src/bufio/scan.go - go - Git at Google

 // Copyright 2013 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 bufio

 import (
 	"bytes"
 	"errors"
 	"io"
 	"unicode/utf8"
 )

 // Scanner provides a convenient interface for reading data such as
 // a file of newline-delimited lines of text. Successive calls to
 // the Scan method will step through the 'tokens' of a file, skipping
 // the bytes between the tokens. The specification of a token is
 // defined by a split function of type SplitFunc; the default split
 // function breaks the input into lines with line termination stripped. Split
 // functions are defined in this package for scanning a file into
 // lines, bytes, UTF-8-encoded runes, and space-delimited words. The
 // client may instead provide a custom split function.
 //
 // Scanning stops unrecoverably at EOF, the first I/O error, or a token too
 // large to fit in the buffer. When a scan stops, the reader may have
 // advanced arbitrarily far past the last token. Programs that need more
 // control over error handling or large tokens, or must run sequential scans
 // on a reader, should use bufio.Reader instead.
 //
 type Scanner struct {
 	r            io.Reader // The reader provided by the client.
 	split        SplitFunc // The function to split the tokens.
 	maxTokenSize int       // Maximum size of a token; modified by tests.
 	token        []byte    // Last token returned by split.
 	buf          []byte    // Buffer used as argument to split.
 	start        int       // First non-processed byte in buf.
 	end          int       // End of data in buf.
 	err          error     // Sticky error.
 	empties      int       // Count of successive empty tokens.
 	scanCalled   bool      // Scan has been called; buffer is in use.
 	done         bool      // Scan has finished.
 }

 // SplitFunc is the signature of the split function used to tokenize the
 // input. The arguments are an initial substring of the remaining unprocessed
 // data and a flag, atEOF, that reports whether the Reader has no more data
 // to give. The return values are the number of bytes to advance the input
 // and the next token to return to the user, if any, plus an error, if any.
 //
 // Scanning stops if the function returns an error, in which case some of
 // the input may be discarded. If that error is ErrFinalToken, scanning
 // stops with no error.
 //
 // Otherwise, the Scanner advances the input. If the token is not nil,
 // the Scanner returns it to the user. If the token is nil, the
 // Scanner reads more data and continues scanning; if there is no more
 // data--if atEOF was true--the Scanner returns. If the data does not
 // yet hold a complete token, for instance if it has no newline while
 // scanning lines, a SplitFunc can return (0, nil, nil) to signal the
 // Scanner to read more data into the slice and try again with a
 // longer slice starting at the same point in the input.
 //
 // The function is never called with an empty data slice unless atEOF
 // is true. If atEOF is true, however, data may be non-empty and,
 // as always, holds unprocessed text.
 type SplitFunc func(data []byte, atEOF bool) (advance int, token []byte, err error)

 // Errors returned by Scanner.
 var (
 	ErrTooLong         = errors.New("bufio.Scanner: token too long")
 	ErrNegativeAdvance = errors.New("bufio.Scanner: SplitFunc returns negative advance count")
 	ErrAdvanceTooFar   = errors.New("bufio.Scanner: SplitFunc returns advance count beyond input")
 	ErrBadReadCount    = errors.New("bufio.Scanner: Read returned impossible count")
 )

 const (
 	// MaxScanTokenSize is the maximum size used to buffer a token
 	// unless the user provides an explicit buffer with Scanner.Buffer.
 	// The actual maximum token size may be smaller as the buffer
 	// may need to include, for instance, a newline.
 	MaxScanTokenSize = 64 * 1024

 	startBufSize = 4096 // Size of initial allocation for buffer.
 )

 // NewScanner returns a new Scanner to read from r.
 // The split function defaults to ScanLines.
 func NewScanner(r io.Reader) *Scanner {
 	return &Scanner{
 		r:            r,
 		split:        ScanLines,
 		maxTokenSize: MaxScanTokenSize,
 	}
 }

 // Err returns the first non-EOF error that was encountered by the Scanner.
 func (s *Scanner) Err() error {
 	if s.err == io.EOF {
 		return nil
 	}
 	return s.err
 }

