Source: scan.go in package fmt


package fmt

import (
	"errors"
	"io"
	"math"
	"os"
	"reflect"
	"strconv"
	"sync"
	"unicode/utf8"
)

ScanState represents the scanner state passed to custom scanners. Scanners may do rune-at-a-time scanning or ask the ScanState to discover the next space-delimited token.

ReadRune reads the next rune (Unicode code point) from the input. If invoked during Scanln, Fscanln, or Sscanln, ReadRune() will return EOF after returning the first '\n' or when reading beyond the specified width.

UnreadRune causes the next call to ReadRune to return the same rune.

SkipSpace skips space in the input. Newlines are treated appropriately for the operation being performed; see the package documentation for more information.

Token skips space in the input if skipSpace is true, then returns the run of Unicode code points c satisfying f(c). If f is nil, !unicode.IsSpace(c) is used; that is, the token will hold non-space characters. Newlines are treated appropriately for the operation being performed; see the package documentation for more information. The returned slice points to shared data that may be overwritten by the next call to Token, a call to a Scan function using the ScanState as input, or when the calling Scan method returns.

Width returns the value of the width option and whether it has been set. The unit is Unicode code points.

Because ReadRune is implemented by the interface, Read should never be called by the scanning routines and a valid implementation of ScanState may choose always to return an error from Read.

	Read(buf []byte) (n int, err error)
}

Scanner is implemented by any value that has a Scan method, which scans the input for the representation of a value and stores the result in the receiver, which must be a pointer to be useful. The Scan method is called for any argument to Scan, Scanf, or Scanln that implements it.

type Scanner interface {
	Scan(state ScanState, verb rune) error
}

Scan scans text read from standard input, storing successive space-separated values into successive arguments. Newlines count as space. It returns the number of items successfully scanned. If that is less than the number of arguments, err will report why.

func Scan(a ...interface{}) (n int, err error) {
	return Fscan(os.Stdin, a...)
}

Scanln is similar to Scan, but stops scanning at a newline and after the final item there must be a newline or EOF.

func Scanln(a ...interface{}) (n int, err error) {
	return Fscanln(os.Stdin, a...)
}

Scanf scans text read from standard input, storing successive space-separated values into successive arguments as determined by the format. It returns the number of items successfully scanned. If that is less than the number of arguments, err will report why. Newlines in the input must match newlines in the format. The one exception: the verb %c always scans the next rune in the input, even if it is a space (or tab etc.) or newline.

func Scanf(format string, a ...interface{}) (n int, err error) {
	return Fscanf(os.Stdin, format, a...)
}

type stringReader string

func (r *stringReader) Read(b []byte) (n int, err error) {
	n = copy(b, *r)
	*r = (*r)[n:]
	if n == 0 {
		err = io.EOF
	}
	return
}

Sscan scans the argument string, storing successive space-separated values into successive arguments. Newlines count as space. It returns the number of items successfully scanned. If that is less than the number of arguments, err will report why.

func Sscan(str string, a ...interface{}) (n int, err error) {
	return Fscan((*stringReader)(&str), a...)
}

Sscanln is similar to Sscan, but stops scanning at a newline and after the final item there must be a newline or EOF.

func Sscanln(str string, a ...interface{}) (n int, err error) {
	return Fscanln((*stringReader)(&str), a...)
}

Sscanf scans the argument string, storing successive space-separated values into successive arguments as determined by the format. It returns the number of items successfully parsed. Newlines in the input must match newlines in the format.

func Sscanf(str string, format string, a ...interface{}) (n int, err error) {
	return Fscanf((*stringReader)(&str), format, a...)
}

Fscan scans text read from r, storing successive space-separated values into successive arguments. Newlines count as space. It returns the number of items successfully scanned. If that is less than the number of arguments, err will report why.

func Fscan(r io.Reader, a ...interface{}) (n int, err error) {
	s, old := newScanState(r, true, false)
	n, err = s.doScan(a)
	s.free(old)
	return
}

Fscanln is similar to Fscan, but stops scanning at a newline and after the final item there must be a newline or EOF.

func Fscanln(r io.Reader, a ...interface{}) (n int, err error) {
	s, old := newScanState(r, false, true)
	n, err = s.doScan(a)
	s.free(old)
	return
}

