Source file src/encoding/json/encode.go

Documentation: encoding/json

     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  	"slices"
    21  	"sort"
    22  	"strconv"
    23  	"strings"
    24  	"sync"
    25  	"unicode"
    26  	"unicode/utf8"
    27  )
    28  
    29  // Marshal returns the JSON encoding of v.
    30  //
    31  // Marshal traverses the value v recursively.
    32  // If an encountered value implements [Marshaler]
    33  // and is not a nil pointer, Marshal calls [Marshaler.MarshalJSON]
    34  // to produce JSON. If no [Marshaler.MarshalJSON] method is present but the
    35  // value implements [encoding.TextMarshaler] instead, Marshal calls
    36  // [encoding.TextMarshaler.MarshalText] and encodes the result as a JSON string.
    37  // The nil pointer exception is not strictly necessary
    38  // but mimics a similar, necessary exception in the behavior of
    39  // [Unmarshaler.UnmarshalJSON].
    40  //
    41  // Otherwise, Marshal uses the following type-dependent default encodings:
    42  //
    43  // Boolean values encode as JSON booleans.
    44  //
    45  // Floating point, integer, and [Number] values encode as JSON numbers.
    46  // NaN and +/-Inf values will return an [UnsupportedValueError].
    47  //
    48  // String values encode as JSON strings coerced to valid UTF-8,
    49  // replacing invalid bytes with the Unicode replacement rune.
    50  // So that the JSON will be safe to embed inside HTML <script> tags,
    51  // the string is encoded using [HTMLEscape],
    52  // which replaces "<", ">", "&", U+2028, and U+2029 are escaped
    53  // to "\u003c","\u003e", "\u0026", "\u2028", and "\u2029".
    54  // This replacement can be disabled when using an [Encoder],
    55  // by calling [Encoder.SetEscapeHTML](false).
    56  //
    57  // Array and slice values encode as JSON arrays, except that
    58  // []byte encodes as a base64-encoded string, and a nil slice
    59  // encodes as the null JSON value.
    60  //
    61  // Struct values encode as JSON objects.
    62  // Each exported struct field becomes a member of the object, using the
    63  // field name as the object key, unless the field is omitted for one of the
    64  // reasons given below.
    65  //
    66  // The encoding of each struct field can be customized by the format string
    67  // stored under the "json" key in the struct field's tag.
    68  // The format string gives the name of the field, possibly followed by a
    69  // comma-separated list of options. The name may be empty in order to
    70  // specify options without overriding the default field name.
    71  //
    72  // The "omitempty" option specifies that the field should be omitted
    73  // from the encoding if the field has an empty value, defined as
    74  // false, 0, a nil pointer, a nil interface value, and any empty array,
    75  // slice, map, or string.
    76  //
    77  // As a special case, if the field tag is "-", the field is always omitted.
    78  // Note that a field with name "-" can still be generated using the tag "-,".
    79  //
    80  // Examples of struct field tags and their meanings:
    81  //
    82  //	// Field appears in JSON as key "myName".
    83  //	Field int `json:"myName"`
    84  //
    85  //	// Field appears in JSON as key "myName" and
    86  //	// the field is omitted from the object if its value is empty,
    87  //	// as defined above.
    88  //	Field int `json:"myName,omitempty"`
    89  //
    90  //	// Field appears in JSON as key "Field" (the default), but
    91  //	// the field is skipped if empty.
    92  //	// Note the leading comma.
    93  //	Field int `json:",omitempty"`
    94  //
    95  //	// Field is ignored by this package.
    96  //	Field int `json:"-"`
    97  //
    98  //	// Field appears in JSON as key "-".
    99  //	Field int `json:"-,"`
   100  //
   101  // The "string" option signals that a field is stored as JSON inside a
   102  // JSON-encoded string. It applies only to fields of string, floating point,
   103  // integer, or boolean types. This extra level of encoding is sometimes used
   104  // when communicating with JavaScript programs:
   105  //
   106  //	Int64String int64 `json:",string"`
   107  //
   108  // The key name will be used if it's a non-empty string consisting of
   109  // only Unicode letters, digits, and ASCII punctuation except quotation
   110  // marks, backslash, and comma.
   111  //
   112  // Embedded struct fields are usually marshaled as if their inner exported fields
   113  // were fields in the outer struct, subject to the usual Go visibility rules amended
   114  // as described in the next paragraph.
   115  // An anonymous struct field with a name given in its JSON tag is treated as
   116  // having that name, rather than being anonymous.
   117  // An anonymous struct field of interface type is treated the same as having
   118  // that type as its name, rather than being anonymous.
   119  //
   120  // The Go visibility rules for struct fields are amended for JSON when
   121  // deciding which field to marshal or unmarshal. If there are
   122  // multiple fields at the same level, and that level is the least
   123  // nested (and would therefore be the nesting level selected by the
   124  // usual Go rules), the following extra rules apply:
   125  //
   126  // 1) Of those fields, if any are JSON-tagged, only tagged fields are considered,
   127  // even if there are multiple untagged fields that would otherwise conflict.
   128  //
   129  // 2) If there is exactly one field (tagged or not according to the first rule), that is selected.
   130  //
   131  // 3) Otherwise there are multiple fields, and all are ignored; no error occurs.
   132  //
   133  // Handling of anonymous struct fields is new in Go 1.1.
   134  // Prior to Go 1.1, anonymous struct fields were ignored. To force ignoring of
   135  // an anonymous struct field in both current and earlier versions, give the field
   136  // a JSON tag of "-".
   137  //
   138  // Map values encode as JSON objects. The map's key type must either be a
   139  // string, an integer type, or implement [encoding.TextMarshaler]. The map keys
   140  // are sorted and used as JSON object keys by applying the following rules,
   141  // subject to the UTF-8 coercion described for string values above:
   142  //   - keys of any string type are used directly
   143  //   - [encoding.TextMarshalers] are marshaled
   144  //   - integer keys are converted to strings
   145  //
   146  // Pointer values encode as the value pointed to.
   147  // A nil pointer encodes as the null JSON value.
   148  //
   149  // Interface values encode as the value contained in the interface.
   150  // A nil interface value encodes as the null JSON value.
   151  //
   152  // Channel, complex, and function values cannot be encoded in JSON.
   153  // Attempting to encode such a value causes Marshal to return
   154  // an [UnsupportedTypeError].
