src/pkg/scanner/scanner.go - go - Git at Google

 // Copyright 2009 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 // A scanner and tokenizer for UTF-8-encoded text.  Takes an io.Reader
 // providing the source, which then can be tokenized through repeated calls
 // to the Scan function.  For compatibility with existing tools, the NUL
 // character is not allowed (implementation restriction).
 //
 // By default, a Scanner skips white space and Go comments and recognizes all
 // literals as defined by the Go language specification.  It may be
 // customized to recognize only a subset of those literals and to recognize
 // different white space characters.
 //
 // Basic usage pattern:
 //
 //	var s scanner.Scanner
 //	s.Init(src)
 //	tok := s.Scan()
 //	for tok != scanner.EOF {
 //		// do something with tok
 //		tok = s.Scan()
 //	}
 //
 package scanner

 import (
 	"bytes"
 	"fmt"
 	"io"
 	"os"
 	"unicode"
 	"utf8"
 )


 // TODO(gri): Consider changing this to use the new (token) Position package.

 // A source position is represented by a Position value.
 // A position is valid if Line > 0.
 type Position struct {
 	Filename string // filename, if any
 	Offset   int    // byte offset, starting at 0
 	Line     int    // line number, starting at 1
 	Column   int    // column number, starting at 1 (character count per line)
 }


 // IsValid returns true if the position is valid.
 func (pos *Position) IsValid() bool { return pos.Line > 0 }


 func (pos Position) String() string {
 	s := pos.Filename
 	if pos.IsValid() {
 		if s != "" {
 			s += ":"
 		}
 		s += fmt.Sprintf("%d:%d", pos.Line, pos.Column)
 	}
 	if s == "" {
 		s = "???"
 	}
 	return s
 }


 // Predefined mode bits to control recognition of tokens. For instance,
 // to configure a Scanner such that it only recognizes (Go) identifiers,
 // integers, and skips comments, set the Scanner's Mode field to:
 //
 //	ScanIdents | ScanInts | SkipComments
 //
 const (
 	ScanIdents     = 1 << -Ident
 	ScanInts       = 1 << -Int
 	ScanFloats     = 1 << -Float // includes Ints
 	ScanChars      = 1 << -Char
 	ScanStrings    = 1 << -String
 	ScanRawStrings = 1 << -RawString
 	ScanComments   = 1 << -Comment
 	SkipComments   = 1 << -skipComment // if set with ScanComments, comments become white space
 	GoTokens       = ScanIdents | ScanFloats | ScanChars | ScanStrings | ScanRawStrings | ScanComments | SkipComments
 )


 // The result of Scan is one of the following tokens or a Unicode character.
 const (
 	EOF = -(iota + 1)
 	Ident
 	Int
 	Float
 	Char
 	String
 	RawString
 	Comment
 	skipComment
 )


 var tokenString = map[int]string{
 	EOF:       "EOF",
 	Ident:     "Ident",
 	Int:       "Int",
 	Float:     "Float",
 	Char:      "Char",
 	String:    "String",
 	RawString: "RawString",
 	Comment:   "Comment",
 }


 // TokenString returns a (visible) string for a token or Unicode character.
 func TokenString(tok int) string {
 	if s, found := tokenString[tok]; found {
 		return s
 	}
 	return fmt.Sprintf("U+%04X", tok)
 }


 // GoWhitespace is the default value for the Scanner's Whitespace field.
 // Its value selects Go's white space characters.
 const GoWhitespace = 1<<'\t' | 1<<'\n' | 1<<'\r' | 1<<' '


 const bufLen = 1024 // at least utf8.UTFMax

 // A Scanner implements reading of Unicode characters and tokens from an io.Reader.
 type Scanner struct {
 	// Input
 	src io.Reader

 	// Source buffer
 	srcBuf [bufLen + 1]byte // +1 for sentinel for common case of s.next()
 	srcPos int              // reading position (srcBuf index)
 	srcEnd int              // source end (srcBuf index)

 	// Source position
 	srcBufOffset int // byte offset of srcBuf[0] in source
 	line         int // line count
 	column       int // character count
 	lastLineLen  int // length of last line in characters (for correct column reporting)
 	lastCharLen  int // length of last character in bytes

 	// Token text buffer
 	// Typically, token text is stored completely in srcBuf, but in general
 	// the token text's head may be buffered in tokBuf while the token text's
 	// tail is stored in srcBuf.
 	tokBuf bytes.Buffer // token text head that is not in srcBuf anymore
 	tokPos int          // token text tail position (srcBuf index); valid if >= 0
 	tokEnd int          // token text tail end (srcBuf index)

 	// One character look-ahead
 	ch int // character before current srcPos

 	// Error is called for each error encountered. If no Error
 	// function is set, the error is reported to os.Stderr.
 	Error func(s *Scanner, msg string)