Fscanf scans text read from r, storing successive space-separated values into successive arguments as determined by the format. It returns the number of items successfully parsed. Newlines in the input must match newlines in the format.

func Fscanf(r io.Reader, format string, a ...interface{}) (n int, err error) {
	s, old := newScanState(r, false, false)
	n, err = s.doScanf(format, a)
	s.free(old)
	return
}

scanError represents an error generated by the scanning software. It's used as a unique signature to identify such errors when recovering.

type scanError struct {
	err error
}

const eof = -1

ss is the internal implementation of ScanState.

type ss struct {
	rs    io.RuneScanner // where to read input
	buf   buffer         // token accumulator
	count int            // runes consumed so far.
	atEOF bool           // already read EOF
	ssave
}

ssave holds the parts of ss that need to be saved and restored on recursive scans.

type ssave struct {
	validSave bool // is or was a part of an actual ss.
	nlIsEnd   bool // whether newline terminates scan
	nlIsSpace bool // whether newline counts as white space
	argLimit  int  // max value of ss.count for this arg; argLimit <= limit
	limit     int  // max value of ss.count.
	maxWid    int  // width of this arg.
}

The Read method is only in ScanState so that ScanState satisfies io.Reader. It will never be called when used as intended, so there is no need to make it actually work.

func (s *ss) Read(buf []byte) (n int, err error) {
	return 0, errors.New("ScanState's Read should not be called. Use ReadRune")
}

func (s *ss) ReadRune() (r rune, size int, err error) {
	if s.atEOF || s.count >= s.argLimit {
		err = io.EOF
		return
	}

	r, size, err = s.rs.ReadRune()
	if err == nil {
		s.count++
		if s.nlIsEnd && r == '\n' {
			s.atEOF = true
		}
	} else if err == io.EOF {
		s.atEOF = true
	}
	return
}

func (s *ss) Width() (wid int, ok bool) {
	if s.maxWid == hugeWid {
		return 0, false
	}
	return s.maxWid, true
}

The public method returns an error; this private one panics. If getRune reaches EOF, the return value is EOF (-1).

func (s *ss) getRune() (r rune) {
	r, _, err := s.ReadRune()
	if err != nil {
		if err == io.EOF {
			return eof
		}
		s.error(err)
	}
	return
}

mustReadRune turns io.EOF into a panic(io.ErrUnexpectedEOF). It is called in cases such as string scanning where an EOF is a syntax error.

func (s *ss) mustReadRune() (r rune) {
	r = s.getRune()
	if r == eof {
		s.error(io.ErrUnexpectedEOF)
	}
	return
}

func (s *ss) UnreadRune() error {
	s.rs.UnreadRune()
	s.atEOF = false
	s.count--
	return nil
}

func (s *ss) error(err error) {
	panic(scanError{err})
}

func (s *ss) errorString(err string) {
	panic(scanError{errors.New(err)})
}

func (s *ss) Token(skipSpace bool, f func(rune) bool) (tok []byte, err error) {
	defer func() {
		if e := recover(); e != nil {
			if se, ok := e.(scanError); ok {
				err = se.err
			} else {
				panic(e)
			}
		}
	}()
	if f == nil {
		f = notSpace
	}
	s.buf = s.buf[:0]
	tok = s.token(skipSpace, f)
	return
}

space is a copy of the unicode.White_Space ranges, to avoid depending on package unicode.

var space = [][2]uint16{
	{0x0009, 0x000d},
	{0x0020, 0x0020},
	{0x0085, 0x0085},
	{0x00a0, 0x00a0},
	{0x1680, 0x1680},
	{0x2000, 0x200a},
	{0x2028, 0x2029},
	{0x202f, 0x202f},
	{0x205f, 0x205f},
	{0x3000, 0x3000},
}

func isSpace(r rune) bool {
	if r >= 1<<16 {
		return false
	}
	rx := uint16(r)
	for _, rng := range space {
		if rx < rng[0] {
			return false
		}
		if rx <= rng[1] {
			return true
		}
	}
	return false
}

notSpace is the default scanning function used in Token.

func notSpace(r rune) bool {
	return !isSpace(r)
}

readRune is a structure to enable reading UTF-8 encoded code points from an io.Reader. It is used if the Reader given to the scanner does not already implement io.RuneScanner.