   155  //
   156  // JSON cannot represent cyclic data structures and Marshal does not
   157  // handle them. Passing cyclic structures to Marshal will result in
   158  // an error.
   159  func Marshal(v any) ([]byte, error) {
   160  	e := newEncodeState()
   161  	defer encodeStatePool.Put(e)
   162  
   163  	err := e.marshal(v, encOpts{escapeHTML: true})
   164  	if err != nil {
   165  		return nil, err
   166  	}
   167  	buf := append([]byte(nil), e.Bytes()...)
   168  
   169  	return buf, nil
   170  }
   171  
   172  // MarshalIndent is like [Marshal] but applies [Indent] to format the output.
   173  // Each JSON element in the output will begin on a new line beginning with prefix
   174  // followed by one or more copies of indent according to the indentation nesting.
   175  func MarshalIndent(v any, prefix, indent string) ([]byte, error) {
   176  	b, err := Marshal(v)
   177  	if err != nil {
   178  		return nil, err
   179  	}
   180  	b2 := make([]byte, 0, indentGrowthFactor*len(b))
   181  	b2, err = appendIndent(b2, b, prefix, indent)
   182  	if err != nil {
   183  		return nil, err
   184  	}
   185  	return b2, nil
   186  }
   187  
   188  // Marshaler is the interface implemented by types that
   189  // can marshal themselves into valid JSON.
   190  type Marshaler interface {
   191  	MarshalJSON() ([]byte, error)
   192  }
   193  
   194  // An UnsupportedTypeError is returned by [Marshal] when attempting
   195  // to encode an unsupported value type.
   196  type UnsupportedTypeError struct {
   197  	Type reflect.Type
   198  }
   199  
   200  func (e *UnsupportedTypeError) Error() string {
   201  	return "json: unsupported type: " + e.Type.String()
   202  }
   203  
   204  // An UnsupportedValueError is returned by [Marshal] when attempting
   205  // to encode an unsupported value.
   206  type UnsupportedValueError struct {
   207  	Value reflect.Value
   208  	Str   string
   209  }
   210  
   211  func (e *UnsupportedValueError) Error() string {
   212  	return "json: unsupported value: " + e.Str
   213  }
   214  
   215  // Before Go 1.2, an InvalidUTF8Error was returned by [Marshal] when
   216  // attempting to encode a string value with invalid UTF-8 sequences.
   217  // As of Go 1.2, [Marshal] instead coerces the string to valid UTF-8 by
   218  // replacing invalid bytes with the Unicode replacement rune U+FFFD.
   219  //
   220  // Deprecated: No longer used; kept for compatibility.
   221  type InvalidUTF8Error struct {
   222  	S string // the whole string value that caused the error
   223  }
   224  
   225  func (e *InvalidUTF8Error) Error() string {
   226  	return "json: invalid UTF-8 in string: " + strconv.Quote(e.S)
   227  }
   228  
   229  // A MarshalerError represents an error from calling a
   230  // [Marshaler.MarshalJSON] or [encoding.TextMarshaler.MarshalText] method.
   231  type MarshalerError struct {
   232  	Type       reflect.Type
   233  	Err        error
   234  	sourceFunc string
   235  }
   236  
   237  func (e *MarshalerError) Error() string {
   238  	srcFunc := e.sourceFunc
   239  	if srcFunc == "" {
   240  		srcFunc = "MarshalJSON"
   241  	}
   242  	return "json: error calling " + srcFunc +
   243  		" for type " + e.Type.String() +
   244  		": " + e.Err.Error()
   245  }
   246  
   247  // Unwrap returns the underlying error.
   248  func (e *MarshalerError) Unwrap() error { return e.Err }
   249  
   250  const hex = "0123456789abcdef"
   251  
   252  // An encodeState encodes JSON into a bytes.Buffer.
   253  type encodeState struct {
   254  	bytes.Buffer // accumulated output
   255  
   256  	// Keep track of what pointers we've seen in the current recursive call
   257  	// path, to avoid cycles that could lead to a stack overflow. Only do
   258  	// the relatively expensive map operations if ptrLevel is larger than
   259  	// startDetectingCyclesAfter, so that we skip the work if we're within a
   260  	// reasonable amount of nested pointers deep.
   261  	ptrLevel uint
   262  	ptrSeen  map[any]struct{}
   263  }
   264  
   265  const startDetectingCyclesAfter = 1000
   266  
   267  var encodeStatePool sync.Pool
   268  
   269  func newEncodeState() *encodeState {
   270  	if v := encodeStatePool.Get(); v != nil {
   271  		e := v.(*encodeState)
   272  		e.Reset()
   273  		if len(e.ptrSeen) > 0 {
   274  			panic("ptrEncoder.encode should have emptied ptrSeen via defers")
   275  		}
   276  		e.ptrLevel = 0
   277  		return e
   278  	}
   279  	return &encodeState{ptrSeen: make(map[any]struct{})}
   280  }
   281  
   282  // jsonError is an error wrapper type for internal use only.
   283  // Panics with errors are wrapped in jsonError so that the top-level recover
   284  // can distinguish intentional panics from this package.
   285  type jsonError struct{ error }
   286  
   287  func (e *encodeState) marshal(v any, opts encOpts) (err error) {
   288  	defer func() {
   289  		if r := recover(); r != nil {
   290  			if je, ok := r.(jsonError); ok {
   291  				err = je.error
   292  			} else {
   293  				panic(r)
   294  			}
   295  		}
   296  	}()
   297  	e.reflectValue(reflect.ValueOf(v), opts)
   298  	return nil
   299  }
   300  
   301  // error aborts the encoding by panicking with err wrapped in jsonError.
   302  func (e *encodeState) error(err error) {
   303  	panic(jsonError{err})
   304  }
   305  
   306  func isEmptyValue(v reflect.Value) bool {
   307  	switch v.Kind() {
   308  	case reflect.Array, reflect.Map, reflect.Slice, reflect.String:
   309  		return v.Len() == 0
   310  	case reflect.Bool,
   311  		reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64,
   312  		reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr,
   313  		reflect.Float32, reflect.Float64,
   314  		reflect.Interface, reflect.Pointer:
   315  		return v.IsZero()
   316  	}
   317  	return false
   318  }
   319  
   320  func (e *encodeState) reflectValue(v reflect.Value, opts encOpts) {
   321  	valueEncoder(v)(e, v, opts)
   322  }
   323  
   324  type encOpts struct {
   325  	// quoted causes primitive fields to be encoded inside JSON strings.
