Source file src/pkg/math/big/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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