[GitHub] [arrow] icexelloss commented on a diff in pull request #34912: GH-34911: [C++] [WIP] Add first and last aggregator

["icexelloss (via GitHub)" <gi...@apache.org>]

icexelloss commented on code in PR #34912:
URL: https://github.com/apache/arrow/pull/34912#discussion_r1159059588


##########
cpp/src/arrow/compute/kernels/aggregate_basic_internal.h:
##########
@@ -272,8 +273,120 @@ struct MeanKernelInit : public SumLikeInit<KernelClass> {
 };
 
 // ----------------------------------------------------------------------
-// MinMax implementation
+// Last implementation
+template <typename ArrowType, SimdLevel::type SimdLevel, typename Enable = void>
+struct FirstLastState {};
+
+template <typename ArrowType, SimdLevel::type SimdLevel>
+struct FirstLastState<ArrowType, SimdLevel, enable_if_floating_point<ArrowType>> {
+  using ThisType = FirstLastState<ArrowType, SimdLevel>;
+  using T = typename ArrowType::c_type;
+  using ScalarType = typename TypeTraits<ArrowType>::ScalarType;
+
+  ThisType& operator+=(const ThisType& rhs) {
+    this->has_nulls |= rhs.has_nulls;
+    this->first = this->first.has_value() ? this->first : rhs.first;
+    this->last = rhs.last.has_value() ? rhs.last : this->last;
+    return *this;
+  }
+
+  void MergeOne(T value) {
+    if (!this->first.has_value()) {
+      this->first = value;
+    }
+    this->last = value;
+  }
+
+  std::optional<T> first = std::nullopt;

Review Comment:
   WIP: Probably want to make this just T and have a bool `has_values` similar to the hash aggregator version.



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