type readRune struct {
	reader   io.Reader
	buf      [utf8.UTFMax]byte // used only inside ReadRune
	pending  int               // number of bytes in pendBuf; only >0 for bad UTF-8
	pendBuf  [utf8.UTFMax]byte // bytes left over
	peekRune rune              // if >=0 next rune; when <0 is ^(previous Rune)
}

readByte returns the next byte from the input, which may be left over from a previous read if the UTF-8 was ill-formed.

func (r *readRune) readByte() (b byte, err error) {
	if r.pending > 0 {
		b = r.pendBuf[0]
		copy(r.pendBuf[0:], r.pendBuf[1:])
		r.pending--
		return
	}
	n, err := io.ReadFull(r.reader, r.pendBuf[:1])
	if n != 1 {
		return 0, err
	}
	return r.pendBuf[0], err
}

ReadRune returns the next UTF-8 encoded code point from the io.Reader inside r.

func (r *readRune) ReadRune() (rr rune, size int, err error) {
	if r.peekRune >= 0 {
		rr = r.peekRune
		r.peekRune = ^r.peekRune
		size = utf8.RuneLen(rr)
		return
	}
	r.buf[0], err = r.readByte()
	if err != nil {
		return
	}
	if r.buf[0] < utf8.RuneSelf { // fast check for common ASCII case
		rr = rune(r.buf[0])

Flip the bits of the rune so it's available to UnreadRune.

		r.peekRune = ^rr
		return
	}
	var n int
	for n = 1; !utf8.FullRune(r.buf[:n]); n++ {
		r.buf[n], err = r.readByte()
		if err != nil {
			if err == io.EOF {
				err = nil
				break
			}
			return
		}
	}
	rr, size = utf8.DecodeRune(r.buf[:n])
	if size < n { // an error, save the bytes for the next read
		copy(r.pendBuf[r.pending:], r.buf[size:n])
		r.pending += n - size

Flip the bits of the rune so it's available to UnreadRune.

	r.peekRune = ^rr
	return
}

func (r *readRune) UnreadRune() error {
	if r.peekRune >= 0 {
		return errors.New("fmt: scanning called UnreadRune with no rune available")

Reverse bit flip of previously read rune to obtain valid >=0 state.

	r.peekRune = ^r.peekRune
	return nil
}

var ssFree = sync.Pool{
	New: func() interface{} { return new(ss) },
}

newScanState allocates a new ss struct or grab a cached one.

func newScanState(r io.Reader, nlIsSpace, nlIsEnd bool) (s *ss, old ssave) {
	s = ssFree.Get().(*ss)
	if rs, ok := r.(io.RuneScanner); ok {
		s.rs = rs
	} else {
		s.rs = &readRune{reader: r, peekRune: -1}
	}
	s.nlIsSpace = nlIsSpace
	s.nlIsEnd = nlIsEnd
	s.atEOF = false
	s.limit = hugeWid
	s.argLimit = hugeWid
	s.maxWid = hugeWid
	s.validSave = true
	s.count = 0
	return
}

free saves used ss structs in ssFree; avoid an allocation per invocation.

If it was used recursively, just restore the old state.

	if old.validSave {
		s.ssave = old
		return

Don't hold on to ss structs with large buffers.

	if cap(s.buf) > 1024 {
		return
	}
	s.buf = s.buf[:0]
	s.rs = nil
	ssFree.Put(s)
}

SkipSpace provides Scan methods the ability to skip space and newline characters in keeping with the current scanning mode set by format strings and Scan/Scanln.

func (s *ss) SkipSpace() {
	for {
		r := s.getRune()
		if r == eof {
			return
		}
		if r == '\r' && s.peek("\n") {
			continue
		}
		if r == '\n' {
			if s.nlIsSpace {
				continue
			}
			s.errorString("unexpected newline")
			return
		}
		if !isSpace(r) {
			s.UnreadRune()
			break
		}
	}
}

token returns the next space-delimited string from the input. It skips white space. For Scanln, it stops at newlines. For Scan, newlines are treated as spaces.