 // Bytes returns the most recent token generated by a call to Scan.
 // The underlying array may point to data that will be overwritten
 // by a subsequent call to Scan. It does no allocation.
 func (s *Scanner) Bytes() []byte {
 	return s.token
 }

 // Text returns the most recent token generated by a call to Scan
 // as a newly allocated string holding its bytes.
 func (s *Scanner) Text() string {
 	return string(s.token)
 }

 // ErrFinalToken is a special sentinel error value. It is intended to be
 // returned by a Split function to indicate that the token being delivered
 // with the error is the last token and scanning should stop after this one.
 // After ErrFinalToken is received by Scan, scanning stops with no error.
 // The value is useful to stop processing early or when it is necessary to
 // deliver a final empty token. One could achieve the same behavior
 // with a custom error value but providing one here is tidier.
 // See the emptyFinalToken example for a use of this value.
 var ErrFinalToken = errors.New("final token")

 // Scan advances the Scanner to the next token, which will then be
 // available through the Bytes or Text method. It returns false when the
 // scan stops, either by reaching the end of the input or an error.
 // After Scan returns false, the Err method will return any error that
 // occurred during scanning, except that if it was io.EOF, Err
 // will return nil.
 // Scan panics if the split function returns too many empty
 // tokens without advancing the input. This is a common error mode for
 // scanners.
 func (s *Scanner) Scan() bool {
 	if s.done {
 		return false
 	}
 	s.scanCalled = true
 	// Loop until we have a token.
 	for {
 		// See if we can get a token with what we already have.
 		// If we've run out of data but have an error, give the split function
 		// a chance to recover any remaining, possibly empty token.
 		if s.end > s.start || s.err != nil {
 			advance, token, err := s.split(s.buf[s.start:s.end], s.err != nil)
 			if err != nil {
 				if err == ErrFinalToken {
 					s.token = token
 					s.done = true
 					return true
 				}
 				s.setErr(err)
 				return false
 			}
 			if !s.advance(advance) {
 				return false
 			}
 			s.token = token
 			if token != nil {
 				if s.err == nil || advance > 0 {
 					s.empties = 0
 				} else {
 					// Returning tokens not advancing input at EOF.
 					s.empties++
 					if s.empties > maxConsecutiveEmptyReads {
 						panic("bufio.Scan: too many empty tokens without progressing")
 					}
 				}
 				return true
 			}
 		}
 		// We cannot generate a token with what we are holding.
 		// If we've already hit EOF or an I/O error, we are done.
 		if s.err != nil {
 			// Shut it down.
 			s.start = 0
 			s.end = 0
 			return false
 		}
 		// Must read more data.
 		// First, shift data to beginning of buffer if there's lots of empty space
 		// or space is needed.
 		if s.start > 0 && (s.end == len(s.buf) || s.start > len(s.buf)/2) {
 			copy(s.buf, s.buf[s.start:s.end])
 			s.end -= s.start
 			s.start = 0
 		}
 		// Is the buffer full? If so, resize.
 		if s.end == len(s.buf) {
 			// Guarantee no overflow in the multiplication below.
 			const maxInt = int(^uint(0) >> 1)
 			if len(s.buf) >= s.maxTokenSize || len(s.buf) > maxInt/2 {
 				s.setErr(ErrTooLong)
 				return false
 			}
 			newSize := len(s.buf) * 2
 			if newSize == 0 {
 				newSize = startBufSize
 			}
 			if newSize > s.maxTokenSize {
 				newSize = s.maxTokenSize
 			}
 			newBuf := make([]byte, newSize)
 			copy(newBuf, s.buf[s.start:s.end])
 			s.buf = newBuf
 			s.end -= s.start
 			s.start = 0
 		}
 		// Finally we can read some input. Make sure we don't get stuck with
 		// a misbehaving Reader. Officially we don't need to do this, but let's
 		// be extra careful: Scanner is for safe, simple jobs.
 		for loop := 0; ; {
 			n, err := s.r.Read(s.buf[s.end:len(s.buf)])
 			if n < 0 || len(s.buf)-s.end < n {
 				s.setErr(ErrBadReadCount)
 				break
 			}
 			s.end += n
 			if err != nil {
 				s.setErr(err)
 				break
 			}
 			if n > 0 {
 				s.empties = 0
 				break
 			}
 			loop++
 			if loop > maxConsecutiveEmptyReads {
 				s.setErr(io.ErrNoProgress)
 				break
 			}
 		}
 	}
 }

