floatmarsh.go

Documentation: math/big

		 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.
		 4  
		 5  // This file implements encoding/decoding of Floats.
		 6  
		 7  package big
		 8  
		 9  import (
		10  	"encoding/binary"
		11  	"errors"
		12  	"fmt"
		13  )
		14  
		15  // Gob codec version. Permits backward-compatible changes to the encoding.
		16  const floatGobVersion byte = 1
		17  
		18  // GobEncode implements the gob.GobEncoder interface.
		19  // The Float value and all its attributes (precision,
		20  // rounding mode, accuracy) are marshaled.
		21  func (x *Float) GobEncode() ([]byte, error) {
		22  	if x == nil {
		23  		return nil, nil
		24  	}
		25  
		26  	// determine max. space (bytes) required for encoding
		27  	sz := 1 + 1 + 4 // version + mode|acc|form|neg (3+2+2+1bit) + prec
		28  	n := 0					// number of mantissa words
		29  	if x.form == finite {
		30  		// add space for mantissa and exponent
		31  		n = int((x.prec + (_W - 1)) / _W) // required mantissa length in words for given precision
		32  		// actual mantissa slice could be shorter (trailing 0's) or longer (unused bits):
		33  		// - if shorter, only encode the words present
		34  		// - if longer, cut off unused words when encoding in bytes
		35  		//	 (in practice, this should never happen since rounding
		36  		//	 takes care of it, but be safe and do it always)
		37  		if len(x.mant) < n {
		38  			n = len(x.mant)
		39  		}
		40  		// len(x.mant) >= n
		41  		sz += 4 + n*_S // exp + mant
		42  	}
		43  	buf := make([]byte, sz)
		44  
		45  	buf[0] = floatGobVersion
		46  	b := byte(x.mode&7)<<5 | byte((x.acc+1)&3)<<3 | byte(x.form&3)<<1
		47  	if x.neg {
		48  		b |= 1
		49  	}
		50  	buf[1] = b
		51  	binary.BigEndian.PutUint32(buf[2:], x.prec)
		52  
		53  	if x.form == finite {
		54  		binary.BigEndian.PutUint32(buf[6:], uint32(x.exp))
		55  		x.mant[len(x.mant)-n:].bytes(buf[10:]) // cut off unused trailing words
		56  	}
		57  
		58  	return buf, nil
		59  }
		60  
		61  // GobDecode implements the gob.GobDecoder interface.
		62  // The result is rounded per the precision and rounding mode of
		63  // z unless z's precision is 0, in which case z is set exactly
		64  // to the decoded value.
		65  func (z *Float) GobDecode(buf []byte) error {
		66  	if len(buf) == 0 {
		67  		// Other side sent a nil or default value.
		68  		*z = Float{}
		69  		return nil
		70  	}
		71  	if len(buf) < 6 {
		72  		return errors.New("Float.GobDecode: buffer too small")
		73  	}
		74  
		75  	if buf[0] != floatGobVersion {
		76  		return fmt.Errorf("Float.GobDecode: encoding version %d not supported", buf[0])
		77  	}
		78  
		79  	oldPrec := z.prec
		80  	oldMode := z.mode
		81  
		82  	b := buf[1]
		83  	z.mode = RoundingMode((b >> 5) & 7)
		84  	z.acc = Accuracy((b>>3)&3) - 1
		85  	z.form = form((b >> 1) & 3)
		86  	z.neg = b&1 != 0
		87  	z.prec = binary.BigEndian.Uint32(buf[2:])
		88  
		89  	if z.form == finite {
		90  		if len(buf) < 10 {
		91  			return errors.New("Float.GobDecode: buffer too small for finite form float")
		92  		}
		93  		z.exp = int32(binary.BigEndian.Uint32(buf[6:]))
		94  		z.mant = z.mant.setBytes(buf[10:])
		95  	}
		96  
		97  	if oldPrec != 0 {
		98  		z.mode = oldMode
		99  		z.SetPrec(uint(oldPrec))
	 100  	}
	 101  
	 102  	return nil
	 103  }
	 104  
	 105  // MarshalText implements the encoding.TextMarshaler interface.
	 106  // Only the Float value is marshaled (in full precision), other
	 107  // attributes such as precision or accuracy are ignored.
	 108  func (x *Float) MarshalText() (text []byte, err error) {
	 109  	if x == nil {
	 110  		return []byte("<nil>"), nil
	 111  	}
	 112  	var buf []byte
	 113  	return x.Append(buf, 'g', -1), nil
	 114  }
	 115  
	 116  // UnmarshalText implements the encoding.TextUnmarshaler interface.
	 117  // The result is rounded per the precision and rounding mode of z.
	 118  // If z's precision is 0, it is changed to 64 before rounding takes
	 119  // effect.
	 120  func (z *Float) UnmarshalText(text []byte) error {
	 121  	// TODO(gri): get rid of the []byte/string conversion
	 122  	_, _, err := z.Parse(string(text), 0)
	 123  	if err != nil {
	 124  		err = fmt.Errorf("math/big: cannot unmarshal %q into a *big.Float (%v)", text, err)
	 125  	}
	 126  	return err
	 127  }
	 128
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