func (s *ss) token(skipSpace bool, f func(rune) bool) []byte {
	if skipSpace {
		s.SkipSpace()

read until white space or newline

	for {
		r := s.getRune()
		if r == eof {
			break
		}
		if !f(r) {
			s.UnreadRune()
			break
		}
		s.buf.writeRune(r)
	}
	return s.buf
}

var complexError = errors.New("syntax error scanning complex number")
var boolError = errors.New("syntax error scanning boolean")

func indexRune(s string, r rune) int {
	for i, c := range s {
		if c == r {
			return i
		}
	}
	return -1
}

consume reads the next rune in the input and reports whether it is in the ok string. If accept is true, it puts the character into the input token.

func (s *ss) consume(ok string, accept bool) bool {
	r := s.getRune()
	if r == eof {
		return false
	}
	if indexRune(ok, r) >= 0 {
		if accept {
			s.buf.writeRune(r)
		}
		return true
	}
	if r != eof && accept {
		s.UnreadRune()
	}
	return false
}

peek reports whether the next character is in the ok string, without consuming it.

func (s *ss) peek(ok string) bool {
	r := s.getRune()
	if r != eof {
		s.UnreadRune()
	}
	return indexRune(ok, r) >= 0
}

Guarantee there is data to be read.

	if r := s.getRune(); r == eof {
		panic(io.EOF)
	}
	s.UnreadRune()
}

accept checks the next rune in the input. If it's a byte (sic) in the string, it puts it in the buffer and returns true. Otherwise it return false.

func (s *ss) accept(ok string) bool {
	return s.consume(ok, true)
}

okVerb verifies that the verb is present in the list, setting s.err appropriately if not.

func (s *ss) okVerb(verb rune, okVerbs, typ string) bool {
	for _, v := range okVerbs {
		if v == verb {
			return true
		}
	}
	s.errorString("bad verb '%" + string(verb) + "' for " + typ)
	return false
}

scanBool returns the value of the boolean represented by the next token.

func (s *ss) scanBool(verb rune) bool {
	s.SkipSpace()
	s.notEOF()
	if !s.okVerb(verb, "tv", "boolean") {
		return false

Syntax-checking a boolean is annoying. We're not fastidious about case.

	switch s.getRune() {
	case '0':
		return false
	case '1':
		return true
	case 't', 'T':
		if s.accept("rR") && (!s.accept("uU") || !s.accept("eE")) {
			s.error(boolError)
		}
		return true
	case 'f', 'F':
		if s.accept("aA") && (!s.accept("lL") || !s.accept("sS") || !s.accept("eE")) {
			s.error(boolError)
		}
		return false
	}
	return false
}

Numerical elements

const (
	binaryDigits      = "01"
	octalDigits       = "01234567"
	decimalDigits     = "0123456789"
	hexadecimalDigits = "0123456789aAbBcCdDeEfF"
	sign              = "+-"
	period            = "."
	exponent          = "eEpP"
)

getBase returns the numeric base represented by the verb and its digit string.

func (s *ss) getBase(verb rune) (base int, digits string) {
	s.okVerb(verb, "bdoUxXv", "integer") // sets s.err
	base = 10
	digits = decimalDigits
	switch verb {
	case 'b':
		base = 2
		digits = binaryDigits
	case 'o':
		base = 8
		digits = octalDigits
	case 'x', 'X', 'U':
		base = 16
		digits = hexadecimalDigits
	}
	return
}

scanNumber returns the numerical string with specified digits starting here.

func (s *ss) scanNumber(digits string, haveDigits bool) string {
	if !haveDigits {
		s.notEOF()
		if !s.accept(digits) {
			s.errorString("expected integer")
		}
	}
	for s.accept(digits) {
	}
	return string(s.buf)
}

scanRune returns the next rune value in the input.

func (s *ss) scanRune(bitSize int) int64 {
	s.notEOF()
	r := s.getRune()
	n := uint(bitSize)
	x := (int64(r) << (64 - n)) >> (64 - n)
	if x != int64(r) {
		s.errorString("overflow on character value " + string(r))
	}
	return int64(r)
}

scanBasePrefix reports whether the integer begins with a base prefix and returns the base, digit string, and whether a zero was found. It is called only if the verb is %v.

func (s *ss) scanBasePrefix() (base int, digits string, zeroFound bool) {
	if !s.peek("0") {
		return 0, decimalDigits + "_", false
	}

Special cases for 0, 0b, 0o, 0x.

