  	if next == '{' {
   763  		e.WriteString("{}")
   764  	} else {
   765  		e.WriteByte('}')
   766  	}
   767  }
   768  
   769  func newStructEncoder(t reflect.Type) encoderFunc {
   770  	se := structEncoder{fields: cachedTypeFields(t)}
   771  	return se.encode
   772  }
   773  
   774  type mapEncoder struct {
   775  	elemEnc encoderFunc
   776  }
   777  
   778  func (me mapEncoder) encode(e *encodeState, v reflect.Value, opts encOpts) {
   779  	if v.IsNil() {
   780  		e.WriteString("null")
   781  		return
   782  	}
   783  	if e.ptrLevel++; e.ptrLevel > startDetectingCyclesAfter {
   784  		// We're a large number of nested ptrEncoder.encode calls deep;
   785  		// start checking if we've run into a pointer cycle.
   786  		ptr := v.UnsafePointer()
   787  		if _, ok := e.ptrSeen[ptr]; ok {
   788  			e.error(&UnsupportedValueError{v, fmt.Sprintf("encountered a cycle via %s", v.Type())})
   789  		}
   790  		e.ptrSeen[ptr] = struct{}{}
   791  		defer delete(e.ptrSeen, ptr)
   792  	}
   793  	e.WriteByte('{')
   794  
   795  	// Extract and sort the keys.
   796  	sv := make([]reflectWithString, v.Len())
   797  	mi := v.MapRange()
   798  	for i := 0; mi.Next(); i++ {
   799  		sv[i].k = mi.Key()
   800  		sv[i].v = mi.Value()
   801  		if err := sv[i].resolve(); err != nil {
   802  			e.error(fmt.Errorf("json: encoding error for type %q: %q", v.Type().String(), err.Error()))
   803  		}
   804  	}
   805  	sort.Slice(sv, func(i, j int) bool { return sv[i].ks < sv[j].ks })
   806  
   807  	for i, kv := range sv {
   808  		if i > 0 {
   809  			e.WriteByte(',')
   810  		}
   811  		e.string(kv.ks, opts.escapeHTML)
   812  		e.WriteByte(':')
   813  		me.elemEnc(e, kv.v, opts)
   814  	}
   815  	e.WriteByte('}')
   816  	e.ptrLevel--
   817  }
   818  
   819  func newMapEncoder(t reflect.Type) encoderFunc {
   820  	switch t.Key().Kind() {
   821  	case reflect.String,
   822  		reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64,
   823  		reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
   824  	default:
   825  		if !t.Key().Implements(textMarshalerType) {
   826  			return unsupportedTypeEncoder
   827  		}
   828  	}
   829  	me := mapEncoder{typeEncoder(t.Elem())}
   830  	return me.encode
   831  }
   832  
   833  func encodeByteSlice(e *encodeState, v reflect.Value, _ encOpts) {
   834  	if v.IsNil() {
   835  		e.WriteString("null")
   836  		return
   837  	}
   838  	s := v.Bytes()
   839  	e.WriteByte('"')
   840  	encodedLen := base64.StdEncoding.EncodedLen(len(s))
   841  	if encodedLen <= len(e.scratch) {
   842  		// If the encoded bytes fit in e.scratch, avoid an extra
   843  		// allocation and use the cheaper Encoding.Encode.
   844  		dst := e.scratch[:encodedLen]
   845  		base64.StdEncoding.Encode(dst, s)
   846  		e.Write(dst)
   847  	} else if encodedLen <= 1024 {
   848  		// The encoded bytes are short enough to allocate for, and
   849  		// Encoding.Encode is still cheaper.
   850  		dst := make([]byte, encodedLen)
   851  		base64.StdEncoding.Encode(dst, s)
   852  		e.Write(dst)
   853  	} else {
   854  		// The encoded bytes are too long to cheaply allocate, and
   855  		// Encoding.Encode is no longer noticeably cheaper.
   856  		enc := base64.NewEncoder(base64.StdEncoding, e)
   857  		enc.Write(s)
   858  		enc.Close()
   859  	}
   860  	e.WriteByte('"')
   861  }
   862  
   863  // sliceEncoder just wraps an arrayEncoder, checking to make sure the value isn't nil.
