Source file src/encoding/json/jsontext/decode.go

     1  // Copyright 2020 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  //go:build goexperiment.jsonv2
     6  
     7  package jsontext
     8  
     9  import (
    10  	"bytes"
    11  	"errors"
    12  	"io"
    13  
    14  	"encoding/json/internal/jsonflags"
    15  	"encoding/json/internal/jsonopts"
    16  	"encoding/json/internal/jsonwire"
    17  )
    18  
    19  // NOTE: The logic for decoding is complicated by the fact that reading from
    20  // an io.Reader into a temporary buffer means that the buffer may contain a
    21  // truncated portion of some valid input, requiring the need to fetch more data.
    22  //
    23  // This file is structured in the following way:
    24  //
    25  //   - consumeXXX functions parse an exact JSON token from a []byte.
    26  //     If the buffer appears truncated, then it returns io.ErrUnexpectedEOF.
    27  //     The consumeSimpleXXX functions are so named because they only handle
    28  //     a subset of the grammar for the JSON token being parsed.
    29  //     They do not handle the full grammar to keep these functions inlinable.
    30  //
    31  //   - Decoder.consumeXXX methods parse the next JSON token from Decoder.buf,
    32  //     automatically fetching more input if necessary. These methods take
    33  //     a position relative to the start of Decoder.buf as an argument and
    34  //     return the end of the consumed JSON token as a position,
    35  //     also relative to the start of Decoder.buf.
    36  //
    37  //   - In the event of an I/O errors or state machine violations,
    38  //     the implementation avoids mutating the state of Decoder
    39  //     (aside from the book-keeping needed to implement Decoder.fetch).
    40  //     For this reason, only Decoder.ReadToken and Decoder.ReadValue are
    41  //     responsible for updated Decoder.prevStart and Decoder.prevEnd.
    42  //
    43  //   - For performance, much of the implementation uses the pattern of calling
    44  //     the inlinable consumeXXX functions first, and if more work is necessary,
    45  //     then it calls the slower Decoder.consumeXXX methods.
    46  //     TODO: Revisit this pattern if the Go compiler provides finer control
    47  //     over exactly which calls are inlined or not.
    48  
    49  // Decoder is a streaming decoder for raw JSON tokens and values.
    50  // It is used to read a stream of top-level JSON values,
    51  // each separated by optional whitespace characters.
    52  //
    53  // [Decoder.ReadToken] and [Decoder.ReadValue] calls may be interleaved.
    54  // For example, the following JSON value:
    55  //
    56  //	{"name":"value","array":[null,false,true,3.14159],"object":{"k":"v"}}
    57  //
    58  // can be parsed with the following calls (ignoring errors for brevity):
    59  //
    60  //	d.ReadToken() // {
    61  //	d.ReadToken() // "name"
    62  //	d.ReadToken() // "value"
    63  //	d.ReadValue() // "array"
    64  //	d.ReadToken() // [
    65  //	d.ReadToken() // null
    66  //	d.ReadToken() // false
    67  //	d.ReadValue() // true
    68  //	d.ReadToken() // 3.14159
    69  //	d.ReadToken() // ]
    70  //	d.ReadValue() // "object"
    71  //	d.ReadValue() // {"k":"v"}
    72  //	d.ReadToken() // }
    73  //
    74  // The above is one of many possible sequence of calls and
    75  // may not represent the most sensible method to call for any given token/value.
    76  // For example, it is probably more common to call [Decoder.ReadToken] to obtain a
    77  // string token for object names.
    78  type Decoder struct {
    79  	s decoderState
    80  }
    81  
    82  // decoderState is the low-level state of Decoder.
    83  // It has exported fields and method for use by the "json" package.
    84  type decoderState struct {
    85  	state
    86  	decodeBuffer
    87  	jsonopts.Struct
    88  
    89  	StringCache *[256]string // only used when unmarshaling; identical to json.stringCache
    90  }
    91  
    92  // decodeBuffer is a buffer split into 4 segments:
    93  //
    94  //   - buf[0:prevEnd]         // already read portion of the buffer
    95  //   - buf[prevStart:prevEnd] // previously read value
    96  //   - buf[prevEnd:len(buf)]  // unread portion of the buffer
    97  //   - buf[len(buf):cap(buf)] // unused portion of the buffer
    98  //
    99  // Invariants:
   100  //
   101  //	0 ≤ prevStart ≤ prevEnd ≤ len(buf) ≤ cap(buf)
   102  type decodeBuffer struct {
   103  	peekPos int   // non-zero if valid offset into buf for start of next token
   104  	peekErr error // implies peekPos is -1
   105  
   106  	buf       []byte // may alias rd if it is a bytes.Buffer
   107  	prevStart int
   108  	prevEnd   int
   109  
   110  	// baseOffset is added to prevStart and prevEnd to obtain
   111  	// the absolute offset relative to the start of io.Reader stream.
   112  	baseOffset int64
   113  
   114  	rd io.Reader
   115  }
   116  
   117  // NewDecoder constructs a new streaming decoder reading from r.
   118  //
   119  // If r is a [bytes.Buffer], then the decoder parses directly from the buffer
   120  // without first copying the contents to an intermediate buffer.
   121  // Additional writes to the buffer must not occur while the decoder is in use.
   122  func NewDecoder(r io.Reader, opts ...Options) *Decoder {
   123  	d := new(Decoder)
   124  	d.Reset(r, opts...)
   125  	return d
   126  }
   127  
   128  // Reset resets a decoder such that it is reading afresh from r and
   129  // configured with the provided options. Reset must not be called on an
   130  // a Decoder passed to the [encoding/json/v2.UnmarshalerFrom.UnmarshalJSONFrom] method
   131  // or the [encoding/json/v2.UnmarshalFromFunc] function.
   132  func (d *Decoder) Reset(r io.Reader, opts ...Options) {
   133  	switch {
   134  	case d == nil:
   135  		panic("jsontext: invalid nil Decoder")
   136  	case r == nil:
   137  		panic("jsontext: invalid nil io.Reader")
   138  	case d.s.Flags.Get(jsonflags.WithinArshalCall):
   139  		panic("jsontext: cannot reset Decoder passed to json.UnmarshalerFrom")
   140  	}
   141  	// Reuse the buffer if it does not alias a previous [bytes.Buffer].
   142  	b := d.s.buf[:0]
   143  	if _, ok := d.s.rd.(*bytes.Buffer); ok {
   144  		b = nil
   145  	}
   146  	d.s.reset(b, r, opts...)
   147  }
   148  
   149  func (d *decoderState) reset(b []byte, r io.Reader, opts ...Options) {
   150  	d.state.reset()
   151  	d.decodeBuffer = decodeBuffer{buf: b, rd: r}
   152  	opts2 := jsonopts.Struct{} // avoid mutating d.Struct in case it is part of opts
   153  	opts2.Join(opts...)
