Fix numerous typos in comments

[official-gcc.git] / libgo / go / fmt / scan.go

// Copyright 2010 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 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.
type ScanState interface {
        // 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.
        ReadRune() (r rune, size int, err error)
        // UnreadRune causes the next call to ReadRune to return the same rune.
        UnreadRune() error
        // SkipSpace skips space in the input. Newlines are treated appropriately
        // for the operation being performed; see the package documentation
        // for more information.
        SkipSpace()
        // 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.
        Token(skipSpace bool, f func(rune) bool) (token []byte, err error)
        // Width returns the value of the width option and whether it has been set.
        // The unit is Unicode code points.
        Width() (wid int, ok bool)
        // 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)
}

// 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() {
        s.skipSpace(false)
}

// 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])
                size = 1 // Known to be 1.
                // 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.
func (s *ss) free(old ssave) {
        // 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 skips spaces and maybe newlines.
func (s *ss) skipSpace(stopAtNewline bool) {
        for {
                r := s.getRune()
                if r == eof {
                        return
                }
                if r == '\r' && s.peek("\n") {
                        continue
                }
                if r == '\n' {
                        if stopAtNewline {
                                break
                        }
                        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(false)
        }
        // 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
}

func (s *ss) notEOF() {
        // 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(false)
        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          = "eEp"
)

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

// scanBasePrefix reports whether the integer begins with a 0 or 0x,
// 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, found bool) {
        if !s.peek("0") {
                return 10, decimalDigits, false
        }
        s.accept("0")
        found = true // We've put a digit into the token buffer.
        // Special cases for '0' && '0x'
        base, digits = 8, octalDigits
        if s.peek("xX") {
                s.consume("xX", false)
                base, digits = 16, hexadecimalDigits
        }
        return
}

// 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(false)
        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(false)
        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 {
        s.buf = s.buf[:0]
        // NaN?
        if s.accept("nN") && s.accept("aA") && s.accept("nN") {
                return string(s.buf)
        }
        // leading sign?
        s.accept(sign)
        // Inf?
        if s.accept("iI") && s.accept("nN") && s.accept("fF") {
                return string(s.buf)
        }
        // digits?
        for s.accept(decimalDigits) {
        }
        // decimal point?
        if s.accept(period) {
                // fraction?
                for s.accept(decimalDigits) {
                }
        }
        // exponent?
        if s.accept(exponent) {
                // leading sign?
                s.accept(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.
func (s *ss) complexTokens() (real, imag string) {
        // TODO: accept N and Ni independently?
        parens := s.accept("(")
        real = s.floatToken()
        s.buf = s.buf[:0]
        // 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
}

// convertFloat converts the string to a float64value.
func (s *ss) convertFloat(str string, n int) float64 {
        if p := indexRune(str, 'p'); p >= 0 {
                // Atof doesn't handle power-of-2 exponents,
                // but they're easy to evaluate.
                f, err := strconv.ParseFloat(str[:p], n)
                if err != nil {
                        // 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:])
                if err != nil {
                        // 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(false)
        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(false)
        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 {
        case '`':
                // 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)
        case '"':
                // Double-quoted: Include the quotes and let strconv.Unquote do the backslash escapes.
                s.buf.WriteByte('"')
                for {
                        r := s.mustReadRune()
                        s.buf.WriteRune(r)
                        if 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)
)

// 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]
        var err error
        // 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:
                *v = uintptr(s.scanUint(verb, uintptrBits))
        // 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(false)
                        s.notEOF()
                        *v = float32(s.convertFloat(s.floatToken(), 32))
                }
        case *float64:
                if s.okVerb(verb, floatVerbs, "float64") {
                        s.skipSpace(false)
                        s.notEOF()
                        *v = s.convertFloat(s.floatToken(), 64)
                }
        case *string:
                *v = s.convertString(verb)
        case *[]byte:
                // 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))
                case reflect.Slice:
                        // 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(false)
                        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 newlines == 0 {
                                        // 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.
                if fmtc == '%' {
                        // % 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)
        end := len(format) - 1
        // 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.
                if format[i] != '%' {
                        // 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()
                }
                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
}