   326  	quoted bool
   327  	// escapeHTML causes '<', '>', and '&' to be escaped in JSON strings.
   328  	escapeHTML bool
   329  }
   330  
   331  type encoderFunc func(e *encodeState, v reflect.Value, opts encOpts)
   332  
   333  var encoderCache sync.Map // map[reflect.Type]encoderFunc
   334  
   335  func valueEncoder(v reflect.Value) encoderFunc {
   336  	if !v.IsValid() {
   337  		return invalidValueEncoder
   338  	}
   339  	return typeEncoder(v.Type())
   340  }
   341  
   342  func typeEncoder(t reflect.Type) encoderFunc {
   343  	if fi, ok := encoderCache.Load(t); ok {
   344  		return fi.(encoderFunc)
   345  	}
   346  
   347  	// To deal with recursive types, populate the map with an
   348  	// indirect func before we build it. This type waits on the
   349  	// real func (f) to be ready and then calls it. This indirect
   350  	// func is only used for recursive types.
   351  	var (
   352  		wg sync.WaitGroup
   353  		f  encoderFunc
   354  	)
   355  	wg.Add(1)
   356  	fi, loaded := encoderCache.LoadOrStore(t, encoderFunc(func(e *encodeState, v reflect.Value, opts encOpts) {
   357  		wg.Wait()
   358  		f(e, v, opts)
   359  	}))
   360  	if loaded {
   361  		return fi.(encoderFunc)
   362  	}
   363  
   364  	// Compute the real encoder and replace the indirect func with it.
   365  	f = newTypeEncoder(t, true)
   366  	wg.Done()
   367  	encoderCache.Store(t, f)
   368  	return f
   369  }
   370  
   371  var (
   372  	marshalerType     = reflect.TypeFor[Marshaler]()
   373  	textMarshalerType = reflect.TypeFor[encoding.TextMarshaler]()
   374  )
   375  
   376  // newTypeEncoder constructs an encoderFunc for a type.
   377  // The returned encoder only checks CanAddr when allowAddr is true.
   378  func newTypeEncoder(t reflect.Type, allowAddr bool) encoderFunc {
   379  	// If we have a non-pointer value whose type implements
   380  	// Marshaler with a value receiver, then we're better off taking
   381  	// the address of the value - otherwise we end up with an
   382  	// allocation as we cast the value to an interface.
   383  	if t.Kind() != reflect.Pointer && allowAddr && reflect.PointerTo(t).Implements(marshalerType) {
   384  		return newCondAddrEncoder(addrMarshalerEncoder, newTypeEncoder(t, false))
   385  	}
   386  	if t.Implements(marshalerType) {
   387  		return marshalerEncoder
   388  	}
   389  	if t.Kind() != reflect.Pointer && allowAddr && reflect.PointerTo(t).Implements(textMarshalerType) {
   390  		return newCondAddrEncoder(addrTextMarshalerEncoder, newTypeEncoder(t, false))
   391  	}
   392  	if t.Implements(textMarshalerType) {
   393  		return textMarshalerEncoder
   394  	}
   395  
   396  	switch t.Kind() {
   397  	case reflect.Bool:
   398  		return boolEncoder
   399  	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
   400  		return intEncoder
   401  	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
   402  		return uintEncoder
   403  	case reflect.Float32:
   404  		return float32Encoder
   405  	case reflect.Float64:
   406  		return float64Encoder
   407  	case reflect.String:
   408  		return stringEncoder
   409  	case reflect.Interface:
   410  		return interfaceEncoder
   411  	case reflect.Struct:
   412  		return newStructEncoder(t)
   413  	case reflect.Map:
   414  		return newMapEncoder(t)
   415  	case reflect.Slice:
   416  		return newSliceEncoder(t)
   417  	case reflect.Array:
   418  		return newArrayEncoder(t)
   419  	case reflect.Pointer:
   420  		return newPtrEncoder(t)
   421  	default:
   422  		return unsupportedTypeEncoder
   423  	}
   424  }
   425  
   426  func invalidValueEncoder(e *encodeState, v reflect.Value, _ encOpts) {
   427  	e.WriteString("null")
   428  }
   429  
   430  func marshalerEncoder(e *encodeState, v reflect.Value, opts encOpts) {
   431  	if v.Kind() == reflect.Pointer && v.IsNil() {
   432  		e.WriteString("null")
   433  		return
   434  	}
   435  	m, ok := v.Interface().(Marshaler)
   436  	if !ok {
   437  		e.WriteString("null")
   438  		return
   439  	}
   440  	b, err := m.MarshalJSON()
   441  	if err == nil {
   442  		e.Grow(len(b))
   443  		out := e.AvailableBuffer()
   444  		out, err = appendCompact(out, b, opts.escapeHTML)
   445  		e.Buffer.Write(out)
   446  	}
   447  	if err != nil {
   448  		e.error(&MarshalerError{v.Type(), err, "MarshalJSON"})
   449  	}
   450  }
   451  
   452  func addrMarshalerEncoder(e *encodeState, v reflect.Value, opts encOpts) {
   453  	va := v.Addr()
   454  	if va.IsNil() {
   455  		e.WriteString("null")
   456  		return
   457  	}
   458  	m := va.Interface().(Marshaler)
   459  	b, err := m.MarshalJSON()
   460  	if err == nil {
   461  		e.Grow(len(b))
   462  		out := e.AvailableBuffer()
   463  		out, err = appendCompact(out, b, opts.escapeHTML)
   464  		e.Buffer.Write(out)
   465  	}
   466  	if err != nil {
   467  		e.error(&MarshalerError{v.Type(), err, "MarshalJSON"})
   468  	}
   469  }
   470  
   471  func textMarshalerEncoder(e *encodeState, v reflect.Value, opts encOpts) {
   472  	if v.Kind() == reflect.Pointer && v.IsNil() {
   473  		e.WriteString("null")
   474  		return
   475  	}
   476  	m, ok := v.Interface().(encoding.TextMarshaler)
   477  	if !ok {
