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GH-50641: [C++][Compute] Fix correctness error in decimal round_binary kernel #50642

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GH-50641: [C++][Compute] Fix correctness error in decimal round_binary kernel #50642

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24 changes: 9 additions & 15 deletions cpp/src/arrow/compute/kernels/scalar_round.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -391,7 +391,7 @@ struct RoundImpl<Type, RoundMode::HALF_TO_ODD> {
const T& pow10, const int32_t scale) {
auto scaled = val->ReduceScaleBy(scale, /*round=*/false);
if (scaled.low_bits() % 2 == 0) {
scaled += remainder.Sign() ? 1 : -1;
scaled += remainder.Sign() >= 0 ? 1 : -1;
}
*val = scaled.IncreaseScaleBy(scale);
}
Expand Down Expand Up @@ -915,29 +915,20 @@ struct RoundBinary<ArrowType, kRoundMode, enable_if_decimal<ArrowType>> {
using CType = typename TypeTraits<ArrowType>::CType;
using State = RoundOptionsWrapper<RoundBinaryOptions, double>;
const ArrowType& ty;
int32_t pow;
// pow10 is "1" for the given decimal scale. Similarly half_pow10 is "0.5".
CType half_pow10, neg_half_pow10;

explicit RoundBinary(const State& state, const DataType& out_ty)
: RoundBinary(out_ty) {}

explicit RoundBinary(const DataType& out_ty)
: ty(checked_cast<const ArrowType&>(out_ty)),
pow(static_cast<int32_t>(ty.scale() - 0)) {
if (pow >= ty.precision() || pow < 0) {
half_pow10 = neg_half_pow10 = 0;
} else {
half_pow10 = CType::GetHalfScaleMultiplier(pow);
neg_half_pow10 = -half_pow10;
}
}
: ty(checked_cast<const ArrowType&>(out_ty)) {}

template <typename T = ArrowType, typename CType0 = typename TypeTraits<T>::CType0,
typename CType1 = typename TypeTraits<T>::CType1>
enable_if_decimal_value<CType> Call(KernelContext* ctx, CType0 arg0, CType1 arg1,
Status* st) const {
if (pow - arg1 >= ty.precision()) {
int32_t pow = static_cast<int32_t>(ty.scale() - arg1);

if (pow >= ty.precision()) {
*st = Status::Invalid("Rounding to ", arg1, " digits will not fit in precision of ",
ty);
return 0;
Expand All @@ -946,7 +937,8 @@ struct RoundBinary<ArrowType, kRoundMode, enable_if_decimal<ArrowType>> {
return arg0;
}

CType0 pow10 = CType0::GetScaleMultiplier(static_cast<int32_t>(ty.scale() - arg1));
// pow10 is "1" for the given decimal scale. Similarly half_pow10 is "0.5".
CType0 pow10 = CType0::GetScaleMultiplier(pow);

std::pair<CType, CType> pair;
*st = arg0.Divide(pow10).Value(&pair);
Expand All @@ -955,6 +947,8 @@ struct RoundBinary<ArrowType, kRoundMode, enable_if_decimal<ArrowType>> {
const auto& remainder = pair.second;
if (remainder == 0) return arg0;
if (kRoundMode >= RoundMode::HALF_DOWN) {
CType0 half_pow10 = CType::GetHalfScaleMultiplier(pow);
CType0 neg_half_pow10 = -half_pow10;
if (remainder == half_pow10 || remainder == neg_half_pow10) {
// On the halfway point, use tiebreaker
RoundImpl<CType0, kRoundMode>::Round(&arg0, remainder, pow10, pow);
Expand Down
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