 // advance consumes n bytes of the buffer. It reports whether the advance was legal.
 func (s *Scanner) advance(n int) bool {
 	if n < 0 {
 		s.setErr(ErrNegativeAdvance)
 		return false
 	}
 	if n > s.end-s.start {
 		s.setErr(ErrAdvanceTooFar)
 		return false
 	}
 	s.start += n
 	return true
 }

 // setErr records the first error encountered.
 func (s *Scanner) setErr(err error) {
 	if s.err == nil || s.err == io.EOF {
 		s.err = err
 	}
 }

 // Buffer sets the initial buffer to use when scanning and the maximum
 // size of buffer that may be allocated during scanning. The maximum
 // token size is the larger of max and cap(buf). If max <= cap(buf),
 // Scan will use this buffer only and do no allocation.
 //
 // By default, Scan uses an internal buffer and sets the
 // maximum token size to MaxScanTokenSize.
 //
 // Buffer panics if it is called after scanning has started.
 func (s *Scanner) Buffer(buf []byte, max int) {
 	if s.scanCalled {
 		panic("Buffer called after Scan")
 	}
 	s.buf = buf[0:cap(buf)]
 	s.maxTokenSize = max
 }

 // Split sets the split function for the Scanner.
 // The default split function is ScanLines.
 //
 // Split panics if it is called after scanning has started.
 func (s *Scanner) Split(split SplitFunc) {
 	if s.scanCalled {
 		panic("Split called after Scan")
 	}
 	s.split = split
 }

 // Split functions

 // ScanBytes is a split function for a Scanner that returns each byte as a token.
 func ScanBytes(data []byte, atEOF bool) (advance int, token []byte, err error) {
 	if atEOF && len(data) == 0 {
 		return 0, nil, nil
 	}
 	return 1, data[0:1], nil
 }

 var errorRune = []byte(string(utf8.RuneError))

 // ScanRunes is a split function for a Scanner that returns each
 // UTF-8-encoded rune as a token. The sequence of runes returned is
 // equivalent to that from a range loop over the input as a string, which
 // means that erroneous UTF-8 encodings translate to U+FFFD = "\xef\xbf\xbd".
 // Because of the Scan interface, this makes it impossible for the client to
 // distinguish correctly encoded replacement runes from encoding errors.
 func ScanRunes(data []byte, atEOF bool) (advance int, token []byte, err error) {
 	if atEOF && len(data) == 0 {
 		return 0, nil, nil
 	}

 	// Fast path 1: ASCII.
 	if data[0] < utf8.RuneSelf {
 		return 1, data[0:1], nil
 	}

 	// Fast path 2: Correct UTF-8 decode without error.
 	_, width := utf8.DecodeRune(data)
 	if width > 1 {
 		// It's a valid encoding. Width cannot be one for a correctly encoded
 		// non-ASCII rune.
 		return width, data[0:width], nil
 	}

 	// We know it's an error: we have width==1 and implicitly r==utf8.RuneError.
 	// Is the error because there wasn't a full rune to be decoded?
 	// FullRune distinguishes correctly between erroneous and incomplete encodings.
 	if !atEOF && !utf8.FullRune(data) {
 		// Incomplete; get more bytes.
 		return 0, nil, nil
 	}

 	// We have a real UTF-8 encoding error. Return a properly encoded error rune
 	// but advance only one byte. This matches the behavior of a range loop over
 	// an incorrectly encoded string.
 	return 1, errorRune, nil
 }

 // dropCR drops a terminal \r from the data.
 func dropCR(data []byte) []byte {
 	if len(data) > 0 && data[len(data)-1] == '\r' {
 		return data[0 : len(data)-1]
 	}
 	return data
 }

