Source file src/pkg/fmt/scan.go

     1	// Copyright 2010 The Go Authors. All rights reserved.
     2	// Use of this source code is governed by a BSD-style
     3	// license that can be found in the LICENSE file.
     4	
     5	package fmt
     6	
     7	import (
     8		"errors"
     9		"io"
    10		"math"
    11		"os"
    12		"reflect"
    13		"strconv"
    14		"unicode/utf8"
    15	)
    16	
    17	// runeUnreader is the interface to something that can unread runes.
    18	// If the object provided to Scan does not satisfy this interface,
    19	// a local buffer will be used to back up the input, but its contents
    20	// will be lost when Scan returns.
    21	type runeUnreader interface {
    22		UnreadRune() error
    23	}
    24	
    25	// ScanState represents the scanner state passed to custom scanners.
    26	// Scanners may do rune-at-a-time scanning or ask the ScanState
    27	// to discover the next space-delimited token.
    28	type ScanState interface {
    29		// ReadRune reads the next rune (Unicode code point) from the input.
    30		// If invoked during Scanln, Fscanln, or Sscanln, ReadRune() will
    31		// return EOF after returning the first '\n' or when reading beyond
    32		// the specified width.
    33		ReadRune() (r rune, size int, err error)
    34		// UnreadRune causes the next call to ReadRune to return the same rune.
    35		UnreadRune() error
    36		// SkipSpace skips space in the input. Newlines are treated as space 
    37		// unless the scan operation is Scanln, Fscanln or Sscanln, in which case 
    38		// a newline is treated as EOF.
    39		SkipSpace()
    40		// Token skips space in the input if skipSpace is true, then returns the
    41		// run of Unicode code points c satisfying f(c).  If f is nil,
    42		// !unicode.IsSpace(c) is used; that is, the token will hold non-space
    43		// characters.  Newlines are treated as space unless the scan operation
    44		// is Scanln, Fscanln or Sscanln, in which case a newline is treated as
    45		// EOF.  The returned slice points to shared data that may be overwritten
    46		// by the next call to Token, a call to a Scan function using the ScanState
    47		// as input, or when the calling Scan method returns.
    48		Token(skipSpace bool, f func(rune) bool) (token []byte, err error)
    49		// Width returns the value of the width option and whether it has been set.
    50		// The unit is Unicode code points.
    51		Width() (wid int, ok bool)
    52		// Because ReadRune is implemented by the interface, Read should never be
    53		// called by the scanning routines and a valid implementation of
    54		// ScanState may choose always to return an error from Read.
    55		Read(buf []byte) (n int, err error)
    56	}
    57	
    58	// Scanner is implemented by any value that has a Scan method, which scans
    59	// the input for the representation of a value and stores the result in the
    60	// receiver, which must be a pointer to be useful.  The Scan method is called
    61	// for any argument to Scan, Scanf, or Scanln that implements it.
    62	type Scanner interface {
    63		Scan(state ScanState, verb rune) error
    64	}
    65	
    66	// Scan scans text read from standard input, storing successive
    67	// space-separated values into successive arguments.  Newlines count
    68	// as space.  It returns the number of items successfully scanned.
    69	// If that is less than the number of arguments, err will report why.
    70	func Scan(a ...interface{}) (n int, err error) {
    71		return Fscan(os.Stdin, a...)
    72	}
    73	
    74	// Scanln is similar to Scan, but stops scanning at a newline and
    75	// after the final item there must be a newline or EOF.
    76	func Scanln(a ...interface{}) (n int, err error) {
    77		return Fscanln(os.Stdin, a...)
    78	}
    79	
    80	// Scanf scans text read from standard input, storing successive
    81	// space-separated values into successive arguments as determined by
    82	// the format.  It returns the number of items successfully scanned.
    83	func Scanf(format string, a ...interface{}) (n int, err error) {
    84		return Fscanf(os.Stdin, format, a...)
    85	}
    86	
    87	type stringReader string
    88	
    89	func (r *stringReader) Read(b []byte) (n int, err error) {
    90		n = copy(b, *r)
    91		*r = (*r)[n:]
    92		if n == 0 {
    93			err = io.EOF
    94		}
    95		return
    96	}
    97	
    98	// Sscan scans the argument string, storing successive space-separated
    99	// values into successive arguments.  Newlines count as space.  It
   100	// returns the number of items successfully scanned.  If that is less
   101	// than the number of arguments, err will report why.
   102	func Sscan(str string, a ...interface{}) (n int, err error) {
   103		return Fscan((*stringReader)(&str), a...)
   104	}
   105	
   106	// Sscanln is similar to Sscan, but stops scanning at a newline and
   107	// after the final item there must be a newline or EOF.
   108	func Sscanln(str string, a ...interface{}) (n int, err error) {
   109		return Fscanln((*stringReader)(&str), a...)
   110	}
   111	
   112	// Sscanf scans the argument string, storing successive space-separated
   113	// values into successive arguments as determined by the format.  It
   114	// returns the number of items successfully parsed.
   115	func Sscanf(str string, format string, a ...interface{}) (n int, err error) {
   116		return Fscanf((*stringReader)(&str), format, a...)