   478  		e.WriteString("null")
   479  		return
   480  	}
   481  	b, err := m.MarshalText()
   482  	if err != nil {
   483  		e.error(&MarshalerError{v.Type(), err, "MarshalText"})
   484  	}
   485  	e.Write(appendString(e.AvailableBuffer(), b, opts.escapeHTML))
   486  }
   487  
   488  func addrTextMarshalerEncoder(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().(encoding.TextMarshaler)
   495  	b, err := m.MarshalText()
   496  	if err != nil {
   497  		e.error(&MarshalerError{v.Type(), err, "MarshalText"})
   498  	}
   499  	e.Write(appendString(e.AvailableBuffer(), b, opts.escapeHTML))
   500  }
   501  
   502  func boolEncoder(e *encodeState, v reflect.Value, opts encOpts) {
   503  	b := e.AvailableBuffer()
   504  	b = mayAppendQuote(b, opts.quoted)
   505  	b = strconv.AppendBool(b, v.Bool())
   506  	b = mayAppendQuote(b, opts.quoted)
   507  	e.Write(b)
   508  }
   509  
   510  func intEncoder(e *encodeState, v reflect.Value, opts encOpts) {
   511  	b := e.AvailableBuffer()
   512  	b = mayAppendQuote(b, opts.quoted)
   513  	b = strconv.AppendInt(b, v.Int(), 10)
   514  	b = mayAppendQuote(b, opts.quoted)
   515  	e.Write(b)
   516  }
   517  
   518  func uintEncoder(e *encodeState, v reflect.Value, opts encOpts) {
   519  	b := e.AvailableBuffer()
   520  	b = mayAppendQuote(b, opts.quoted)
   521  	b = strconv.AppendUint(b, v.Uint(), 10)
   522  	b = mayAppendQuote(b, opts.quoted)
   523  	e.Write(b)
   524  }
   525  
   526  type floatEncoder int // number of bits
   527  
   528  func (bits floatEncoder) encode(e *encodeState, v reflect.Value, opts encOpts) {
   529  	f := v.Float()
   530  	if math.IsInf(f, 0) || math.IsNaN(f) {
   531  		e.error(&UnsupportedValueError{v, strconv.FormatFloat(f, 'g', -1, int(bits))})
   532  	}
   533  
   534  	// Convert as if by ES6 number to string conversion.
   535  	// This matches most other JSON generators.
   536  	// See golang.org/issue/6384 and golang.org/issue/14135.
   537  	// Like fmt %g, but the exponent cutoffs are different
   538  	// and exponents themselves are not padded to two digits.
   539  	b := e.AvailableBuffer()
   540  	b = mayAppendQuote(b, opts.quoted)
   541  	abs := math.Abs(f)
   542  	fmt := byte('f')
   543  	// Note: Must use float32 comparisons for underlying float32 value to get precise cutoffs right.
   544  	if abs != 0 {
   545  		if bits == 64 && (abs < 1e-6 || abs >= 1e21) || bits == 32 && (float32(abs) < 1e-6 || float32(abs) >= 1e21) {
   546  			fmt = 'e'
   547  		}
   548  	}
   549  	b = strconv.AppendFloat(b, f, fmt, -1, int(bits))
   550  	if fmt == 'e' {
   551  		// clean up e-09 to e-9
   552  		n := len(b)
   553  		if n >= 4 && b[n-4] == 'e' && b[n-3] == '-' && b[n-2] == '0' {
   554  			b[n-2] = b[n-1]
   555  			b = b[:n-1]
   556  		}
   557  	}
   558  	b = mayAppendQuote(b, opts.quoted)
   559  	e.Write(b)
   560  }
   561  
   562  var (
   563  	float32Encoder = (floatEncoder(32)).encode
   564  	float64Encoder = (floatEncoder(64)).encode
   565  )
   566  
   567  func stringEncoder(e *encodeState, v reflect.Value, opts encOpts) {
   568  	if v.Type() == numberType {
   569  		numStr := v.String()
   570  		// In Go1.5 the empty string encodes to "0", while this is not a valid number literal
   571  		// we keep compatibility so check validity after this.
   572  		if numStr == "" {
   573  			numStr = "0" // Number's zero-val
   574  		}
   575  		if !isValidNumber(numStr) {
   576  			e.error(fmt.Errorf("json: invalid number literal %q", numStr))
   577  		}
   578  		b := e.AvailableBuffer()
   579  		b = mayAppendQuote(b, opts.quoted)
   580  		b = append(b, numStr...)
   581  		b = mayAppendQuote(b, opts.quoted)
   582  		e.Write(b)
   583  		return
   584  	}
   585  	if opts.quoted {
   586  		b := appendString(nil, v.String(), opts.escapeHTML)
   587  		e.Write(appendString(e.AvailableBuffer(), b, false)) // no need to escape again since it is already escaped
   588  	} else {
   589  		e.Write(appendString(e.AvailableBuffer(), v.String(), opts.escapeHTML))
   590  	}
   591  }
   592  
   593  // isValidNumber reports whether s is a valid JSON number literal.
   594  func isValidNumber(s string) bool {
   595  	// This function implements the JSON numbers grammar.
   596  	// See https://tools.ietf.org/html/rfc7159#section-6
   597  	// and https://www.json.org/img/number.png
   598  
   599  	if s == "" {
   600  		return false
   601  	}
   602  
   603  	// Optional -
   604  	if s[0] == '-' {
   605  		s = s[1:]
   606  		if s == "" {
   607  			return false
   608  		}
   609  	}
   610  
   611  	// Digits
   612  	switch {
   613  	default:
   614  		return false
   615  
   616  	case s[0] == '0':
   617  		s = s[1:]
   618  
   619  	case '1' <= s[0] && s[0] <= '9':
   620  		s = s[1:]
   621  		for len(s) > 0 && '0' <= s[0] && s[0] <= '9' {
   622  			s = s[1:]
   623  		}
   624  	}
   625  
   626  	// . followed by 1 or more digits.
   627  	if len(s) >= 2 && s[0] == '.' && '0' <= s[1] && s[1] <= '9' {
   628  		s = s[2:]
   629  		for len(s) > 0 && '0' <= s[0] && s[0] <= '9' {
   630  			s = s[1:]
   631  		}
   632  	}
   633  
   634  	// e or E followed by an optional - or + and
   635  	// 1 or more digits.
