From 4505c5399a7aea119e07dded7b54084be713e985 Mon Sep 17 00:00:00 2001
From: David Wendt <45795991+davidwendt@users.noreply.github.com>
Date: Wed, 4 Dec 2024 13:52:38 -0500
Subject: [PATCH] Return empty result for segmented_reduce if input and offsets
 are both empty (#17437)

Changes the behavior of `cudf::segmented_reduce` to return an empty column if both the input and the offsets parameter are empty.
Closes #17433

Authors:
  - David Wendt (https://github.com/davidwendt)

Approvers:
  - Karthikeyan (https://github.com/karthikeyann)
  - Bradley Dice (https://github.com/bdice)
  - Basit Ayantunde (https://github.com/lamarrr)
  - Nghia Truong (https://github.com/ttnghia)

URL: https://github.com/rapidsai/cudf/pull/17437
---
 cpp/src/reductions/segmented/reductions.cpp   |  6 ++++++
 .../reductions/segmented_reduction_tests.cpp  | 20 +++++++++++++++++++
 2 files changed, 26 insertions(+)

diff --git a/cpp/src/reductions/segmented/reductions.cpp b/cpp/src/reductions/segmented/reductions.cpp
index dedfc4b0734..1c3a2b0c0f3 100644
--- a/cpp/src/reductions/segmented/reductions.cpp
+++ b/cpp/src/reductions/segmented/reductions.cpp
@@ -13,6 +13,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+#include <cudf/column/column_factories.hpp>
 #include <cudf/detail/aggregation/aggregation.hpp>
 #include <cudf/detail/nvtx/ranges.hpp>
 #include <cudf/reduction.hpp>
@@ -120,6 +121,11 @@ std::unique_ptr<column> segmented_reduce(column_view const& segmented_values,
     CUDF_FAIL(
       "Initial value is only supported for SUM, PRODUCT, MIN, MAX, ANY, and ALL aggregation types");
   }
+
+  if (segmented_values.is_empty() && offsets.empty()) {
+    return cudf::make_empty_column(output_dtype);
+  }
+
   CUDF_EXPECTS(offsets.size() > 0, "`offsets` should have at least 1 element.");
 
   return cudf::detail::aggregation_dispatcher(
diff --git a/cpp/tests/reductions/segmented_reduction_tests.cpp b/cpp/tests/reductions/segmented_reduction_tests.cpp
index bc0321bd40a..2281a517aa6 100644
--- a/cpp/tests/reductions/segmented_reduction_tests.cpp
+++ b/cpp/tests/reductions/segmented_reduction_tests.cpp
@@ -1122,6 +1122,26 @@ TEST_F(SegmentedReductionTestUntyped, EmptyInputWithOffsets)
   CUDF_TEST_EXPECT_COLUMNS_EQUAL(*result, expect_bool);
 }
 
+TEST_F(SegmentedReductionTestUntyped, EmptyInputEmptyOffsets)
+{
+  auto const str_empty = cudf::test::strings_column_wrapper{};
+  auto const int_empty = cudf::test::fixed_width_column_wrapper<cudf::size_type>{};
+  auto result =
+    cudf::segmented_reduce(str_empty,
+                           cudf::column_view{int_empty},
+                           *cudf::make_max_aggregation<cudf::segmented_reduce_aggregation>(),
+                           cudf::data_type{cudf::type_id::STRING},
+                           cudf::null_policy::EXCLUDE);
+  CUDF_TEST_EXPECT_COLUMNS_EQUAL(*result, str_empty);
+
+  result = cudf::segmented_reduce(int_empty,
+                                  cudf::column_view{int_empty},
+                                  *cudf::make_min_aggregation<cudf::segmented_reduce_aggregation>(),
+                                  cudf::data_type{cudf::type_id::INT32},
+                                  cudf::null_policy::INCLUDE);
+  CUDF_TEST_EXPECT_COLUMNS_EQUAL(*result, int_empty);
+}
+
 template <typename T>
 struct SegmentedReductionFixedPointTest : public cudf::test::BaseFixture {};