 	// ErrorCount is incremented by one for each error encountered.
 	ErrorCount int

 	// The Mode field controls which tokens are recognized. For instance,
 	// to recognize Ints, set the ScanInts bit in Mode. The field may be
 	// changed at any time.
 	Mode uint

 	// The Whitespace field controls which characters are recognized
 	// as white space. To recognize a character ch <= ' ' as white space,
 	// set the ch'th bit in Whitespace (the Scanner's behavior is undefined
 	// for values ch > ' '). The field may be changed at any time.
 	Whitespace uint64

 	// Current token position. The Offset, Line, and Column fields
 	// are set by Scan(); the Filename field is left untouched by the
 	// Scanner.
 	Position
 }


 // Init initializes a Scanner with a new source and returns s.
 // Error is set to nil, ErrorCount is set to 0, Mode is set to GoTokens,
 // and Whitespace is set to GoWhitespace.
 func (s *Scanner) Init(src io.Reader) *Scanner {
 	s.src = src

 	// initialize source buffer
 	// (the first call to next() will fill it by calling src.Read)
 	s.srcBuf[0] = utf8.RuneSelf // sentinel
 	s.srcPos = 0
 	s.srcEnd = 0

 	// initialize source position
 	s.srcBufOffset = 0
 	s.line = 1
 	s.column = 0
 	s.lastLineLen = 0
 	s.lastCharLen = 0

 	// initialize token text buffer
 	// (required for first call to next()).
 	s.tokPos = -1

 	// initialize one character look-ahead
 	s.ch = -1 // no char read yet

 	// initialize public fields
 	s.Error = nil
 	s.ErrorCount = 0
 	s.Mode = GoTokens
 	s.Whitespace = GoWhitespace

 	return s
 }


 // TODO(gri): The code for next() and the internal scanner state could benefit
 //            from a rethink. While next() is optimized for the common ASCII
 //            case, the "corrections" needed for proper position tracking undo
 //            some of the attempts for fast-path optimization.

 // next reads and returns the next Unicode character. It is designed such
 // that only a minimal amount of work needs to be done in the common ASCII
 // case (one test to check for both ASCII and end-of-buffer, and one test
 // to check for newlines).
 func (s *Scanner) next() int {
 	ch, width := int(s.srcBuf[s.srcPos]), 1

 	if ch >= utf8.RuneSelf {
 		// uncommon case: not ASCII or not enough bytes
 		for s.srcPos+utf8.UTFMax > s.srcEnd && !utf8.FullRune(s.srcBuf[s.srcPos:s.srcEnd]) {
 			// not enough bytes: read some more, but first
 			// save away token text if any
 			if s.tokPos >= 0 {
 				s.tokBuf.Write(s.srcBuf[s.tokPos:s.srcPos])
 				s.tokPos = 0
 				// s.tokEnd is set by Scan()
 			}
 			// move unread bytes to beginning of buffer
 			copy(s.srcBuf[0:], s.srcBuf[s.srcPos:s.srcEnd])
 			s.srcBufOffset += s.srcPos
 			// read more bytes
 			// (an io.Reader must return os.EOF when it reaches
 			// the end of what it is reading - simply returning
 			// n == 0 will make this loop retry forever; but the
 			// error is in the reader implementation in that case)
 			i := s.srcEnd - s.srcPos
 			n, err := s.src.Read(s.srcBuf[i:bufLen])
 			s.srcPos = 0
 			s.srcEnd = i + n
 			s.srcBuf[s.srcEnd] = utf8.RuneSelf // sentinel
 			if err != nil {
 				if s.srcEnd == 0 {
 					if s.lastCharLen > 0 {
 						// previous character was not EOF
 						s.column++
 					}
 					s.lastCharLen = 0
 					return EOF
 				}
 				if err != os.EOF {
 					s.error(err.String())
 				}
 				// If err == EOF, we won't be getting more
 				// bytes; break to avoid infinite loop. If
 				// err is something else, we don't know if
 				// we can get more bytes; thus also break.
 				break
 			}
 		}
 		// at least one byte
 		ch = int(s.srcBuf[s.srcPos])
 		if ch >= utf8.RuneSelf {
 			// uncommon case: not ASCII
 			ch, width = utf8.DecodeRune(s.srcBuf[s.srcPos:s.srcEnd])
 			if ch == utf8.RuneError && width == 1 {
 				s.error("illegal UTF-8 encoding")
 			}
 		}
 	}

 	// advance
 	s.srcPos += width
 	s.lastCharLen = width
 	s.column++

 	// special situations
 	switch ch {
 	case 0:
 		// implementation restriction for compatibility with other tools
 		s.error("illegal character NUL")
 	case '\n':
 		s.line++
 		s.lastLineLen = s.column
 		s.column = 0
 	}

 	return ch
 }


 // Next reads and returns the next Unicode character.
 // It returns EOF at the end of the source. It reports
 // a read error by calling s.Error, if not nil; otherwise
 // it prints an error message to os.Stderr. Next does not
 // update the Scanner's Position field; use Pos() to
 // get the current position.
 func (s *Scanner) Next() int {
 	s.tokPos = -1 // don't collect token text
 	ch := s.Peek()
 	s.ch = s.next()
 	return ch
 }