   636  	if len(s) >= 2 && (s[0] == 'e' || s[0] == 'E') {
   637  		s = s[1:]
   638  		if s[0] == '+' || s[0] == '-' {
   639  			s = s[1:]
   640  			if s == "" {
   641  				return false
   642  			}
   643  		}
   644  		for len(s) > 0 && '0' <= s[0] && s[0] <= '9' {
   645  			s = s[1:]
   646  		}
   647  	}
   648  
   649  	// Make sure we are at the end.
   650  	return s == ""
   651  }
   652  
   653  func interfaceEncoder(e *encodeState, v reflect.Value, opts encOpts) {
   654  	if v.IsNil() {
   655  		e.WriteString("null")
   656  		return
   657  	}
   658  	e.reflectValue(v.Elem(), opts)
   659  }
   660  
   661  func unsupportedTypeEncoder(e *encodeState, v reflect.Value, _ encOpts) {
   662  	e.error(&UnsupportedTypeError{v.Type()})
   663  }
   664  
   665  type structEncoder struct {
   666  	fields structFields
   667  }
   668  
   669  type structFields struct {
   670  	list         []field
   671  	byExactName  map[string]*field
   672  	byFoldedName map[string]*field
   673  }
   674  
   675  func (se structEncoder) encode(e *encodeState, v reflect.Value, opts encOpts) {
   676  	next := byte('{')
   677  FieldLoop:
   678  	for i := range se.fields.list {
   679  		f := &se.fields.list[i]
   680  
   681  		// Find the nested struct field by following f.index.
   682  		fv := v
   683  		for _, i := range f.index {
   684  			if fv.Kind() == reflect.Pointer {
   685  				if fv.IsNil() {
   686  					continue FieldLoop
   687  				}
   688  				fv = fv.Elem()
   689  			}
   690  			fv = fv.Field(i)
   691  		}
   692  
   693  		if f.omitEmpty && isEmptyValue(fv) {
   694  			continue
   695  		}
   696  		e.WriteByte(next)
   697  		next = ','
   698  		if opts.escapeHTML {
   699  			e.WriteString(f.nameEscHTML)
   700  		} else {
   701  			e.WriteString(f.nameNonEsc)
   702  		}
   703  		opts.quoted = f.quoted
   704  		f.encoder(e, fv, opts)
   705  	}
   706  	if next == '{' {
   707  		e.WriteString("{}")
   708  	} else {
   709  		e.WriteByte('}')
   710  	}
   711  }
   712  
   713  func newStructEncoder(t reflect.Type) encoderFunc {
   714  	se := structEncoder{fields: cachedTypeFields(t)}
   715  	return se.encode
   716  }
   717  
   718  type mapEncoder struct {
   719  	elemEnc encoderFunc
   720  }
   721  
   722  func (me mapEncoder) encode(e *encodeState, v reflect.Value, opts encOpts) {
   723  	if v.IsNil() {
   724  		e.WriteString("null")
   725  		return
   726  	}
   727  	if e.ptrLevel++; e.ptrLevel > startDetectingCyclesAfter {
   728  		// We're a large number of nested ptrEncoder.encode calls deep;
   729  		// start checking if we've run into a pointer cycle.
   730  		ptr := v.UnsafePointer()
   731  		if _, ok := e.ptrSeen[ptr]; ok {
   732  			e.error(&UnsupportedValueError{v, fmt.Sprintf("encountered a cycle via %s", v.Type())})
   733  		}
   734  		e.ptrSeen[ptr] = struct{}{}
   735  		defer delete(e.ptrSeen, ptr)
   736  	}
   737  	e.WriteByte('{')
   738  
   739  	// Extract and sort the keys.
   740  	var (
   741  		sv  = make([]reflectWithString, v.Len())
   742  		mi  = v.MapRange()
   743  		err error
   744  	)
   745  	for i := 0; mi.Next(); i++ {
   746  		if sv[i].ks, err = resolveKeyName(mi.Key()); err != nil {
   747  			e.error(fmt.Errorf("json: encoding error for type %q: %q", v.Type().String(), err.Error()))
   748  		}
   749  		sv[i].v = mi.Value()
   750  	}
   751  	slices.SortFunc(sv, func(i, j reflectWithString) int {
   752  		return strings.Compare(i.ks, j.ks)
   753  	})
   754  
   755  	for i, kv := range sv {
   756  		if i > 0 {
   757  			e.WriteByte(',')
   758  		}
   759  		e.Write(appendString(e.AvailableBuffer(), kv.ks, opts.escapeHTML))
   760  		e.WriteByte(':')
   761  		me.elemEnc(e, kv.v, opts)
   762  	}
   763  	e.WriteByte('}')
   764  	e.ptrLevel--
   765  }
   766  
   767  func newMapEncoder(t reflect.Type) encoderFunc {
   768  	switch t.Key().Kind() {
   769  	case reflect.String,
   770  		reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64,
   771  		reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
   772  	default:
   773  		if !t.Key().Implements(textMarshalerType) {
   774  			return unsupportedTypeEncoder
   775  		}
   776  	}
   777  	me := mapEncoder{typeEncoder(t.Elem())}
   778  	return me.encode
   779  }
   780  
   781  func encodeByteSlice(e *encodeState, v reflect.Value, _ encOpts) {
   782  	if v.IsNil() {
   783  		e.WriteString("null")
   784  		return
   785  	}
   786  
   787  	s := v.Bytes()
   788  	b := e.AvailableBuffer()
   789  	b = append(b, '"')
   790  	b = base64.StdEncoding.AppendEncode(b, s)
   791  	b = append(b, '"')
   792  	e.Write(b)
   793  }
   794  
   795  // sliceEncoder just wraps an arrayEncoder, checking to make sure the value isn't nil.
   796  type sliceEncoder struct {
   797  	arrayEnc encoderFunc
   798  }
   799  
   800  func (se sliceEncoder) encode(e *encodeState, v reflect.Value, opts encOpts) {
   801  	if v.IsNil() {
   802  		e.WriteString("null")
   803  		return
   804  	}
   805  	if e.ptrLevel++; e.ptrLevel > startDetectingCyclesAfter {
   806  		// We're a large number of nested ptrEncoder.encode calls deep;
   807  		// start checking if we've run into a pointer cycle.
   808  		// Here we use a struct to memorize the pointer to the first element of the slice
   809  		// and its length.
