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GH-53288: [C++][Python] Preserve full coefficients in safe Decimal32 casts #55246

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GH-53288: [C++][Python] Preserve full coefficients in safe Decimal32 casts #55246

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32 changes: 31 additions & 1 deletion cpp/src/arrow/compute/kernels/scalar_cast_numeric.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -490,6 +490,36 @@ struct DecimalConversions<Decimal32, InDecimal> {
static Decimal32 ConvertOutput(Decimal32&& val) { return val; }
};

// Safe casts to Decimal32 need to rescale the full input coefficient before
// narrowing it. Otherwise, high bits can be lost before overflow and precision
// checks run.
template <typename OutDecimal, typename InDecimal>
struct SafeDecimalConversions : DecimalConversions<OutDecimal, InDecimal> {};

template <>
struct SafeDecimalConversions<Decimal32, Decimal64> {
static Decimal64 ConvertInput(Decimal64&& val) { return val; }
static Decimal32 ConvertOutput(Decimal64&& val) {
return Decimal32(static_cast<int32_t>(val.low_bits()));
}
};

template <>
struct SafeDecimalConversions<Decimal32, Decimal128> {
static Decimal128 ConvertInput(Decimal128&& val) { return val; }
static Decimal32 ConvertOutput(Decimal128&& val) {
return Decimal32(static_cast<int32_t>(val.low_bits()));
}
};

template <>
struct SafeDecimalConversions<Decimal32, Decimal256> {
static Decimal256 ConvertInput(Decimal256&& val) { return val; }
static Decimal32 ConvertOutput(Decimal256&& val) {
return Decimal32(static_cast<int32_t>(val.low_bits()));
}
};

template <>
struct DecimalConversions<Decimal64, Decimal32> {
// Convert then scale
Expand Down Expand Up @@ -573,7 +603,7 @@ struct UnsafeDownscaleDecimal {
struct SafeRescaleDecimal {
template <typename OutValue, typename Arg0Value>
OutValue Call(KernelContext*, Arg0Value val, Status* st) const {
using Conv = DecimalConversions<OutValue, Arg0Value>;
using Conv = SafeDecimalConversions<OutValue, Arg0Value>;
auto maybe_rescaled =
Conv::ConvertInput(std::move(val)).Rescale(in_scale_, out_scale_);
if (ARROW_PREDICT_FALSE(!maybe_rescaled.ok())) {
Expand Down
18 changes: 18 additions & 0 deletions cpp/src/arrow/compute/kernels/scalar_cast_test.cc
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Expand Up @@ -1646,6 +1646,24 @@ TEST(Cast, Decimal64ToDecimal256) {
TEST(Cast, Decimal128ToDecimal32) {
CastOptions options;

// Safe downcasts must check the value after rescaling the full input
// coefficient, before narrowing it to Decimal32.
auto d128_12_0_overflow = ArrayFromJSON(decimal128(12, 0), R"(["5000000000"])");
options.to_type = decimal32(9, 0);
CheckCastFails(d128_12_0_overflow, options);

auto d128_12_0_rescaled = ArrayFromJSON(decimal128(12, 0), R"(["536870912000"])");
Decimal32Builder d32_9_neg3_builder(decimal32(9, -3));
ASSERT_OK(d32_9_neg3_builder.Append(Decimal32(536870912)));
ASSERT_OK_AND_ASSIGN(auto d32_9_neg3, d32_9_neg3_builder.Finish());
options.to_type = d32_9_neg3->type();
CheckCast(d128_12_0_rescaled, d32_9_neg3, options);

auto d128_10_2_rescaled = ArrayFromJSON(decimal128(10, 2), R"(["30000000.00"])");
auto d32_9_0 = ArrayFromJSON(decimal32(9, 0), R"(["30000000"])");
options.to_type = d32_9_0->type();
CheckCast(d128_10_2_rescaled, d32_9_0, options);

for (bool allow_decimal_truncate : {false, true}) {
options.allow_decimal_truncate = allow_decimal_truncate;

Expand Down
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