From 1339df54b2a9c01a2ff9e5b6fd2297f0f5012528 Mon Sep 17 00:00:00 2001
From: Kevin H Wilson <khwilson@gmail.com>
Date: Mon, 30 Sep 2024 10:04:31 -0400
Subject: [PATCH] extract common code

---
 .../arrow/compute/kernels/aggregate_basic.cc  | 12 +----------
 .../compute/kernels/aggregate_basic.inc.cc    | 21 ++-----------------
 .../arrow/compute/kernels/codegen_internal.cc | 11 ++++++++++
 .../arrow/compute/kernels/codegen_internal.h  |  3 +++
 4 files changed, 17 insertions(+), 30 deletions(-)

diff --git a/cpp/src/arrow/compute/kernels/aggregate_basic.cc b/cpp/src/arrow/compute/kernels/aggregate_basic.cc
index 042eca4747520..00be838550b9d 100644
--- a/cpp/src/arrow/compute/kernels/aggregate_basic.cc
+++ b/cpp/src/arrow/compute/kernels/aggregate_basic.cc
@@ -366,17 +366,7 @@ struct ProductImpl : public ScalarAggregator {
 
   Status Finalize(KernelContext*, Datum* out) override {
     std::shared_ptr<DataType> out_type_;
-    if (out_type->id() == Type::DECIMAL128) {
-      auto cast_type = checked_pointer_cast<Decimal128Type>(this->out_type);
-      ARROW_ASSIGN_OR_RAISE(out_type_, Decimal128Type::Make(Decimal128Type::kMaxPrecision,
-                                                            cast_type->scale()));
-    } else if (out_type->id() == Type::DECIMAL256) {
-      auto cast_type = checked_pointer_cast<Decimal256Type>(this->out_type);
-      ARROW_ASSIGN_OR_RAISE(out_type_, Decimal256Type::Make(Decimal256Type::kMaxPrecision,
-                                                            cast_type->scale()));
-    } else {
-      out_type_ = out_type;
-    }
+    ARROW_ASSIGN_OR_RAISE(out_type_, WidenDecimalToMaxPrecision(this->out_type));
 
     if ((!options.skip_nulls && this->nulls_observed) ||
         (this->count < options.min_count)) {
diff --git a/cpp/src/arrow/compute/kernels/aggregate_basic.inc.cc b/cpp/src/arrow/compute/kernels/aggregate_basic.inc.cc
index d06f83910bf8a..4fa32edd50929 100644
--- a/cpp/src/arrow/compute/kernels/aggregate_basic.inc.cc
+++ b/cpp/src/arrow/compute/kernels/aggregate_basic.inc.cc
@@ -93,17 +93,7 @@ struct SumImpl : public ScalarAggregator {
 
   Status Finalize(KernelContext*, Datum* out) override {
     std::shared_ptr<DataType> out_type_;
-    if (out_type->id() == Type::DECIMAL128) {
-      auto cast_type = checked_pointer_cast<Decimal128Type>(this->out_type);
-      ARROW_ASSIGN_OR_RAISE(out_type_, Decimal128Type::Make(Decimal128Type::kMaxPrecision,
-                                                            cast_type->scale()));
-    } else if (out_type->id() == Type::DECIMAL256) {
-      auto cast_type = checked_pointer_cast<Decimal256Type>(this->out_type);
-      ARROW_ASSIGN_OR_RAISE(out_type_, Decimal256Type::Make(Decimal256Type::kMaxPrecision,
-                                                            cast_type->scale()));
-    } else {
-      out_type_ = out_type;
-    }
+    ARROW_ASSIGN_OR_RAISE(out_type_, WidenDecimalToMaxPrecision(this->out_type));
 
     if ((!options.skip_nulls && this->nulls_observed) ||
         (this->count < options.min_count)) {
@@ -233,14 +223,7 @@ struct MeanImpl<ArrowType, SimdLevel, enable_if_decimal<ArrowType>>
   template <typename T = ArrowType>
   Status FinalizeImpl(Datum* out) {
     std::shared_ptr<DataType> out_type_;
-    auto decimal_type = checked_pointer_cast<DecimalType>(this->out_type);
-    if (decimal_type->id() == Type::DECIMAL128) {
-      ARROW_ASSIGN_OR_RAISE(out_type_, Decimal128Type::Make(Decimal128Type::kMaxPrecision,
-                                                            decimal_type->scale()));
-    } else {
-      ARROW_ASSIGN_OR_RAISE(out_type_, Decimal256Type::Make(Decimal256Type::kMaxPrecision,
-                                                            decimal_type->scale()));
-    }
+    ARROW_ASSIGN_OR_RAISE(out_type_, WidenDecimalToMaxPrecision(this->out_type));
 
     if ((!options.skip_nulls && this->nulls_observed) ||
         (this->count < options.min_count) || (this->count == 0)) {
diff --git a/cpp/src/arrow/compute/kernels/codegen_internal.cc b/cpp/src/arrow/compute/kernels/codegen_internal.cc
index fa29c622e7241..fe46fc9da031b 100644
--- a/cpp/src/arrow/compute/kernels/codegen_internal.cc
+++ b/cpp/src/arrow/compute/kernels/codegen_internal.cc
@@ -547,6 +547,17 @@ Status CastDecimalArgs(TypeHolder* begin, size_t count) {
   return Status::OK();
 }
 
+Result<std::shared_ptr<DataType>> WidenDecimalToMaxPrecision(std::shared_ptr<DataType> type) {
+  if (type->id() == Type::DECIMAL128) {
+    auto cast_type = checked_pointer_cast<Decimal128Type>(type);
+    return Decimal128Type::Make(Decimal128Type::kMaxPrecision, cast_type->scale());
+  } else if (type->id() == Type::DECIMAL256) {
+    auto cast_type = checked_pointer_cast<Decimal256Type>(type);
+    return Decimal256Type::Make(Decimal256Type::kMaxPrecision, cast_type->scale());
+  }
+  return type;
+}
+
 bool HasDecimal(const std::vector<TypeHolder>& types) {
   for (const auto& th : types) {
     if (is_decimal(th.id())) {
diff --git a/cpp/src/arrow/compute/kernels/codegen_internal.h b/cpp/src/arrow/compute/kernels/codegen_internal.h
index 29ab2c70e46c4..6eee2cac40eef 100644
--- a/cpp/src/arrow/compute/kernels/codegen_internal.h
+++ b/cpp/src/arrow/compute/kernels/codegen_internal.h
@@ -55,6 +55,7 @@ using internal::BinaryBitBlockCounter;
 using internal::BitBlockCount;
 using internal::BitmapReader;
 using internal::checked_cast;
+using internal::checked_pointer_cast;
 using internal::FirstTimeBitmapWriter;
 using internal::GenerateBitsUnrolled;
 using internal::VisitBitBlocks;
@@ -1382,6 +1383,8 @@ Status CastBinaryDecimalArgs(DecimalPromotion promotion, std::vector<TypeHolder>
 ARROW_EXPORT
 Status CastDecimalArgs(TypeHolder* begin, size_t count);
 
+Result<std::shared_ptr<DataType>> WidenDecimalToMaxPrecision(std::shared_ptr<DataType> type);
+
 ARROW_EXPORT
 bool HasDecimal(const std::vector<TypeHolder>& types);