sstemmle commented on PR #38478:
URL: https://github.com/apache/arrow/pull/38478#issuecomment-1782505446
Thanks for working on this @zeroshade. The current version is still failing
for e.g. `9323406071781562130.6457232358109488923`. The following piece of code
can be used to check a range of different input values.
```
func TestFromStringDecimal128(t *testing.T) {
rq := require.New(t)
const maxprec = 38
for {
scale := int(rand.Int31()) % (maxprec + 1)
s := ""
if maxprec-scale > 0 {
for i := 0; i < maxprec-scale; i++ {
s += randDigit()
}
} else {
s += "0"
}
if scale > 0 {
s += "."
for i := 0; i < scale; i++ {
s += randDigit()
}
}
num, err := decimal128.FromString(s, maxprec, int32(scale))
rq.NoErrorf(err, "s=%s, scale=%d", s, scale)
actualCoeff := num.BigInt()
expectedCoeff, _ := (&big.Int{}).SetString(strings.Replace(s,
".", "", -1), 10)
rq.Equal(expectedCoeff.Bytes(), actualCoeff.Bytes(), s)
}
}
func randDigit() string {
return strconv.Itoa(int(rand.Int31() % 10))
}
```
I think one needs to adjust the precision provided to `big.ParseFloat()` to
128. See
https://stackoverflow.com/questions/30688422/is-the-most-significant-decimal-digits-precision-that-can-be-converted-to-binary
.
The significant (in the above sense) decimal digits giving a floating point
precision are given by ⌊(N-1) log10(2)⌋.
To get 38 digits, 127 is not enough, but 128 is needed. For 76 digits 255 is
enough.
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