   810  		ptr := struct {
   811  			ptr interface{} // always an unsafe.Pointer, but avoids a dependency on package unsafe
   812  			len int
   813  		}{v.UnsafePointer(), v.Len()}
   814  		if _, ok := e.ptrSeen[ptr]; ok {
   815  			e.error(&UnsupportedValueError{v, fmt.Sprintf("encountered a cycle via %s", v.Type())})
   816  		}
   817  		e.ptrSeen[ptr] = struct{}{}
   818  		defer delete(e.ptrSeen, ptr)
   819  	}
   820  	se.arrayEnc(e, v, opts)
   821  	e.ptrLevel--
   822  }
   823  
   824  func newSliceEncoder(t reflect.Type) encoderFunc {
   825  	// Byte slices get special treatment; arrays don't.
   826  	if t.Elem().Kind() == reflect.Uint8 {
   827  		p := reflect.PointerTo(t.Elem())
   828  		if !p.Implements(marshalerType) && !p.Implements(textMarshalerType) {
   829  			return encodeByteSlice
   830  		}
   831  	}
   832  	enc := sliceEncoder{newArrayEncoder(t)}
   833  	return enc.encode
   834  }
   835  
   836  type arrayEncoder struct {
   837  	elemEnc encoderFunc
   838  }
   839  
   840  func (ae arrayEncoder) encode(e *encodeState, v reflect.Value, opts encOpts) {
   841  	e.WriteByte('[')
   842  	n := v.Len()
   843  	for i := 0; i < n; i++ {
   844  		if i > 0 {
   845  			e.WriteByte(',')
   846  		}
   847  		ae.elemEnc(e, v.Index(i), opts)
   848  	}
   849  	e.WriteByte(']')
   850  }
   851  
   852  func newArrayEncoder(t reflect.Type) encoderFunc {
   853  	enc := arrayEncoder{typeEncoder(t.Elem())}
   854  	return enc.encode
   855  }
   856  
   857  type ptrEncoder struct {
   858  	elemEnc encoderFunc
   859  }
   860  
   861  func (pe ptrEncoder) encode(e *encodeState, v reflect.Value, opts encOpts) {
   862  	if v.IsNil() {
   863  		e.WriteString("null")
   864  		return
   865  	}
   866  	if e.ptrLevel++; e.ptrLevel > startDetectingCyclesAfter {
   867  		// We're a large number of nested ptrEncoder.encode calls deep;
   868  		// start checking if we've run into a pointer cycle.
   869  		ptr := v.Interface()
   870  		if _, ok := e.ptrSeen[ptr]; ok {
   871  			e.error(&UnsupportedValueError{v, fmt.Sprintf("encountered a cycle via %s", v.Type())})
   872  		}
   873  		e.ptrSeen[ptr] = struct{}{}
   874  		defer delete(e.ptrSeen, ptr)
   875  	}
   876  	pe.elemEnc(e, v.Elem(), opts)
   877  	e.ptrLevel--
   878  }
   879  
   880  func newPtrEncoder(t reflect.Type) encoderFunc {
   881  	enc := ptrEncoder{typeEncoder(t.Elem())}
   882  	return enc.encode
   883  }
   884  
   885  type condAddrEncoder struct {
   886  	canAddrEnc, elseEnc encoderFunc
   887  }
   888  
   889  func (ce condAddrEncoder) encode(e *encodeState, v reflect.Value, opts encOpts) {
   890  	if v.CanAddr() {
   891  		ce.canAddrEnc(e, v, opts)
   892  	} else {
   893  		ce.elseEnc(e, v, opts)
   894  	}
   895  }
   896  
   897  // newCondAddrEncoder returns an encoder that checks whether its value
   898  // CanAddr and delegates to canAddrEnc if so, else to elseEnc.
   899  func newCondAddrEncoder(canAddrEnc, elseEnc encoderFunc) encoderFunc {
   900  	enc := condAddrEncoder{canAddrEnc: canAddrEnc, elseEnc: elseEnc}
   901  	return enc.encode
   902  }
   903  
   904  func isValidTag(s string) bool {
   905  	if s == "" {
   906  		return false
   907  	}
   908  	for _, c := range s {
   909  		switch {
   910  		case strings.ContainsRune("!#$%&()*+-./:;<=>?@[]^_{|}~ ", c):
   911  			// Backslash and quote chars are reserved, but
   912  			// otherwise any punctuation chars are allowed
   913  			// in a tag name.
   914  		case !unicode.IsLetter(c) && !unicode.IsDigit(c):
   915  			return false
   916  		}
   917  	}
   918  	return true
   919  }
   920  
   921  func typeByIndex(t reflect.Type, index []int) reflect.Type {
   922  	for _, i := range index {
   923  		if t.Kind() == reflect.Pointer {
   924  			t = t.Elem()
   925  		}
   926  		t = t.Field(i).Type
   927  	}
   928  	return t
   929  }
   930  
   931  type reflectWithString struct {
   932  	v  reflect.Value
   933  	ks string
   934  }
   935  
   936  func resolveKeyName(k reflect.Value) (string, error) {
   937  	if k.Kind() == reflect.String {
   938  		return k.String(), nil
   939  	}
   940  	if tm, ok := k.Interface().(encoding.TextMarshaler); ok {
   941  		if k.Kind() == reflect.Pointer && k.IsNil() {
   942  			return "", nil
   943  		}
   944  		buf, err := tm.MarshalText()
   945  		return string(buf), err
   946  	}
   947  	switch k.Kind() {
   948  	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
   949  		return strconv.FormatInt(k.Int(), 10), nil
   950  	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
   951  		return strconv.FormatUint(k.Uint(), 10), nil
   952  	}
   953  	panic("unexpected map key type")
   954  }
   955  
   956  func appendString[Bytes []byte | string](dst []byte, src Bytes, escapeHTML bool) []byte {
   957  	dst = append(dst, '"')
   958  	start := 0
   959  	for i := 0; i < len(src); {
   960  		if b := src[i]; b < utf8.RuneSelf {
   961  			if htmlSafeSet[b] || (!escapeHTML && safeSet[b]) {
   962  				i++
   963  				continue
   964  			}
   965  			dst = append(dst, src[start:i]...)
