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Source file 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"
  	"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 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 {
  		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
  }
  

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