   154  	d.Struct = opts2
   155  }
   156  
   157  // Options returns the options used to construct the encoder and
   158  // may additionally contain semantic options passed to a
   159  // [encoding/json/v2.UnmarshalDecode] call.
   160  //
   161  // If operating within
   162  // a [encoding/json/v2.UnmarshalerFrom.UnmarshalJSONFrom] method call or
   163  // a [encoding/json/v2.UnmarshalFromFunc] function call,
   164  // then the returned options are only valid within the call.
   165  func (d *Decoder) Options() Options {
   166  	return &d.s.Struct
   167  }
   168  
   169  var errBufferWriteAfterNext = errors.New("invalid bytes.Buffer.Write call after calling bytes.Buffer.Next")
   170  
   171  // fetch reads at least 1 byte from the underlying io.Reader.
   172  // It returns io.ErrUnexpectedEOF if zero bytes were read and io.EOF was seen.
   173  func (d *decoderState) fetch() error {
   174  	if d.rd == nil {
   175  		return io.ErrUnexpectedEOF
   176  	}
   177  
   178  	// Inform objectNameStack that we are about to fetch new buffer content.
   179  	d.Names.copyQuotedBuffer(d.buf)
   180  
   181  	// Specialize bytes.Buffer for better performance.
   182  	if bb, ok := d.rd.(*bytes.Buffer); ok {
   183  		switch {
   184  		case bb.Len() == 0:
   185  			return io.ErrUnexpectedEOF
   186  		case len(d.buf) == 0:
   187  			d.buf = bb.Next(bb.Len()) // "read" all data in the buffer
   188  			return nil
   189  		default:
   190  			// This only occurs if a partially filled bytes.Buffer was provided
   191  			// and more data is written to it while Decoder is reading from it.
   192  			// This practice will lead to data corruption since future writes
   193  			// may overwrite the contents of the current buffer.
   194  			//
   195  			// The user is trying to use a bytes.Buffer as a pipe,
   196  			// but a bytes.Buffer is poor implementation of a pipe,
   197  			// the purpose-built io.Pipe should be used instead.
   198  			return &ioError{action: "read", err: errBufferWriteAfterNext}
   199  		}
   200  	}
   201  
   202  	// Allocate initial buffer if empty.
   203  	if cap(d.buf) == 0 {
   204  		d.buf = make([]byte, 0, 64)
   205  	}
   206  
   207  	// Check whether to grow the buffer.
   208  	const maxBufferSize = 4 << 10
   209  	const growthSizeFactor = 2 // higher value is faster
   210  	const growthRateFactor = 2 // higher value is slower
   211  	// By default, grow if below the maximum buffer size.
   212  	grow := cap(d.buf) <= maxBufferSize/growthSizeFactor
   213  	// Growing can be expensive, so only grow
   214  	// if a sufficient number of bytes have been processed.
   215  	grow = grow && int64(cap(d.buf)) < d.previousOffsetEnd()/growthRateFactor
   216  	// If prevStart==0, then fetch was called in order to fetch more data
   217  	// to finish consuming a large JSON value contiguously.
   218  	// Grow if less than 25% of the remaining capacity is available.
   219  	// Note that this may cause the input buffer to exceed maxBufferSize.
   220  	grow = grow || (d.prevStart == 0 && len(d.buf) >= 3*cap(d.buf)/4)
   221  
   222  	if grow {
   223  		// Allocate a new buffer and copy the contents of the old buffer over.
   224  		// TODO: Provide a hard limit on the maximum internal buffer size?
   225  		buf := make([]byte, 0, cap(d.buf)*growthSizeFactor)
   226  		d.buf = append(buf, d.buf[d.prevStart:]...)
   227  	} else {
   228  		// Move unread portion of the data to the front.
   229  		n := copy(d.buf[:cap(d.buf)], d.buf[d.prevStart:])
   230  		d.buf = d.buf[:n]
   231  	}
   232  	d.baseOffset += int64(d.prevStart)
   233  	d.prevEnd -= d.prevStart
   234  	d.prevStart = 0
   235  
   236  	// Read more data into the internal buffer.
   237  	for {
   238  		n, err := d.rd.Read(d.buf[len(d.buf):cap(d.buf)])
   239  		switch {
   240  		case n > 0:
   241  			d.buf = d.buf[:len(d.buf)+n]
   242  			return nil // ignore errors if any bytes are read
   243  		case err == io.EOF:
   244  			return io.ErrUnexpectedEOF
   245  		case err != nil:
   246  			return &ioError{action: "read", err: err}
   247  		default:
   248  			continue // Read returned (0, nil)
   249  		}
   250  	}
   251  }
   252  
   253  const invalidateBufferByte = '#' // invalid starting character for JSON grammar
   254  
   255  // invalidatePreviousRead invalidates buffers returned by Peek and Read calls
   256  // so that the first byte is an invalid character.
   257  // This Hyrum-proofs the API against faulty application code that assumes
   258  // values returned by ReadValue remain valid past subsequent Read calls.
   259  func (d *decodeBuffer) invalidatePreviousRead() {
   260  	// Avoid mutating the buffer if d.rd is nil which implies that d.buf
   261  	// is provided by the user code and may not expect mutations.
   262  	isBytesBuffer := func(r io.Reader) bool {
   263  		_, ok := r.(*bytes.Buffer)
   264  		return ok
   265  	}
   266  	if d.rd != nil && !isBytesBuffer(d.rd) && d.prevStart < d.prevEnd && uint(d.prevStart) < uint(len(d.buf)) {
   267  		d.buf[d.prevStart] = invalidateBufferByte
   268  		d.prevStart = d.prevEnd
   269  	}
   270  }
   271  
   272  // needMore reports whether there are no more unread bytes.
   273  func (d *decodeBuffer) needMore(pos int) bool {
   274  	// NOTE: The arguments and logic are kept simple to keep this inlinable.
   275  	return pos == len(d.buf)
   276  }
   277  
   278  func (d *decodeBuffer) offsetAt(pos int) int64     { return d.baseOffset + int64(pos) }
   279  func (d *decodeBuffer) previousOffsetStart() int64 { return d.baseOffset + int64(d.prevStart) }
   280  func (d *decodeBuffer) previousOffsetEnd() int64   { return d.baseOffset + int64(d.prevEnd) }
   281  func (d *decodeBuffer) previousBuffer() []byte     { return d.buf[d.prevStart:d.prevEnd] }
   282  func (d *decodeBuffer) unreadBuffer() []byte       { return d.buf[d.prevEnd:len(d.buf)] }
   283  
   284  // PreviousTokenOrValue returns the previously read token or value
   285  // unless it has been invalidated by a call to PeekKind.