 // Peek returns the next Unicode character in the source without advancing
 // the scanner. It returns EOF if the scanner's position is at the last
 // character of the source.
 func (s *Scanner) Peek() int {
 	if s.ch < 0 {
 		s.ch = s.next()
 	}
 	return s.ch
 }


 func (s *Scanner) error(msg string) {
 	s.ErrorCount++
 	if s.Error != nil {
 		s.Error(s, msg)
 		return
 	}
 	fmt.Fprintf(os.Stderr, "%s: %s", s.Position, msg)
 }


 func (s *Scanner) scanIdentifier() int {
 	ch := s.next() // read character after first '_' or letter
 	for ch == '_' || unicode.IsLetter(ch) || unicode.IsDigit(ch) {
 		ch = s.next()
 	}
 	return ch
 }


 func digitVal(ch int) int {
 	switch {
 	case '0' <= ch && ch <= '9':
 		return ch - '0'
 	case 'a' <= ch && ch <= 'f':
 		return ch - 'a' + 10
 	case 'A' <= ch && ch <= 'F':
 		return ch - 'A' + 10
 	}
 	return 16 // larger than any legal digit val
 }


 func isDecimal(ch int) bool { return '0' <= ch && ch <= '9' }


 func (s *Scanner) scanMantissa(ch int) int {
 	for isDecimal(ch) {
 		ch = s.next()
 	}
 	return ch
 }


 func (s *Scanner) scanFraction(ch int) int {
 	if ch == '.' {
 		ch = s.scanMantissa(s.next())
 	}
 	return ch
 }


 func (s *Scanner) scanExponent(ch int) int {
 	if ch == 'e' || ch == 'E' {
 		ch = s.next()
 		if ch == '-' || ch == '+' {
 			ch = s.next()
 		}
 		ch = s.scanMantissa(ch)
 	}
 	return ch
 }


 func (s *Scanner) scanNumber(ch int) (int, int) {
 	// isDecimal(ch)
 	if ch == '0' {
 		// int or float
 		ch = s.next()
 		if ch == 'x' || ch == 'X' {
 			// hexadecimal int
 			ch = s.next()
 			for digitVal(ch) < 16 {
 				ch = s.next()
 			}
 		} else {
 			// octal int or float
 			seenDecimalDigit := false
 			for isDecimal(ch) {
 				if ch > '7' {
 					seenDecimalDigit = true
 				}
 				ch = s.next()
 			}
 			if s.Mode&ScanFloats != 0 && (ch == '.' || ch == 'e' || ch == 'E') {
 				// float
 				ch = s.scanFraction(ch)
 				ch = s.scanExponent(ch)
 				return Float, ch
 			}
 			// octal int
 			if seenDecimalDigit {
 				s.error("illegal octal number")
 			}
 		}
 		return Int, ch
 	}
 	// decimal int or float
 	ch = s.scanMantissa(ch)
 	if s.Mode&ScanFloats != 0 && (ch == '.' || ch == 'e' || ch == 'E') {
 		// float
 		ch = s.scanFraction(ch)
 		ch = s.scanExponent(ch)
 		return Float, ch
 	}
 	return Int, ch
 }


 func (s *Scanner) scanDigits(ch, base, n int) int {
 	for n > 0 && digitVal(ch) < base {
 		ch = s.next()
 		n--
 	}
 	if n > 0 {
 		s.error("illegal char escape")
 	}
 	return ch
 }


 func (s *Scanner) scanEscape(quote int) int {
 	ch := s.next() // read character after '/'
 	switch ch {
 	case 'a', 'b', 'f', 'n', 'r', 't', 'v', '\\', quote:
 		// nothing to do
 		ch = s.next()
 	case '0', '1', '2', '3', '4', '5', '6', '7':
 		ch = s.scanDigits(ch, 8, 3)
 	case 'x':
 		ch = s.scanDigits(s.next(), 16, 2)
 	case 'u':
 		ch = s.scanDigits(s.next(), 16, 4)
 	case 'U':
 		ch = s.scanDigits(s.next(), 16, 8)
 	default:
 		s.error("illegal char escape")
 	}
 	return ch
 }


 func (s *Scanner) scanString(quote int) (n int) {
 	ch := s.next() // read character after quote
 	for ch != quote {
 		if ch == '\n' || ch < 0 {
 			s.error("literal not terminated")
 			return
 		}
 		if ch == '\\' {
 			ch = s.scanEscape(quote)
 		} else {
 			ch = s.next()
 		}
 		n++
 	}
 	return
 }


 func (s *Scanner) scanRawString() {
 	ch := s.next() // read character after '`'
 	for ch != '`' {
 		if ch < 0 {
 			s.error("literal not terminated")
 			return
 		}
 		ch = s.next()
 	}
 }


