src/text/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.

 // Package scanner provides a scanner and tokenizer for UTF-8-encoded text.
 // It 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. If the first character
 // in the source is a UTF-8 encoded byte order mark (BOM), it is discarded.
 //
 // 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 identifier and white space characters.
 package scanner

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

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

 func (pos Position) String() string {
 	s := pos.Filename
 	if s == "" {
 		s = "<input>"
 	}
 	if pos.IsValid() {
 		s += fmt.Sprintf(":%d:%d", pos.Line, pos.Column)
 	}
 	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
 //
 // With the exceptions of comments, which are skipped if SkipComments is
 // set, unrecognized tokens are not ignored. Instead, the scanner simply
 // returns the respective individual characters (or possibly sub-tokens).
 // For instance, if the mode is ScanIdents (not ScanStrings), the string
 // "foo" is scanned as the token sequence '"' Ident '"'.
 //
 // Use GoTokens to configure the Scanner such that it accepts all Go
 // literal tokens including Go identifiers. Comments will be skipped.
 //
 const (
 	ScanIdents     = 1 << -Ident
 	ScanInts       = 1 << -Int
 	ScanFloats     = 1 << -Float // includes Ints and hexadecimal floats
 	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 these tokens or a Unicode character.
 const (
 	EOF = -(iota + 1)
 	Ident
 	Int
 	Float
 	Char
 	String
 	RawString
 	Comment

 	// internal use only
 	skipComment
 )

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

 // TokenString returns a printable string for a token or Unicode character.
 func TokenString(tok rune) string {
 	if s, found := tokenString[tok]; found {
 		return s
 	}
 	return fmt.Sprintf("%q", string(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 rune // 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

 	// IsIdentRune is a predicate controlling the characters accepted
 	// as the ith rune in an identifier. The set of valid characters
 	// must not intersect with the set of white space characters.
 	// If no IsIdentRune function is set, regular Go identifiers are
 	// accepted instead. The field may be changed at any time.
 	IsIdentRune func(ch rune, i int) bool

 	// Start position of most recently scanned token; set by Scan.
 	// Calling Init or Next invalidates the position (Line == 0).
 	// The Filename field is always left untouched by the Scanner.
 	// If an error is reported (via Error) and Position is invalid,
 	// the scanner is not inside a token. Call Pos to obtain an error
 	// position in that case, or to obtain the position immediately
 	// after the most recently scanned token.
 	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 = -2 // no char read yet, not EOF

 	// initialize public fields
 	s.Error = nil
 	s.ErrorCount = 0
 	s.Mode = GoTokens
 	s.Whitespace = GoWhitespace
 	s.Line = 0 // invalidate token position

 	return s
 }

 // 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() rune {
 	ch, width := rune(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 io.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 err != io.EOF {
 					s.error(err.Error())
 				}
 				if s.srcEnd == 0 {
 					if s.lastCharLen > 0 {
 						// previous character was not EOF
 						s.column++
 					}
 					s.lastCharLen = 0
 					return EOF
 				}
 				// 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 = rune(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 {
 				// advance for correct error position
 				s.srcPos += width
 				s.lastCharLen = width
 				s.column++
 				s.error("invalid UTF-8 encoding")
 				return ch
 			}
 		}
 	}

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

 	// special situations
 	switch ch {
 	case 0:
 		// for compatibility with other tools
 		s.error("invalid 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() rune {
 	s.tokPos = -1 // don't collect token text
 	s.Line = 0    // invalidate token position
 	ch := s.Peek()
 	if ch != EOF {
 		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() rune {
 	if s.ch == -2 {
 		// this code is only run for the very first character
 		s.ch = s.next()
 		if s.ch == '\uFEFF' {
 			s.ch = s.next() // ignore BOM
 		}
 	}
 	return s.ch
 }