   966  			switch b {
   967  			case '\\', '"':
   968  				dst = append(dst, '\\', b)
   969  			case '\b':
   970  				dst = append(dst, '\\', 'b')
   971  			case '\f':
   972  				dst = append(dst, '\\', 'f')
   973  			case '\n':
   974  				dst = append(dst, '\\', 'n')
   975  			case '\r':
   976  				dst = append(dst, '\\', 'r')
   977  			case '\t':
   978  				dst = append(dst, '\\', 't')
   979  			default:
   980  				// This encodes bytes < 0x20 except for \b, \f, \n, \r and \t.
   981  				// If escapeHTML is set, it also escapes <, >, and &
   982  				// because they can lead to security holes when
   983  				// user-controlled strings are rendered into JSON
   984  				// and served to some browsers.
   985  				dst = append(dst, '\\', 'u', '0', '0', hex[b>>4], hex[b&0xF])
   986  			}
   987  			i++
   988  			start = i
   989  			continue
   990  		}
   991  		// TODO(https://go.dev/issue/56948): Use generic utf8 functionality.
   992  		// For now, cast only a small portion of byte slices to a string
   993  		// so that it can be stack allocated. This slows down []byte slightly
   994  		// due to the extra copy, but keeps string performance roughly the same.
   995  		n := len(src) - i
   996  		if n > utf8.UTFMax {
   997  			n = utf8.UTFMax
   998  		}
   999  		c, size := utf8.DecodeRuneInString(string(src[i : i+n]))
  1000  		if c == utf8.RuneError && size == 1 {
  1001  			dst = append(dst, src[start:i]...)
  1002  			dst = append(dst, `\ufffd`...)
  1003  			i += size
  1004  			start = i
  1005  			continue
  1006  		}
  1007  		// U+2028 is LINE SEPARATOR.
  1008  		// U+2029 is PARAGRAPH SEPARATOR.
  1009  		// They are both technically valid characters in JSON strings,
  1010  		// but don't work in JSONP, which has to be evaluated as JavaScript,
  1011  		// and can lead to security holes there. It is valid JSON to
  1012  		// escape them, so we do so unconditionally.
  1013  		// See https://en.wikipedia.org/wiki/JSON#Safety.
  1014  		if c == '\u2028' || c == '\u2029' {
  1015  			dst = append(dst, src[start:i]...)
  1016  			dst = append(dst, '\\', 'u', '2', '0', '2', hex[c&0xF])
  1017  			i += size
  1018  			start = i
  1019  			continue
  1020  		}
  1021  		i += size
  1022  	}
  1023  	dst = append(dst, src[start:]...)
  1024  	dst = append(dst, '"')
  1025  	return dst
  1026  }
  1027  
  1028  // A field represents a single field found in a struct.
  1029  type field struct {
  1030  	name      string
  1031  	nameBytes []byte // []byte(name)
  1032  
  1033  	nameNonEsc  string // `"` + name + `":`
  1034  	nameEscHTML string // `"` + HTMLEscape(name) + `":`
  1035  
  1036  	tag       bool
  1037  	index     []int
  1038  	typ       reflect.Type
  1039  	omitEmpty bool
  1040  	quoted    bool
  1041  
  1042  	encoder encoderFunc
  1043  }
  1044  
  1045  // byIndex sorts field by index sequence.
  1046  type byIndex []field
  1047  
  1048  func (x byIndex) Len() int { return len(x) }
  1049  
  1050  func (x byIndex) Swap(i, j int) { x[i], x[j] = x[j], x[i] }
  1051  
  1052  func (x byIndex) Less(i, j int) bool {
  1053  	for k, xik := range x[i].index {
  1054  		if k >= len(x[j].index) {
  1055  			return false
  1056  		}
  1057  		if xik != x[j].index[k] {
  1058  			return xik < x[j].index[k]
  1059  		}
  1060  	}
  1061  	return len(x[i].index) < len(x[j].index)
  1062  }
  1063  
  1064  // typeFields returns a list of fields that JSON should recognize for the given type.
  1065  // The algorithm is breadth-first search over the set of structs to include - the top struct
  1066  // and then any reachable anonymous structs.
  1067  func typeFields(t reflect.Type) structFields {
  1068  	// Anonymous fields to explore at the current level and the next.
  1069  	current := []field{}
  1070  	next := []field{{typ: t}}
  1071  
  1072  	// Count of queued names for current level and the next.
  1073  	var count, nextCount map[reflect.Type]int
  1074  
  1075  	// Types already visited at an earlier level.
  1076  	visited := map[reflect.Type]bool{}
  1077  
  1078  	// Fields found.
  1079  	var fields []field
  1080  
  1081  	// Buffer to run appendHTMLEscape on field names.
  1082  	var nameEscBuf []byte
  1083  
  1084  	for len(next) > 0 {
  1085  		current, next = next, current[:0]
  1086  		count, nextCount = nextCount, map[reflect.Type]int{}
  1087  
  1088  		for _, f := range current {
  1089  			if visited[f.typ] {
  1090  				continue
  1091  			}
  1092  			visited[f.typ] = true
  1093  
  1094  			// Scan f.typ for fields to include.
  1095  			for i := 0; i < f.typ.NumField(); i++ {
  1096  				sf := f.typ.Field(i)
  1097  				if sf.Anonymous {
  1098  					t := sf.Type
  1099  					if t.Kind() == reflect.Pointer {
  1100  						t = t.Elem()
  1101  					}
  1102  					if !sf.IsExported() && t.Kind() != reflect.Struct {
  1103  						// Ignore embedded fields of unexported non-struct types.
  1104  						continue
  1105  					}
  1106  					// Do not ignore embedded fields of unexported struct types
  1107  					// since they may have exported fields.
  1108  				} else if !sf.IsExported() {
  1109  					// Ignore unexported non-embedded fields.
  1110  					continue
  1111  				}
  1112  				tag := sf.Tag.Get("json")
  1113  				if tag == "-" {
  1114  					continue
  1115  				}
  1116  				name, opts := parseTag(tag)
  1117  				if !isValidTag(name) {
  1118  					name = ""
  1119  				}
  1120  				index := make([]int, len(f.index)+1)
  1121  				copy(index, f.index)
  1122  				index[len(f.index)] = i
  1123  
  1124  				ft := sf.Type
  1125  				if ft.Name() == "" && ft.Kind() == reflect.Pointer {
  1126  					// Follow pointer.
  1127  					ft = ft.Elem()
  1128  				}
  1129  
  1130  				// Only strings, floats, integers, and booleans can be quoted.