 func (s *Scanner) scanChar() {
 	if s.scanString('\'') != 1 {
 		s.error("illegal char literal")
 	}
 }


 func (s *Scanner) scanLineComment() {
 	ch := s.next() // read character after "//"
 	for ch != '\n' {
 		if ch < 0 {
 			s.error("comment not terminated")
 			return
 		}
 		ch = s.next()
 	}
 }


 func (s *Scanner) scanGeneralComment() {
 	ch := s.next() // read character after "/*"
 	for {
 		if ch < 0 {
 			s.error("comment not terminated")
 			return
 		}
 		ch0 := ch
 		ch = s.next()
 		if ch0 == '*' && ch == '/' {
 			break
 		}
 	}
 }


 func (s *Scanner) scanComment(ch int) {
 	// ch == '/' || ch == '*'
 	if ch == '/' {
 		s.scanLineComment()
 		return
 	}
 	s.scanGeneralComment()
 }


 // Scan reads the next token or Unicode character from source and returns it.
 // It only recognizes tokens t for which the respective Mode bit (1<<-t) is set.
 // It returns EOF at the end of the source. It reports scanner errors (read and
 // token errors) by calling s.Error, if not nil; otherwise it prints an error
 // message to os.Stderr.
 func (s *Scanner) Scan() int {
 	ch := s.Peek()

 	// reset token text position
 	s.tokPos = -1

 redo:
 	// skip white space
 	for s.Whitespace&(1<<uint(ch)) != 0 {
 		ch = s.next()
 	}

 	// start collecting token text
 	s.tokBuf.Reset()
 	s.tokPos = s.srcPos - s.lastCharLen

 	// set token position
 	// (this is a slightly optimized version of the code in Pos())
 	s.Offset = s.srcBufOffset + s.tokPos
 	if s.column > 0 {
 		// common case: last character was not a '\n'
 		s.Line = s.line
 		s.Column = s.column
 	} else {
 		// last character was a '\n'
 		// (we cannot be at the beginning of the source
 		// since we have called next() at least once)
 		s.Line = s.line - 1
 		s.Column = s.lastLineLen
 	}

 	// determine token value
 	tok := ch
 	switch {
 	case unicode.IsLetter(ch) || ch == '_':
 		if s.Mode&ScanIdents != 0 {
 			tok = Ident
 			ch = s.scanIdentifier()
 		} else {
 			ch = s.next()
 		}
 	case isDecimal(ch):
 		if s.Mode&(ScanInts|ScanFloats) != 0 {
 			tok, ch = s.scanNumber(ch)
 		} else {
 			ch = s.next()
 		}
 	default:
 		switch ch {
 		case '"':
 			if s.Mode&ScanStrings != 0 {
 				s.scanString('"')
 				tok = String
 			}
 			ch = s.next()
 		case '\'':
 			if s.Mode&ScanChars != 0 {
 				s.scanChar()
 				tok = Char
 			}
 			ch = s.next()
 		case '.':
 			ch = s.next()
 			if isDecimal(ch) && s.Mode&ScanFloats != 0 {
 				tok = Float
 				ch = s.scanMantissa(ch)
 				ch = s.scanExponent(ch)
 			}
 		case '/':
 			ch = s.next()
 			if (ch == '/' || ch == '*') && s.Mode&ScanComments != 0 {
 				if s.Mode&SkipComments != 0 {
 					s.tokPos = -1 // don't collect token text
 					s.scanComment(ch)
 					ch = s.next()
 					goto redo
 				}
 				s.scanComment(ch)
 				tok = Comment
 				ch = s.next()
 			}
 		case '`':
 			if s.Mode&ScanRawStrings != 0 {
 				s.scanRawString()
 				tok = String
 			}
 			ch = s.next()
 		default:
 			ch = s.next()
 		}
 	}

 	// end of token text
 	s.tokEnd = s.srcPos - s.lastCharLen

 	s.ch = ch
 	return tok
 }


 // Pos returns the position of the character immediately after
 // the character or token returned by the last call to Next or Scan.
 func (s *Scanner) Pos() (pos Position) {
 	pos.Filename = s.Filename
 	pos.Offset = s.srcBufOffset + s.srcPos - s.lastCharLen
 	switch {
 	case s.column > 0:
 		// common case: last character was not a '\n'
 		pos.Line = s.line
 		pos.Column = s.column
 	case s.lastLineLen > 0:
 		// last character was a '\n'
 		pos.Line = s.line - 1
 		pos.Column = s.lastLineLen
 	default:
 		// at the beginning of the source
 		pos.Line = 1
 		pos.Column = 1
 	}
 	return
 }


 // TokenText returns the string corresponding to the most recently scanned token.
 // Valid after calling Scan().
 func (s *Scanner) TokenText() string {
 	if s.tokPos < 0 {
 		// no token text
 		return ""
 	}

 	if s.tokEnd < 0 {
 		// if EOF was reached, s.tokEnd is set to -1 (s.srcPos == 0)
 		s.tokEnd = s.tokPos
 	}

 	if s.tokBuf.Len() == 0 {
 		// common case: the entire token text is still in srcBuf
 		return string(s.srcBuf[s.tokPos:s.tokEnd])
 	}