 func (s *Scanner) error(msg string) {
 	s.tokEnd = s.srcPos - s.lastCharLen // make sure token text is terminated
 	s.ErrorCount++
 	if s.Error != nil {
 		s.Error(s, msg)
 		return
 	}
 	pos := s.Position
 	if !pos.IsValid() {
 		pos = s.Pos()
 	}
 	fmt.Fprintf(os.Stderr, "%s: %s\n", pos, msg)
 }

 func (s *Scanner) errorf(format string, args ...interface{}) {
 	s.error(fmt.Sprintf(format, args...))
 }

 func (s *Scanner) isIdentRune(ch rune, i int) bool {
 	if s.IsIdentRune != nil {
 		return s.IsIdentRune(ch, i)
 	}
 	return ch == '_' || unicode.IsLetter(ch) || unicode.IsDigit(ch) && i > 0
 }

 func (s *Scanner) scanIdentifier() rune {
 	// we know the zero'th rune is OK; start scanning at the next one
 	ch := s.next()
 	for i := 1; s.isIdentRune(ch, i); i++ {
 		ch = s.next()
 	}
 	return ch
 }

 func lower(ch rune) rune     { return ('a' - 'A') | ch } // returns lower-case ch iff ch is ASCII letter
 func isDecimal(ch rune) bool { return '0' <= ch && ch <= '9' }
 func isHex(ch rune) bool     { return '0' <= ch && ch <= '9' || 'a' <= lower(ch) && lower(ch) <= 'f' }

 // digits accepts the sequence { digit | '_' } starting with ch0.
 // If base <= 10, digits accepts any decimal digit but records
 // the first invalid digit >= base in *invalid if *invalid == 0.
 // digits returns the first rune that is not part of the sequence
 // anymore, and a bitset describing whether the sequence contained
 // digits (bit 0 is set), or separators '_' (bit 1 is set).
 func (s *Scanner) digits(ch0 rune, base int, invalid *rune) (ch rune, digsep int) {
 	ch = ch0
 	if base <= 10 {
 		max := rune('0' + base)
 		for isDecimal(ch) || ch == '_' {
 			ds := 1
 			if ch == '_' {
 				ds = 2
 			} else if ch >= max && *invalid == 0 {
 				*invalid = ch
 			}
 			digsep |= ds
 			ch = s.next()
 		}
 	} else {
 		for isHex(ch) || ch == '_' {
 			ds := 1
 			if ch == '_' {
 				ds = 2
 			}
 			digsep |= ds
 			ch = s.next()
 		}
 	}
 	return
 }

 func (s *Scanner) scanNumber(ch rune, seenDot bool) (rune, rune) {
 	base := 10         // number base
 	prefix := rune(0)  // one of 0 (decimal), '0' (0-octal), 'x', 'o', or 'b'
 	digsep := 0        // bit 0: digit present, bit 1: '_' present
 	invalid := rune(0) // invalid digit in literal, or 0

 	// integer part
 	var tok rune
 	var ds int
 	if !seenDot {
 		tok = Int
 		if ch == '0' {
 			ch = s.next()
 			switch lower(ch) {
 			case 'x':
 				ch = s.next()
 				base, prefix = 16, 'x'
 			case 'o':
 				ch = s.next()
 				base, prefix = 8, 'o'
 			case 'b':
 				ch = s.next()
 				base, prefix = 2, 'b'
 			default:
 				base, prefix = 8, '0'
 				digsep = 1 // leading 0
 			}
 		}
 		ch, ds = s.digits(ch, base, &invalid)
 		digsep |= ds
 		if ch == '.' && s.Mode&ScanFloats != 0 {
 			ch = s.next()
 			seenDot = true
 		}
 	}