   864  type sliceEncoder struct {
   865  	arrayEnc encoderFunc
   866  }
   867  
   868  func (se sliceEncoder) encode(e *encodeState, v reflect.Value, opts encOpts) {
   869  	if v.IsNil() {
   870  		e.WriteString("null")
   871  		return
   872  	}
   873  	if e.ptrLevel++; e.ptrLevel > startDetectingCyclesAfter {
   874  		// We're a large number of nested ptrEncoder.encode calls deep;
   875  		// start checking if we've run into a pointer cycle.
   876  		// Here we use a struct to memorize the pointer to the first element of the slice
   877  		// and its length.
   878  		ptr := struct {
   879  			ptr interface{} // always an unsafe.Pointer, but avoids a dependency on package unsafe
   880  			len int
   881  		}{v.UnsafePointer(), v.Len()}
   882  		if _, ok := e.ptrSeen[ptr]; ok {
   883  			e.error(&UnsupportedValueError{v, fmt.Sprintf("encountered a cycle via %s", v.Type())})
   884  		}
   885  		e.ptrSeen[ptr] = struct{}{}
   886  		defer delete(e.ptrSeen, ptr)
   887  	}
   888  	se.arrayEnc(e, v, opts)
   889  	e.ptrLevel--
   890  }
   891  
   892  func newSliceEncoder(t reflect.Type) encoderFunc {
   893  	// Byte slices get special treatment; arrays don't.
   894  	if t.Elem().Kind() == reflect.Uint8 {
   895  		p := reflect.PointerTo(t.Elem())
   896  		if !p.Implements(marshalerType) && !p.Implements(textMarshalerType) {
   897  			return encodeByteSlice
   898  		}
   899  	}
   900  	enc := sliceEncoder{newArrayEncoder(t)}
   901  	return enc.encode
   902  }
   903  
   904  type arrayEncoder struct {
   905  	elemEnc encoderFunc
   906  }
   907  
   908  func (ae arrayEncoder) encode(e *encodeState, v reflect.Value, opts encOpts) {
   909  	e.WriteByte('[')
   910  	n := v.Len()
   911  	for i := 0; i < n; i++ {
   912  		if i > 0 {
   913  			e.WriteByte(',')
   914  		}
   915  		ae.elemEnc(e, v.Index(i), opts)
   916  	}
   917  	e.WriteByte(']')
   918  }
   919  
   920  func newArrayEncoder(t reflect.Type) encoderFunc {
   921  	enc := arrayEncoder{typeEncoder(t.Elem())}
   922  	return enc.encode
   923  }
   924  
   925  type ptrEncoder struct {
   926  	elemEnc encoderFunc
   927  }
   928  
   929  func (pe ptrEncoder) encode(e *encodeState, v reflect.Value, opts encOpts) {
   930  	if v.IsNil() {
   931  		e.WriteString("null")
   932  		return
   933  	}
   934  	if e.ptrLevel++; e.ptrLevel > startDetectingCyclesAfter {
   935  		// We're a large number of nested ptrEncoder.encode calls deep;
   936  		// start checking if we've run into a pointer cycle.
   937  		ptr := v.Interface()
   938  		if _, ok := e.ptrSeen[ptr]; ok {
   939  			e.error(&UnsupportedValueError{v, fmt.Sprintf("encountered a cycle via %s", v.Type())})
   940  		}
   941  		e.ptrSeen[ptr] = struct{}{}
   942  		defer delete(e.ptrSeen, ptr)
   943  	}
   944  	pe.elemEnc(e, v.Elem(), opts)
   945  	e.ptrLevel--
   946  }
   947  
   948  func newPtrEncoder(t reflect.Type) encoderFunc {
   949  	enc := ptrEncoder{typeEncoder(t.Elem())}
   950  	return enc.encode
   951  }
   952  
   953  type condAddrEncoder struct {
   954  	canAddrEnc, elseEnc encoderFunc
   955  }
   956  
   957  func (ce condAddrEncoder) encode(e *encodeState, v reflect.Value, opts encOpts) {
   958  	if v.CanAddr() {
   959  		ce.canAddrEnc(e, v, opts)
   960  	} else {
   961  		ce.elseEnc(e, v, opts)
   962  	}
   963  }
   964  
   965  // newCondAddrEncoder returns an encoder that checks whether its value
   966  // CanAddr and delegates to canAddrEnc if so, else to elseEnc.
