hcrosse opened a new issue, #1507:
URL: https://github.com/apache/arrow-go/issues/1507
### Describe the bug, including details regarding any error messages,
version, and platform.
`decimal128.Num.ToBigFloat` and `decimal128.FromString` panic with `index
out of range` when the scale's absolute value exceeds 38. The corresponding
`decimal256` functions panic beyond 76. Both panic even for zero.
The panic reaches array `ValueStr` and JSON output, scalar `String` and
string casts, builders, `array.FromJSON`, `scalar.ParseScalar` and the CSV
reader.
I reproduced this on main at `32947b25`, on macOS arm64 with Go 1.26.10.
**Reproduction**
Save this as `main.go` in a new subdirectory of an arrow-go checkout at
`32947b259414a77ae6347d3d29bd69ec00b9d560`, then run `go run . format128`, `go
run . format256`, `go run . scalar128`, `go run . parse128` and `go run .
parse256` from that directory:
```go
package main
import (
"fmt"
"os"
"github.com/apache/arrow-go/v18/arrow"
"github.com/apache/arrow-go/v18/arrow/decimal128"
"github.com/apache/arrow-go/v18/arrow/decimal256"
"github.com/apache/arrow-go/v18/arrow/scalar"
)
func main() {
switch os.Args[1] {
case "format128":
fmt.Println(decimal128.FromI64(12345).ToBigFloat(39).Text('g',
38))
case "format256":
fmt.Println(decimal256.FromI64(12345).ToBigFloat(77).Text('g',
76))
case "scalar128":
fmt.Println(scalar.NewDecimal128Scalar(decimal128.FromI64(12345),
&arrow.Decimal128Type{Precision: 38, Scale: 100000}).String())
case "parse128":
n, err := decimal128.FromString("1e-39", 38, 39)
fmt.Println(n.BigInt(), err)
case "parse256":
n, err := decimal256.FromString("1e-77", 76, 77)
fmt.Println(n.BigInt(), err)
}
}
```
Actual output starts with this for `format128` and `parse128`:
```text
panic: runtime error: index out of range [39] with length 39
```
For `format256` and `parse256`:
```text
panic: runtime error: index out of range [77] with length 77
```
For `scalar128`:
```text
panic: runtime error: index out of range [100000] with length 39
```
Expected output for `format128`:
```text
1.2345e-35
```
Expected output for `format256`:
```text
1.2345e-73
```
Expected output for `scalar128`:
```text
1.2345e-99996
```
Expected output for either parse command:
```text
1 <nil>
```
**Cause**
`ToBigFloat` and `FromString` index fixed tables of powers of ten without
handling scales outside them.
https://github.com/apache/arrow-go/blob/32947b259414a77ae6347d3d29bd69ec00b9d560/arrow/decimal128/decimal128.go#L339-L347
https://github.com/apache/arrow-go/blob/32947b259414a77ae6347d3d29bd69ec00b9d560/arrow/decimal256/decimal256.go#L158-L168
I have a patch with tests and can open a PR.
### Component(s)
Other
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