 	// fractional part
 	if seenDot {
 		tok = Float
 		if prefix == 'o' || prefix == 'b' {
 			s.error("invalid radix point in " + litname(prefix))
 		}
 		ch, ds = s.digits(ch, base, &invalid)
 		digsep |= ds
 	}

 	if digsep&1 == 0 {
 		s.error(litname(prefix) + " has no digits")
 	}

 	// exponent
 	if e := lower(ch); (e == 'e' || e == 'p') && s.Mode&ScanFloats != 0 {
 		switch {
 		case e == 'e' && prefix != 0 && prefix != '0':
 			s.errorf("%q exponent requires decimal mantissa", ch)
 		case e == 'p' && prefix != 'x':
 			s.errorf("%q exponent requires hexadecimal mantissa", ch)
 		}
 		ch = s.next()
 		tok = Float
 		if ch == '+' || ch == '-' {
 			ch = s.next()
 		}
 		ch, ds = s.digits(ch, 10, nil)
 		digsep |= ds
 		if ds&1 == 0 {
 			s.error("exponent has no digits")
 		}
 	} else if prefix == 'x' && tok == Float {
 		s.error("hexadecimal mantissa requires a 'p' exponent")
 	}

 	if tok == Int && invalid != 0 {
 		s.errorf("invalid digit %q in %s", invalid, litname(prefix))
 	}

 	if digsep&2 != 0 {
 		s.tokEnd = s.srcPos - s.lastCharLen // make sure token text is terminated
 		if i := invalidSep(s.TokenText()); i >= 0 {
 			s.error("'_' must separate successive digits")
 		}
 	}

 	return tok, ch
 }

 func litname(prefix rune) string {
 	switch prefix {
 	default:
 		return "decimal literal"
 	case 'x':
 		return "hexadecimal literal"
 	case 'o', '0':
 		return "octal literal"
 	case 'b':
 		return "binary literal"
 	}
 }

 // invalidSep returns the index of the first invalid separator in x, or -1.
 func invalidSep(x string) int {
 	x1 := ' ' // prefix char, we only care if it's 'x'
 	d := '.'  // digit, one of '_', '0' (a digit), or '.' (anything else)
 	i := 0

 	// a prefix counts as a digit
 	if len(x) >= 2 && x[0] == '0' {
 		x1 = lower(rune(x[1]))
 		if x1 == 'x' || x1 == 'o' || x1 == 'b' {
 			d = '0'
 			i = 2
 		}
 	}

 	// mantissa and exponent
 	for ; i < len(x); i++ {
 		p := d // previous digit
 		d = rune(x[i])
 		switch {
 		case d == '_':
 			if p != '0' {
 				return i
 			}
 		case isDecimal(d) || x1 == 'x' && isHex(d):
 			d = '0'
 		default:
 			if p == '_' {
 				return i - 1
 			}
 			d = '.'
 		}
 	}
 	if d == '_' {
 		return len(x) - 1
 	}

 	return -1
 }

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

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

 func (s *Scanner) scanEscape(quote rune) rune {
 	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("invalid char escape")
 	}
 	return ch
 }

 func (s *Scanner) scanString(quote rune) (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("invalid char literal")
 	}
 }

 func (s *Scanner) scanComment(ch rune) rune {
 	// ch == '/' || ch == '*'
 	if ch == '/' {
 		// line comment
 		ch = s.next() // read character after "//"
 		for ch != '\n' && ch >= 0 {
 			ch = s.next()
 		}
 		return ch
 	}

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

 // 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() rune {
 	ch := s.Peek()

 	// reset token text position
 	s.tokPos = -1
 	s.Line = 0

 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 s.isIdentRune(ch, 0):
 		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, false)
 		} else {
 			ch = s.next()
 		}
 	default:
 		switch ch {
 		case EOF:
 			break
 		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, ch = s.scanNumber(ch, true)
 			}
 		case '/':
 			ch = s.next()
 			if (ch == '/' || ch == '*') && s.Mode&ScanComments != 0 {
 				if s.Mode&SkipComments != 0 {
 					s.tokPos = -1 // don't collect token text
 					ch = s.scanComment(ch)
 					goto redo
 				}
 				ch = s.scanComment(ch)
 				tok = Comment
 			}
 		case '`':
 			if s.Mode&ScanRawStrings != 0 {
 				s.scanRawString()
 				tok = RawString
 			}
 			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.
 // Use the Scanner's Position field for the start position of the most
 // recently scanned token.
 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 and in calls of Scanner.Error.
 func (s *Scanner) TokenText() string {
 	if s.tokPos < 0 {
 		// no token text
 		return ""
 	}