   117	}
   118	
   119	// Fscan scans text read from r, storing successive space-separated
   120	// values into successive arguments.  Newlines count as space.  It
   121	// returns the number of items successfully scanned.  If that is less
   122	// than the number of arguments, err will report why.
   123	func Fscan(r io.Reader, a ...interface{}) (n int, err error) {
   124		s, old := newScanState(r, true, false)
   125		n, err = s.doScan(a)
   126		s.free(old)
   127		return
   128	}
   129	
   130	// Fscanln is similar to Fscan, but stops scanning at a newline and
   131	// after the final item there must be a newline or EOF.
   132	func Fscanln(r io.Reader, a ...interface{}) (n int, err error) {
   133		s, old := newScanState(r, false, true)
   134		n, err = s.doScan(a)
   135		s.free(old)
   136		return
   137	}
   138	
   139	// Fscanf scans text read from r, storing successive space-separated
   140	// values into successive arguments as determined by the format.  It
   141	// returns the number of items successfully parsed.
   142	func Fscanf(r io.Reader, format string, a ...interface{}) (n int, err error) {
   143		s, old := newScanState(r, false, false)
   144		n, err = s.doScanf(format, a)
   145		s.free(old)
   146		return
   147	}
   148	
   149	// scanError represents an error generated by the scanning software.
   150	// It's used as a unique signature to identify such errors when recovering.
   151	type scanError struct {
   152		err error
   153	}
   154	
   155	const eof = -1
   156	
   157	// ss is the internal implementation of ScanState.
   158	type ss struct {
   159		rr       io.RuneReader // where to read input
   160		buf      buffer        // token accumulator
   161		peekRune rune          // one-rune lookahead
   162		prevRune rune          // last rune returned by ReadRune
   163		count    int           // runes consumed so far.
   164		atEOF    bool          // already read EOF
   165		ssave
   166	}
   167	
   168	// ssave holds the parts of ss that need to be
   169	// saved and restored on recursive scans.
   170	type ssave struct {
   171		validSave  bool // is or was a part of an actual ss.
   172		nlIsEnd    bool // whether newline terminates scan
   173		nlIsSpace  bool // whether newline counts as white space
   174		fieldLimit int  // max value of ss.count for this field; fieldLimit <= limit
   175		limit      int  // max value of ss.count.
   176		maxWid     int  // width of this field.
   177	}
   178	
   179	// The Read method is only in ScanState so that ScanState
   180	// satisfies io.Reader. It will never be called when used as
   181	// intended, so there is no need to make it actually work.
   182	func (s *ss) Read(buf []byte) (n int, err error) {
   183		return 0, errors.New("ScanState's Read should not be called. Use ReadRune")
   184	}
   185	
   186	func (s *ss) ReadRune() (r rune, size int, err error) {
   187		if s.peekRune >= 0 {
   188			s.count++
   189			r = s.peekRune
   190			size = utf8.RuneLen(r)
   191			s.prevRune = r
   192			s.peekRune = -1
   193			return
   194		}
   195		if s.atEOF || s.nlIsEnd && s.prevRune == '\n' || s.count >= s.fieldLimit {
   196			err = io.EOF
   197			return
   198		}
   199	
   200		r, size, err = s.rr.ReadRune()
   201		if err == nil {
   202			s.count++
   203			s.prevRune = r
   204		} else if err == io.EOF {
   205			s.atEOF = true
   206		}
   207		return
   208	}
   209	
   210	func (s *ss) Width() (wid int, ok bool) {
   211		if s.maxWid == hugeWid {
   212			return 0, false
   213		}
   214		return s.maxWid, true
   215	}
   216	
   217	// The public method returns an error; this private one panics.
   218	// If getRune reaches EOF, the return value is EOF (-1).
   219	func (s *ss) getRune() (r rune) {
   220		r, _, err := s.ReadRune()
   221		if err != nil {
   222			if err == io.EOF {
   223				return eof
   224			}
   225			s.error(err)
   226		}
   227		return
   228	}
   229	
   230	// mustReadRune turns io.EOF into a panic(io.ErrUnexpectedEOF).
   231	// It is called in cases such as string scanning where an EOF is a
   232	// syntax error.
   233	func (s *ss) mustReadRune() (r rune) {
   234		r = s.getRune()
   235		if r == eof {
   236			s.error(io.ErrUnexpectedEOF)
   237		}
   238		return
   239	}
   240	
   241	func (s *ss) UnreadRune() error {
   242		if u, ok := s.rr.(runeUnreader); ok {
   243			u.UnreadRune()
   244		} else {
   245			s.peekRune = s.prevRune
   246		}
   247		s.prevRune = -1
   248		s.count--
   249		return nil
   250	}
   251	
   252	func (s *ss) error(err error) {
   253		panic(scanError{err})
   254	}
   255	
   256	func (s *ss) errorString(err string) {
   257		panic(scanError{errors.New(err)})
   258	}
   259	
   260	func (s *ss) Token(skipSpace bool, f func(rune) bool) (tok []byte, err error) {
   261		defer func() {
   262			if e := recover(); e != nil {
   263				if se, ok := e.(scanError); ok {
   264					err = se.err
   265				} else {
   266					panic(e)
   267				}
   268			}
   269		}()
   270		if f == nil {
   271			f = notSpace
   272		}
   273		s.buf = s.buf[:0]
   274		tok = s.token(skipSpace, f)
   275		return
   276	}
   277	
   278	// space is a copy of the unicode.White_Space ranges,
   279	// to avoid depending on package unicode.