   967  func newCondAddrEncoder(canAddrEnc, elseEnc encoderFunc) encoderFunc {
   968  	enc := condAddrEncoder{canAddrEnc: canAddrEnc, elseEnc: elseEnc}
   969  	return enc.encode
   970  }
   971  
   972  func isValidTag(s string) bool {
   973  	if s == "" {
   974  		return false
   975  	}
   976  	for _, c := range s {
   977  		switch {
   978  		case strings.ContainsRune("!#$%&()*+-./:;<=>?@[]^_{|}~ ", c):
   979  			// Backslash and quote chars are reserved, but
   980  			// otherwise any punctuation chars are allowed
   981  			// in a tag name.
   982  		case !unicode.IsLetter(c) && !unicode.IsDigit(c):
   983  			return false
   984  		}
   985  	}
   986  	return true
   987  }
   988  
   989  func typeByIndex(t reflect.Type, index []int) reflect.Type {
   990  	for _, i := range index {
   991  		if t.Kind() == reflect.Pointer {
   992  			t = t.Elem()
   993  		}
   994  		t = t.Field(i).Type
   995  	}
   996  	return t
   997  }
   998  
   999  type reflectWithString struct {
  1000  	k  reflect.Value
  1001  	v  reflect.Value
  1002  	ks string
  1003  }
  1004  
  1005  func (w *reflectWithString) resolve() error {
  1006  	if w.k.Kind() == reflect.String {
  1007  		w.ks = w.k.String()
  1008  		return nil
  1009  	}
  1010  	if tm, ok := w.k.Interface().(encoding.TextMarshaler); ok {
  1011  		if w.k.Kind() == reflect.Pointer && w.k.IsNil() {
  1012  			return nil
  1013  		}
  1014  		buf, err := tm.MarshalText()
  1015  		w.ks = string(buf)
  1016  		return err
  1017  	}
  1018  	switch w.k.Kind() {
  1019  	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
  1020  		w.ks = strconv.FormatInt(w.k.Int(), 10)
  1021  		return nil
  1022  	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
  1023  		w.ks = strconv.FormatUint(w.k.Uint(), 10)
  1024  		return nil
  1025  	}
  1026  	panic("unexpected map key type")
  1027  }
  1028  
  1029  // NOTE: keep in sync with stringBytes below.
  1030  func (e *encodeState) string(s string, escapeHTML bool) {
  1031  	e.WriteByte('"')
  1032  	start := 0
  1033  	for i := 0; i < len(s); {
  1034  		if b := s[i]; b < utf8.RuneSelf {
  1035  			if htmlSafeSet[b] || (!escapeHTML && safeSet[b]) {
  1036  				i++
  1037  				continue
  1038  			}
  1039  			if start < i {
  1040  				e.WriteString(s[start:i])
  1041  			}
  1042  			e.WriteByte('\\')
  1043  			switch b {
  1044  			case '\\', '"':
  1045  				e.WriteByte(b)
  1046  			case '\n':
  1047  				e.WriteByte('n')
  1048  			case '\r':
  1049  				e.WriteByte('r')
  1050  			case '\t':
  1051  				e.WriteByte('t')
  1052  			default:
  1053  				// This encodes bytes < 0x20 except for \t, \n and \r.
  1054  				// If escapeHTML is set, it also escapes <, >, and &
  1055  				// because they can lead to security holes when
  1056  				// user-controlled strings are rendered into JSON
  1057  				// and served to some browsers.
  1058  				e.WriteString(`u00`)
  1059  				e.WriteByte(hex[b>>4])
  1060  				e.WriteByte(hex[b&0xF])
  1061  			}
  1062  			i++
  1063  			start = i
  1064  			continue
  1065  		}
  1066  		c, size := utf8.DecodeRuneInString(s[i:])
  1067  		if c == utf8.RuneError && size == 1 {
  1068  			if start < i {
  1069  				e.WriteString(s[start:i])
  1070  			}
  1071  			e.WriteString(`\ufffd`)
  1072  			i += size
  1073  			start = i
  1074  			continue
  1075  		}
  1076  		// U+2028 is LINE SEPARATOR.
  1077  		// U+2029 is PARAGRAPH SEPARATOR.
  1078  		// They are both technically valid characters in JSON strings,
  1079  		// but don't work in JSONP, which has to be evaluated as JavaScript,
  1080  		// and can lead to security holes there. It is valid JSON to
  1081  		// escape them, so we do so unconditionally.
  1082  		// See http://timelessrepo.com/json-isnt-a-javascript-subset for discussion.