   286  // If a token is just a delimiter, then this returns a 1-byte buffer.
   287  // This method is used for error reporting at the semantic layer.
   288  func (d *decodeBuffer) PreviousTokenOrValue() []byte {
   289  	b := d.previousBuffer()
   290  	// If peek was called, then the previous token or buffer is invalidated.
   291  	if d.peekPos > 0 || len(b) > 0 && b[0] == invalidateBufferByte {
   292  		return nil
   293  	}
   294  	// ReadToken does not preserve the buffer for null, bools, or delimiters.
   295  	// Manually re-construct that buffer.
   296  	if len(b) == 0 {
   297  		b = d.buf[:d.prevEnd] // entirety of the previous buffer
   298  		for _, tok := range []string{"null", "false", "true", "{", "}", "[", "]"} {
   299  			if len(b) >= len(tok) && string(b[len(b)-len(tok):]) == tok {
   300  				return b[len(b)-len(tok):]
   301  			}
   302  		}
   303  	}
   304  	return b
   305  }
   306  
   307  // PeekKind retrieves the next token kind, but does not advance the read offset.
   308  //
   309  // It returns [KindInvalid] if an error occurs. Any such error is cached until
   310  // the next read call and it is the caller's responsibility to eventually
   311  // follow up a PeekKind call with a read call.
   312  func (d *Decoder) PeekKind() Kind {
   313  	return d.s.PeekKind()
   314  }
   315  func (d *decoderState) PeekKind() Kind {
   316  	// Check whether we have a cached peek result.
   317  	if d.peekPos > 0 {
   318  		return Kind(d.buf[d.peekPos]).normalize()
   319  	}
   320  
   321  	var err error
   322  	d.invalidatePreviousRead()
   323  	pos := d.prevEnd
   324  
   325  	// Consume leading whitespace.
   326  	pos += jsonwire.ConsumeWhitespace(d.buf[pos:])
   327  	if d.needMore(pos) {
   328  		if pos, err = d.consumeWhitespace(pos); err != nil {
   329  			if err == io.ErrUnexpectedEOF && d.Tokens.Depth() == 1 {
   330  				err = io.EOF // EOF possibly if no Tokens present after top-level value
   331  			}
   332  			d.peekPos, d.peekErr = -1, wrapSyntacticError(d, err, pos, 0)
   333  			return invalidKind
   334  		}
   335  	}
   336  
   337  	// Consume colon or comma.
   338  	var delim byte
   339  	if c := d.buf[pos]; c == ':' || c == ',' {
   340  		delim = c
   341  		pos += 1
   342  		pos += jsonwire.ConsumeWhitespace(d.buf[pos:])
   343  		if d.needMore(pos) {
   344  			if pos, err = d.consumeWhitespace(pos); err != nil {
   345  				err = wrapSyntacticError(d, err, pos, 0)
   346  				d.peekPos, d.peekErr = -1, d.checkDelimBeforeIOError(delim, err)
   347  				return invalidKind
   348  			}
   349  		}
   350  	}
   351  	next := Kind(d.buf[pos]).normalize()
   352  	if d.Tokens.needDelim(next) != delim {
   353  		d.peekPos, d.peekErr = -1, d.checkDelim(delim, next)
   354  		return invalidKind
   355  	}
   356  
   357  	// This may set peekPos to zero, which is indistinguishable from
   358  	// the uninitialized state. While a small hit to performance, it is correct
   359  	// since ReadValue and ReadToken will disregard the cached result and
   360  	// recompute the next kind.
   361  	d.peekPos, d.peekErr = pos, nil
   362  	return next
   363  }
   364  
   365  // checkDelimBeforeIOError checks whether the delim is even valid
   366  // before returning an IO error, which occurs after the delim.
   367  func (d *decoderState) checkDelimBeforeIOError(delim byte, err error) error {
   368  	// Since an IO error occurred, we do not know what the next kind is.
   369  	// However, knowing the next kind is necessary to validate
   370  	// whether the current delim is at least potentially valid.
   371  	// Since a JSON string is always valid as the next token,
   372  	// conservatively assume that is the next kind for validation.
   373  	const next = Kind('"')
   374  	if d.Tokens.needDelim(next) != delim {
   375  		err = d.checkDelim(delim, next)
   376  	}
   377  	return err
   378  }
   379  
   380  // CountNextDelimWhitespace counts the number of upcoming bytes of
   381  // delimiter or whitespace characters.
   382  // This method is used for error reporting at the semantic layer.
   383  func (d *decoderState) CountNextDelimWhitespace() int {
   384  	d.PeekKind() // populate unreadBuffer
   385  	return len(d.unreadBuffer()) - len(bytes.TrimLeft(d.unreadBuffer(), ",: \n\r\t"))
   386  }
   387  
   388  // checkDelim checks whether delim is valid for the given next kind.
   389  func (d *decoderState) checkDelim(delim byte, next Kind) error {
   390  	where := "at start of value"
   391  	switch d.Tokens.needDelim(next) {
   392  	case delim:
   393  		return nil
   394  	case ':':
   395  		where = "after object name (expecting ':')"
   396  	case ',':
   397  		if d.Tokens.Last.isObject() {
   398  			where = "after object value (expecting ',' or '}')"
   399  		} else {
   400  			where = "after array element (expecting ',' or ']')"
   401  		}
   402  	}
   403  	pos := d.prevEnd // restore position to right after leading whitespace
   404  	pos += jsonwire.ConsumeWhitespace(d.buf[pos:])
   405  	err := jsonwire.NewInvalidCharacterError(d.buf[pos:], where)
   406  	return wrapSyntacticError(d, err, pos, 0)
   407  }
   408  
   409  // SkipValue is semantically equivalent to calling [Decoder.ReadValue] and discarding
   410  // the result except that memory is not wasted trying to hold the entire result.
   411  func (d *Decoder) SkipValue() error {
   412  	return d.s.SkipValue()
   413  }
   414  func (d *decoderState) SkipValue() error {
   415  	switch d.PeekKind() {
   416  	case '{', '[':
   417  		// For JSON objects and arrays, keep skipping all tokens
   418  		// until the depth matches the starting depth.
   419  		depth := d.Tokens.Depth()
   420  		for {
   421  			if _, err := d.ReadToken(); err != nil {
   422  				return err
   423  			}
   424  			if depth >= d.Tokens.Depth() {
   425  				return nil
   426  			}
   427  		}
   428  	default:
   429  		// Trying to skip a value when the next token is a '}' or ']'
   430  		// will result in an error being returned here.
