// makeDecimalFromMandE reconstructs the decimal from the mantissa M and
// exponent E.
func makeDecimalFromMandE(negative bool, e int, m []byte, tmp []byte) decimal.Decimal {
// ±dddd.
b := tmp[:0]
if n := len(m)*2 + 1; cap(b) < n {
b = make([]byte, 0, n)
}
if negative {
b = append(b, '-')
}
for i, v := range m {
t := int(v)
if i == len(m) {
t--
}
t /= 2
b = append(b, byte(t/10)+'0', byte(t%10)+'0')
}
// We unsafely convert the []byte to a string to avoid the usual allocation
// when converting to a string.
bi := new(big.Int)
bi, ok := bi.SetString(*(*string)(unsafe.Pointer(&b)), 10)
if !ok {
panic("could not set big.Int's string value")
}
exp := 2*e - len(b)
if negative {
exp++
}
return decimal.NewFromBigInt(bi, int32(exp))
}
// GetDecimal decodes a decimal value from the bytes of the receiver. If the
// tag is not DECIMAL an error will be returned.
func (v Value) GetDecimal() (decimal.Decimal, error) {
if tag := v.GetTag(); tag != ValueType_DECIMAL {
return decimal.Decimal{}, fmt.Errorf("value type is not %s: %s", ValueType_DECIMAL, tag)
}
data := v.dataBytes()
i, n := binary.Varint(data)
if n <= 0 {
return decimal.Decimal{}, fmt.Errorf("int64 varint decoding failed: %d", n)
}
bi := new(big.Int)
err := bi.GobDecode(data[n:])
if err != nil {
return decimal.Decimal{}, err
}
return decimal.NewFromBigInt(bi, int32(i)), nil
}