ratmarsh.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 Rats.
     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 ratGobVersion byte = 1
    17  
    18  // GobEncode implements the gob.GobEncoder interface.
    19  func (x *Rat) GobEncode() ([]byte, error) {
    20  	if x == nil {
    21  		return nil, nil
    22  	}
    23  	buf := make([]byte, 1+4+(len(x.a.abs)+len(x.b.abs))*_S) // extra bytes for version and sign bit (1), and numerator length (4)
    24  	i := x.b.abs.bytes(buf)
    25  	j := x.a.abs.bytes(buf[:i])
    26  	n := i - j
    27  	if int(uint32(n)) != n {
    28  		// this should never happen
    29  		return nil, errors.New("Rat.GobEncode: numerator too large")
    30  	}
    31  	binary.BigEndian.PutUint32(buf[j-4:j], uint32(n))
    32  	j -= 1 + 4
    33  	b := ratGobVersion << 1 // make space for sign bit
    34  	if x.a.neg {
    35  		b |= 1
    36  	}
    37  	buf[j] = b
    38  	return buf[j:], nil
    39  }
    40  
    41  // GobDecode implements the gob.GobDecoder interface.
    42  func (z *Rat) GobDecode(buf []byte) error {
    43  	if len(buf) == 0 {
    44  		// Other side sent a nil or default value.
    45  		*z = Rat{}
    46  		return nil
    47  	}
    48  	b := buf[0]
    49  	if b>>1 != ratGobVersion {
    50  		return fmt.Errorf("Rat.GobDecode: encoding version %d not supported", b>>1)
    51  	}
    52  	const j = 1 + 4
    53  	i := j + binary.BigEndian.Uint32(buf[j-4:j])
    54  	z.a.neg = b&1 != 0
    55  	z.a.abs = z.a.abs.setBytes(buf[j:i])
    56  	z.b.abs = z.b.abs.setBytes(buf[i:])
    57  	return nil
    58  }
    59  
    60  // MarshalText implements the encoding.TextMarshaler interface.
    61  func (x *Rat) MarshalText() (text []byte, err error) {
    62  	if x.IsInt() {
    63  		return x.a.MarshalText()
    64  	}
    65  	return x.marshal(), nil
    66  }
    67  
    68  // UnmarshalText implements the encoding.TextUnmarshaler interface.
    69  func (z *Rat) UnmarshalText(text []byte) error {
    70  	// TODO(gri): get rid of the []byte/string conversion
    71  	if _, ok := z.SetString(string(text)); !ok {
    72  		return fmt.Errorf("math/big: cannot unmarshal %q into a *big.Rat", text)
    73  	}
    74  	return nil
    75  }
    76
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