   431  		var flags jsonwire.ValueFlags
   432  		if _, err := d.ReadValue(&flags); err != nil {
   433  			return err
   434  		}
   435  		return nil
   436  	}
   437  }
   438  
   439  // SkipValueRemainder skips the remainder of a value
   440  // after reading a '{' or '[' token.
   441  func (d *decoderState) SkipValueRemainder() error {
   442  	if d.Tokens.Depth()-1 > 0 && d.Tokens.Last.Length() == 0 {
   443  		for n := d.Tokens.Depth(); d.Tokens.Depth() >= n; {
   444  			if _, err := d.ReadToken(); err != nil {
   445  				return err
   446  			}
   447  		}
   448  	}
   449  	return nil
   450  }
   451  
   452  // SkipUntil skips all tokens until the state machine
   453  // is at or past the specified depth and length.
   454  func (d *decoderState) SkipUntil(depth int, length int64) error {
   455  	for d.Tokens.Depth() > depth || (d.Tokens.Depth() == depth && d.Tokens.Last.Length() < length) {
   456  		if _, err := d.ReadToken(); err != nil {
   457  			return err
   458  		}
   459  	}
   460  	return nil
   461  }
   462  
   463  // ReadToken reads the next [Token], advancing the read offset.
   464  // The returned token is only valid until the next Peek, Read, or Skip call.
   465  // It returns [io.EOF] if there are no more tokens.
   466  func (d *Decoder) ReadToken() (Token, error) {
   467  	return d.s.ReadToken()
   468  }
   469  func (d *decoderState) ReadToken() (Token, error) {
   470  	// Determine the next kind.
   471  	var err error
   472  	var next Kind
   473  	pos := d.peekPos
   474  	if pos != 0 {
   475  		// Use cached peek result.
   476  		if d.peekErr != nil {
   477  			err := d.peekErr
   478  			d.peekPos, d.peekErr = 0, nil // possibly a transient I/O error
   479  			return Token{}, err
   480  		}
   481  		next = Kind(d.buf[pos]).normalize()
   482  		d.peekPos = 0 // reset cache
   483  	} else {
   484  		d.invalidatePreviousRead()
   485  		pos = d.prevEnd
   486  
   487  		// Consume leading whitespace.
   488  		pos += jsonwire.ConsumeWhitespace(d.buf[pos:])
   489  		if d.needMore(pos) {
   490  			if pos, err = d.consumeWhitespace(pos); err != nil {
   491  				if err == io.ErrUnexpectedEOF && d.Tokens.Depth() == 1 {
   492  					err = io.EOF // EOF possibly if no Tokens present after top-level value
   493  				}
   494  				return Token{}, wrapSyntacticError(d, err, pos, 0)
   495  			}
   496  		}
   497  
   498  		// Consume colon or comma.
   499  		var delim byte
   500  		if c := d.buf[pos]; c == ':' || c == ',' {
   501  			delim = c
   502  			pos += 1
   503  			pos += jsonwire.ConsumeWhitespace(d.buf[pos:])
   504  			if d.needMore(pos) {
   505  				if pos, err = d.consumeWhitespace(pos); err != nil {
   506  					err = wrapSyntacticError(d, err, pos, 0)
   507  					return Token{}, d.checkDelimBeforeIOError(delim, err)
   508  				}
   509  			}
   510  		}
   511  		next = Kind(d.buf[pos]).normalize()
   512  		if d.Tokens.needDelim(next) != delim {
   513  			return Token{}, d.checkDelim(delim, next)
   514  		}
   515  	}
   516  
   517  	// Handle the next token.
   518  	var n int
   519  	switch next {
   520  	case 'n':
   521  		if jsonwire.ConsumeNull(d.buf[pos:]) == 0 {
   522  			pos, err = d.consumeLiteral(pos, "null")
   523  			if err != nil {
   524  				return Token{}, wrapSyntacticError(d, err, pos, +1)
   525  			}
   526  		} else {
   527  			pos += len("null")
   528  		}
   529  		if err = d.Tokens.appendLiteral(); err != nil {
   530  			return Token{}, wrapSyntacticError(d, err, pos-len("null"), +1) // report position at start of literal
   531  		}
   532  		d.prevStart, d.prevEnd = pos, pos
   533  		return Null, nil
   534  
   535  	case 'f':
   536  		if jsonwire.ConsumeFalse(d.buf[pos:]) == 0 {
   537  			pos, err = d.consumeLiteral(pos, "false")
   538  			if err != nil {
   539  				return Token{}, wrapSyntacticError(d, err, pos, +1)
   540  			}
   541  		} else {
   542  			pos += len("false")
   543  		}
   544  		if err = d.Tokens.appendLiteral(); err != nil {
   545  			return Token{}, wrapSyntacticError(d, err, pos-len("false"), +1) // report position at start of literal
   546  		}
   547  		d.prevStart, d.prevEnd = pos, pos
   548  		return False, nil
   549  
   550  	case 't':
   551  		if jsonwire.ConsumeTrue(d.buf[pos:]) == 0 {
   552  			pos, err = d.consumeLiteral(pos, "true")
   553  			if err != nil {
   554  				return Token{}, wrapSyntacticError(d, err, pos, +1)
   555  			}
   556  		} else {
   557  			pos += len("true")
   558  		}
   559  		if err = d.Tokens.appendLiteral(); err != nil {
   560  			return Token{}, wrapSyntacticError(d, err, pos-len("true"), +1) // report position at start of literal
   561  		}
   562  		d.prevStart, d.prevEnd = pos, pos
   563  		return True, nil
   564  
   565  	case '"':
   566  		var flags jsonwire.ValueFlags // TODO: Preserve this in Token?
   567  		if n = jsonwire.ConsumeSimpleString(d.buf[pos:]); n == 0 {
   568  			oldAbsPos := d.baseOffset + int64(pos)
   569  			pos, err = d.consumeString(&flags, pos)
   570  			newAbsPos := d.baseOffset + int64(pos)
   571  			n = int(newAbsPos - oldAbsPos)
   572  			if err != nil {
   573  				return Token{}, wrapSyntacticError(d, err, pos, +1)
   574  			}
   575  		} else {
   576  			pos += n
   577  		}
   578  		if d.Tokens.Last.NeedObjectName() {
   579  			if !d.Flags.Get(jsonflags.AllowDuplicateNames) {
   580  				if !d.Tokens.Last.isValidNamespace() {
   581  					return Token{}, wrapSyntacticError(d, errInvalidNamespace, pos-n, +1)
   582  				}
   583  				if d.Tokens.Last.isActiveNamespace() && !d.Namespaces.Last().insertQuoted(d.buf[pos-n:pos], flags.IsVerbatim()) {
   584  					err = wrapWithObjectName(ErrDuplicateName, d.buf[pos-n:pos])
   585  					return Token{}, wrapSyntacticError(d, err, pos-n, +1) // report position at start of string
   586  				}
   587  			}
   588  			d.Names.ReplaceLastQuotedOffset(pos - n) // only replace if insertQuoted succeeds
   589  		}
   590  		if err = d.Tokens.appendString(); err != nil {
   591  			return Token{}, wrapSyntacticError(d, err, pos-n, +1) // report position at start of string
   592  		}
   593  		d.prevStart, d.prevEnd = pos-n, pos
   594  		return Token{raw: &d.decodeBuffer, num: uint64(d.previousOffsetStart())}, nil
   595  
   596  	case '0':
   597  		// NOTE: Since JSON numbers are not self-terminating,
   598  		// we need to make sure that the next byte is not part of a number.