  1083  		if c == '\u2028' || c == '\u2029' {
  1084  			if start < i {
  1085  				e.WriteString(s[start:i])
  1086  			}
  1087  			e.WriteString(`\u202`)
  1088  			e.WriteByte(hex[c&0xF])
  1089  			i += size
  1090  			start = i
  1091  			continue
  1092  		}
  1093  		i += size
  1094  	}
  1095  	if start < len(s) {
  1096  		e.WriteString(s[start:])
  1097  	}
  1098  	e.WriteByte('"')
  1099  }
  1100  
  1101  // NOTE: keep in sync with string above.
  1102  func (e *encodeState) stringBytes(s []byte, escapeHTML bool) {
  1103  	e.WriteByte('"')
  1104  	start := 0
  1105  	for i := 0; i < len(s); {
  1106  		if b := s[i]; b < utf8.RuneSelf {
  1107  			if htmlSafeSet[b] || (!escapeHTML && safeSet[b]) {
  1108  				i++
  1109  				continue
  1110  			}
  1111  			if start < i {
  1112  				e.Write(s[start:i])
  1113  			}
  1114  			e.WriteByte('\\')
  1115  			switch b {
  1116  			case '\\', '"':
  1117  				e.WriteByte(b)
  1118  			case '\n':
  1119  				e.WriteByte('n')
  1120  			case '\r':
  1121  				e.WriteByte('r')
  1122  			case '\t':
  1123  				e.WriteByte('t')
  1124  			default:
  1125  				// This encodes bytes < 0x20 except for \t, \n and \r.
  1126  				// If escapeHTML is set, it also escapes <, >, and &
  1127  				// because they can lead to security holes when
  1128  				// user-controlled strings are rendered into JSON
  1129  				// and served to some browsers.
  1130  				e.WriteString(`u00`)
  1131  				e.WriteByte(hex[b>>4])
  1132  				e.WriteByte(hex[b&0xF])
  1133  			}
  1134  			i++
  1135  			start = i
  1136  			continue
  1137  		}
  1138  		c, size := utf8.DecodeRune(s[i:])
  1139  		if c == utf8.RuneError && size == 1 {
  1140  			if start < i {
  1141  				e.Write(s[start:i])
  1142  			}
  1143  			e.WriteString(`\ufffd`)
  1144  			i += size
  1145  			start = i
  1146  			continue
  1147  		}
  1148  		// U+2028 is LINE SEPARATOR.
  1149  		// U+2029 is PARAGRAPH SEPARATOR.
  1150  		// They are both technically valid characters in JSON strings,
  1151  		// but don't work in JSONP, which has to be evaluated as JavaScript,
  1152  		// and can lead to security holes there. It is valid JSON to
  1153  		// escape them, so we do so unconditionally.
  1154  		// See http://timelessrepo.com/json-isnt-a-javascript-subset for discussion.
  1155  		if c == '\u2028' || c == '\u2029' {
  1156  			if start < i {
  1157  				e.Write(s[start:i])
  1158  			}
  1159  			e.WriteString(`\u202`)
  1160  			e.WriteByte(hex[c&0xF])
  1161  			i += size
  1162  			start = i
  1163  			continue
  1164  		}
  1165  		i += size
  1166  	}
  1167  	if start < len(s) {
  1168  		e.Write(s[start:])
  1169  	}
  1170  	e.WriteByte('"')
  1171  }
  1172  
  1173  // A field represents a single field found in a struct.
  1174  type field struct {
  1175  	name      string
  1176  	nameBytes []byte                 // []byte(name)
  1177  	equalFold func(s, t []byte) bool // bytes.EqualFold or equivalent
  1178  
  1179  	nameNonEsc  string // `"` + name + `":`
  1180  	nameEscHTML string // `"` + HTMLEscape(name) + `":`
  1181  
  1182  	tag       bool
  1183  	index     []int
  1184  	typ       reflect.Type
  1185  	omitEmpty bool
  1186  	quoted    bool
  1187  
  1188  	encoder encoderFunc
  1189  }
  1190  
  1191  // byIndex sorts field by index sequence.
  1192  type byIndex []field
  1193  
  1194  func (x byIndex) Len() int { return len(x) }
  1195  
  1196  func (x byIndex) Swap(i, j int) { x[i], x[j] = x[j], x[i] }
  1197  
  1198  func (x byIndex) Less(i, j int) bool {
  1199  	for k, xik := range x[i].index {
  1200  		if k >= len(x[j].index) {
  1201  			return false
  1202  		}
  1203  		if xik != x[j].index[k] {
  1204  			return xik < x[j].index[k]
  1205  		}
  1206  	}
  1207  	return len(x[i].index) < len(x[j].index)
  1208  }
  1209  
  1210  // typeFields returns a list of fields that JSON should recognize for the given type.