 // ScanLines is a split function for a Scanner that returns each line of
 // text, stripped of any trailing end-of-line marker. The returned line may
 // be empty. The end-of-line marker is one optional carriage return followed
 // by one mandatory newline. In regular expression notation, it is `\r?\n`.
 // The last non-empty line of input will be returned even if it has no
 // newline.
 func ScanLines(data []byte, atEOF bool) (advance int, token []byte, err error) {
 	if atEOF && len(data) == 0 {
 		return 0, nil, nil
 	}
 	if i := bytes.IndexByte(data, '\n'); i >= 0 {
 		// We have a full newline-terminated line.
 		return i + 1, dropCR(data[0:i]), nil
 	}
 	// If we're at EOF, we have a final, non-terminated line. Return it.
 	if atEOF {
 		return len(data), dropCR(data), nil
 	}
 	// Request more data.
 	return 0, nil, nil
 }

 // isSpace reports whether the character is a Unicode white space character.
 // We avoid dependency on the unicode package, but check validity of the implementation
 // in the tests.
 func isSpace(r rune) bool {
 	if r <= '\u00FF' {
 		// Obvious ASCII ones: \t through \r plus space. Plus two Latin-1 oddballs.
 		switch r {
 		case ' ', '\t', '\n', '\v', '\f', '\r':
 			return true
 		case '\u0085', '\u00A0':
 			return true
 		}
 		return false
 	}
 	// High-valued ones.
 	if '\u2000' <= r && r <= '\u200a' {
 		return true
 	}
 	switch r {
 	case '\u1680', '\u2028', '\u2029', '\u202f', '\u205f', '\u3000':
 		return true
 	}
 	return false
 }

 // ScanWords is a split function for a Scanner that returns each
 // space-separated word of text, with surrounding spaces deleted. It will
 // never return an empty string. The definition of space is set by
 // unicode.IsSpace.
 func ScanWords(data []byte, atEOF bool) (advance int, token []byte, err error) {
 	// Skip leading spaces.
 	start := 0
 	for width := 0; start < len(data); start += width {
 		var r rune
 		r, width = utf8.DecodeRune(data[start:])
 		if !isSpace(r) {
 			break
 		}
 	}
 	// Scan until space, marking end of word.
 	for width, i := 0, start; i < len(data); i += width {
 		var r rune
 		r, width = utf8.DecodeRune(data[i:])
 		if isSpace(r) {
 			return i + width, data[start:i], nil
 		}
 	}
 	// If we're at EOF, we have a final, non-empty, non-terminated word. Return it.
 	if atEOF && len(data) > start {
 		return len(data), data[start:], nil
 	}
 	// Request more data.
 	return start, nil, nil
 }
	// Copyright 2013 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 bufio

	import (
	"bytes"
	"errors"
	"io"
	"unicode/utf8"
	)

	// Scanner provides a convenient interface for reading data such as
	// a file of newline-delimited lines of text. Successive calls to
	// the Scan method will step through the 'tokens' of a file, skipping
	// the bytes between the tokens. The specification of a token is
	// defined by a split function of type SplitFunc; the default split
	// function breaks the input into lines with line termination stripped. Split
	// functions are defined in this package for scanning a file into
	// lines, bytes, UTF-8-encoded runes, and space-delimited words. The
	// client may instead provide a custom split function.
	//
	// Scanning stops unrecoverably at EOF, the first I/O error, or a token too
	// large to fit in the buffer. When a scan stops, the reader may have
	// advanced arbitrarily far past the last token. Programs that need more
	// control over error handling or large tokens, or must run sequential scans
	// on a reader, should use bufio.Reader instead.
	//
	type Scanner struct {
	r io.Reader // The reader provided by the client.
	split SplitFunc // The function to split the tokens.
	maxTokenSize int // Maximum size of a token; modified by tests.
	token []byte // Last token returned by split.
	buf []byte // Buffer used as argument to split.
	start int // First non-processed byte in buf.
	end int // End of data in buf.
	err error // Sticky error.
	empties int // Count of successive empty tokens.
	scanCalled bool // Scan has been called; buffer is in use.
	done bool // Scan has finished.
	}