   599  		if n = jsonwire.ConsumeSimpleNumber(d.buf[pos:]); n == 0 || d.needMore(pos+n) {
   600  			oldAbsPos := d.baseOffset + int64(pos)
   601  			pos, err = d.consumeNumber(pos)
   602  			newAbsPos := d.baseOffset + int64(pos)
   603  			n = int(newAbsPos - oldAbsPos)
   604  			if err != nil {
   605  				return Token{}, wrapSyntacticError(d, err, pos, +1)
   606  			}
   607  		} else {
   608  			pos += n
   609  		}
   610  		if err = d.Tokens.appendNumber(); err != nil {
   611  			return Token{}, wrapSyntacticError(d, err, pos-n, +1) // report position at start of number
   612  		}
   613  		d.prevStart, d.prevEnd = pos-n, pos
   614  		return Token{raw: &d.decodeBuffer, num: uint64(d.previousOffsetStart())}, nil
   615  
   616  	case '{':
   617  		if err = d.Tokens.pushObject(); err != nil {
   618  			return Token{}, wrapSyntacticError(d, err, pos, +1)
   619  		}
   620  		d.Names.push()
   621  		if !d.Flags.Get(jsonflags.AllowDuplicateNames) {
   622  			d.Namespaces.push()
   623  		}
   624  		pos += 1
   625  		d.prevStart, d.prevEnd = pos, pos
   626  		return BeginObject, nil
   627  
   628  	case '}':
   629  		if err = d.Tokens.popObject(); err != nil {
   630  			return Token{}, wrapSyntacticError(d, err, pos, +1)
   631  		}
   632  		d.Names.pop()
   633  		if !d.Flags.Get(jsonflags.AllowDuplicateNames) {
   634  			d.Namespaces.pop()
   635  		}
   636  		pos += 1
   637  		d.prevStart, d.prevEnd = pos, pos
   638  		return EndObject, nil
   639  
   640  	case '[':
   641  		if err = d.Tokens.pushArray(); err != nil {
   642  			return Token{}, wrapSyntacticError(d, err, pos, +1)
   643  		}
   644  		pos += 1
   645  		d.prevStart, d.prevEnd = pos, pos
   646  		return BeginArray, nil
   647  
   648  	case ']':
   649  		if err = d.Tokens.popArray(); err != nil {
   650  			return Token{}, wrapSyntacticError(d, err, pos, +1)
   651  		}
   652  		pos += 1
   653  		d.prevStart, d.prevEnd = pos, pos
   654  		return EndArray, nil
   655  
   656  	default:
   657  		err = jsonwire.NewInvalidCharacterError(d.buf[pos:], "at start of value")
   658  		return Token{}, wrapSyntacticError(d, err, pos, +1)
   659  	}
   660  }
   661  
   662  // ReadValue returns the next raw JSON value, advancing the read offset.
   663  // The value is stripped of any leading or trailing whitespace and
   664  // contains the exact bytes of the input, which may contain invalid UTF-8
   665  // if [AllowInvalidUTF8] is specified.
   666  //
   667  // The returned value is only valid until the next Peek, Read, or Skip call and
   668  // may not be mutated while the Decoder remains in use.
   669  // If the decoder is currently at the end token for an object or array,
   670  // then it reports a [SyntacticError] and the internal state remains unchanged.
   671  // It returns [io.EOF] if there are no more values.
   672  func (d *Decoder) ReadValue() (Value, error) {
   673  	var flags jsonwire.ValueFlags
   674  	return d.s.ReadValue(&flags)
   675  }
   676  func (d *decoderState) ReadValue(flags *jsonwire.ValueFlags) (Value, error) {
   677  	// Determine the next kind.
   678  	var err error
   679  	var next Kind
   680  	pos := d.peekPos
   681  	if pos != 0 {
   682  		// Use cached peek result.
   683  		if d.peekErr != nil {
   684  			err := d.peekErr
   685  			d.peekPos, d.peekErr = 0, nil // possibly a transient I/O error
   686  			return nil, err
   687  		}
   688  		next = Kind(d.buf[pos]).normalize()
   689  		d.peekPos = 0 // reset cache
   690  	} else {
   691  		d.invalidatePreviousRead()
   692  		pos = d.prevEnd
   693  
   694  		// Consume leading whitespace.
   695  		pos += jsonwire.ConsumeWhitespace(d.buf[pos:])
   696  		if d.needMore(pos) {
   697  			if pos, err = d.consumeWhitespace(pos); err != nil {
   698  				if err == io.ErrUnexpectedEOF && d.Tokens.Depth() == 1 {
   699  					err = io.EOF // EOF possibly if no Tokens present after top-level value
   700  				}
   701  				return nil, wrapSyntacticError(d, err, pos, 0)
   702  			}
   703  		}
   704  
   705  		// Consume colon or comma.
   706  		var delim byte
   707  		if c := d.buf[pos]; c == ':' || c == ',' {
   708  			delim = c
   709  			pos += 1
   710  			pos += jsonwire.ConsumeWhitespace(d.buf[pos:])
   711  			if d.needMore(pos) {
   712  				if pos, err = d.consumeWhitespace(pos); err != nil {
   713  					err = wrapSyntacticError(d, err, pos, 0)
   714  					return nil, d.checkDelimBeforeIOError(delim, err)
   715  				}
   716  			}
   717  		}
   718  		next = Kind(d.buf[pos]).normalize()
   719  		if d.Tokens.needDelim(next) != delim {
   720  			return nil, d.checkDelim(delim, next)
   721  		}
   722  	}
   723  
   724  	// Handle the next value.