  1211  // The algorithm is breadth-first search over the set of structs to include - the top struct
  1212  // and then any reachable anonymous structs.
  1213  func typeFields(t reflect.Type) structFields {
  1214  	// Anonymous fields to explore at the current level and the next.
  1215  	current := []field{}
  1216  	next := []field{{typ: t}}
  1217  
  1218  	// Count of queued names for current level and the next.
  1219  	var count, nextCount map[reflect.Type]int
  1220  
  1221  	// Types already visited at an earlier level.
  1222  	visited := map[reflect.Type]bool{}
  1223  
  1224  	// Fields found.
  1225  	var fields []field
  1226  
  1227  	// Buffer to run HTMLEscape on field names.
  1228  	var nameEscBuf bytes.Buffer
  1229  
  1230  	for len(next) > 0 {
  1231  		current, next = next, current[:0]
  1232  		count, nextCount = nextCount, map[reflect.Type]int{}
  1233  
  1234  		for _, f := range current {
  1235  			if visited[f.typ] {
  1236  				continue
  1237  			}
  1238  			visited[f.typ] = true
  1239  
  1240  			// Scan f.typ for fields to include.
  1241  			for i := 0; i < f.typ.NumField(); i++ {
  1242  				sf := f.typ.Field(i)
  1243  				if sf.Anonymous {
  1244  					t := sf.Type
  1245  					if t.Kind() == reflect.Pointer {
  1246  						t = t.Elem()
  1247  					}
  1248  					if !sf.IsExported() && t.Kind() != reflect.Struct {
  1249  						// Ignore embedded fields of unexported non-struct types.
  1250  						continue
  1251  					}
  1252  					// Do not ignore embedded fields of unexported struct types
  1253  					// since they may have exported fields.
  1254  				} else if !sf.IsExported() {
  1255  					// Ignore unexported non-embedded fields.
  1256  					continue
  1257  				}
  1258  				tag := sf.Tag.Get("json")
  1259  				if tag == "-" {
  1260  					continue
  1261  				}
  1262  				name, opts := parseTag(tag)
  1263  				if !isValidTag(name) {
  1264  					name = ""
  1265  				}
  1266  				index := make([]int, len(f.index)+1)
  1267  				copy(index, f.index)
  1268  				index[len(f.index)] = i
  1269  
  1270  				ft := sf.Type
  1271  				if ft.Name() == "" && ft.Kind() == reflect.Pointer {
  1272  					// Follow pointer.
  1273  					ft = ft.Elem()
  1274  				}
  1275  
  1276  				// Only strings, floats, integers, and booleans can be quoted.
  1277  				quoted := false
  1278  				if opts.Contains("string") {
  1279  					switch ft.Kind() {
  1280  					case reflect.Bool,
  1281  						reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64,
  1282  						reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr,
  1283  						reflect.Float32, reflect.Float64,
  1284  						reflect.String:
  1285  						quoted = true
  1286  					}
  1287  				}
  1288  
  1289  				// Record found field and index sequence.
  1290  				if name != "" || !sf.Anonymous || ft.Kind() != reflect.Struct {
  1291  					tagged := name != ""
  1292  					if name == "" {
  1293  						name = sf.Name
  1294  					}
  1295  					field := field{
  1296  						name:      name,
  1297  						tag:       tagged,
  1298  						index:     index,
  1299  						typ:       ft,
  1300  						omitEmpty: opts.Contains("omitempty"),
  1301  						quoted:    quoted,
  1302  					}
  1303  					field.nameBytes = []byte(field.name)
  1304  					field.equalFold = foldFunc(field.nameBytes)
  1305  
  1306  					// Build nameEscHTML and nameNonEsc ahead of time.
  1307  					nameEscBuf.Reset()
  1308  					nameEscBuf.WriteString(`"`)
  1309  					HTMLEscape(&nameEscBuf, field.nameBytes)
  1310  					nameEscBuf.WriteString(`":`)
  1311  					field.nameEscHTML = nameEscBuf.String()
  1312  					field.nameNonEsc = `"` + field.name + `":`
  1313  
  1314  					fields = append(fields, field)
  1315  					if count[f.typ] > 1 {
  1316  						// If there were multiple instances, add a second,
  1317  						// so that the annihilation code will see a duplicate.
