1 // Copyright 2015 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.
5 // This file implements rat-to-string conversion functions.
17 func ratTok(ch rune) bool {
18 return strings.ContainsRune("+-/0123456789.eE", ch)
22 var _ fmt.Scanner = &ratZero // *Rat must implement fmt.Scanner
24 // Scan is a support routine for fmt.Scanner. It accepts the formats
25 // 'e', 'E', 'f', 'F', 'g', 'G', and 'v'. All formats are equivalent.
26 func (z *Rat) Scan(s fmt.ScanState, ch rune) error {
27 tok, err := s.Token(true, ratTok)
31 if !strings.ContainsRune("efgEFGv", ch) {
32 return errors.New("Rat.Scan: invalid verb")
34 if _, ok := z.SetString(string(tok)); !ok {
35 return errors.New("Rat.Scan: invalid syntax")
40 // SetString sets z to the value of s and returns z and a boolean indicating
41 // success. s can be given as a fraction "a/b" or as a floating-point number
42 // optionally followed by an exponent. The entire string (not just a prefix)
43 // must be valid for success. If the operation failed, the value of z is
44 // undefined but the returned value is nil.
45 func (z *Rat) SetString(s string) (*Rat, bool) {
51 // parse fraction a/b, if any
52 if sep := strings.Index(s, "/"); sep >= 0 {
53 if _, ok := z.a.SetString(s[:sep], 0); !ok {
56 r := strings.NewReader(s[sep+1:])
58 if z.b.abs, _, _, err = z.b.abs.scan(r, 0, false); err != nil {
61 // entire string must have been consumed
62 if _, err = r.ReadByte(); err != io.EOF {
65 if len(z.b.abs) == 0 {
71 // parse floating-point number
72 r := strings.NewReader(s)
75 neg, err := scanSign(r)
82 z.a.abs, _, ecorr, err = z.a.abs.scan(r, 10, true)
89 exp, _, err = scanExponent(r, false)
94 // there should be no unread characters left
95 if _, err = r.ReadByte(); err != io.EOF {
99 // special-case 0 (see also issue #16176)
100 if len(z.a.abs) == 0 {
110 // compute exponent power
115 powTen := nat(nil).expNN(natTen, nat(nil).setWord(Word(expabs)), nil)
122 z.a.abs = z.a.abs.mul(z.a.abs, powTen)
123 z.b.abs = z.b.abs[:0]
126 z.a.neg = neg && len(z.a.abs) > 0 // 0 has no sign
131 // scanExponent scans the longest possible prefix of r representing a decimal
132 // ('e', 'E') or binary ('p') exponent, if any. It returns the exponent, the
133 // exponent base (10 or 2), or a read or syntax error, if any.
135 // exponent = ( "E" | "e" | "p" ) [ sign ] digits .
136 // sign = "+" | "-" .
137 // digits = digit { digit } .
138 // digit = "0" ... "9" .
140 // A binary exponent is only permitted if binExpOk is set.
141 func scanExponent(r io.ByteScanner, binExpOk bool) (exp int64, base int, err error) {
145 if ch, err = r.ReadByte(); err != nil {
147 err = nil // no exponent; same as e0
160 fallthrough // binary exponent not permitted
163 return // no exponent; same as e0
167 if neg, err = scanSign(r); err != nil {
173 digits = append(digits, '-')
176 // no need to use nat.scan for exponent digits
177 // since we only care about int64 values - the
178 // from-scratch scan is easy enough and faster
180 if ch, err = r.ReadByte(); err != nil {
181 if err != io.EOF || i == 0 {
187 if ch < '0' || '9' < ch {
190 err = fmt.Errorf("invalid exponent (missing digits)")
195 digits = append(digits, ch)
197 // i > 0 => we have at least one digit
199 exp, err = strconv.ParseInt(string(digits), 10, 64)
203 // String returns a string representation of x in the form "a/b" (even if b == 1).
204 func (x *Rat) String() string {
205 return string(x.marshal())
208 // marshal implements String returning a slice of bytes
209 func (x *Rat) marshal() []byte {
211 buf = x.a.Append(buf, 10)
212 buf = append(buf, '/')
213 if len(x.b.abs) != 0 {
214 buf = x.b.Append(buf, 10)
216 buf = append(buf, '1')
221 // RatString returns a string representation of x in the form "a/b" if b != 1,
222 // and in the form "a" if b == 1.
223 func (x *Rat) RatString() string {
230 // FloatString returns a string representation of x in decimal form with prec
231 // digits of precision after the decimal point. The last digit is rounded to
232 // nearest, with halves rounded away from zero.
233 func (x *Rat) FloatString(prec int) string {
237 buf = x.a.Append(buf, 10)
239 buf = append(buf, '.')
240 for i := prec; i > 0; i-- {
241 buf = append(buf, '0')
248 q, r := nat(nil).div(nat(nil), x.a.abs, x.b.abs)
252 p = nat(nil).expNN(natTen, nat(nil).setUint64(uint64(prec)), nil)
256 r, r2 := r.div(nat(nil), r, x.b.abs)
258 // see if we need to round up
260 if x.b.abs.cmp(r2) <= 0 {
263 q = nat(nil).add(q, natOne)
264 r = nat(nil).sub(r, p)
269 buf = append(buf, '-')
271 buf = append(buf, q.utoa(10)...) // itoa ignores sign if q == 0
274 buf = append(buf, '.')
276 for i := prec - len(rs); i > 0; i-- {
277 buf = append(buf, '0')
279 buf = append(buf, rs...)