zeroshade commented on code in PR #38426:
URL: https://github.com/apache/arrow/pull/38426#discussion_r1370587873
##########
go/arrow/decimal128/decimal128.go:
##########
@@ -266,23 +266,36 @@ func FromString(v string, prec, scale int32) (n Num, err
error) {
return
}
- // Since we're going to truncate this to get an integer, we need to
round
- // the value instead because of edge cases so that we match how other
implementations
- // (e.g. C++) handles Decimal values. So if we're negative we'll
subtract 0.5 and if
- // we're positive we'll add 0.5.
- out.Mul(out, big.NewFloat(math.Pow10(int(scale)))).SetPrec(precInBits)
- if out.Signbit() {
- out.Sub(out, pt5)
+ if scale < 0 {
+ var tmp big.Int
+ val, _ := out.Int(&tmp)
+ if val.BitLen() > 127 {
+ return Num{}, errors.New("bitlen too large for
decimal128")
+ }
+ n = FromBigInt(val)
+ n, _ = n.Div(scaleMultipliers[-scale])
} else {
- out.Add(out, pt5)
- }
- var tmp big.Int
- val, _ := out.Int(&tmp)
- if val.BitLen() > 127 {
- return Num{}, errors.New("bitlen too large for decimal128")
+ // Since we're going to truncate this to get an integer, we
need to round
+ // the value instead because of edge cases so that we match how
other implementations
+ // (e.g. C++) handles Decimal values. So if we're negative
we'll subtract 0.5 and if
+ // we're positive we'll add 0.5.
+ p := (&big.Float{}).SetFloat64(float64PowersOfTen[scale+38])
+ out.Mul(out, p).SetPrec(precInBits)
+ if out.Signbit() {
+ out.Sub(out, pt5)
+ } else {
+ out.Add(out, pt5)
+ }
Review Comment:
Not quite, check the comment above the spot: we truncate the value to get an
integer. So -7.0 with a scale of 0 becomes -7.5 and then we truncate it to -7.
-7.5 becomes -8.0 and then we truncate it to -8.
A better example would be if we're given the value -0.75 with a scale of 1:
The scale of 1 means that our final Decimal value is going to be the integer -8
because of rounding.
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