	switch {
	case s.peek("bB"):
		s.consume("bB", true)
		return 0, binaryDigits + "_", true
	case s.peek("oO"):
		s.consume("oO", true)
		return 0, octalDigits + "_", true
	case s.peek("xX"):
		s.consume("xX", true)
		return 0, hexadecimalDigits + "_", true
	default:
		return 0, octalDigits + "_", true
	}
}

scanInt returns the value of the integer represented by the next token, checking for overflow. Any error is stored in s.err.

func (s *ss) scanInt(verb rune, bitSize int) int64 {
	if verb == 'c' {
		return s.scanRune(bitSize)
	}
	s.SkipSpace()
	s.notEOF()
	base, digits := s.getBase(verb)
	haveDigits := false
	if verb == 'U' {
		if !s.consume("U", false) || !s.consume("+", false) {
			s.errorString("bad unicode format ")
		}
	} else {
		s.accept(sign) // If there's a sign, it will be left in the token buffer.
		if verb == 'v' {
			base, digits, haveDigits = s.scanBasePrefix()
		}
	}
	tok := s.scanNumber(digits, haveDigits)
	i, err := strconv.ParseInt(tok, base, 64)
	if err != nil {
		s.error(err)
	}
	n := uint(bitSize)
	x := (i << (64 - n)) >> (64 - n)
	if x != i {
		s.errorString("integer overflow on token " + tok)
	}
	return i
}

scanUint returns the value of the unsigned integer represented by the next token, checking for overflow. Any error is stored in s.err.

func (s *ss) scanUint(verb rune, bitSize int) uint64 {
	if verb == 'c' {
		return uint64(s.scanRune(bitSize))
	}
	s.SkipSpace()
	s.notEOF()
	base, digits := s.getBase(verb)
	haveDigits := false
	if verb == 'U' {
		if !s.consume("U", false) || !s.consume("+", false) {
			s.errorString("bad unicode format ")
		}
	} else if verb == 'v' {
		base, digits, haveDigits = s.scanBasePrefix()
	}
	tok := s.scanNumber(digits, haveDigits)
	i, err := strconv.ParseUint(tok, base, 64)
	if err != nil {
		s.error(err)
	}
	n := uint(bitSize)
	x := (i << (64 - n)) >> (64 - n)
	if x != i {
		s.errorString("unsigned integer overflow on token " + tok)
	}
	return i
}

floatToken returns the floating-point number starting here, no longer than swid if the width is specified. It's not rigorous about syntax because it doesn't check that we have at least some digits, but Atof will do that.

func (s *ss) floatToken() string {

NaN?

	if s.accept("nN") && s.accept("aA") && s.accept("nN") {
		return string(s.buf)

leading sign?

Inf?

	if s.accept("iI") && s.accept("nN") && s.accept("fF") {
		return string(s.buf)
	}
	digits := decimalDigits + "_"
	exp := exponent
	if s.accept("0") && s.accept("xX") {
		digits = hexadecimalDigits + "_"
		exp = "pP"

digits?

	for s.accept(digits) {

decimal point?

fraction?

		for s.accept(digits) {
		}

exponent?

leading sign?

digits?

		for s.accept(decimalDigits + "_") {
		}
	}
	return string(s.buf)
}

complexTokens returns the real and imaginary parts of the complex number starting here. The number might be parenthesized and has the format (N+Ni) where N is a floating-point number and there are no spaces within.

TODO: accept N and Ni independently?

	parens := s.accept("(")
	real = s.floatToken()

Must now have a sign.

	if !s.accept("+-") {
		s.error(complexError)

Sign is now in buffer

	imagSign := string(s.buf)
	imag = s.floatToken()
	if !s.accept("i") {
		s.error(complexError)
	}
	if parens && !s.accept(")") {
		s.error(complexError)
	}
	return real, imagSign + imag
}

func hasX(s string) bool {
	for i := 0; i < len(s); i++ {
		if s[i] == 'x' || s[i] == 'X' {
			return true
		}
	}
	return false
}

convertFloat converts the string to a float64value.

strconv.ParseFloat will handle "+0x1.fp+2", but we have to implement our non-standard decimal+binary exponent mix (1.2p4) ourselves.

