Fix segmented_reduce on empty column with non-empty offsets

cpp/src/reductions/segmented_reductions.cpp

@@ -81,7 +81,6 @@ std::unique_ptr<column> segmented_reduce(column_view const& segmented_values,
                                          rmm::mr::device_memory_resource* mr)
 {
   CUDF_EXPECTS(offsets.size() > 0, "`offsets` should have at least 1 element.");
-  if (segmented_values.is_empty()) { return empty_like(segmented_values); }
 
   return aggregation_dispatcher(
     agg.kind,

cpp/tests/reductions/rank_tests.cpp

@@ -55,8 +55,6 @@ struct TypedRankScanTest : BaseScanTest<T> {
                                        std::unique_ptr<scan_aggregation> const& agg)
   {
     auto col_out = cudf::scan(input, agg, INCLUSIVE_SCAN, INCLUDE_NULLS);
-    std::cout << "expect type: " << static_cast<int>(expect_vals.type().id()) << std::endl;
-    std::cout << "out type: " << static_cast<int>(col_out->type().id()) << std::endl;
     CUDF_TEST_EXPECT_COLUMNS_EQUIVALENT(expect_vals, col_out->view());
   }
 };

cpp/tests/reductions/segmented_reduction_tests.cpp

@@ -323,7 +323,7 @@ TYPED_TEST(SegmentedReductionTest, AllIncludeNulls)
   CUDF_TEST_EXPECT_COLUMNS_EQUAL(*res, expect);
 }
 
-TEST_F(SegmentedReductionTestUntyped, PartialSegmentReudction)
+TEST_F(SegmentedReductionTestUntyped, PartialSegmentReduction)
 {
   // Segmented reduction allows offsets only specify part of the input columns.
   // [1], [2, 3], [4]
@@ -387,6 +387,47 @@ TEST_F(SegmentedReductionTestUntyped, ReduceEmptyColumn)
   CUDF_TEST_EXPECT_COLUMNS_EQUAL(*res, expect);
 }
 
+TEST_F(SegmentedReductionTestUntyped, EmptyInputWithOffsets)
+{
+  // values:    {}
+  // offsets:   {0, 0}
+  // outputs:   {XXX}
+  // output nullmask: {0}
+  auto input     = fixed_width_column_wrapper<int32_t>{};
+  auto offsets   = std::vector<size_type>{0, 0};
+  auto d_offsets = thrust::device_vector<size_type>(offsets);
+  auto expect    = fixed_width_column_wrapper<int32_t>{{XXX}, {0}};
+
+  auto aggregates =
+    std::vector<std::unique_ptr<cudf::segmented_reduce_aggregation,
+                                std::default_delete<cudf::segmented_reduce_aggregation>>>();
+  aggregates.push_back(std::move(make_max_aggregation<segmented_reduce_aggregation>()));
+  aggregates.push_back(std::move(make_min_aggregation<segmented_reduce_aggregation>()));
+  aggregates.push_back(std::move(make_sum_aggregation<segmented_reduce_aggregation>()));
+  aggregates.push_back(std::move(make_product_aggregation<segmented_reduce_aggregation>()));
+
+  auto output_type = data_type{type_to_id<int32_t>()};
+  for (auto&& agg : aggregates) {
+    auto result = segmented_reduce(input, d_offsets, *agg, output_type, null_policy::EXCLUDE);
+    CUDF_TEST_EXPECT_COLUMNS_EQUAL(*result, expect);
+  }
+
+  auto expect_bool = fixed_width_column_wrapper<bool>{{XXX}, {0}};
+
+  auto result = segmented_reduce(input,
+                                 d_offsets,
+                                 *make_any_aggregation<segmented_reduce_aggregation>(),
+                                 data_type{type_id::BOOL8},
+                                 null_policy::INCLUDE);
+  CUDF_TEST_EXPECT_COLUMNS_EQUAL(*result, expect_bool);
+  result = segmented_reduce(input,
+                            d_offsets,
+                            *make_all_aggregation<segmented_reduce_aggregation>(),
+                            data_type{type_id::BOOL8},
+                            null_policy::INCLUDE);
+  CUDF_TEST_EXPECT_COLUMNS_EQUAL(*result, expect_bool);
+}
+
 template <typename T>
 struct SegmentedReductionFixedPointTest : public cudf::test::BaseFixture {
 };
@@ -714,6 +755,27 @@ TEST_F(SegmentedReductionStringTest, MinExcludeNulls)
   CUDF_TEST_EXPECT_COLUMNS_EQUAL(*res, expect);
 }
 
+TEST_F(SegmentedReductionStringTest, EmptyInputWithOffsets)
+{
+  auto input     = strings_column_wrapper{};
+  auto offsets   = std::vector<size_type>{0, 0, 0, 0};
+  auto d_offsets = thrust::device_vector<size_type>(offsets);
+  auto expect    = strings_column_wrapper({XXX, XXX, XXX}, {0, 0, 0});
+
+  auto result = segmented_reduce(input,
+                                 d_offsets,
+                                 *make_max_aggregation<segmented_reduce_aggregation>(),
+                                 data_type{type_id::STRING},
+                                 null_policy::EXCLUDE);
+  CUDF_TEST_EXPECT_COLUMNS_EQUAL(*result, expect);
+  result = segmented_reduce(input,
+                            d_offsets,
+                            *make_min_aggregation<segmented_reduce_aggregation>(),
+                            data_type{type_id::STRING},
+                            null_policy::INCLUDE);
+  CUDF_TEST_EXPECT_COLUMNS_EQUAL(*result, expect);
+}
+
 #undef XXX
 
 }  // namespace test