sjhddh opened a new pull request, #10059:
URL: https://github.com/apache/arrow-rs/pull/10059
# Which issue does this PR close?
- Closes #7216.
# Rationale for this change
Dividing two `Decimal256` values whose result is representable can still
fail with
`ArithmeticOverflow`. The `Op::Div` path pre-scales the numerator by
`10^mul_pow`
(to add the +4 fractional digits Hive/postgres semantics require) before
dividing.
For e.g. `Decimal256(38, 37) / Decimal256(38, 37)` this intermediate `l *
10^41`
exceeds `i256::MAX` even though the true quotient (~1.0) easily fits, so a
valid
division spuriously errors.
# What changes are included in this PR?
- Adds `decimal_div_scaled`, which computes `trunc(l * 10^pow / r)`:
- Fast path: the existing `(l * 10^pow) / r` formula when it doesn't
overflow
(common case, behavior unchanged).
- Slow path (only on overflow): schoolbook base-10 long division on
magnitudes that
brings down one digit at a time and computes each digit via repeated
modular
addition, so no intermediate value ever exceeds the divisor. This avoids
the
intermediate overflow while preserving truncate-toward-zero semantics,
and is sound
even when the divisor is close to `i256::MAX` (where `remainder * 10`
itself would
overflow).
- Rewires the `Op::Div` arm to use the helper for the scale-up case; the
divisor-only-scaled and no-scaling cases keep the original direct formula.
# Are these changes tested?
Yes. Added `test_decimal256_div` (the repro from the issue) plus assertions
for all four
sign combinations and a near-`i256::MAX` divisor edge case. The test
overflows on `main`
and passes with this change; existing decimal
add/sub/mul/div/rem/divide-by-zero tests
still pass (`cargo test -p arrow-arith`, clippy `-D warnings`, fmt all
clean).
# Are there any user-facing changes?
No API changes. `Decimal128`/`Decimal256` division that previously returned
a spurious
`ArithmeticOverflow` now returns the correct quotient. Results that
genuinely don't fit
the native type still return `ArithmeticOverflow`, and divide-by-zero still
returns
`DivideByZero`.
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