 	// part of the token text was saved in tokBuf: save the rest in
 	// tokBuf as well and return its content
 	s.tokBuf.Write(s.srcBuf[s.tokPos:s.tokEnd])
 	s.tokPos = s.tokEnd // ensure idempotency of TokenText() call
 	return s.tokBuf.String()
 }
	// Copyright 2009 The Go Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	// A scanner and tokenizer for UTF-8-encoded text. Takes an io.Reader
	// providing the source, which then can be tokenized through repeated calls
	// to the Scan function. For compatibility with existing tools, the NUL
	// character is not allowed (implementation restriction).
	//
	// By default, a Scanner skips white space and Go comments and recognizes all
	// literals as defined by the Go language specification. It may be
	// customized to recognize only a subset of those literals and to recognize
	// different white space characters.
	//
	// Basic usage pattern:
	//
	// var s scanner.Scanner
	// s.Init(src)
	// tok := s.Scan()
	// for tok != scanner.EOF {
	// // do something with tok
	// tok = s.Scan()
	// }
	//
	package scanner

	import (
	"bytes"
	"fmt"
	"io"
	"os"
	"unicode"
	"utf8"
	)


	// TODO(gri): Consider changing this to use the new (token) Position package.

	// A source position is represented by a Position value.
	// A position is valid if Line > 0.
	type Position struct {
	Filename string // filename, if any
	Offset int // byte offset, starting at 0
	Line int // line number, starting at 1
	Column int // column number, starting at 1 (character count per line)
	}


	// IsValid returns true if the position is valid.
	func (pos *Position) IsValid() bool { return pos.Line > 0 }


	func (pos Position) String() string {
	s := pos.Filename
	if pos.IsValid() {
	if s != "" {
	s += ":"
	}
	s += fmt.Sprintf("%d:%d", pos.Line, pos.Column)
	}
	if s == "" {
	s = "???"
	}
	return s
	}


	// Predefined mode bits to control recognition of tokens. For instance,
	// to configure a Scanner such that it only recognizes (Go) identifiers,
	// integers, and skips comments, set the Scanner's Mode field to:
	//
	// ScanIdents \| ScanInts \| SkipComments
	//
	const (
	ScanIdents = 1 << -Ident
	ScanInts = 1 << -Int
	ScanFloats = 1 << -Float // includes Ints
	ScanChars = 1 << -Char
	ScanStrings = 1 << -String
	ScanRawStrings = 1 << -RawString
	ScanComments = 1 << -Comment
	SkipComments = 1 << -skipComment // if set with ScanComments, comments become white space
	GoTokens = ScanIdents \| ScanFloats \| ScanChars \| ScanStrings \| ScanRawStrings \| ScanComments \| SkipComments
	)


	// The result of Scan is one of the following tokens or a Unicode character.
	const (
	EOF = -(iota + 1)
	Ident
	Int
	Float
	Char
	String
	RawString
	Comment
	skipComment
	)


	var tokenString = map[int]string{
	EOF: "EOF",
	Ident: "Ident",
	Int: "Int",
	Float: "Float",
	Char: "Char",
	String: "String",
	RawString: "RawString",
	Comment: "Comment",
	}


	// TokenString returns a (visible) string for a token or Unicode character.
	func TokenString(tok int) string {
	if s, found := tokenString[tok]; found {
	return s
	}
	return fmt.Sprintf("U+%04X", tok)
	}


	// GoWhitespace is the default value for the Scanner's Whitespace field.
	// Its value selects Go's white space characters.
	const GoWhitespace = 1<<'\t' \| 1<<'\n' \| 1<<'\r' \| 1<<' '


	const bufLen = 1024 // at least utf8.UTFMax

	// A Scanner implements reading of Unicode characters and tokens from an io.Reader.
	type Scanner struct {
	// Input
	src io.Reader

	// Source buffer
	srcBuf [bufLen + 1]byte // +1 for sentinel for common case of s.next()
	srcPos int // reading position (srcBuf index)
	srcEnd int // source end (srcBuf index)

	// Source position
	srcBufOffset int // byte offset of srcBuf[0] in source
	line int // line count
	column int // character count
	lastLineLen int // length of last line in characters (for correct column reporting)
	lastCharLen int // length of last character in bytes

	// Token text buffer
	// Typically, token text is stored completely in srcBuf, but in general
	// the token text's head may be buffered in tokBuf while the token text's
	// tail is stored in srcBuf.
	tokBuf bytes.Buffer // token text head that is not in srcBuf anymore
	tokPos int // token text tail position (srcBuf index); valid if >= 0
	tokEnd int // token text tail end (srcBuf index)

