floatmarsh.go

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