   280	var space = [][2]uint16{
   281		{0x0009, 0x000d},
   282		{0x0020, 0x0020},
   283		{0x0085, 0x0085},
   284		{0x00a0, 0x00a0},
   285		{0x1680, 0x1680},
   286		{0x180e, 0x180e},
   287		{0x2000, 0x200a},
   288		{0x2028, 0x2029},
   289		{0x202f, 0x202f},
   290		{0x205f, 0x205f},
   291		{0x3000, 0x3000},
   292	}
   293	
   294	func isSpace(r rune) bool {
   295		if r >= 1<<16 {
   296			return false
   297		}
   298		rx := uint16(r)
   299		for _, rng := range space {
   300			if rx < rng[0] {
   301				return false
   302			}
   303			if rx <= rng[1] {
   304				return true
   305			}
   306		}
   307		return false
   308	}
   309	
   310	// notSpace is the default scanning function used in Token.
   311	func notSpace(r rune) bool {
   312		return !isSpace(r)
   313	}
   314	
   315	// skipSpace provides Scan() methods the ability to skip space and newline characters 
   316	// in keeping with the current scanning mode set by format strings and Scan()/Scanln().
   317	func (s *ss) SkipSpace() {
   318		s.skipSpace(false)
   319	}
   320	
   321	// readRune is a structure to enable reading UTF-8 encoded code points
   322	// from an io.Reader.  It is used if the Reader given to the scanner does
   323	// not already implement io.RuneReader.
   324	type readRune struct {
   325		reader  io.Reader
   326		buf     [utf8.UTFMax]byte // used only inside ReadRune
   327		pending int               // number of bytes in pendBuf; only >0 for bad UTF-8
   328		pendBuf [utf8.UTFMax]byte // bytes left over
   329	}
   330	
   331	// readByte returns the next byte from the input, which may be
   332	// left over from a previous read if the UTF-8 was ill-formed.
   333	func (r *readRune) readByte() (b byte, err error) {
   334		if r.pending > 0 {
   335			b = r.pendBuf[0]
   336			copy(r.pendBuf[0:], r.pendBuf[1:])
   337			r.pending--
   338			return
   339		}
   340		_, err = r.reader.Read(r.pendBuf[0:1])
   341		return r.pendBuf[0], err
   342	}
   343	
   344	// unread saves the bytes for the next read.
   345	func (r *readRune) unread(buf []byte) {
   346		copy(r.pendBuf[r.pending:], buf)
   347		r.pending += len(buf)
   348	}
   349	
   350	// ReadRune returns the next UTF-8 encoded code point from the
   351	// io.Reader inside r.
   352	func (r *readRune) ReadRune() (rr rune, size int, err error) {
   353		r.buf[0], err = r.readByte()
   354		if err != nil {
   355			return 0, 0, err
   356		}
   357		if r.buf[0] < utf8.RuneSelf { // fast check for common ASCII case
   358			rr = rune(r.buf[0])
   359			return
   360		}
   361		var n int
   362		for n = 1; !utf8.FullRune(r.buf[0:n]); n++ {
   363			r.buf[n], err = r.readByte()
   364			if err != nil {
   365				if err == io.EOF {
   366					err = nil
   367					break
   368				}
   369				return
   370			}
   371		}
   372		rr, size = utf8.DecodeRune(r.buf[0:n])
   373		if size < n { // an error
   374			r.unread(r.buf[size:n])
   375		}
   376		return
   377	}
   378	
   379	var ssFree = newCache(func() interface{} { return new(ss) })
   380	
   381	// Allocate a new ss struct or grab a cached one.
   382	func newScanState(r io.Reader, nlIsSpace, nlIsEnd bool) (s *ss, old ssave) {
   383		// If the reader is a *ss, then we've got a recursive
   384		// call to Scan, so re-use the scan state.
   385		s, ok := r.(*ss)
   386		if ok {
   387			old = s.ssave
   388			s.limit = s.fieldLimit
   389			s.nlIsEnd = nlIsEnd || s.nlIsEnd
   390			s.nlIsSpace = nlIsSpace
   391			return
   392		}
   393	
   394		s = ssFree.get().(*ss)
   395		if rr, ok := r.(io.RuneReader); ok {
   396			s.rr = rr
   397		} else {
   398			s.rr = &readRune{reader: r}
   399		}
   400		s.nlIsSpace = nlIsSpace
   401		s.nlIsEnd = nlIsEnd
   402		s.prevRune = -1
   403		s.peekRune = -1
   404		s.atEOF = false
   405		s.limit = hugeWid
   406		s.fieldLimit = hugeWid
   407		s.maxWid = hugeWid
   408		s.validSave = true
   409		s.count = 0
   410		return
   411	}
   412	
   413	// Save used ss structs in ssFree; avoid an allocation per invocation.
   414	func (s *ss) free(old ssave) {
   415		// If it was used recursively, just restore the old state.
