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gitea/vendor/github.com/pingcap/tidb/util/codec/decimal.go

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// Copyright 2015 PingCAP, Inc.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.
package codec
import (
"bytes"
"math/big"
"github.com/juju/errors"
"github.com/pingcap/tidb/mysql"
)
const (
negativeSign int64 = 8
zeroSign int64 = 16
positiveSign int64 = 24
)
func codecSign(value int64) int64 {
if value < 0 {
return negativeSign
}
return positiveSign
}
func encodeExp(expValue int64, expSign int64, valSign int64) int64 {
if expSign == negativeSign {
expValue = -expValue
}
if expSign != valSign {
expValue = ^expValue
}
return expValue
}
func decodeExp(expValue int64, expSign int64, valSign int64) int64 {
if expSign != valSign {
expValue = ^expValue
}
if expSign == negativeSign {
expValue = -expValue
}
return expValue
}
func codecValue(value []byte, valSign int64) {
if valSign == negativeSign {
reverseBytes(value)
}
}
// EncodeDecimal encodes a decimal d into a byte slice which can be sorted lexicographically later.
// EncodeDecimal guarantees that the encoded value is in ascending order for comparison.
// Decimal encoding:
// Byte -> value sign
// Byte -> exp sign
// EncodeInt -> exp value
// EncodeBytes -> abs value bytes
func EncodeDecimal(b []byte, d mysql.Decimal) []byte {
if d.Equals(mysql.ZeroDecimal) {
return append(b, byte(zeroSign))
}
v := d.BigIntValue()
valSign := codecSign(int64(v.Sign()))
absVal := new(big.Int)
absVal.Abs(v)
value := []byte(absVal.String())
// Trim right side "0", like "12.34000" -> "12.34" or "0.1234000" -> "0.1234".
if d.Exponent() != 0 {
value = bytes.TrimRight(value, "0")
}
// Get exp and value, format is "value":"exp".
// like "12.34" -> "0.1234":"2".
// like "-0.01234" -> "-0.1234":"-1".
exp := int64(0)
div := big.NewInt(10)
for ; ; exp++ {
if absVal.Sign() == 0 {
break
}
absVal = absVal.Div(absVal, div)
}
expVal := exp + int64(d.Exponent())
expSign := codecSign(expVal)
// For negtive exp, do bit reverse for exp.
// For negtive decimal, do bit reverse for exp and value.
expVal = encodeExp(expVal, expSign, valSign)
codecValue(value, valSign)
b = append(b, byte(valSign))
b = append(b, byte(expSign))
b = EncodeInt(b, expVal)
b = EncodeBytes(b, value)
return b
}
// DecodeDecimal decodes bytes to decimal.
// DecodeFloat decodes a float from a byte slice
// Decimal decoding:
// Byte -> value sign
// Byte -> exp sign
// DecodeInt -> exp value
// DecodeBytes -> abs value bytes
func DecodeDecimal(b []byte) ([]byte, mysql.Decimal, error) {
var (
r = b
d mysql.Decimal
err error
)
// Decode value sign.
valSign := int64(r[0])
r = r[1:]
if valSign == zeroSign {
d, err = mysql.ParseDecimal("0")
return r, d, errors.Trace(err)
}
// Decode exp sign.
expSign := int64(r[0])
r = r[1:]
// Decode exp value.
expVal := int64(0)
r, expVal, err = DecodeInt(r)
if err != nil {
return r, d, errors.Trace(err)
}
expVal = decodeExp(expVal, expSign, valSign)
// Decode abs value bytes.
value := []byte{}
r, value, err = DecodeBytes(r)
if err != nil {
return r, d, errors.Trace(err)
}
codecValue(value, valSign)
// Generate decimal string value.
var decimalStr []byte
if valSign == negativeSign {
decimalStr = append(decimalStr, '-')
}
if expVal <= 0 {
// Like decimal "0.1234" or "0.01234".
decimalStr = append(decimalStr, '0')
decimalStr = append(decimalStr, '.')
decimalStr = append(decimalStr, bytes.Repeat([]byte{'0'}, -int(expVal))...)
decimalStr = append(decimalStr, value...)
} else {
// Like decimal "12.34".
decimalStr = append(decimalStr, value[:expVal]...)
decimalStr = append(decimalStr, '.')
decimalStr = append(decimalStr, value[expVal:]...)
}
d, err = mysql.ParseDecimal(string(decimalStr))
return r, d, errors.Trace(err)
}