Atof doesn't handle power-of-2 exponents, but they're easy to evaluate.

		f, err := strconv.ParseFloat(str[:p], n)

Put full string into error.

			if e, ok := err.(*strconv.NumError); ok {
				e.Num = str
			}
			s.error(err)
		}
		m, err := strconv.Atoi(str[p+1:])

Put full string into error.

			if e, ok := err.(*strconv.NumError); ok {
				e.Num = str
			}
			s.error(err)
		}
		return math.Ldexp(f, m)
	}
	f, err := strconv.ParseFloat(str, n)
	if err != nil {
		s.error(err)
	}
	return f
}

convertComplex converts the next token to a complex128 value. The atof argument is a type-specific reader for the underlying type. If we're reading complex64, atof will parse float32s and convert them to float64's to avoid reproducing this code for each complex type.

func (s *ss) scanComplex(verb rune, n int) complex128 {
	if !s.okVerb(verb, floatVerbs, "complex") {
		return 0
	}
	s.SkipSpace()
	s.notEOF()
	sreal, simag := s.complexTokens()
	real := s.convertFloat(sreal, n/2)
	imag := s.convertFloat(simag, n/2)
	return complex(real, imag)
}

convertString returns the string represented by the next input characters. The format of the input is determined by the verb.

func (s *ss) convertString(verb rune) (str string) {
	if !s.okVerb(verb, "svqxX", "string") {
		return ""
	}
	s.SkipSpace()
	s.notEOF()
	switch verb {
	case 'q':
		str = s.quotedString()
	case 'x', 'X':
		str = s.hexString()
	default:
		str = string(s.token(true, notSpace)) // %s and %v just return the next word
	}
	return
}

quotedString returns the double- or back-quoted string represented by the next input characters.

func (s *ss) quotedString() string {
	s.notEOF()
	quote := s.getRune()
	switch quote {

Back-quoted: Anything goes until EOF or back quote.

		for {
			r := s.mustReadRune()
			if r == quote {
				break
			}
			s.buf.writeRune(r)
		}
		return string(s.buf)

Double-quoted: Include the quotes and let strconv.Unquote do the backslash escapes.

		s.buf.writeByte('"')
		for {
			r := s.mustReadRune()
			s.buf.writeRune(r)

In a legal backslash escape, no matter how long, only the character immediately after the escape can itself be a backslash or quote. Thus we only need to protect the first character after the backslash.

				s.buf.writeRune(s.mustReadRune())
			} else if r == '"' {
				break
			}
		}
		result, err := strconv.Unquote(string(s.buf))
		if err != nil {
			s.error(err)
		}
		return result
	default:
		s.errorString("expected quoted string")
	}
	return ""
}

hexDigit returns the value of the hexadecimal digit.

func hexDigit(d rune) (int, bool) {
	digit := int(d)
	switch digit {
	case '0', '1', '2', '3', '4', '5', '6', '7', '8', '9':
		return digit - '0', true
	case 'a', 'b', 'c', 'd', 'e', 'f':
		return 10 + digit - 'a', true
	case 'A', 'B', 'C', 'D', 'E', 'F':
		return 10 + digit - 'A', true
	}
	return -1, false
}

hexByte returns the next hex-encoded (two-character) byte from the input. It returns ok==false if the next bytes in the input do not encode a hex byte. If the first byte is hex and the second is not, processing stops.

func (s *ss) hexByte() (b byte, ok bool) {
	rune1 := s.getRune()
	if rune1 == eof {
		return
	}
	value1, ok := hexDigit(rune1)
	if !ok {
		s.UnreadRune()
		return
	}
	value2, ok := hexDigit(s.mustReadRune())
	if !ok {
		s.errorString("illegal hex digit")
		return
	}
	return byte(value1<<4 | value2), true
}

hexString returns the space-delimited hexpair-encoded string.

func (s *ss) hexString() string {
	s.notEOF()
	for {
		b, ok := s.hexByte()
		if !ok {
			break
		}
		s.buf.writeByte(b)
	}
	if len(s.buf) == 0 {
		s.errorString("no hex data for %x string")
		return ""
	}
	return string(s.buf)
}

const (
	floatVerbs = "beEfFgGv"

	hugeWid = 1 << 30

	intBits     = 32 << (^uint(0) >> 63)
	uintptrBits = 32 << (^uintptr(0) >> 63)
)

scanPercent scans a literal percent character.

