Whatsonyourmind commented on issue #9422:
URL: https://github.com/apache/arrow-rs/issues/9422#issuecomment-4182733753
The fix needs to implement half-up rounding (banker's rounding or
round-half-to-even would also be defensible, but Postgres and DuckDB both use
half-up).
The core logic: when parsing a decimal string with more fractional digits
than the target scale allows, you need to look at the first truncated digit to
decide whether to round up.
```rust
// After parsing the significant digits and determining where truncation
occurs:
fn round_decimal_parse(digits: &[u8], truncation_point: usize, scale: i8) ->
i128 {
let mut result = parse_digits_up_to(digits, truncation_point);
if truncation_point < digits.len() {
let next_digit = digits[truncation_point] - b'0';
if next_digit >= 5 {
// Round away from zero (half-up for positive, half-down for
negative)
if result >= 0 {
result += 1;
} else {
result -= 1;
}
}
}
result
}
```
For the specific case of scale=0, parsing "1.9" should give:
1. Parse integer part: 1
2. First truncated fractional digit: 9 >= 5, so round up
3. Result: 2
One edge case to watch: rounding can cause overflow. Parsing "99.5" as
`numeric(2, 0)` should round to 100, which exceeds the precision. Postgres
raises an error here. The implementation should check `result < 10^precision`
after rounding and return an overflow error if violated.
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