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

	// Package scanner provides a scanner and tokenizer for UTF-8-encoded text.
	// It 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. If the first character
	// in the source is a UTF-8 encoded byte order mark (BOM), it is discarded.
	//
	// 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 identifier and white space characters.
	package scanner

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

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

	func (pos Position) String() string {
	s := pos.Filename
	if s == "" {
	s = "<input>"
	}
	if pos.IsValid() {
	s += fmt.Sprintf(":%d:%d", pos.Line, pos.Column)
	}
	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
	//
	// With the exceptions of comments, which are skipped if SkipComments is
	// set, unrecognized tokens are not ignored. Instead, the scanner simply
	// returns the respective individual characters (or possibly sub-tokens).
	// For instance, if the mode is ScanIdents (not ScanStrings), the string
	// "foo" is scanned as the token sequence '"' Ident '"'.
	//
	// Use GoTokens to configure the Scanner such that it accepts all Go
	// literal tokens including Go identifiers. Comments will be skipped.
	//
	const (
	ScanIdents = 1 << -Ident
	ScanInts = 1 << -Int
	ScanFloats = 1 << -Float // includes Ints and hexadecimal floats
	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 these tokens or a Unicode character.
	const (
	EOF = -(iota + 1)
	Ident
	Int
	Float
	Char
	String
	RawString
	Comment

	// internal use only
	skipComment
	)

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

	// TokenString returns a printable string for a token or Unicode character.
	func TokenString(tok rune) string {
	if s, found := tokenString[tok]; found {
	return s
	}
	return fmt.Sprintf("%q", string(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 rune // 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

	// IsIdentRune is a predicate controlling the characters accepted
	// as the ith rune in an identifier. The set of valid characters
	// must not intersect with the set of white space characters.
	// If no IsIdentRune function is set, regular Go identifiers are
	// accepted instead. The field may be changed at any time.
	IsIdentRune func(ch rune, i int) bool

	// Start position of most recently scanned token; set by Scan.
	// Calling Init or Next invalidates the position (Line == 0).
	// The Filename field is always left untouched by the Scanner.
	// If an error is reported (via Error) and Position is invalid,
	// the scanner is not inside a token. Call Pos to obtain an error
	// position in that case, or to obtain the position immediately
	// after the most recently scanned token.
	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 = -2 // no char read yet, not EOF

	// initialize public fields
	s.Error = nil
	s.ErrorCount = 0
	s.Mode = GoTokens
	s.Whitespace = GoWhitespace
	s.Line = 0 // invalidate token position

	return s
	}

	// 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() rune {
	ch, width := rune(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 io.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 err != io.EOF {
	s.error(err.Error())
	}
	if s.srcEnd == 0 {
	if s.lastCharLen > 0 {
	// previous character was not EOF
	s.column++
	}
	s.lastCharLen = 0
	return EOF
	}
	// 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 = rune(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 {
	// advance for correct error position
	s.srcPos += width
	s.lastCharLen = width
	s.column++
	s.error("invalid UTF-8 encoding")
	return ch
	}
	}
	}