   725  	oldAbsPos := d.baseOffset + int64(pos)
   726  	pos, err = d.consumeValue(flags, pos, d.Tokens.Depth())
   727  	newAbsPos := d.baseOffset + int64(pos)
   728  	n := int(newAbsPos - oldAbsPos)
   729  	if err != nil {
   730  		return nil, wrapSyntacticError(d, err, pos, +1)
   731  	}
   732  	switch next {
   733  	case 'n', 't', 'f':
   734  		err = d.Tokens.appendLiteral()
   735  	case '"':
   736  		if d.Tokens.Last.NeedObjectName() {
   737  			if !d.Flags.Get(jsonflags.AllowDuplicateNames) {
   738  				if !d.Tokens.Last.isValidNamespace() {
   739  					err = errInvalidNamespace
   740  					break
   741  				}
   742  				if d.Tokens.Last.isActiveNamespace() && !d.Namespaces.Last().insertQuoted(d.buf[pos-n:pos], flags.IsVerbatim()) {
   743  					err = wrapWithObjectName(ErrDuplicateName, d.buf[pos-n:pos])
   744  					break
   745  				}
   746  			}
   747  			d.Names.ReplaceLastQuotedOffset(pos - n) // only replace if insertQuoted succeeds
   748  		}
   749  		err = d.Tokens.appendString()
   750  	case '0':
   751  		err = d.Tokens.appendNumber()
   752  	case '{':
   753  		if err = d.Tokens.pushObject(); err != nil {
   754  			break
   755  		}
   756  		if err = d.Tokens.popObject(); err != nil {
   757  			panic("BUG: popObject should never fail immediately after pushObject: " + err.Error())
   758  		}
   759  	case '[':
   760  		if err = d.Tokens.pushArray(); err != nil {
   761  			break
   762  		}
   763  		if err = d.Tokens.popArray(); err != nil {
   764  			panic("BUG: popArray should never fail immediately after pushArray: " + err.Error())
   765  		}
   766  	}
   767  	if err != nil {
   768  		return nil, wrapSyntacticError(d, err, pos-n, +1) // report position at start of value
   769  	}
   770  	d.prevEnd = pos
   771  	d.prevStart = pos - n
   772  	return d.buf[pos-n : pos : pos], nil
   773  }
   774  
   775  // CheckNextValue checks whether the next value is syntactically valid,
   776  // but does not advance the read offset.
   777  // If last, it verifies that the stream cleanly terminates with [io.EOF].
   778  func (d *decoderState) CheckNextValue(last bool) error {
   779  	d.PeekKind() // populates d.peekPos and d.peekErr
   780  	pos, err := d.peekPos, d.peekErr
   781  	d.peekPos, d.peekErr = 0, nil
   782  	if err != nil {
   783  		return err
   784  	}
   785  
   786  	var flags jsonwire.ValueFlags
   787  	if pos, err := d.consumeValue(&flags, pos, d.Tokens.Depth()); err != nil {
   788  		return wrapSyntacticError(d, err, pos, +1)
   789  	} else if last {
   790  		return d.checkEOF(pos)
   791  	}
   792  	return nil
   793  }
   794  
   795  // AtEOF reports whether the decoder is at EOF.
   796  func (d *decoderState) AtEOF() bool {
   797  	_, err := d.consumeWhitespace(d.prevEnd)
   798  	return err == io.ErrUnexpectedEOF
   799  }
   800  
   801  // CheckEOF verifies that the input has no more data.
   802  func (d *decoderState) CheckEOF() error {
   803  	return d.checkEOF(d.prevEnd)
   804  }
   805  func (d *decoderState) checkEOF(pos int) error {
   806  	switch pos, err := d.consumeWhitespace(pos); err {
   807  	case nil:
   808  		err := jsonwire.NewInvalidCharacterError(d.buf[pos:], "after top-level value")
   809  		return wrapSyntacticError(d, err, pos, 0)
   810  	case io.ErrUnexpectedEOF:
   811  		return nil
   812  	default:
   813  		return err
   814  	}
   815  }
   816  
   817  // consumeWhitespace consumes all whitespace starting at d.buf[pos:].
   818  // It returns the new position in d.buf immediately after the last whitespace.
   819  // If it returns nil, there is guaranteed to at least be one unread byte.
   820  //
   821  // The following pattern is common in this implementation:
   822  //
   823  //	pos += jsonwire.ConsumeWhitespace(d.buf[pos:])
   824  //	if d.needMore(pos) {
   825  //		if pos, err = d.consumeWhitespace(pos); err != nil {
   826  //			return ...
   827  //		}
   828  //	}
   829  //
   830  // It is difficult to simplify this without sacrificing performance since
   831  // consumeWhitespace must be inlined. The body of the if statement is
   832  // executed only in rare situations where we need to fetch more data.
   833  // Since fetching may return an error, we also need to check the error.
   834  func (d *decoderState) consumeWhitespace(pos int) (newPos int, err error) {
   835  	for {
   836  		pos += jsonwire.ConsumeWhitespace(d.buf[pos:])
   837  		if d.needMore(pos) {
   838  			absPos := d.baseOffset + int64(pos)
   839  			err = d.fetch() // will mutate d.buf and invalidate pos
   840  			pos = int(absPos - d.baseOffset)
   841  			if err != nil {
   842  				return pos, err
   843  			}
   844  			continue
   845  		}
   846  		return pos, nil
   847  	}
   848  }
   849  
   850  // consumeValue consumes a single JSON value starting at d.buf[pos:].
   851  // It returns the new position in d.buf immediately after the value.
   852  func (d *decoderState) consumeValue(flags *jsonwire.ValueFlags, pos, depth int) (newPos int, err error) {
   853  	for {
   854  		var n int
   855  		var err error
   856  		switch next := Kind(d.buf[pos]).normalize(); next {
   857  		case 'n':
   858  			if n = jsonwire.ConsumeNull(d.buf[pos:]); n == 0 {
   859  				n, err = jsonwire.ConsumeLiteral(d.buf[pos:], "null")
   860  			}
   861  		case 'f':
   862  			if n = jsonwire.ConsumeFalse(d.buf[pos:]); n == 0 {
   863  				n, err = jsonwire.ConsumeLiteral(d.buf[pos:], "false")
   864  			}
   865  		case 't':
   866  			if n = jsonwire.ConsumeTrue(d.buf[pos:]); n == 0 {
   867  				n, err = jsonwire.ConsumeLiteral(d.buf[pos:], "true")
   868  			}
   869  		case '"':
   870  			if n = jsonwire.ConsumeSimpleString(d.buf[pos:]); n == 0 {
   871  				return d.consumeString(flags, pos)
   872  			}
   873  		case '0':
   874  			// NOTE: Since JSON numbers are not self-terminating,
   875  			// we need to make sure that the next byte is not part of a number.