	// SplitFunc is the signature of the split function used to tokenize the
	// input. The arguments are an initial substring of the remaining unprocessed
	// data and a flag, atEOF, that reports whether the Reader has no more data
	// to give. The return values are the number of bytes to advance the input
	// and the next token to return to the user, if any, plus an error, if any.
	//
	// Scanning stops if the function returns an error, in which case some of
	// the input may be discarded. If that error is ErrFinalToken, scanning
	// stops with no error.
	//
	// Otherwise, the Scanner advances the input. If the token is not nil,
	// the Scanner returns it to the user. If the token is nil, the
	// Scanner reads more data and continues scanning; if there is no more
	// data--if atEOF was true--the Scanner returns. If the data does not
	// yet hold a complete token, for instance if it has no newline while
	// scanning lines, a SplitFunc can return (0, nil, nil) to signal the
	// Scanner to read more data into the slice and try again with a
	// longer slice starting at the same point in the input.
	//
	// The function is never called with an empty data slice unless atEOF
	// is true. If atEOF is true, however, data may be non-empty and,
	// as always, holds unprocessed text.
	type SplitFunc func(data []byte, atEOF bool) (advance int, token []byte, err error)

	// Errors returned by Scanner.
	var (
	ErrTooLong = errors.New("bufio.Scanner: token too long")
	ErrNegativeAdvance = errors.New("bufio.Scanner: SplitFunc returns negative advance count")
	ErrAdvanceTooFar = errors.New("bufio.Scanner: SplitFunc returns advance count beyond input")
	ErrBadReadCount = errors.New("bufio.Scanner: Read returned impossible count")
	)

	const (
	// MaxScanTokenSize is the maximum size used to buffer a token
	// unless the user provides an explicit buffer with Scanner.Buffer.
	// The actual maximum token size may be smaller as the buffer
	// may need to include, for instance, a newline.
	MaxScanTokenSize = 64 * 1024

	startBufSize = 4096 // Size of initial allocation for buffer.
	)

	// NewScanner returns a new Scanner to read from r.
	// The split function defaults to ScanLines.
	func NewScanner(r io.Reader) *Scanner {
	return &Scanner{
	r: r,
	split: ScanLines,
	maxTokenSize: MaxScanTokenSize,
	}
	}

	// Err returns the first non-EOF error that was encountered by the Scanner.
	func (s *Scanner) Err() error {
	if s.err == io.EOF {
	return nil
	}
	return s.err
	}

	// Bytes returns the most recent token generated by a call to Scan.
	// The underlying array may point to data that will be overwritten
	// by a subsequent call to Scan. It does no allocation.
	func (s *Scanner) Bytes() []byte {
	return s.token
	}

	// Text returns the most recent token generated by a call to Scan
	// as a newly allocated string holding its bytes.
	func (s *Scanner) Text() string {
	return string(s.token)
	}

	// ErrFinalToken is a special sentinel error value. It is intended to be
	// returned by a Split function to indicate that the token being delivered
	// with the error is the last token and scanning should stop after this one.
	// After ErrFinalToken is received by Scan, scanning stops with no error.
	// The value is useful to stop processing early or when it is necessary to
	// deliver a final empty token. One could achieve the same behavior
	// with a custom error value but providing one here is tidier.
	// See the emptyFinalToken example for a use of this value.
	var ErrFinalToken = errors.New("final token")