   416		if old.validSave {
   417			s.ssave = old
   418			return
   419		}
   420		// Don't hold on to ss structs with large buffers.
   421		if cap(s.buf) > 1024 {
   422			return
   423		}
   424		s.buf = s.buf[:0]
   425		s.rr = nil
   426		ssFree.put(s)
   427	}
   428	
   429	// skipSpace skips spaces and maybe newlines.
   430	func (s *ss) skipSpace(stopAtNewline bool) {
   431		for {
   432			r := s.getRune()
   433			if r == eof {
   434				return
   435			}
   436			if r == '\n' {
   437				if stopAtNewline {
   438					break
   439				}
   440				if s.nlIsSpace {
   441					continue
   442				}
   443				s.errorString("unexpected newline")
   444				return
   445			}
   446			if !isSpace(r) {
   447				s.UnreadRune()
   448				break
   449			}
   450		}
   451	}
   452	
   453	// token returns the next space-delimited string from the input.  It
   454	// skips white space.  For Scanln, it stops at newlines.  For Scan,
   455	// newlines are treated as spaces.
   456	func (s *ss) token(skipSpace bool, f func(rune) bool) []byte {
   457		if skipSpace {
   458			s.skipSpace(false)
   459		}
   460		// read until white space or newline
   461		for {
   462			r := s.getRune()
   463			if r == eof {
   464				break
   465			}
   466			if !f(r) {
   467				s.UnreadRune()
   468				break
   469			}
   470			s.buf.WriteRune(r)
   471		}
   472		return s.buf
   473	}
   474	
   475	// typeError indicates that the type of the operand did not match the format
   476	func (s *ss) typeError(field interface{}, expected string) {
   477		s.errorString("expected field of type pointer to " + expected + "; found " + reflect.TypeOf(field).String())
   478	}
   479	
   480	var complexError = errors.New("syntax error scanning complex number")
   481	var boolError = errors.New("syntax error scanning boolean")
   482	
   483	func indexRune(s string, r rune) int {
   484		for i, c := range s {
   485			if c == r {
   486				return i
   487			}
   488		}
   489		return -1
   490	}
   491	
   492	// consume reads the next rune in the input and reports whether it is in the ok string.
   493	// If accept is true, it puts the character into the input token.
   494	func (s *ss) consume(ok string, accept bool) bool {
   495		r := s.getRune()
   496		if r == eof {
   497			return false
   498		}
   499		if indexRune(ok, r) >= 0 {
   500			if accept {
   501				s.buf.WriteRune(r)
   502			}
   503			return true
   504		}
   505		if r != eof && accept {
   506			s.UnreadRune()
   507		}
   508		return false
   509	}
   510	
   511	// peek reports whether the next character is in the ok string, without consuming it.
   512	func (s *ss) peek(ok string) bool {
   513		r := s.getRune()
   514		if r != eof {
   515			s.UnreadRune()
   516		}
   517		return indexRune(ok, r) >= 0
   518	}
   519	
   520	func (s *ss) notEOF() {
   521		// Guarantee there is data to be read.
   522		if r := s.getRune(); r == eof {
   523			panic(io.EOF)
   524		}
   525		s.UnreadRune()
   526	}
   527	
   528	// accept checks the next rune in the input.  If it's a byte (sic) in the string, it puts it in the
   529	// buffer and returns true. Otherwise it return false.
   530	func (s *ss) accept(ok string) bool {
   531		return s.consume(ok, true)
   532	}
   533	
   534	// okVerb verifies that the verb is present in the list, setting s.err appropriately if not.
   535	func (s *ss) okVerb(verb rune, okVerbs, typ string) bool {
   536		for _, v := range okVerbs {
   537			if v == verb {
   538				return true
   539			}
   540		}
   541		s.errorString("bad verb %" + string(verb) + " for " + typ)
   542		return false
   543	}
   544	
   545	// scanBool returns the value of the boolean represented by the next token.
   546	func (s *ss) scanBool(verb rune) bool {
   547		s.skipSpace(false)
   548		s.notEOF()
   549		if !s.okVerb(verb, "tv", "boolean") {
   550			return false
   551		}
   552		// Syntax-checking a boolean is annoying.  We're not fastidious about case.
   553		switch s.getRune() {
   554		case '0':
   555			return false
   556		case '1':
   557			return true
   558		case 't', 'T':
   559			if s.accept("rR") && (!s.accept("uU") || !s.accept("eE")) {
   560				s.error(boolError)
   561			}
   562			return true
   563		case 'f', 'F':
   564			if s.accept("aA") && (!s.accept("lL") || !s.accept("sS") || !s.accept("eE")) {
   565				s.error(boolError)
   566			}
   567			return false
   568		}
   569		return false
   570	}
   571	
   572	// Numerical elements
   573	const (
   574		binaryDigits      = "01"
   575		octalDigits       = "01234567"
   576		decimalDigits     = "0123456789"
   577		hexadecimalDigits = "0123456789aAbBcCdDeEfF"
   578		sign              = "+-"
   579		period            = "."
   580		exponent          = "eEp"
   581	)
   582	
   583	// getBase returns the numeric base represented by the verb and its digit string.