func (s *ss) scanPercent() {
	s.SkipSpace()
	s.notEOF()
	if !s.accept("%") {
		s.errorString("missing literal %")
	}
}

scanOne scans a single value, deriving the scanner from the type of the argument.

func (s *ss) scanOne(verb rune, arg interface{}) {
	s.buf = s.buf[:0]

If the parameter has its own Scan method, use that.

	if v, ok := arg.(Scanner); ok {
		err = v.Scan(s, verb)
		if err != nil {
			if err == io.EOF {
				err = io.ErrUnexpectedEOF
			}
			s.error(err)
		}
		return
	}

	switch v := arg.(type) {
	case *bool:
		*v = s.scanBool(verb)
	case *complex64:
		*v = complex64(s.scanComplex(verb, 64))
	case *complex128:
		*v = s.scanComplex(verb, 128)
	case *int:
		*v = int(s.scanInt(verb, intBits))
	case *int8:
		*v = int8(s.scanInt(verb, 8))
	case *int16:
		*v = int16(s.scanInt(verb, 16))
	case *int32:
		*v = int32(s.scanInt(verb, 32))
	case *int64:
		*v = s.scanInt(verb, 64)
	case *uint:
		*v = uint(s.scanUint(verb, intBits))
	case *uint8:
		*v = uint8(s.scanUint(verb, 8))
	case *uint16:
		*v = uint16(s.scanUint(verb, 16))
	case *uint32:
		*v = uint32(s.scanUint(verb, 32))
	case *uint64:
		*v = s.scanUint(verb, 64)
	case *uintptr:

Floats are tricky because you want to scan in the precision of the result, not scan in high precision and convert, in order to preserve the correct error condition.

	case *float32:
		if s.okVerb(verb, floatVerbs, "float32") {
			s.SkipSpace()
			s.notEOF()
			*v = float32(s.convertFloat(s.floatToken(), 32))
		}
	case *float64:
		if s.okVerb(verb, floatVerbs, "float64") {
			s.SkipSpace()
			s.notEOF()
			*v = s.convertFloat(s.floatToken(), 64)
		}
	case *string:
		*v = s.convertString(verb)

We scan to string and convert so we get a copy of the data. If we scanned to bytes, the slice would point at the buffer.

		*v = []byte(s.convertString(verb))
	default:
		val := reflect.ValueOf(v)
		ptr := val
		if ptr.Kind() != reflect.Ptr {
			s.errorString("type not a pointer: " + val.Type().String())
			return
		}
		switch v := ptr.Elem(); v.Kind() {
		case reflect.Bool:
			v.SetBool(s.scanBool(verb))
		case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
			v.SetInt(s.scanInt(verb, v.Type().Bits()))
		case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
			v.SetUint(s.scanUint(verb, v.Type().Bits()))
		case reflect.String:
			v.SetString(s.convertString(verb))

For now, can only handle (renamed) []byte.

			typ := v.Type()
			if typ.Elem().Kind() != reflect.Uint8 {
				s.errorString("can't scan type: " + val.Type().String())
			}
			str := s.convertString(verb)
			v.Set(reflect.MakeSlice(typ, len(str), len(str)))
			for i := 0; i < len(str); i++ {
				v.Index(i).SetUint(uint64(str[i]))
			}
		case reflect.Float32, reflect.Float64:
			s.SkipSpace()
			s.notEOF()
			v.SetFloat(s.convertFloat(s.floatToken(), v.Type().Bits()))
		case reflect.Complex64, reflect.Complex128:
			v.SetComplex(s.scanComplex(verb, v.Type().Bits()))
		default:
			s.errorString("can't scan type: " + val.Type().String())
		}
	}
}

errorHandler turns local panics into error returns.

func errorHandler(errp *error) {
	if e := recover(); e != nil {
		if se, ok := e.(scanError); ok { // catch local error
			*errp = se.err
		} else if eof, ok := e.(error); ok && eof == io.EOF { // out of input
			*errp = eof
		} else {
			panic(e)
		}
	}
}

doScan does the real work for scanning without a format string.

func (s *ss) doScan(a []interface{}) (numProcessed int, err error) {
	defer errorHandler(&err)
	for _, arg := range a {
		s.scanOne('v', arg)
		numProcessed++