	// Scan advances the Scanner to the next token, which will then be
	// available through the Bytes or Text method. It returns false when the
	// scan stops, either by reaching the end of the input or an error.
	// After Scan returns false, the Err method will return any error that
	// occurred during scanning, except that if it was io.EOF, Err
	// will return nil.
	// Scan panics if the split function returns too many empty
	// tokens without advancing the input. This is a common error mode for
	// scanners.
	func (s *Scanner) Scan() bool {
	if s.done {
	return false
	}
	s.scanCalled = true
	// Loop until we have a token.
	for {
	// See if we can get a token with what we already have.
	// If we've run out of data but have an error, give the split function
	// a chance to recover any remaining, possibly empty token.
	if s.end > s.start \|\| s.err != nil {
	advance, token, err := s.split(s.buf[s.start:s.end], s.err != nil)
	if err != nil {
	if err == ErrFinalToken {
	s.token = token
	s.done = true
	return true
	}
	s.setErr(err)
	return false
	}
	if !s.advance(advance) {
	return false
	}
	s.token = token
	if token != nil {
	if s.err == nil \|\| advance > 0 {
	s.empties = 0
	} else {
	// Returning tokens not advancing input at EOF.
	s.empties++
	if s.empties > maxConsecutiveEmptyReads {
	panic("bufio.Scan: too many empty tokens without progressing")
	}
	}
	return true
	}
	}
	// We cannot generate a token with what we are holding.
	// If we've already hit EOF or an I/O error, we are done.
	if s.err != nil {
	// Shut it down.
	s.start = 0
	s.end = 0
	return false
	}
	// Must read more data.
	// First, shift data to beginning of buffer if there's lots of empty space
	// or space is needed.
	if s.start > 0 && (s.end == len(s.buf) \|\| s.start > len(s.buf)/2) {
	copy(s.buf, s.buf[s.start:s.end])
	s.end -= s.start
	s.start = 0
	}
	// Is the buffer full? If so, resize.
	if s.end == len(s.buf) {
	// Guarantee no overflow in the multiplication below.
	const maxInt = int(^uint(0) >> 1)
	if len(s.buf) >= s.maxTokenSize \|\| len(s.buf) > maxInt/2 {
	s.setErr(ErrTooLong)
	return false
	}
	newSize := len(s.buf) * 2
	if newSize == 0 {
	newSize = startBufSize
	}
	if newSize > s.maxTokenSize {
	newSize = s.maxTokenSize
	}
	newBuf := make([]byte, newSize)
	copy(newBuf, s.buf[s.start:s.end])
	s.buf = newBuf
	s.end -= s.start
	s.start = 0
	}
	// Finally we can read some input. Make sure we don't get stuck with
	// a misbehaving Reader. Officially we don't need to do this, but let's
	// be extra careful: Scanner is for safe, simple jobs.
	for loop := 0; ; {
	n, err := s.r.Read(s.buf[s.end:len(s.buf)])
	if n < 0 \|\| len(s.buf)-s.end < n {
	s.setErr(ErrBadReadCount)
	break
	}
	s.end += n
	if err != nil {
	s.setErr(err)
	break
	}
	if n > 0 {
	s.empties = 0
	break
	}
	loop++
	if loop > maxConsecutiveEmptyReads {
	s.setErr(io.ErrNoProgress)
	break
	}
	}
	}
	}

	// advance consumes n bytes of the buffer. It reports whether the advance was legal.
	func (s *Scanner) advance(n int) bool {
	if n < 0 {
	s.setErr(ErrNegativeAdvance)
	return false
	}
	if n > s.end-s.start {
	s.setErr(ErrAdvanceTooFar)
	return false
	}
	s.start += n
	return true
	}

	// setErr records the first error encountered.
	func (s *Scanner) setErr(err error) {
	if s.err == nil \|\| s.err == io.EOF {
	s.err = err
	}
	}

	// Buffer sets the initial buffer to use when scanning and the maximum
	// size of buffer that may be allocated during scanning. The maximum
	// token size is the larger of max and cap(buf). If max <= cap(buf),
	// Scan will use this buffer only and do no allocation.
	//
	// By default, Scan uses an internal buffer and sets the
	// maximum token size to MaxScanTokenSize.
	//
	// Buffer panics if it is called after scanning has started.
	func (s *Scanner) Buffer(buf []byte, max int) {
	if s.scanCalled {
	panic("Buffer called after Scan")
	}
	s.buf = buf[0:cap(buf)]
	s.maxTokenSize = max
	}

	// Split sets the split function for the Scanner.
	// The default split function is ScanLines.
	//
	// Split panics if it is called after scanning has started.
	func (s *Scanner) Split(split SplitFunc) {
	if s.scanCalled {
	panic("Split called after Scan")
	}
	s.split = split
	}

	// Split functions

	// ScanBytes is a split function for a Scanner that returns each byte as a token.
	func ScanBytes(data []byte, atEOF bool) (advance int, token []byte, err error) {
	if atEOF && len(data) == 0 {
	return 0, nil, nil
	}
	return 1, data[0:1], nil
	}

	var errorRune = []byte(string(utf8.RuneError))