   584	func (s *ss) getBase(verb rune) (base int, digits string) {
   585		s.okVerb(verb, "bdoUxXv", "integer") // sets s.err
   586		base = 10
   587		digits = decimalDigits
   588		switch verb {
   589		case 'b':
   590			base = 2
   591			digits = binaryDigits
   592		case 'o':
   593			base = 8
   594			digits = octalDigits
   595		case 'x', 'X', 'U':
   596			base = 16
   597			digits = hexadecimalDigits
   598		}
   599		return
   600	}
   601	
   602	// scanNumber returns the numerical string with specified digits starting here.
   603	func (s *ss) scanNumber(digits string, haveDigits bool) string {
   604		if !haveDigits {
   605			s.notEOF()
   606			if !s.accept(digits) {
   607				s.errorString("expected integer")
   608			}
   609		}
   610		for s.accept(digits) {
   611		}
   612		return string(s.buf)
   613	}
   614	
   615	// scanRune returns the next rune value in the input.
   616	func (s *ss) scanRune(bitSize int) int64 {
   617		s.notEOF()
   618		r := int64(s.getRune())
   619		n := uint(bitSize)
   620		x := (r << (64 - n)) >> (64 - n)
   621		if x != r {
   622			s.errorString("overflow on character value " + string(r))
   623		}
   624		return r
   625	}
   626	
   627	// scanBasePrefix reports whether the integer begins with a 0 or 0x,
   628	// and returns the base, digit string, and whether a zero was found.
   629	// It is called only if the verb is %v.
   630	func (s *ss) scanBasePrefix() (base int, digits string, found bool) {
   631		if !s.peek("0") {
   632			return 10, decimalDigits, false
   633		}
   634		s.accept("0")
   635		found = true // We've put a digit into the token buffer.
   636		// Special cases for '0' && '0x'
   637		base, digits = 8, octalDigits
   638		if s.peek("xX") {
   639			s.consume("xX", false)
   640			base, digits = 16, hexadecimalDigits
   641		}
   642		return
   643	}
   644	
   645	// scanInt returns the value of the integer represented by the next
   646	// token, checking for overflow.  Any error is stored in s.err.
   647	func (s *ss) scanInt(verb rune, bitSize int) int64 {
   648		if verb == 'c' {
   649			return s.scanRune(bitSize)
   650		}
   651		s.skipSpace(false)
   652		s.notEOF()
   653		base, digits := s.getBase(verb)
   654		haveDigits := false
   655		if verb == 'U' {
   656			if !s.consume("U", false) || !s.consume("+", false) {
   657				s.errorString("bad unicode format ")
   658			}
   659		} else {
   660			s.accept(sign) // If there's a sign, it will be left in the token buffer.
   661			if verb == 'v' {
   662				base, digits, haveDigits = s.scanBasePrefix()
   663			}
   664		}
   665		tok := s.scanNumber(digits, haveDigits)
   666		i, err := strconv.ParseInt(tok, base, 64)
   667		if err != nil {
   668			s.error(err)
   669		}
   670		n := uint(bitSize)
   671		x := (i << (64 - n)) >> (64 - n)
   672		if x != i {
   673			s.errorString("integer overflow on token " + tok)
   674		}
   675		return i
   676	}
   677	
   678	// scanUint returns the value of the unsigned integer represented
   679	// by the next token, checking for overflow.  Any error is stored in s.err.
   680	func (s *ss) scanUint(verb rune, bitSize int) uint64 {
   681		if verb == 'c' {
   682			return uint64(s.scanRune(bitSize))
   683		}
   684		s.skipSpace(false)
   685		s.notEOF()
   686		base, digits := s.getBase(verb)
   687		haveDigits := false
   688		if verb == 'U' {
   689			if !s.consume("U", false) || !s.consume("+", false) {
   690				s.errorString("bad unicode format ")
   691			}
   692		} else if verb == 'v' {
   693			base, digits, haveDigits = s.scanBasePrefix()
   694		}
   695		tok := s.scanNumber(digits, haveDigits)
   696		i, err := strconv.ParseUint(tok, base, 64)
   697		if err != nil {
   698			s.error(err)
   699		}
   700		n := uint(bitSize)
   701		x := (i << (64 - n)) >> (64 - n)
   702		if x != i {
   703			s.errorString("unsigned integer overflow on token " + tok)
   704		}
   705		return i
   706	}
   707	
   708	// floatToken returns the floating-point number starting here, no longer than swid
   709	// if the width is specified. It's not rigorous about syntax because it doesn't check that
   710	// we have at least some digits, but Atof will do that.
   711	func (s *ss) floatToken() string {
   712		s.buf = s.buf[:0]
   713		// NaN?
   714		if s.accept("nN") && s.accept("aA") && s.accept("nN") {
   715			return string(s.buf)
   716		}
   717		// leading sign?
   718		s.accept(sign)
   719		// Inf?
   720		if s.accept("iI") && s.accept("nN") && s.accept("fF") {
   721			return string(s.buf)
   722		}
   723		// digits?
   724		for s.accept(decimalDigits) {
   725		}
   726		// decimal point?
   727		if s.accept(period) {
   728			// fraction?
   729			for s.accept(decimalDigits) {
   730			}
   731		}
   732		// exponent?
