tustvold commented on code in PR #3943:
URL: https://github.com/apache/arrow-rs/pull/3943#discussion_r1148448583
##########
arrow-buffer/src/bigint.rs:
##########
@@ -243,9 +243,8 @@ impl i256 {
/// Performs checked addition
#[inline]
pub fn checked_add(self, other: Self) -> Option<Self> {
- let (low, carry) = self.low.overflowing_add(other.low);
- let high = self.high.checked_add(other.high)?.checked_add(carry as _)?;
Review Comment:
The mistake this makes is assuming that addition is transitive, in the case
of overflow it is not.
In particular consider the case
```
i256::from_parts(u128::MAX, i128::MIN)
.checked_add(i256::from_parts(1, -1))
.unwrap();
```
The carry is enough to increase `i128::MIN` enough that adding `-1` doesn't
result in overflow
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