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GH-40911: [C++] Remove decimal division's precision and scale calculate logic from implicit casts #40969

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GH-40911: [C++] Remove decimal division's precision and scale calculate logic from implicit casts #40969

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35 changes: 35 additions & 0 deletions cpp/src/arrow/compute/expression_test.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -944,6 +944,41 @@ TEST(Expression, ExecuteDictionaryTransparent) {
])"));
}

TEST(Expression, ExecuteDecimalDivide) {
// GH-40911 Decimal divide promotion rule output wrong precision and scale
// in expression Bind when scale1 >= scale2
//
// scale1 < scale2
ExpectExecute(
call("divide", {field_ref("a"), field_ref("b")}),
ArrayFromJSON(struct_({field("a", decimal128(6, 1)), field("b", decimal128(7, 2))}),
R"([
{"a": "0.7", "b": "-2.17"},
{"a": "2.9", "b": "9.11"},
{"a": "3.1", "b" : "12.31"}
])"));

// scale1 == scale2
ExpectExecute(
call("divide", {field_ref("a"), field_ref("b")}),
ArrayFromJSON(struct_({field("a", decimal128(6, 1)), field("b", decimal128(6, 1))}),
R"([
{"a": "0.7", "b": "-2.1"},
{"a": "2.9", "b": "9.1"},
{"a": "3.1", "b" : "12.3"}
])"));

// scale1 > scale2
ExpectExecute(
call("divide", {field_ref("a"), field_ref("b")}),
ArrayFromJSON(struct_({field("a", decimal128(6, 2)), field("b", decimal128(6, 1))}),
R"([
{"a": "0.71", "b": "-2.1"},
{"a": "2.92", "b": "9.1"},
{"a": "3.17", "b" : "12.3"}
])"));
}

void ExpectIdenticalIfUnchanged(Expression modified, Expression original) {
if (modified == original) {
// no change -> must be identical
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8 changes: 2 additions & 6 deletions cpp/src/arrow/compute/kernels/codegen_internal.cc
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Expand Up @@ -428,13 +428,9 @@ Status CastBinaryDecimalArgs(DecimalPromotion promotion, std::vector<TypeHolder>
right_scaleup = std::max(s1, s2) - s2;
break;
}
case DecimalPromotion::kMultiply: {
left_scaleup = right_scaleup = 0;
break;
}
case DecimalPromotion::kMultiply:
case DecimalPromotion::kDivide: {
left_scaleup = std::max(4, s1 + p2 - s2 + 1) + s2 - s1;
right_scaleup = 0;
left_scaleup = right_scaleup = 0;
break;
}
default:
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9 changes: 2 additions & 7 deletions cpp/src/arrow/compute/kernels/scalar_arithmetic.cc
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Expand Up @@ -541,13 +541,8 @@ Result<TypeHolder> ResolveDecimalDivisionOutput(KernelContext*,
types,
[](int32_t p1, int32_t s1, int32_t p2,
int32_t s2) -> Result<std::pair<int32_t, int32_t>> {
if (s1 < s2) {
return Status::Invalid("Division of two decimal types scale1 < scale2. ", "(",
s1, s2, ").");
}
DCHECK_GE(s1, s2);
const int32_t scale = s1 - s2;
const int32_t precision = p1;
const int32_t scale = std::max(4, s1 + p2 - s2 + 1);
const int32_t precision = p1 - s1 + s2 + scale;
return std::make_pair(precision, scale);
});
}
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