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)
21 // Scan is a support routine for fmt.Scanner. It accepts the formats
22 // 'e', 'E', 'f', 'F', 'g', 'G', and 'v'. All formats are equivalent.
23 func (z *Rat) Scan(s fmt.ScanState, ch rune) error {
24 tok, err := s.Token(true, ratTok)
28 if !strings.ContainsRune("efgEFGv", ch) {
29 return errors.New("Rat.Scan: invalid verb")
31 if _, ok := z.SetString(string(tok)); !ok {
32 return errors.New("Rat.Scan: invalid syntax")
37 // SetString sets z to the value of s and returns z and a boolean indicating
38 // success. s can be given as a fraction "a/b" or as a floating-point number
39 // optionally followed by an exponent. If the operation failed, the value of
40 // z is undefined but the returned value is nil.
41 func (z *Rat) SetString(s string) (*Rat, bool) {
47 // parse fraction a/b, if any
48 if sep := strings.Index(s, "/"); sep >= 0 {
49 if _, ok := z.a.SetString(s[:sep], 0); !ok {
54 if z.b.abs, _, _, err = z.b.abs.scan(strings.NewReader(s), 0, false); err != nil {
57 if len(z.b.abs) == 0 {
63 // parse floating-point number
64 r := strings.NewReader(s)
67 neg, err := scanSign(r)
74 z.a.abs, _, ecorr, err = z.a.abs.scan(r, 10, true)
81 exp, _, err = scanExponent(r, false)
86 // there should be no unread characters left
87 if _, err = r.ReadByte(); err != io.EOF {
91 // special-case 0 (see also issue #16176)
92 if len(z.a.abs) == 0 {
102 // compute exponent power
107 powTen := nat(nil).expNN(natTen, nat(nil).setWord(Word(expabs)), nil)
114 z.a.abs = z.a.abs.mul(z.a.abs, powTen)
115 z.b.abs = z.b.abs[:0]
118 z.a.neg = neg && len(z.a.abs) > 0 // 0 has no sign
123 // scanExponent scans the longest possible prefix of r representing a decimal
124 // ('e', 'E') or binary ('p') exponent, if any. It returns the exponent, the
125 // exponent base (10 or 2), or a read or syntax error, if any.
127 // exponent = ( "E" | "e" | "p" ) [ sign ] digits .
128 // sign = "+" | "-" .
129 // digits = digit { digit } .
130 // digit = "0" ... "9" .
132 // A binary exponent is only permitted if binExpOk is set.
133 func scanExponent(r io.ByteScanner, binExpOk bool) (exp int64, base int, err error) {
137 if ch, err = r.ReadByte(); err != nil {
139 err = nil // no exponent; same as e0
152 fallthrough // binary exponent not permitted
155 return // no exponent; same as e0
159 if neg, err = scanSign(r); err != nil {
165 digits = append(digits, '-')
168 // no need to use nat.scan for exponent digits
169 // since we only care about int64 values - the
170 // from-scratch scan is easy enough and faster
172 if ch, err = r.ReadByte(); err != nil {
173 if err != io.EOF || i == 0 {
179 if ch < '0' || '9' < ch {
182 err = fmt.Errorf("invalid exponent (missing digits)")
187 digits = append(digits, ch)
189 // i > 0 => we have at least one digit
191 exp, err = strconv.ParseInt(string(digits), 10, 64)
195 // String returns a string representation of x in the form "a/b" (even if b == 1).
196 func (x *Rat) String() string {
198 buf = x.a.Append(buf, 10)
199 buf = append(buf, '/')
200 if len(x.b.abs) != 0 {
201 buf = x.b.Append(buf, 10)
203 buf = append(buf, '1')
208 // RatString returns a string representation of x in the form "a/b" if b != 1,
209 // and in the form "a" if b == 1.
210 func (x *Rat) RatString() string {
217 // FloatString returns a string representation of x in decimal form with prec
218 // digits of precision after the decimal point. The last digit is rounded to
219 // nearest, with halves rounded away from zero.
220 func (x *Rat) FloatString(prec int) string {
224 buf = x.a.Append(buf, 10)
226 buf = append(buf, '.')
227 for i := prec; i > 0; i-- {
228 buf = append(buf, '0')
235 q, r := nat(nil).div(nat(nil), x.a.abs, x.b.abs)
239 p = nat(nil).expNN(natTen, nat(nil).setUint64(uint64(prec)), nil)
243 r, r2 := r.div(nat(nil), r, x.b.abs)
245 // see if we need to round up
247 if x.b.abs.cmp(r2) <= 0 {
250 q = nat(nil).add(q, natOne)
251 r = nat(nil).sub(r, p)
256 buf = append(buf, '-')
258 buf = append(buf, q.utoa(10)...) // itoa ignores sign if q == 0
261 buf = append(buf, '.')
263 for i := prec - len(rs); i > 0; i-- {
264 buf = append(buf, '0')
266 buf = append(buf, rs...)