arrow-buffer: i256: fix ilog off-by-one when self == base - #1
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Extends the existing `test_ilog` with cases that the current tests do not cover: small results (0 and 1) for non-base-10 bases, including `self == base`. Per the std contract `n.ilog(n) == 1` (e.g. `2u32.ilog(2) == 1`), but `i256::checked_ilog` currently returns 0 for these inputs. Also cross-checks small results against `u128::ilog` as ground truth. These tests fail until the following fix is applied. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
`checked_ilog` returned the early `Some(0)` for `self == base`, but the correct result is 1 (`base^1 == base <= self`), matching the std contract `n.ilog(n) == 1`. The base-10 path was unaffected since it is handled by `checked_ilog10`, so this affected every other base (including `ilog2(2)`). Narrow the early return from `self <= base` to `self < base`; the loop already computes the correct value for `self == base`. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
alamb
commented
Jun 2, 2026
| assert_eq!(i256::ZERO.checked_ilog2(), None); | ||
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| // self == base, matches std: `n.ilog(n) == 1` | ||
| assert_eq!(i256::from(2).checked_ilog(i256::from(2)), Some(1)); |
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these tests fail without the code change
theirix
approved these changes
Jun 2, 2026
theirix
marked this pull request as ready for review
June 2, 2026 18:02
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Rationale for this change
Builds on apache#9453. While reviewing that PR, I found that
i256::checked_ilog(and thereforeilog/checked_ilog2/ilog2) returns0instead of1whenself == base.What changes are included in this PR?
Are these changes tested?
Yes — see the first commit. The added assertions fail before the fix and pass after.
Are there any user-facing changes?
Fixes incorrect results from
i256::ilog/checked_ilog/ilog2/checked_ilog2whenself == base. No API changes.