   733		if s.accept(exponent) {
   734			// leading sign?
   735			s.accept(sign)
   736			// digits?
   737			for s.accept(decimalDigits) {
   738			}
   739		}
   740		return string(s.buf)
   741	}
   742	
   743	// complexTokens returns the real and imaginary parts of the complex number starting here.
   744	// The number might be parenthesized and has the format (N+Ni) where N is a floating-point
   745	// number and there are no spaces within.
   746	func (s *ss) complexTokens() (real, imag string) {
   747		// TODO: accept N and Ni independently?
   748		parens := s.accept("(")
   749		real = s.floatToken()
   750		s.buf = s.buf[:0]
   751		// Must now have a sign.
   752		if !s.accept("+-") {
   753			s.error(complexError)
   754		}
   755		// Sign is now in buffer
   756		imagSign := string(s.buf)
   757		imag = s.floatToken()
   758		if !s.accept("i") {
   759			s.error(complexError)
   760		}
   761		if parens && !s.accept(")") {
   762			s.error(complexError)
   763		}
   764		return real, imagSign + imag
   765	}
   766	
   767	// convertFloat converts the string to a float64value.
   768	func (s *ss) convertFloat(str string, n int) float64 {
   769		if p := indexRune(str, 'p'); p >= 0 {
   770			// Atof doesn't handle power-of-2 exponents,
   771			// but they're easy to evaluate.
   772			f, err := strconv.ParseFloat(str[:p], n)
   773			if err != nil {
   774				// Put full string into error.
   775				if e, ok := err.(*strconv.NumError); ok {
   776					e.Num = str
   777				}
   778				s.error(err)
   779			}
   780			n, err := strconv.Atoi(str[p+1:])
   781			if err != nil {
   782				// Put full string into error.
   783				if e, ok := err.(*strconv.NumError); ok {
   784					e.Num = str
   785				}
   786				s.error(err)
   787			}
   788			return math.Ldexp(f, n)
   789		}
   790		f, err := strconv.ParseFloat(str, n)
   791		if err != nil {
   792			s.error(err)
   793		}
   794		return f
   795	}
   796	
   797	// convertComplex converts the next token to a complex128 value.
   798	// The atof argument is a type-specific reader for the underlying type.
   799	// If we're reading complex64, atof will parse float32s and convert them
   800	// to float64's to avoid reproducing this code for each complex type.
   801	func (s *ss) scanComplex(verb rune, n int) complex128 {
   802		if !s.okVerb(verb, floatVerbs, "complex") {
   803			return 0
   804		}
   805		s.skipSpace(false)
   806		s.notEOF()
   807		sreal, simag := s.complexTokens()
   808		real := s.convertFloat(sreal, n/2)
   809		imag := s.convertFloat(simag, n/2)
   810		return complex(real, imag)
   811	}
   812	
   813	// convertString returns the string represented by the next input characters.
   814	// The format of the input is determined by the verb.
   815	func (s *ss) convertString(verb rune) (str string) {
   816		if !s.okVerb(verb, "svqx", "string") {
   817			return ""
   818		}
   819		s.skipSpace(false)
   820		s.notEOF()
   821		switch verb {
   822		case 'q':
   823			str = s.quotedString()
   824		case 'x':
   825			str = s.hexString()
   826		default:
   827			str = string(s.token(true, notSpace)) // %s and %v just return the next word
   828		}
   829		return
   830	}
   831	
   832	// quotedString returns the double- or back-quoted string represented by the next input characters.
   833	func (s *ss) quotedString() string {
   834		s.notEOF()
   835		quote := s.getRune()
   836		switch quote {
   837		case '`':
   838			// Back-quoted: Anything goes until EOF or back quote.
   839			for {
   840				r := s.mustReadRune()
   841				if r == quote {
   842					break
   843				}
   844				s.buf.WriteRune(r)
   845			}
   846			return string(s.buf)
   847		case '"':
   848			// Double-quoted: Include the quotes and let strconv.Unquote do the backslash escapes.
   849			s.buf.WriteRune(quote)
   850			for {
   851				r := s.mustReadRune()
   852				s.buf.WriteRune(r)
   853				if r == '\\' {
   854					// In a legal backslash escape, no matter how long, only the character
   855					// immediately after the escape can itself be a backslash or quote.
   856					// Thus we only need to protect the first character after the backslash.
   857					r := s.mustReadRune()
   858					s.buf.WriteRune(r)
   859				} else if r == '"' {
   860					break
   861				}
   862			}
   863			result, err := strconv.Unquote(string(s.buf))
   864			if err != nil {
   865				s.error(err)
   866			}
   867			return result
   868		default:
   869			s.errorString("expected quoted string")
   870		}
   871		return ""
   872	}
   873	
   874	// hexDigit returns the value of the hexadecimal digit
   875	func (s *ss) hexDigit(d rune) int {
   876		digit := int(d)
   877		switch digit {
   878		case '0', '1', '2', '3', '4', '5', '6', '7', '8', '9':
   879			return digit - '0'
   880		case 'a', 'b', 'c', 'd', 'e', 'f':
   881			return 10 + digit - 'a'
   882		case 'A', 'B', 'C', 'D', 'E', 'F':
   883			return 10 + digit - 'A'
   884		}
   885		s.errorString("Scan: illegal hex digit")
   886		return 0
   887	}
   888	
   889	// hexByte returns the next hex-encoded (two-character) byte from the input.