  1318  						// It only cares about the distinction between 1 or 2,
  1319  						// so don't bother generating any more copies.
  1320  						fields = append(fields, fields[len(fields)-1])
  1321  					}
  1322  					continue
  1323  				}
  1324  
  1325  				// Record new anonymous struct to explore in next round.
  1326  				nextCount[ft]++
  1327  				if nextCount[ft] == 1 {
  1328  					next = append(next, field{name: ft.Name(), index: index, typ: ft})
  1329  				}
  1330  			}
  1331  		}
  1332  	}
  1333  
  1334  	sort.Slice(fields, func(i, j int) bool {
  1335  		x := fields
  1336  		// sort field by name, breaking ties with depth, then
  1337  		// breaking ties with "name came from json tag", then
  1338  		// breaking ties with index sequence.
  1339  		if x[i].name != x[j].name {
  1340  			return x[i].name < x[j].name
  1341  		}
  1342  		if len(x[i].index) != len(x[j].index) {
  1343  			return len(x[i].index) < len(x[j].index)
  1344  		}
  1345  		if x[i].tag != x[j].tag {
  1346  			return x[i].tag
  1347  		}
  1348  		return byIndex(x).Less(i, j)
  1349  	})
  1350  
  1351  	// Delete all fields that are hidden by the Go rules for embedded fields,
  1352  	// except that fields with JSON tags are promoted.
  1353  
  1354  	// The fields are sorted in primary order of name, secondary order
  1355  	// of field index length. Loop over names; for each name, delete
  1356  	// hidden fields by choosing the one dominant field that survives.
  1357  	out := fields[:0]
  1358  	for advance, i := 0, 0; i < len(fields); i += advance {
  1359  		// One iteration per name.
  1360  		// Find the sequence of fields with the name of this first field.
  1361  		fi := fields[i]
  1362  		name := fi.name
  1363  		for advance = 1; i+advance < len(fields); advance++ {
  1364  			fj := fields[i+advance]
  1365  			if fj.name != name {
  1366  				break
  1367  			}
  1368  		}
  1369  		if advance == 1 { // Only one field with this name
  1370  			out = append(out, fi)
  1371  			continue
  1372  		}
  1373  		dominant, ok := dominantField(fields[i : i+advance])
  1374  		if ok {
  1375  			out = append(out, dominant)
  1376  		}
  1377  	}
  1378  
  1379  	fields = out
  1380  	sort.Sort(byIndex(fields))
  1381  
  1382  	for i := range fields {
  1383  		f := &fields[i]
  1384  		f.encoder = typeEncoder(typeByIndex(t, f.index))
  1385  	}
  1386  	nameIndex := make(map[string]int, len(fields))
  1387  	for i, field := range fields {
  1388  		nameIndex[field.name] = i
  1389  	}
  1390  	return structFields{fields, nameIndex}
  1391  }
  1392  
  1393  // dominantField looks through the fields, all of which are known to
  1394  // have the same name, to find the single field that dominates the
  1395  // others using Go's embedding rules, modified by the presence of
  1396  // JSON tags. If there are multiple top-level fields, the boolean
  1397  // will be false: This condition is an error in Go and we skip all
  1398  // the fields.
  1399  func dominantField(fields []field) (field, bool) {
  1400  	// The fields are sorted in increasing index-length order, then by presence of tag.
  1401  	// That means that the first field is the dominant one. We need only check
  1402  	// for error cases: two fields at top level, either both tagged or neither tagged.
  1403  	if len(fields) > 1 && len(fields[0].index) == len(fields[1].index) && fields[0].tag == fields[1].tag {
  1404  		return field{}, false
  1405  	}
  1406  	return fields[0], true
  1407  }
  1408  
  1409  var fieldCache sync.Map // map[reflect.Type]structFields
  1410  
  1411  // cachedTypeFields is like typeFields but uses a cache to avoid repeated work.
  1412  func cachedTypeFields(t reflect.Type) structFields {
  1413  	if f, ok := fieldCache.Load(t); ok {
  1414  		return f.(structFields)
  1415  	}
  1416  	f, _ := fieldCache.LoadOrStore(t, typeFields(t))
  1417  	return f.(structFields)
  1418  }
  1419  

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