   876  			if n = jsonwire.ConsumeSimpleNumber(d.buf[pos:]); n == 0 || d.needMore(pos+n) {
   877  				return d.consumeNumber(pos)
   878  			}
   879  		case '{':
   880  			return d.consumeObject(flags, pos, depth)
   881  		case '[':
   882  			return d.consumeArray(flags, pos, depth)
   883  		default:
   884  			if (d.Tokens.Last.isObject() && next == ']') || (d.Tokens.Last.isArray() && next == '}') {
   885  				return pos, errMismatchDelim
   886  			}
   887  			return pos, jsonwire.NewInvalidCharacterError(d.buf[pos:], "at start of value")
   888  		}
   889  		if err == io.ErrUnexpectedEOF {
   890  			absPos := d.baseOffset + int64(pos)
   891  			err = d.fetch() // will mutate d.buf and invalidate pos
   892  			pos = int(absPos - d.baseOffset)
   893  			if err != nil {
   894  				return pos + n, err
   895  			}
   896  			continue
   897  		}
   898  		return pos + n, err
   899  	}
   900  }
   901  
   902  // consumeLiteral consumes a single JSON literal starting at d.buf[pos:].
   903  // It returns the new position in d.buf immediately after the literal.
   904  func (d *decoderState) consumeLiteral(pos int, lit string) (newPos int, err error) {
   905  	for {
   906  		n, err := jsonwire.ConsumeLiteral(d.buf[pos:], lit)
   907  		if err == io.ErrUnexpectedEOF {
   908  			absPos := d.baseOffset + int64(pos)
   909  			err = d.fetch() // will mutate d.buf and invalidate pos
   910  			pos = int(absPos - d.baseOffset)
   911  			if err != nil {
   912  				return pos + n, err
   913  			}
   914  			continue
   915  		}
   916  		return pos + n, err
   917  	}
   918  }
   919  
   920  // consumeString consumes a single JSON string starting at d.buf[pos:].
   921  // It returns the new position in d.buf immediately after the string.
   922  func (d *decoderState) consumeString(flags *jsonwire.ValueFlags, pos int) (newPos int, err error) {
   923  	var n int
   924  	for {
   925  		n, err = jsonwire.ConsumeStringResumable(flags, d.buf[pos:], n, !d.Flags.Get(jsonflags.AllowInvalidUTF8))
   926  		if err == io.ErrUnexpectedEOF {
   927  			absPos := d.baseOffset + int64(pos)
   928  			err = d.fetch() // will mutate d.buf and invalidate pos
   929  			pos = int(absPos - d.baseOffset)
   930  			if err != nil {
   931  				return pos + n, err
   932  			}
   933  			continue
   934  		}
   935  		return pos + n, err
   936  	}
   937  }
   938  
   939  // consumeNumber consumes a single JSON number starting at d.buf[pos:].
   940  // It returns the new position in d.buf immediately after the number.
   941  func (d *decoderState) consumeNumber(pos int) (newPos int, err error) {
   942  	var n int
   943  	var state jsonwire.ConsumeNumberState
   944  	for {
   945  		n, state, err = jsonwire.ConsumeNumberResumable(d.buf[pos:], n, state)
   946  		// NOTE: Since JSON numbers are not self-terminating,
   947  		// we need to make sure that the next byte is not part of a number.
   948  		if err == io.ErrUnexpectedEOF || d.needMore(pos+n) {
   949  			mayTerminate := err == nil
   950  			absPos := d.baseOffset + int64(pos)
   951  			err = d.fetch() // will mutate d.buf and invalidate pos
   952  			pos = int(absPos - d.baseOffset)
   953  			if err != nil {
   954  				if mayTerminate && err == io.ErrUnexpectedEOF {
   955  					return pos + n, nil
   956  				}
   957  				return pos, err
   958  			}
   959  			continue
   960  		}
   961  		return pos + n, err
   962  	}
   963  }
   964  
   965  // consumeObject consumes a single JSON object starting at d.buf[pos:].
   966  // It returns the new position in d.buf immediately after the object.
   967  func (d *decoderState) consumeObject(flags *jsonwire.ValueFlags, pos, depth int) (newPos int, err error) {
   968  	var n int
   969  	var names *objectNamespace
   970  	if !d.Flags.Get(jsonflags.AllowDuplicateNames) {
   971  		d.Namespaces.push()
   972  		defer d.Namespaces.pop()
   973  		names = d.Namespaces.Last()
   974  	}
   975  
   976  	// Handle before start.
   977  	if uint(pos) >= uint(len(d.buf)) || d.buf[pos] != '{' {
   978  		panic("BUG: consumeObject must be called with a buffer that starts with '{'")
   979  	} else if depth == maxNestingDepth+1 {
   980  		return pos, errMaxDepth
   981  	}
   982  	pos++
   983  
   984  	// Handle after start.
   985  	pos += jsonwire.ConsumeWhitespace(d.buf[pos:])
   986  	if d.needMore(pos) {
   987  		if pos, err = d.consumeWhitespace(pos); err != nil {
   988  			return pos, err
   989  		}
   990  	}
   991  	if d.buf[pos] == '}' {
   992  		pos++
   993  		return pos, nil
   994  	}
   995  
   996  	depth++
   997  	for {
   998  		// Handle before name.
   999  		pos += jsonwire.ConsumeWhitespace(d.buf[pos:])
  1000  		if d.needMore(pos) {
  1001  			if pos, err = d.consumeWhitespace(pos); err != nil {
  1002  				return pos, err
  1003  			}
  1004  		}
  1005  		var flags2 jsonwire.ValueFlags
  1006  		if n = jsonwire.ConsumeSimpleString(d.buf[pos:]); n == 0 {
  1007  			oldAbsPos := d.baseOffset + int64(pos)
  1008  			pos, err = d.consumeString(&flags2, pos)
  1009  			newAbsPos := d.baseOffset + int64(pos)
  1010  			n = int(newAbsPos - oldAbsPos)
  1011  			flags.Join(flags2)
  1012  			if err != nil {
  1013  				return pos, err
  1014  			}
  1015  		} else {
  1016  			pos += n
  1017  		}
  1018  		quotedName := d.buf[pos-n : pos]
  1019  		if !d.Flags.Get(jsonflags.AllowDuplicateNames) && !names.insertQuoted(quotedName, flags2.IsVerbatim()) {
  1020  			return pos - n, wrapWithObjectName(ErrDuplicateName, quotedName)
  1021  		}
  1022  
  1023  		// Handle after name.
  1024  		pos += jsonwire.ConsumeWhitespace(d.buf[pos:])
  1025  		if d.needMore(pos) {
  1026  			if pos, err = d.consumeWhitespace(pos); err != nil {
  1027  				return pos, wrapWithObjectName(err, quotedName)
  1028  			}
  1029  		}
  1030  		if d.buf[pos] != ':' {
  1031  			err := jsonwire.NewInvalidCharacterError(d.buf[pos:], "after object name (expecting ':')")
  1032  			return pos, wrapWithObjectName(err, quotedName)
  1033  		}
  1034  		pos++
  1035  
  1036  		// Handle before value.