   890	// There must be either two hexadecimal digits or a space character in the input.
   891	func (s *ss) hexByte() (b byte, ok bool) {
   892		rune1 := s.getRune()
   893		if rune1 == eof {
   894			return
   895		}
   896		if isSpace(rune1) {
   897			s.UnreadRune()
   898			return
   899		}
   900		rune2 := s.mustReadRune()
   901		return byte(s.hexDigit(rune1)<<4 | s.hexDigit(rune2)), true
   902	}
   903	
   904	// hexString returns the space-delimited hexpair-encoded string.
   905	func (s *ss) hexString() string {
   906		s.notEOF()
   907		for {
   908			b, ok := s.hexByte()
   909			if !ok {
   910				break
   911			}
   912			s.buf.WriteByte(b)
   913		}
   914		if len(s.buf) == 0 {
   915			s.errorString("Scan: no hex data for %x string")
   916			return ""
   917		}
   918		return string(s.buf)
   919	}
   920	
   921	const floatVerbs = "beEfFgGv"
   922	
   923	const hugeWid = 1 << 30
   924	
   925	// scanOne scans a single value, deriving the scanner from the type of the argument.
   926	func (s *ss) scanOne(verb rune, field interface{}) {
   927		s.buf = s.buf[:0]
   928		var err error
   929		// If the parameter has its own Scan method, use that.
   930		if v, ok := field.(Scanner); ok {
   931			err = v.Scan(s, verb)
   932			if err != nil {
   933				if err == io.EOF {
   934					err = io.ErrUnexpectedEOF
   935				}
   936				s.error(err)
   937			}
   938			return
   939		}
   940	
   941		switch v := field.(type) {
   942		case *bool:
   943			*v = s.scanBool(verb)
   944		case *complex64:
   945			*v = complex64(s.scanComplex(verb, 64))
   946		case *complex128:
   947			*v = s.scanComplex(verb, 128)
   948		case *int:
   949			*v = int(s.scanInt(verb, intBits))
   950		case *int8:
   951			*v = int8(s.scanInt(verb, 8))
   952		case *int16:
   953			*v = int16(s.scanInt(verb, 16))
   954		case *int32:
   955			*v = int32(s.scanInt(verb, 32))
   956		case *int64:
   957			*v = s.scanInt(verb, 64)
   958		case *uint:
   959			*v = uint(s.scanUint(verb, intBits))
   960		case *uint8:
   961			*v = uint8(s.scanUint(verb, 8))
   962		case *uint16:
   963			*v = uint16(s.scanUint(verb, 16))
   964		case *uint32:
   965			*v = uint32(s.scanUint(verb, 32))
   966		case *uint64:
   967			*v = s.scanUint(verb, 64)
   968		case *uintptr:
   969			*v = uintptr(s.scanUint(verb, uintptrBits))
   970		// Floats are tricky because you want to scan in the precision of the result, not
   971		// scan in high precision and convert, in order to preserve the correct error condition.
   972		case *float32:
   973			if s.okVerb(verb, floatVerbs, "float32") {
   974				s.skipSpace(false)
   975				s.notEOF()
   976				*v = float32(s.convertFloat(s.floatToken(), 32))
   977			}
   978		case *float64:
   979			if s.okVerb(verb, floatVerbs, "float64") {
   980				s.skipSpace(false)
   981				s.notEOF()
   982				*v = s.convertFloat(s.floatToken(), 64)
   983			}
   984		case *string:
   985			*v = s.convertString(verb)
   986		case *[]byte:
   987			// We scan to string and convert so we get a copy of the data.
   988			// If we scanned to bytes, the slice would point at the buffer.
   989			*v = []byte(s.convertString(verb))
   990		default:
   991			val := reflect.ValueOf(v)
   992			ptr := val
   993			if ptr.Kind() != reflect.Ptr {
   994				s.errorString("Scan: type not a pointer: " + val.Type().String())
   995				return
   996			}
   997			switch v := ptr.Elem(); v.Kind() {
   998			case reflect.Bool:
   999				v.SetBool(s.scanBool(verb))
  1000			case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
  1001				v.SetInt(s.scanInt(verb, v.Type().Bits()))
  1002			case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
  1003				v.SetUint(s.scanUint(verb, v.Type().Bits()))
  1004			case reflect.String:
  1005				v.SetString(s.convertString(verb))
  1006			case reflect.Slice:
  1007				// For now, can only handle (renamed) []byte.
  1008				typ := v.Type()
  1009				if typ.Elem().Kind() != reflect.Uint8 {
  1010					s.errorString("Scan: can't handle type: " + val.Type().String())
  1011				}
  1012				str := s.convertString(verb)
  1013				v.Set(reflect.MakeSlice(typ, len(str), len(str)))
  1014				for i := 0; i < len(str); i++ {
  1015					v.Index(i).SetUint(uint64(str[i]))
  1016				}
  1017			case reflect.Float32, reflect.Float64:
  1018				s.skipSpace(false)
  1019				s.notEOF()
  1020				v.SetFloat(s.convertFloat(s.floatToken(), v.Type().Bits()))
  1021			case reflect.Complex64, reflect.Complex128:
  1022				v.SetComplex(s.scanComplex(verb, v.Type().Bits()))
  1023			default:
  1024				s.errorString("Scan: can't handle type: " + val.Type().String())
  1025			}
  1026		}
  1027	}
  1028	
  1029	// errorHandler turns local panics into error returns.