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

	// special situations
	switch ch {
	case 0:
	// for compatibility with other tools
	s.error("invalid 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() rune {
	s.tokPos = -1 // don't collect token text
	s.Line = 0 // invalidate token position
	ch := s.Peek()
	if ch != EOF {
	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() rune {
	if s.ch == -2 {
	// this code is only run for the very first character
	s.ch = s.next()
	if s.ch == '\uFEFF' {
	s.ch = s.next() // ignore BOM
	}
	}
	return s.ch
	}

	func (s *Scanner) error(msg string) {
	s.tokEnd = s.srcPos - s.lastCharLen // make sure token text is terminated
	s.ErrorCount++
	if s.Error != nil {
	s.Error(s, msg)
	return
	}
	pos := s.Position
	if !pos.IsValid() {
	pos = s.Pos()
	}
	fmt.Fprintf(os.Stderr, "%s: %s\n", pos, msg)
	}

	func (s *Scanner) errorf(format string, args ...interface{}) {
	s.error(fmt.Sprintf(format, args...))
	}

	func (s *Scanner) isIdentRune(ch rune, i int) bool {
	if s.IsIdentRune != nil {
	return s.IsIdentRune(ch, i)
	}
	return ch == '_' \|\| unicode.IsLetter(ch) \|\| unicode.IsDigit(ch) && i > 0
	}

	func (s *Scanner) scanIdentifier() rune {
	// we know the zero'th rune is OK; start scanning at the next one
	ch := s.next()
	for i := 1; s.isIdentRune(ch, i); i++ {
	ch = s.next()
	}
	return ch
	}

	func lower(ch rune) rune { return ('a' - 'A') \| ch } // returns lower-case ch iff ch is ASCII letter
	func isDecimal(ch rune) bool { return '0' <= ch && ch <= '9' }
	func isHex(ch rune) bool { return '0' <= ch && ch <= '9' \|\| 'a' <= lower(ch) && lower(ch) <= 'f' }

	// digits accepts the sequence { digit \| '_' } starting with ch0.
	// If base <= 10, digits accepts any decimal digit but records
	// the first invalid digit >= base in invalid if invalid == 0.
	// digits returns the first rune that is not part of the sequence
	// anymore, and a bitset describing whether the sequence contained
	// digits (bit 0 is set), or separators '_' (bit 1 is set).
	func (s Scanner) digits(ch0 rune, base int, invalid rune) (ch rune, digsep int) {
	ch = ch0
	if base <= 10 {
	max := rune('0' + base)
	for isDecimal(ch) \|\| ch == '_' {
	ds := 1
	if ch == '_' {
	ds = 2
	} else if ch >= max && *invalid == 0 {
	*invalid = ch
	}
	digsep \|= ds
	ch = s.next()
	}
	} else {
	for isHex(ch) \|\| ch == '_' {
	ds := 1
	if ch == '_' {
	ds = 2
	}
	digsep \|= ds
	ch = s.next()
	}
	}
	return
	}

	func (s *Scanner) scanNumber(ch rune, seenDot bool) (rune, rune) {
	base := 10 // number base
	prefix := rune(0) // one of 0 (decimal), '0' (0-octal), 'x', 'o', or 'b'
	digsep := 0 // bit 0: digit present, bit 1: '_' present
	invalid := rune(0) // invalid digit in literal, or 0

	// integer part
	var tok rune
	var ds int
	if !seenDot {
	tok = Int
	if ch == '0' {
	ch = s.next()
	switch lower(ch) {
	case 'x':
	ch = s.next()
	base, prefix = 16, 'x'
	case 'o':
	ch = s.next()
	base, prefix = 8, 'o'
	case 'b':
	ch = s.next()
	base, prefix = 2, 'b'
	default:
	base, prefix = 8, '0'
	digsep = 1 // leading 0
	}
	}
	ch, ds = s.digits(ch, base, &invalid)
	digsep \|= ds
	if ch == '.' && s.Mode&ScanFloats != 0 {
	ch = s.next()
	seenDot = true
	}
	}