  1037  		pos += jsonwire.ConsumeWhitespace(d.buf[pos:])
  1038  		if d.needMore(pos) {
  1039  			if pos, err = d.consumeWhitespace(pos); err != nil {
  1040  				return pos, wrapWithObjectName(err, quotedName)
  1041  			}
  1042  		}
  1043  		pos, err = d.consumeValue(flags, pos, depth)
  1044  		if err != nil {
  1045  			return pos, wrapWithObjectName(err, quotedName)
  1046  		}
  1047  
  1048  		// Handle after value.
  1049  		pos += jsonwire.ConsumeWhitespace(d.buf[pos:])
  1050  		if d.needMore(pos) {
  1051  			if pos, err = d.consumeWhitespace(pos); err != nil {
  1052  				return pos, err
  1053  			}
  1054  		}
  1055  		switch d.buf[pos] {
  1056  		case ',':
  1057  			pos++
  1058  			continue
  1059  		case '}':
  1060  			pos++
  1061  			return pos, nil
  1062  		default:
  1063  			return pos, jsonwire.NewInvalidCharacterError(d.buf[pos:], "after object value (expecting ',' or '}')")
  1064  		}
  1065  	}
  1066  }
  1067  
  1068  // consumeArray consumes a single JSON array starting at d.buf[pos:].
  1069  // It returns the new position in d.buf immediately after the array.
  1070  func (d *decoderState) consumeArray(flags *jsonwire.ValueFlags, pos, depth int) (newPos int, err error) {
  1071  	// Handle before start.
  1072  	if uint(pos) >= uint(len(d.buf)) || d.buf[pos] != '[' {
  1073  		panic("BUG: consumeArray must be called with a buffer that starts with '['")
  1074  	} else if depth == maxNestingDepth+1 {
  1075  		return pos, errMaxDepth
  1076  	}
  1077  	pos++
  1078  
  1079  	// Handle after start.
  1080  	pos += jsonwire.ConsumeWhitespace(d.buf[pos:])
  1081  	if d.needMore(pos) {
  1082  		if pos, err = d.consumeWhitespace(pos); err != nil {
  1083  			return pos, err
  1084  		}
  1085  	}
  1086  	if d.buf[pos] == ']' {
  1087  		pos++
  1088  		return pos, nil
  1089  	}
  1090  
  1091  	var idx int64
  1092  	depth++
  1093  	for {
  1094  		// Handle before value.
  1095  		pos += jsonwire.ConsumeWhitespace(d.buf[pos:])
  1096  		if d.needMore(pos) {
  1097  			if pos, err = d.consumeWhitespace(pos); err != nil {
  1098  				return pos, err
  1099  			}
  1100  		}
  1101  		pos, err = d.consumeValue(flags, pos, depth)
  1102  		if err != nil {
  1103  			return pos, wrapWithArrayIndex(err, idx)
  1104  		}
  1105  
  1106  		// Handle after value.
  1107  		pos += jsonwire.ConsumeWhitespace(d.buf[pos:])
  1108  		if d.needMore(pos) {
  1109  			if pos, err = d.consumeWhitespace(pos); err != nil {
  1110  				return pos, err
  1111  			}
  1112  		}
  1113  		switch d.buf[pos] {
  1114  		case ',':
  1115  			pos++
  1116  			idx++
  1117  			continue
  1118  		case ']':
  1119  			pos++
  1120  			return pos, nil
  1121  		default:
  1122  			return pos, jsonwire.NewInvalidCharacterError(d.buf[pos:], "after array element (expecting ',' or ']')")
  1123  		}
  1124  	}
  1125  }
  1126  
  1127  // InputOffset returns the current input byte offset. It gives the location
  1128  // of the next byte immediately after the most recently returned token or value.
  1129  // The number of bytes actually read from the underlying [io.Reader] may be more
  1130  // than this offset due to internal buffering effects.
  1131  func (d *Decoder) InputOffset() int64 {
  1132  	return d.s.previousOffsetEnd()
  1133  }
  1134  
  1135  // UnreadBuffer returns the data remaining in the unread buffer,
  1136  // which may contain zero or more bytes.
  1137  // The returned buffer must not be mutated while Decoder continues to be used.
  1138  // The buffer contents are valid until the next Peek, Read, or Skip call.
  1139  func (d *Decoder) UnreadBuffer() []byte {
  1140  	return d.s.unreadBuffer()
  1141  }
  1142  
  1143  // StackDepth returns the depth of the state machine for read JSON data.
  1144  // Each level on the stack represents a nested JSON object or array.
  1145  // It is incremented whenever an [BeginObject] or [BeginArray] token is encountered
  1146  // and decremented whenever an [EndObject] or [EndArray] token is encountered.
  1147  // The depth is zero-indexed, where zero represents the top-level JSON value.
  1148  func (d *Decoder) StackDepth() int {
  1149  	// NOTE: Keep in sync with Encoder.StackDepth.
  1150  	return d.s.Tokens.Depth() - 1
  1151  }
  1152  
  1153  // StackIndex returns information about the specified stack level.
  1154  // It must be a number between 0 and [Decoder.StackDepth], inclusive.
  1155  // For each level, it reports the kind:
  1156  //
  1157  //   - [KindInvalid] for a level of zero,
  1158  //   - [KindBeginObject] for a level representing a JSON object, and
  1159  //   - [KindBeginArray] for a level representing a JSON array.
  1160  //
  1161  // It also reports the length of that JSON object or array.
  1162  // Each name and value in a JSON object is counted separately,
  1163  // so the effective number of members would be half the length.
  1164  // A complete JSON object must have an even length.
  1165  func (d *Decoder) StackIndex(i int) (Kind, int64) {
  1166  	// NOTE: Keep in sync with Encoder.StackIndex.
  1167  	switch s := d.s.Tokens.index(i); {
  1168  	case i > 0 && s.isObject():
  1169  		return '{', s.Length()
  1170  	case i > 0 && s.isArray():
  1171  		return '[', s.Length()
  1172  	default:
  1173  		return 0, s.Length()
  1174  	}
  1175  }
  1176  
  1177  // StackPointer returns a JSON Pointer (RFC 6901) to the most recently read value.
  1178  func (d *Decoder) StackPointer() Pointer {
  1179  	return Pointer(d.s.AppendStackPointer(nil, -1))
  1180  }
  1181  
  1182  func (d *decoderState) AppendStackPointer(b []byte, where int) []byte {
  1183  	d.Names.copyQuotedBuffer(d.buf)
  1184  	return d.state.appendStackPointer(b, where)
  1185  }
  1186  

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