[GitHub] [arrow] JabariBooker commented on a diff in pull request #12460: ARROW-13530: [C++] Implement cumulative sum compute function

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

JabariBooker commented on code in PR #12460:
URL: https://github.com/apache/arrow/pull/12460#discussion_r843360774


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cpp/src/arrow/compute/kernels/vector_cumulative_ops.cc:
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@@ -0,0 +1,171 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+#include "arrow/array/array_base.h"
+#include "arrow/compute/api_scalar.h"
+#include "arrow/compute/api_vector.h"
+#include "arrow/compute/cast.h"
+#include "arrow/compute/kernels/base_arithmetic_internal.h"
+#include "arrow/compute/kernels/codegen_internal.h"
+#include "arrow/compute/kernels/common.h"
+#include "arrow/result.h"
+#include "arrow/util/bit_util.h"
+#include "arrow/visit_type_inline.h"
+
+namespace arrow {
+namespace compute {
+namespace internal {
+
+// NOTE: Missing description of this class
+template <typename OutType, typename ArgType, typename Op, typename OptionsType>
+struct CumulativeGeneric {
+  using OutValue = typename GetOutputType<OutType>::T;
+  using ArgValue = typename GetViewType<ArgType>::T;
+
+  static Status Exec(KernelContext* ctx, const ExecBatch& batch, Datum* out) {
+    const auto& options = OptionsWrapper<OptionsType>::Get(ctx);
+    auto skip_nulls = options.skip_nulls;
+
+    // Cast `start` option to match output type
+    // NOTE: Is at least one descriptor guaranteed? If not, need to check size
+    // before indexing.
+    auto out_type = batch.GetDescriptors()[0].type;
+    ARROW_ASSIGN_OR_RAISE(auto cast_value, Cast(Datum(options.start), out_type));
+    auto start = UnboxScalar<OutType>::Unbox(*cast_value.scalar());
+
+    int64_t base_output_offset = 0;
+    bool encounted_null = false;
+    ArrayData* out_arr = out->mutable_array();
+
+    switch (batch[0].kind()) {
+      case Datum::ARRAY: {
+        auto st = Call(ctx, base_output_offset, *batch[0].array(), out_arr, start,
+                       skip_nulls, encounted_null);
+        out_arr->SetNullCount(arrow::kUnknownNullCount);
+        return st;
+      }
+      case Datum::CHUNKED_ARRAY: {
+        const auto& input = batch[0].chunked_array();
+
+        for (const auto& chunk : input->chunks()) {
+          RETURN_NOT_OK(Call(ctx, base_output_offset, *chunk->data(), out_arr, start,
+                             skip_nulls, encounted_null));
+          base_output_offset += chunk->length();
+        }
+        out_arr->SetNullCount(arrow::kUnknownNullCount);
+        return Status::OK();
+      }
+      default:
+        return Status::NotImplemented(
+            "Unsupported input type for function 'cumulative_<operator>': ",
+            batch[0].ToString());
+    }
+  }
+
+  static Status Call(KernelContext* ctx, int64_t base_output_offset,
+                     const ArrayData& input, ArrayData* output, ArgValue& accumulator,
+                     bool skip_nulls, bool& encounted_null) {
+    Status st = Status::OK();
+    auto out_bitmap = output->GetMutableValues<uint8_t>(0);
+    auto out_data = output->GetMutableValues<OutValue>(1) + base_output_offset;
+    auto start_null_idx = input.offset;
+    int64_t curr = 0;
+
+    VisitArrayValuesInline<ArgType>(
+        input,
+        [&](ArgValue v) {
+          if (!skip_nulls && encounted_null) {
+            *out_data++ = OutValue{};
+          } else {
+            accumulator =
+                Op::template Call<OutValue, ArgValue, ArgValue>(ctx, v, accumulator, &st);
+            *out_data++ = accumulator;
+            ++start_null_idx;
+          }
+          ++curr;
+        },
+        [&]() {
+          // null
+          *out_data++ = OutValue{};
+          encounted_null = true;
+          arrow::bit_util::SetBitsTo(out_bitmap, base_output_offset + curr, 1, false);

Review Comment:
   The bitmap for the output seems be set to all true. Therefore, null bits to corresponding null values need to be reset to false. I wonder if the change of dimension from input to output has an affect on this behavior.



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