Check for newline (or EOF) if required (Scanln etc.).

	if s.nlIsEnd {
		for {
			r := s.getRune()
			if r == '\n' || r == eof {
				break
			}
			if !isSpace(r) {
				s.errorString("expected newline")
				break
			}
		}
	}
	return
}

advance determines whether the next characters in the input match those of the format. It returns the number of bytes (sic) consumed in the format. All runs of space characters in either input or format behave as a single space. Newlines are special, though: newlines in the format must match those in the input and vice versa. This routine also handles the %% case. If the return value is zero, either format starts with a % (with no following %) or the input is empty. If it is negative, the input did not match the string.

func (s *ss) advance(format string) (i int) {
	for i < len(format) {
		fmtc, w := utf8.DecodeRuneInString(format[i:])

Space processing. In the rest of this comment "space" means spaces other than newline. Newline in the format matches input of zero or more spaces and then newline or end-of-input. Spaces in the format before the newline are collapsed into the newline. Spaces in the format after the newline match zero or more spaces after the corresponding input newline. Other spaces in the format match input of one or more spaces or end-of-input.

		if isSpace(fmtc) {
			newlines := 0
			trailingSpace := false
			for isSpace(fmtc) && i < len(format) {
				if fmtc == '\n' {
					newlines++
					trailingSpace = false
				} else {
					trailingSpace = true
				}
				i += w
				fmtc, w = utf8.DecodeRuneInString(format[i:])
			}
			for j := 0; j < newlines; j++ {
				inputc := s.getRune()
				for isSpace(inputc) && inputc != '\n' {
					inputc = s.getRune()
				}
				if inputc != '\n' && inputc != eof {
					s.errorString("newline in format does not match input")
				}
			}
			if trailingSpace {
				inputc := s.getRune()

If the trailing space stood alone (did not follow a newline), it must find at least one space to consume.

					if !isSpace(inputc) && inputc != eof {
						s.errorString("expected space in input to match format")
					}
					if inputc == '\n' {
						s.errorString("newline in input does not match format")
					}
				}
				for isSpace(inputc) && inputc != '\n' {
					inputc = s.getRune()
				}
				if inputc != eof {
					s.UnreadRune()
				}
			}
			continue
		}

Verbs.

% at end of string is an error.

			if i+w == len(format) {
				s.errorString("missing verb: % at end of format string")

%% acts like a real percent

			nextc, _ := utf8.DecodeRuneInString(format[i+w:]) // will not match % if string is empty
			if nextc != '%' {
				return
			}
			i += w // skip the first %
		}

Literals.

		inputc := s.mustReadRune()
		if fmtc != inputc {
			s.UnreadRune()
			return -1
		}
		i += w
	}
	return
}

doScanf does the real work when scanning with a format string. At the moment, it handles only pointers to basic types.

func (s *ss) doScanf(format string, a []interface{}) (numProcessed int, err error) {
	defer errorHandler(&err)

We process one item per non-trivial format

	for i := 0; i <= end; {
		w := s.advance(format[i:])
		if w > 0 {
			i += w
			continue

Either we failed to advance, we have a percent character, or we ran out of input.

Can't advance format. Why not?

			if w < 0 {
				s.errorString("input does not match format")

Otherwise at EOF; "too many operands" error handled below

			break
		}
		i++ // % is one byte

do we have 20 (width)?

		var widPresent bool
		s.maxWid, widPresent, i = parsenum(format, i, end)
		if !widPresent {
			s.maxWid = hugeWid
		}

		c, w := utf8.DecodeRuneInString(format[i:])
		i += w

		if c != 'c' {
			s.SkipSpace()
		}
		if c == '%' {
			s.scanPercent()
			continue // Do not consume an argument.
		}
		s.argLimit = s.limit
		if f := s.count + s.maxWid; f < s.argLimit {
			s.argLimit = f
		}

		if numProcessed >= len(a) { // out of operands
			s.errorString("too few operands for format '%" + format[i-w:] + "'")
			break
		}
		arg := a[numProcessed]

		s.scanOne(c, arg)
		numProcessed++
		s.argLimit = s.limit
	}
	if numProcessed < len(a) {
		s.errorString("too many operands")
	}
	return