golang floatmarsh 代码
文件路径：/src/math/big/floatmarsh.go
/ Copyright 2015 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.
/ This file implements encoding/decoding of Floats.
package big
import (
"encoding/binary"
"errors"
"fmt"
)
/ Gob codec version. Permits backward-compatible changes to the encoding.
const floatGobVersion byte = 1
/ GobEncode implements the gob.GobEncoder interface.
/ The Float value and all its attributes (precision,
/ rounding mode, accuracy) are marshaled.
func (x *Float) GobEncode() ([]byte, error) {
if x == nil {
return nil, nil
}
/ determine max. space (bytes) required for encoding
sz := 1 + 1 + 4 / version + mode|acc|form|neg (3+2+2+1bit) + prec
n := 0 / number of mantissa words
if x.form == finite {
/ add space for mantissa and exponent
n = int((x.prec + (_W - 1)) / _W) / required mantissa length in words for given precision
/ actual mantissa slice could be shorter (trailing 0's) or longer (unused bits):
/ - if shorter, only encode the words present
/ - if longer, cut off unused words when encoding in bytes
/ (in practice, this should never happen since rounding
/ takes care of it, but be safe and do it always)
if len(x.mant) < n {
n = len(x.mant)
}
/ len(x.mant) >= n
sz += 4 + n*_S / exp + mant
}
buf := make([]byte, sz)
buf[0] = floatGobVersion
b := byte(x.mode&7)<<5 | byte((x.acc+1)&3)<<3 | byte(x.form&3)<<1
if x.neg {
b |= 1
}
buf[1] = b
binary.BigEndian.PutUint32(buf[2:], x.prec)
if x.form == finite {
binary.BigEndian.PutUint32(buf[6:], uint32(x.exp))
x.mant[len(x.mant)-n:].bytes(buf[10:]) / cut off unused trailing words
}
return buf, nil
}
/ GobDecode implements the gob.GobDecoder interface.
/ The result is rounded per the precision and rounding mode of
/ z unless z's precision is 0, in which case z is set exactly
/ to the decoded value.
func (z *Float) GobDecode(buf []byte) error {
if len(buf) == 0 {
/ Other side sent a nil or default value.
*z = Float{}
return nil
}
if len(buf) < 6 {
return errors.New("Float.GobDecode: buffer too small")
}
if buf[0] != floatGobVersion {
return fmt.Errorf("Float.GobDecode: encoding version %d not supported", buf[0])
}
oldPrec := z.prec
oldMode := z.mode
b := buf[1]
z.mode = RoundingMode((b >> 5) & 7)
z.acc = Accuracy((b>>3)&3) - 1
z.form = form((b >> 1) & 3)
z.neg = b&1 != 0
z.prec = binary.BigEndian.Uint32(buf[2:])
if z.form == finite {
if len(buf) < 10 {
return errors.New("Float.GobDecode: buffer too small for finite form float")
}
z.exp = int32(binary.BigEndian.Uint32(buf[6:]))
z.mant = z.mant.setBytes(buf[10:])
}
if oldPrec != 0 {
z.mode = oldMode
z.SetPrec(uint(oldPrec))
}
return nil
}
/ MarshalText implements the encoding.TextMarshaler interface.
/ Only the Float value is marshaled (in full precision), other
/ attributes such as precision or accuracy are ignored.
func (x *Float) MarshalText() (text []byte, err error) {
if x == nil {
return []byte("<nil>"), nil
}
var buf []byte
return x.Append(buf, 'g', -1), nil
}
/ UnmarshalText implements the encoding.TextUnmarshaler interface.
/ The result is rounded per the precision and rounding mode of z.
/ If z's precision is 0, it is changed to 64 before rounding takes
/ effect.
func (z *Float) UnmarshalText(text []byte) error {
/ TODO(gri): get rid of the []byte/string conversion
_, _, err := z.Parse(string(text), 0)
if err != nil {
err = fmt.Errorf("math/big: cannot unmarshal %q into a *big.Float (%v)", text, err)
}
return err
}