miguelcorderopamphile opened a new pull request, #11315: URL: https://github.com/apache/arrow-rs/pull/11315
# Which issue does this PR close? - Closes #11314. # Rationale for this change `i256_to_f64` normalizes the input, keeps the top 64 bits and discards the lower 192, so the final `i64 as f64` rounds a value truncated at 64 bits. When the discarded bits put the exact value just past a binary64 midpoint, the result can round to the wrong neighbor: `(1_i128 << 63) + 1024 + 1` returns `0x43e0000000000000` instead of `0x43e0000000000001`. # What changes are included in this PR? Set a sticky bit on the retained value when any discarded bit is non-zero, so midpoint ties round to nearest (ties to even only apply when the exact value is the midpoint). No API changes. A naive `top + 1` variant was also checked and is incorrect, since it can move a value that sits just below a midpoint up to the midpoint; setting the low bit preserves the side of the midpoint. # Are these changes tested? - The reported value plus midpoint neighborhoods for every binary64 exponent in `i256` range, both signs, checked against `to_string().parse::<f64>()`. - `MIN`, `MAX`, `-1`, `0`, `1`, and 1000 deterministic pseudo-random full-range values against the same oracle. - `cargo test -p arrow-buffer` (358 passed) and `cargo test -p arrow-cast --lib test_cast_decimal256` (12 passed); `cargo fmt --check` and `cargo clippy -p arrow-buffer --all-targets -- -D warnings` clean. # Are there any user-facing changes? No API changes. `i256::to_f64` now returns the correctly rounded `f64`, which also fixes `Decimal256 -> Float64` casts that use it. # AI usage disclosure Per CONTRIBUTING: the sticky-bit approach was suggested in the issue. I reproduced the failure, validated the approach at scale against correctly rounded conversions (midpoint sweeps and randomized comparisons, including ruling out the `+1` variant), and wrote the tests. AI assistance was used for analysis and drafting; I have reviewed and verified every change and own the final implementation. -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. To unsubscribe, e-mail: [email protected] For queries about this service, please contact Infrastructure at: [email protected]
