[GitHub] [arrow] bkietz commented on a change in pull request #10579: ARROW-11932: [C++] Implement ArrayBuilder::AppendScalar

[GitBox <gi...@apache.org>]

bkietz commented on a change in pull request #10579:
URL: https://github.com/apache/arrow/pull/10579#discussion_r660698340



##########
File path: cpp/src/arrow/array/builder_base.cc
##########
@@ -92,6 +95,139 @@ Status ArrayBuilder::Advance(int64_t elements) {
   return null_bitmap_builder_.Advance(elements);
 }
 
+namespace {
+struct AppendScalarImpl {
+  template <typename T, typename AppendScalar,
+            typename BuilderType = typename TypeTraits<T>::BuilderType,
+            typename ScalarType = typename TypeTraits<T>::ScalarType>
+  Status UseBuilder(const AppendScalar& append) {
+    for (int64_t i = 0; i < n_repeats_; i++) {
+      for (const std::shared_ptr<Scalar>* scalar = scalars_begin_; scalar != scalars_end_;
+           scalar++) {
+        if ((*scalar)->is_valid) {
+          RETURN_NOT_OK(append(internal::checked_cast<const ScalarType&>(**scalar),
+                               static_cast<BuilderType*>(builder_)));
+        } else {
+          RETURN_NOT_OK(builder_->AppendNull());
+        }
+      }
+    }
+    return Status::OK();
+  }
+
+  struct AppendValue {
+    template <typename BuilderType, typename ScalarType>
+    Status operator()(const ScalarType& s, BuilderType* builder) const {
+      return builder->Append(s.value);
+    }
+  };
+
+  struct AppendBuffer {
+    template <typename BuilderType, typename ScalarType>
+    Status operator()(const ScalarType& s, BuilderType* builder) const {
+      const Buffer& buffer = *s.value;
+      return builder->Append(util::string_view{buffer});
+    }
+  };
+
+  struct AppendList {
+    template <typename BuilderType, typename ScalarType>
+    Status operator()(const ScalarType& s, BuilderType* builder) const {
+      RETURN_NOT_OK(builder->Append());
+      const Array& list = *s.value;
+      for (int64_t i = 0; i < list.length(); i++) {
+        ARROW_ASSIGN_OR_RAISE(auto scalar, list.GetScalar(i));
+        RETURN_NOT_OK(builder->value_builder()->AppendScalar(*scalar));
+      }
+      return Status::OK();
+    }
+  };
+
+  template <typename T>
+  enable_if_has_c_type<T, Status> Visit(const T&) {
+    return UseBuilder<T>(AppendValue{});
+  }
+
+  template <typename T>
+  enable_if_has_string_view<T, Status> Visit(const T&) {
+    return UseBuilder<T>(AppendBuffer{});
+  }
+
+  template <typename T>
+  enable_if_decimal<T, Status> Visit(const T&) {
+    return UseBuilder<T>(AppendValue{});
+  }
+

Review comment:
       +1 for reserving AOT for simple types
   
   ```suggestion
     template <typename T>
     enable_if_fixed_width<T, Status> Visit(const T&) {
       auto builder = checked_cast<typename TypeTraits<T>::BuilderType*>(builder_);
       RETURN_NOT_OK(builder->Reserve(n_repeats_ * (scalar_end_ - scalar_begin_)));
   
       for (int64_t i = 0; i < n_repeats_; i++) {
         for (const std::shared_ptr<Scalar>* raw = scalars_begin_; raw != scalars_end_;
              raw++) {
           auto scalar = checked_cast<const typename TypeTraits<T>::ScalarType*>(raw->get());
           if (scalar->is_valid) {
             builder->UnsafeAppend(scalar->value);
           } else {
             builder->UnsafeAppendNull();
           }
         }
       }
       return Status::OK();
     }
   
     template <typename T>
     enable_if_base_binary<T, Status> Visit(const T&) {
       int64_t data_size = 0;
       for (const std::shared_ptr<Scalar>* raw = scalars_begin_; raw != scalars_end_;
              raw++) {
         auto scalar = checked_cast<const typename TypeTraits<T>::ScalarType*>(raw->get());
         if (scalar->is_valid) {
           data_size += scalar->value->size();
         }
       }
   
       auto builder = checked_cast<typename TypeTraits<T>::BuilderType*>(builder_);
       RETURN_NOT_OK(builder->Reserve(n_repeats_ * (scalar_end_ - scalar_begin_)));
       RETURN_NOT_OK(builder->ReserveData(n_repeats_ * data_size));
   
       for (int64_t i = 0; i < n_repeats_; i++) {
         for (const std::shared_ptr<Scalar>* raw = scalars_begin_; raw != scalars_end_;
              raw++) {
           auto scalar = checked_cast<const typename TypeTraits<T>::ScalarType*>(raw->get());
           if (scalar->is_valid) {
             builder->UnsafeAppend(util::string_view{*scalar->value});
           } else {
             builder->UnsafeAppendNull();
           }
         }
       }
       return Status::OK();
     }
   ```




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