	// fractional part
	if seenDot {
	tok = Float
	if prefix == 'o' \|\| prefix == 'b' {
	s.error("invalid radix point in " + litname(prefix))
	}
	ch, ds = s.digits(ch, base, &invalid)
	digsep \|= ds
	}

	if digsep&1 == 0 {
	s.error(litname(prefix) + " has no digits")
	}

	// exponent
	if e := lower(ch); (e == 'e' \|\| e == 'p') && s.Mode&ScanFloats != 0 {
	switch {
	case e == 'e' && prefix != 0 && prefix != '0':
	s.errorf("%q exponent requires decimal mantissa", ch)
	case e == 'p' && prefix != 'x':
	s.errorf("%q exponent requires hexadecimal mantissa", ch)
	}
	ch = s.next()
	tok = Float
	if ch == '+' \|\| ch == '-' {
	ch = s.next()
	}
	ch, ds = s.digits(ch, 10, nil)
	digsep \|= ds
	if ds&1 == 0 {
	s.error("exponent has no digits")
	}
	} else if prefix == 'x' && tok == Float {
	s.error("hexadecimal mantissa requires a 'p' exponent")
	}

	if tok == Int && invalid != 0 {
	s.errorf("invalid digit %q in %s", invalid, litname(prefix))
	}

	if digsep&2 != 0 {
	s.tokEnd = s.srcPos - s.lastCharLen // make sure token text is terminated
	if i := invalidSep(s.TokenText()); i >= 0 {
	s.error("'_' must separate successive digits")
	}
	}

	return tok, ch
	}

	func litname(prefix rune) string {
	switch prefix {
	default:
	return "decimal literal"
	case 'x':
	return "hexadecimal literal"
	case 'o', '0':
	return "octal literal"
	case 'b':
	return "binary literal"
	}
	}

	// invalidSep returns the index of the first invalid separator in x, or -1.
	func invalidSep(x string) int {
	x1 := ' ' // prefix char, we only care if it's 'x'
	d := '.' // digit, one of '_', '0' (a digit), or '.' (anything else)
	i := 0

	// a prefix counts as a digit
	if len(x) >= 2 && x[0] == '0' {
	x1 = lower(rune(x[1]))
	if x1 == 'x' \|\| x1 == 'o' \|\| x1 == 'b' {
	d = '0'
	i = 2
	}
	}

	// mantissa and exponent
	for ; i < len(x); i++ {
	p := d // previous digit
	d = rune(x[i])
	switch {
	case d == '_':
	if p != '0' {
	return i
	}
	case isDecimal(d) \|\| x1 == 'x' && isHex(d):
	d = '0'
	default:
	if p == '_' {
	return i - 1
	}
	d = '.'
	}
	}
	if d == '_' {
	return len(x) - 1
	}

	return -1
	}

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

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

	func (s *Scanner) scanEscape(quote rune) rune {
	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("invalid char escape")
	}
	return ch
	}

	func (s *Scanner) scanString(quote rune) (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("invalid char literal")
	}
	}

	func (s *Scanner) scanComment(ch rune) rune {
	// ch == '/' \|\| ch == '*'
	if ch == '/' {
	// line comment
	ch = s.next() // read character after "//"
	for ch != '\n' && ch >= 0 {
	ch = s.next()
	}
	return ch
	}

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

	// 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() rune {
	ch := s.Peek()

	// reset token text position
	s.tokPos = -1
	s.Line = 0

	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 s.isIdentRune(ch, 0):
	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, false)
	} else {
	ch = s.next()
	}
	default:
	switch ch {
	case EOF:
	break
	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, ch = s.scanNumber(ch, true)
	}
	case '/':
	ch = s.next()
	if (ch == '/' \|\| ch == '*') && s.Mode&ScanComments != 0 {
	if s.Mode&SkipComments != 0 {
	s.tokPos = -1 // don't collect token text
	ch = s.scanComment(ch)
	goto redo
	}
	ch = s.scanComment(ch)
	tok = Comment
	}
	case '`':
	if s.Mode&ScanRawStrings != 0 {
	s.scanRawString()
	tok = RawString
	}
	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.
	// Use the Scanner's Position field for the start position of the most
	// recently scanned token.
	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 and in calls of Scanner.Error.
	func (s *Scanner) TokenText() string {
	if s.tokPos < 0 {
	// no token text
	return ""
	}

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