	// One character look-ahead
	ch int // character before current srcPos

	// Error is called for each error encountered. If no Error
	// function is set, the error is reported to os.Stderr.
	Error func(s *Scanner, msg string)

	// ErrorCount is incremented by one for each error encountered.
	ErrorCount int

	// The Mode field controls which tokens are recognized. For instance,
	// to recognize Ints, set the ScanInts bit in Mode. The field may be
	// changed at any time.
	Mode uint

	// The Whitespace field controls which characters are recognized
	// as white space. To recognize a character ch <= ' ' as white space,
	// set the ch'th bit in Whitespace (the Scanner's behavior is undefined
	// for values ch > ' '). The field may be changed at any time.
	Whitespace uint64

	// Current token position. The Offset, Line, and Column fields
	// are set by Scan(); the Filename field is left untouched by the
	// Scanner.
	Position
	}


	// Init initializes a Scanner with a new source and returns s.
	// Error is set to nil, ErrorCount is set to 0, Mode is set to GoTokens,
	// and Whitespace is set to GoWhitespace.
	func (s Scanner) Init(src io.Reader) Scanner {
	s.src = src

	// initialize source buffer
	// (the first call to next() will fill it by calling src.Read)
	s.srcBuf[0] = utf8.RuneSelf // sentinel
	s.srcPos = 0
	s.srcEnd = 0

	// initialize source position
	s.srcBufOffset = 0
	s.line = 1
	s.column = 0
	s.lastLineLen = 0
	s.lastCharLen = 0

	// initialize token text buffer
	// (required for first call to next()).
	s.tokPos = -1

	// initialize one character look-ahead
	s.ch = -1 // no char read yet

	// initialize public fields
	s.Error = nil
	s.ErrorCount = 0
	s.Mode = GoTokens
	s.Whitespace = GoWhitespace

	return s
	}


	// TODO(gri): The code for next() and the internal scanner state could benefit
	// from a rethink. While next() is optimized for the common ASCII
	// case, the "corrections" needed for proper position tracking undo
	// some of the attempts for fast-path optimization.

	// next reads and returns the next Unicode character. It is designed such
	// that only a minimal amount of work needs to be done in the common ASCII
	// case (one test to check for both ASCII and end-of-buffer, and one test
	// to check for newlines).
	func (s *Scanner) next() int {
	ch, width := int(s.srcBuf[s.srcPos]), 1

	if ch >= utf8.RuneSelf {
	// uncommon case: not ASCII or not enough bytes
	for s.srcPos+utf8.UTFMax > s.srcEnd && !utf8.FullRune(s.srcBuf[s.srcPos:s.srcEnd]) {
	// not enough bytes: read some more, but first
	// save away token text if any
	if s.tokPos >= 0 {
	s.tokBuf.Write(s.srcBuf[s.tokPos:s.srcPos])
	s.tokPos = 0
	// s.tokEnd is set by Scan()
	}
	// move unread bytes to beginning of buffer
	copy(s.srcBuf[0:], s.srcBuf[s.srcPos:s.srcEnd])
	s.srcBufOffset += s.srcPos
	// read more bytes
	// (an io.Reader must return os.EOF when it reaches
	// the end of what it is reading - simply returning
	// n == 0 will make this loop retry forever; but the
	// error is in the reader implementation in that case)
	i := s.srcEnd - s.srcPos
	n, err := s.src.Read(s.srcBuf[i:bufLen])
	s.srcPos = 0
	s.srcEnd = i + n
	s.srcBuf[s.srcEnd] = utf8.RuneSelf // sentinel
	if err != nil {
	if s.srcEnd == 0 {
	if s.lastCharLen > 0 {
	// previous character was not EOF
	s.column++
	}
	s.lastCharLen = 0
	return EOF
	}
	if err != os.EOF {
	s.error(err.String())
	}
	// If err == EOF, we won't be getting more
	// bytes; break to avoid infinite loop. If
	// err is something else, we don't know if
	// we can get more bytes; thus also break.
	break
	}
	}
	// at least one byte
	ch = int(s.srcBuf[s.srcPos])
	if ch >= utf8.RuneSelf {
	// uncommon case: not ASCII
	ch, width = utf8.DecodeRune(s.srcBuf[s.srcPos:s.srcEnd])
	if ch == utf8.RuneError && width == 1 {
	s.error("illegal UTF-8 encoding")
	}
	}
	}

	// advance
	s.srcPos += width
	s.lastCharLen = width
	s.column++

	// special situations
	switch ch {
	case 0:
	// implementation restriction for compatibility with other tools
	s.error("illegal character NUL")
	case '\n':
	s.line++
	s.lastLineLen = s.column
	s.column = 0
	}

	return ch
	}


	// Next reads and returns the next Unicode character.
	// It returns EOF at the end of the source. It reports
	// a read error by calling s.Error, if not nil; otherwise
	// it prints an error message to os.Stderr. Next does not
	// update the Scanner's Position field; use Pos() to
	// get the current position.
	func (s *Scanner) Next() int {
	s.tokPos = -1 // don't collect token text
	ch := s.Peek()
	s.ch = s.next()
	return ch
	}