	// ScanRunes is a split function for a Scanner that returns each
	// UTF-8-encoded rune as a token. The sequence of runes returned is
	// equivalent to that from a range loop over the input as a string, which
	// means that erroneous UTF-8 encodings translate to U+FFFD = "\xef\xbf\xbd".
	// Because of the Scan interface, this makes it impossible for the client to
	// distinguish correctly encoded replacement runes from encoding errors.
	func ScanRunes(data []byte, atEOF bool) (advance int, token []byte, err error) {
	if atEOF && len(data) == 0 {
	return 0, nil, nil
	}

	// Fast path 1: ASCII.
	if data[0] < utf8.RuneSelf {
	return 1, data[0:1], nil
	}

	// Fast path 2: Correct UTF-8 decode without error.
	_, width := utf8.DecodeRune(data)
	if width > 1 {
	// It's a valid encoding. Width cannot be one for a correctly encoded
	// non-ASCII rune.
	return width, data[0:width], nil
	}

	// We know it's an error: we have width==1 and implicitly r==utf8.RuneError.
	// Is the error because there wasn't a full rune to be decoded?
	// FullRune distinguishes correctly between erroneous and incomplete encodings.
	if !atEOF && !utf8.FullRune(data) {
	// Incomplete; get more bytes.
	return 0, nil, nil
	}

	// We have a real UTF-8 encoding error. Return a properly encoded error rune
	// but advance only one byte. This matches the behavior of a range loop over
	// an incorrectly encoded string.
	return 1, errorRune, nil
	}

	// dropCR drops a terminal \r from the data.
	func dropCR(data []byte) []byte {
	if len(data) > 0 && data[len(data)-1] == '\r' {
	return data[0 : len(data)-1]
	}
	return data
	}

	// ScanLines is a split function for a Scanner that returns each line of
	// text, stripped of any trailing end-of-line marker. The returned line may
	// be empty. The end-of-line marker is one optional carriage return followed
	// by one mandatory newline. In regular expression notation, it is `\r?\n`.
	// The last non-empty line of input will be returned even if it has no
	// newline.
	func ScanLines(data []byte, atEOF bool) (advance int, token []byte, err error) {
	if atEOF && len(data) == 0 {
	return 0, nil, nil
	}
	if i := bytes.IndexByte(data, '\n'); i >= 0 {
	// We have a full newline-terminated line.
	return i + 1, dropCR(data[0:i]), nil
	}
	// If we're at EOF, we have a final, non-terminated line. Return it.
	if atEOF {
	return len(data), dropCR(data), nil
	}
	// Request more data.
	return 0, nil, nil
	}

	// isSpace reports whether the character is a Unicode white space character.
	// We avoid dependency on the unicode package, but check validity of the implementation
	// in the tests.
	func isSpace(r rune) bool {
	if r <= '\u00FF' {
	// Obvious ASCII ones: \t through \r plus space. Plus two Latin-1 oddballs.
	switch r {
	case ' ', '\t', '\n', '\v', '\f', '\r':
	return true
	case '\u0085', '\u00A0':
	return true
	}
	return false
	}
	// High-valued ones.
	if '\u2000' <= r && r <= '\u200a' {
	return true
	}
	switch r {
	case '\u1680', '\u2028', '\u2029', '\u202f', '\u205f', '\u3000':
	return true
	}
	return false
	}

	// ScanWords is a split function for a Scanner that returns each
	// space-separated word of text, with surrounding spaces deleted. It will
	// never return an empty string. The definition of space is set by
	// unicode.IsSpace.
	func ScanWords(data []byte, atEOF bool) (advance int, token []byte, err error) {
	// Skip leading spaces.
	start := 0
	for width := 0; start < len(data); start += width {
	var r rune
	r, width = utf8.DecodeRune(data[start:])
	if !isSpace(r) {
	break
	}
	}
	// Scan until space, marking end of word.
	for width, i := 0, start; i < len(data); i += width {
	var r rune
	r, width = utf8.DecodeRune(data[i:])
	if isSpace(r) {
	return i + width, data[start:i], nil
	}
	}
	// If we're at EOF, we have a final, non-empty, non-terminated word. Return it.
	if atEOF && len(data) > start {
	return len(data), data[start:], nil
	}
	// Request more data.
	return start, nil, nil
	}