  1131  				quoted := false
  1132  				if opts.Contains("string") {
  1133  					switch ft.Kind() {
  1134  					case reflect.Bool,
  1135  						reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64,
  1136  						reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr,
  1137  						reflect.Float32, reflect.Float64,
  1138  						reflect.String:
  1139  						quoted = true
  1140  					}
  1141  				}
  1142  
  1143  				// Record found field and index sequence.
  1144  				if name != "" || !sf.Anonymous || ft.Kind() != reflect.Struct {
  1145  					tagged := name != ""
  1146  					if name == "" {
  1147  						name = sf.Name
  1148  					}
  1149  					field := field{
  1150  						name:      name,
  1151  						tag:       tagged,
  1152  						index:     index,
  1153  						typ:       ft,
  1154  						omitEmpty: opts.Contains("omitempty"),
  1155  						quoted:    quoted,
  1156  					}
  1157  					field.nameBytes = []byte(field.name)
  1158  
  1159  					// Build nameEscHTML and nameNonEsc ahead of time.
  1160  					nameEscBuf = appendHTMLEscape(nameEscBuf[:0], field.nameBytes)
  1161  					field.nameEscHTML = `"` + string(nameEscBuf) + `":`
  1162  					field.nameNonEsc = `"` + field.name + `":`
  1163  
  1164  					fields = append(fields, field)
  1165  					if count[f.typ] > 1 {
  1166  						// If there were multiple instances, add a second,
  1167  						// so that the annihilation code will see a duplicate.
  1168  						// It only cares about the distinction between 1 and 2,
  1169  						// so don't bother generating any more copies.
  1170  						fields = append(fields, fields[len(fields)-1])
  1171  					}
  1172  					continue
  1173  				}
  1174  
  1175  				// Record new anonymous struct to explore in next round.
  1176  				nextCount[ft]++
  1177  				if nextCount[ft] == 1 {
  1178  					next = append(next, field{name: ft.Name(), index: index, typ: ft})
  1179  				}
  1180  			}
  1181  		}
  1182  	}
  1183  
  1184  	sort.Slice(fields, func(i, j int) bool {
  1185  		x := fields
  1186  		// sort field by name, breaking ties with depth, then
  1187  		// breaking ties with "name came from json tag", then
  1188  		// breaking ties with index sequence.
  1189  		if x[i].name != x[j].name {
  1190  			return x[i].name < x[j].name
  1191  		}
  1192  		if len(x[i].index) != len(x[j].index) {
  1193  			return len(x[i].index) < len(x[j].index)
  1194  		}
  1195  		if x[i].tag != x[j].tag {
  1196  			return x[i].tag
  1197  		}
  1198  		return byIndex(x).Less(i, j)
  1199  	})
  1200  
  1201  	// Delete all fields that are hidden by the Go rules for embedded fields,
  1202  	// except that fields with JSON tags are promoted.
  1203  
  1204  	// The fields are sorted in primary order of name, secondary order
  1205  	// of field index length. Loop over names; for each name, delete
  1206  	// hidden fields by choosing the one dominant field that survives.
  1207  	out := fields[:0]
  1208  	for advance, i := 0, 0; i < len(fields); i += advance {
  1209  		// One iteration per name.
  1210  		// Find the sequence of fields with the name of this first field.
  1211  		fi := fields[i]
  1212  		name := fi.name
  1213  		for advance = 1; i+advance < len(fields); advance++ {
  1214  			fj := fields[i+advance]
  1215  			if fj.name != name {
  1216  				break
  1217  			}
  1218  		}
  1219  		if advance == 1 { // Only one field with this name
  1220  			out = append(out, fi)
  1221  			continue
  1222  		}
  1223  		dominant, ok := dominantField(fields[i : i+advance])
  1224  		if ok {
  1225  			out = append(out, dominant)
  1226  		}
  1227  	}
  1228  
  1229  	fields = out
  1230  	sort.Sort(byIndex(fields))
  1231  
  1232  	for i := range fields {
  1233  		f := &fields[i]
  1234  		f.encoder = typeEncoder(typeByIndex(t, f.index))
  1235  	}
  1236  	exactNameIndex := make(map[string]*field, len(fields))
  1237  	foldedNameIndex := make(map[string]*field, len(fields))
  1238  	for i, field := range fields {
  1239  		exactNameIndex[field.name] = &fields[i]
  1240  		// For historical reasons, first folded match takes precedence.
  1241  		if _, ok := foldedNameIndex[string(foldName(field.nameBytes))]; !ok {
  1242  			foldedNameIndex[string(foldName(field.nameBytes))] = &fields[i]
  1243  		}
  1244  	}
  1245  	return structFields{fields, exactNameIndex, foldedNameIndex}
  1246  }
  1247  
  1248  // dominantField looks through the fields, all of which are known to
  1249  // have the same name, to find the single field that dominates the
  1250  // others using Go's embedding rules, modified by the presence of
  1251  // JSON tags. If there are multiple top-level fields, the boolean
  1252  // will be false: This condition is an error in Go and we skip all
  1253  // the fields.
  1254  func dominantField(fields []field) (field, bool) {
  1255  	// The fields are sorted in increasing index-length order, then by presence of tag.
  1256  	// That means that the first field is the dominant one. We need only check
  1257  	// for error cases: two fields at top level, either both tagged or neither tagged.
  1258  	if len(fields) > 1 && len(fields[0].index) == len(fields[1].index) && fields[0].tag == fields[1].tag {
  1259  		return field{}, false
  1260  	}
  1261  	return fields[0], true
  1262  }
  1263  
  1264  var fieldCache sync.Map // map[reflect.Type]structFields
  1265  
  1266  // cachedTypeFields is like typeFields but uses a cache to avoid repeated work.
  1267  func cachedTypeFields(t reflect.Type) structFields {
  1268  	if f, ok := fieldCache.Load(t); ok {
  1269  		return f.(structFields)
  1270  	}
  1271  	f, _ := fieldCache.LoadOrStore(t, typeFields(t))
  1272  	return f.(structFields)
  1273  }
  1274  
  1275  func mayAppendQuote(b []byte, quoted bool) []byte {
  1276  	if quoted {
  1277  		b = append(b, '"')
  1278  	}
  1279  	return b
  1280  }
  1281  

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