	// Peek returns the next Unicode character in the source without advancing
	// the scanner. It returns EOF if the scanner's position is at the last
	// character of the source.
	func (s *Scanner) Peek() int {
	if s.ch < 0 {
	s.ch = s.next()
	}
	return s.ch
	}


	func (s *Scanner) error(msg string) {
	s.ErrorCount++
	if s.Error != nil {
	s.Error(s, msg)
	return
	}
	fmt.Fprintf(os.Stderr, "%s: %s", s.Position, msg)
	}


	func (s *Scanner) scanIdentifier() int {
	ch := s.next() // read character after first '_' or letter
	for ch == '_' \|\| unicode.IsLetter(ch) \|\| unicode.IsDigit(ch) {
	ch = s.next()
	}
	return ch
	}


	func digitVal(ch int) int {
	switch {
	case '0' <= ch && ch <= '9':
	return ch - '0'
	case 'a' <= ch && ch <= 'f':
	return ch - 'a' + 10
	case 'A' <= ch && ch <= 'F':
	return ch - 'A' + 10
	}
	return 16 // larger than any legal digit val
	}


	func isDecimal(ch int) bool { return '0' <= ch && ch <= '9' }


	func (s *Scanner) scanMantissa(ch int) int {
	for isDecimal(ch) {
	ch = s.next()
	}
	return ch
	}


	func (s *Scanner) scanFraction(ch int) int {
	if ch == '.' {
	ch = s.scanMantissa(s.next())
	}
	return ch
	}


	func (s *Scanner) scanExponent(ch int) int {
	if ch == 'e' \|\| ch == 'E' {
	ch = s.next()
	if ch == '-' \|\| ch == '+' {
	ch = s.next()
	}
	ch = s.scanMantissa(ch)
	}
	return ch
	}


	func (s *Scanner) scanNumber(ch int) (int, int) {
	// isDecimal(ch)
	if ch == '0' {
	// int or float
	ch = s.next()
	if ch == 'x' \|\| ch == 'X' {
	// hexadecimal int
	ch = s.next()
	for digitVal(ch) < 16 {
	ch = s.next()
	}
	} else {
	// octal int or float
	seenDecimalDigit := false
	for isDecimal(ch) {
	if ch > '7' {
	seenDecimalDigit = true
	}
	ch = s.next()
	}
	if s.Mode&ScanFloats != 0 && (ch == '.' \|\| ch == 'e' \|\| ch == 'E') {
	// float
	ch = s.scanFraction(ch)
	ch = s.scanExponent(ch)
	return Float, ch
	}
	// octal int
	if seenDecimalDigit {
	s.error("illegal octal number")
	}
	}
	return Int, ch
	}
	// decimal int or float
	ch = s.scanMantissa(ch)
	if s.Mode&ScanFloats != 0 && (ch == '.' \|\| ch == 'e' \|\| ch == 'E') {
	// float
	ch = s.scanFraction(ch)
	ch = s.scanExponent(ch)
	return Float, ch
	}
	return Int, ch
	}


	func (s *Scanner) scanDigits(ch, base, n int) int {
	for n > 0 && digitVal(ch) < base {
	ch = s.next()
	n--
	}
	if n > 0 {
	s.error("illegal char escape")
	}
	return ch
	}


	func (s *Scanner) scanEscape(quote int) int {
	ch := s.next() // read character after '/'
	switch ch {
	case 'a', 'b', 'f', 'n', 'r', 't', 'v', '\\', quote:
	// nothing to do
	ch = s.next()
	case '0', '1', '2', '3', '4', '5', '6', '7':
	ch = s.scanDigits(ch, 8, 3)
	case 'x':
	ch = s.scanDigits(s.next(), 16, 2)
	case 'u':
	ch = s.scanDigits(s.next(), 16, 4)
	case 'U':
	ch = s.scanDigits(s.next(), 16, 8)
	default:
	s.error("illegal char escape")
	}
	return ch
	}


	func (s *Scanner) scanString(quote int) (n int) {
	ch := s.next() // read character after quote
	for ch != quote {
	if ch == '\n' \|\| ch < 0 {
	s.error("literal not terminated")
	return
	}
	if ch == '\\' {
	ch = s.scanEscape(quote)
	} else {
	ch = s.next()
	}
	n++
	}
	return
	}


	func (s *Scanner) scanRawString() {
	ch := s.next() // read character after '`'
	for ch != '`' {
	if ch < 0 {
	s.error("literal not terminated")
	return
	}
	ch = s.next()
	}
	}


	func (s *Scanner) scanChar() {
	if s.scanString('\'') != 1 {
	s.error("illegal char literal")
	}
	}


	func (s *Scanner) scanLineComment() {
	ch := s.next() // read character after "//"
	for ch != '\n' {
	if ch < 0 {
	s.error("comment not terminated")
	return
	}
	ch = s.next()
	}
	}


	func (s *Scanner) scanGeneralComment() {
	ch := s.next() // read character after "/*"
	for {
	if ch < 0 {
	s.error("comment not terminated")
	return
	}
	ch0 := ch
	ch = s.next()
	if ch0 == '*' && ch == '/' {
	break
	}
	}
	}


	func (s *Scanner) scanComment(ch int) {
	// ch == '/' \|\| ch == '*'
	if ch == '/' {
	s.scanLineComment()
	return
	}
	s.scanGeneralComment()
	}