  1030	func errorHandler(errp *error) {
  1031		if e := recover(); e != nil {
  1032			if se, ok := e.(scanError); ok { // catch local error
  1033				*errp = se.err
  1034			} else if eof, ok := e.(error); ok && eof == io.EOF { // out of input
  1035				*errp = eof
  1036			} else {
  1037				panic(e)
  1038			}
  1039		}
  1040	}
  1041	
  1042	// doScan does the real work for scanning without a format string.
  1043	func (s *ss) doScan(a []interface{}) (numProcessed int, err error) {
  1044		defer errorHandler(&err)
  1045		for _, field := range a {
  1046			s.scanOne('v', field)
  1047			numProcessed++
  1048		}
  1049		// Check for newline if required.
  1050		if !s.nlIsSpace {
  1051			for {
  1052				r := s.getRune()
  1053				if r == '\n' || r == eof {
  1054					break
  1055				}
  1056				if !isSpace(r) {
  1057					s.errorString("Scan: expected newline")
  1058					break
  1059				}
  1060			}
  1061		}
  1062		return
  1063	}
  1064	
  1065	// advance determines whether the next characters in the input match
  1066	// those of the format.  It returns the number of bytes (sic) consumed
  1067	// in the format. Newlines included, all runs of space characters in
  1068	// either input or format behave as a single space. This routine also
  1069	// handles the %% case.  If the return value is zero, either format
  1070	// starts with a % (with no following %) or the input is empty.
  1071	// If it is negative, the input did not match the string.
  1072	func (s *ss) advance(format string) (i int) {
  1073		for i < len(format) {
  1074			fmtc, w := utf8.DecodeRuneInString(format[i:])
  1075			if fmtc == '%' {
  1076				// %% acts like a real percent
  1077				nextc, _ := utf8.DecodeRuneInString(format[i+w:]) // will not match % if string is empty
  1078				if nextc != '%' {
  1079					return
  1080				}
  1081				i += w // skip the first %
  1082			}
  1083			sawSpace := false
  1084			for isSpace(fmtc) && i < len(format) {
  1085				sawSpace = true
  1086				i += w
  1087				fmtc, w = utf8.DecodeRuneInString(format[i:])
  1088			}
  1089			if sawSpace {
  1090				// There was space in the format, so there should be space (EOF)
  1091				// in the input.
  1092				inputc := s.getRune()
  1093				if inputc == eof {
  1094					return
  1095				}
  1096				if !isSpace(inputc) {
  1097					// Space in format but not in input: error
  1098					s.errorString("expected space in input to match format")
  1099				}
  1100				s.skipSpace(true)
  1101				continue
  1102			}
  1103			inputc := s.mustReadRune()
  1104			if fmtc != inputc {
  1105				s.UnreadRune()
  1106				return -1
  1107			}
  1108			i += w
  1109		}
  1110		return
  1111	}
  1112	
  1113	// doScanf does the real work when scanning with a format string.
  1114	//  At the moment, it handles only pointers to basic types.
  1115	func (s *ss) doScanf(format string, a []interface{}) (numProcessed int, err error) {
  1116		defer errorHandler(&err)
  1117		end := len(format) - 1
  1118		// We process one item per non-trivial format
  1119		for i := 0; i <= end; {
  1120			w := s.advance(format[i:])
  1121			if w > 0 {
  1122				i += w
  1123				continue
  1124			}
  1125			// Either we failed to advance, we have a percent character, or we ran out of input.
  1126			if format[i] != '%' {
  1127				// Can't advance format.  Why not?
  1128				if w < 0 {
  1129					s.errorString("input does not match format")
  1130				}
  1131				// Otherwise at EOF; "too many operands" error handled below
  1132				break
  1133			}
  1134			i++ // % is one byte
  1135	
  1136			// do we have 20 (width)?
  1137			var widPresent bool
  1138			s.maxWid, widPresent, i = parsenum(format, i, end)
  1139			if !widPresent {
  1140				s.maxWid = hugeWid
  1141			}
  1142			s.fieldLimit = s.limit
  1143			if f := s.count + s.maxWid; f < s.fieldLimit {
  1144				s.fieldLimit = f
  1145			}
  1146	
  1147			c, w := utf8.DecodeRuneInString(format[i:])
  1148			i += w
  1149	
  1150			if numProcessed >= len(a) { // out of operands
  1151				s.errorString("too few operands for format %" + format[i-w:])
  1152				break
  1153			}
  1154			field := a[numProcessed]
  1155	
  1156			s.scanOne(c, field)
  1157			numProcessed++
  1158			s.fieldLimit = s.limit
  1159		}
  1160		if numProcessed < len(a) {
  1161			s.errorString("too many operands")
  1162		}
  1163		return
  1164	}