	// Scan reads the next token or Unicode character from source and returns it.
	// It only recognizes tokens t for which the respective Mode bit (1<<-t) is set.
	// It returns EOF at the end of the source. It reports scanner errors (read and
	// token errors) by calling s.Error, if not nil; otherwise it prints an error
	// message to os.Stderr.
	func (s *Scanner) Scan() int {
	ch := s.Peek()

	// reset token text position
	s.tokPos = -1

	redo:
	// skip white space
	for s.Whitespace&(1<<uint(ch)) != 0 {
	ch = s.next()
	}

	// start collecting token text
	s.tokBuf.Reset()
	s.tokPos = s.srcPos - s.lastCharLen

	// set token position
	// (this is a slightly optimized version of the code in Pos())
	s.Offset = s.srcBufOffset + s.tokPos
	if s.column > 0 {
	// common case: last character was not a '\n'
	s.Line = s.line
	s.Column = s.column
	} else {
	// last character was a '\n'
	// (we cannot be at the beginning of the source
	// since we have called next() at least once)
	s.Line = s.line - 1
	s.Column = s.lastLineLen
	}

	// determine token value
	tok := ch
	switch {
	case unicode.IsLetter(ch) \|\| ch == '_':
	if s.Mode&ScanIdents != 0 {
	tok = Ident
	ch = s.scanIdentifier()
	} else {
	ch = s.next()
	}
	case isDecimal(ch):
	if s.Mode&(ScanInts\|ScanFloats) != 0 {
	tok, ch = s.scanNumber(ch)
	} else {
	ch = s.next()
	}
	default:
	switch ch {
	case '"':
	if s.Mode&ScanStrings != 0 {
	s.scanString('"')
	tok = String
	}
	ch = s.next()
	case '\'':
	if s.Mode&ScanChars != 0 {
	s.scanChar()
	tok = Char
	}
	ch = s.next()
	case '.':
	ch = s.next()
	if isDecimal(ch) && s.Mode&ScanFloats != 0 {
	tok = Float
	ch = s.scanMantissa(ch)
	ch = s.scanExponent(ch)
	}
	case '/':
	ch = s.next()
	if (ch == '/' \|\| ch == '*') && s.Mode&ScanComments != 0 {
	if s.Mode&SkipComments != 0 {
	s.tokPos = -1 // don't collect token text
	s.scanComment(ch)
	ch = s.next()
	goto redo
	}
	s.scanComment(ch)
	tok = Comment
	ch = s.next()
	}
	case '`':
	if s.Mode&ScanRawStrings != 0 {
	s.scanRawString()
	tok = String
	}
	ch = s.next()
	default:
	ch = s.next()
	}
	}

	// end of token text
	s.tokEnd = s.srcPos - s.lastCharLen

	s.ch = ch
	return tok
	}


	// Pos returns the position of the character immediately after
	// the character or token returned by the last call to Next or Scan.
	func (s *Scanner) Pos() (pos Position) {
	pos.Filename = s.Filename
	pos.Offset = s.srcBufOffset + s.srcPos - s.lastCharLen
	switch {
	case s.column > 0:
	// common case: last character was not a '\n'
	pos.Line = s.line
	pos.Column = s.column
	case s.lastLineLen > 0:
	// last character was a '\n'
	pos.Line = s.line - 1
	pos.Column = s.lastLineLen
	default:
	// at the beginning of the source
	pos.Line = 1
	pos.Column = 1
	}
	return
	}


	// TokenText returns the string corresponding to the most recently scanned token.
	// Valid after calling Scan().
	func (s *Scanner) TokenText() string {
	if s.tokPos < 0 {
	// no token text
	return ""
	}

	if s.tokEnd < 0 {
	// if EOF was reached, s.tokEnd is set to -1 (s.srcPos == 0)
	s.tokEnd = s.tokPos
	}

	if s.tokBuf.Len() == 0 {
	// common case: the entire token text is still in srcBuf
	return string(s.srcBuf[s.tokPos:s.tokEnd])
	}

	// part of the token text was saved in tokBuf: save the rest in
	// tokBuf as well and return its content
	s.tokBuf.Write(s.srcBuf[s.tokPos:s.tokEnd])
	s.tokPos = s.tokEnd // ensure idempotency of TokenText() call
	return s.tokBuf.String()
	}