[GitHub] [arrow] bkietz commented on a diff in pull request #35787: GH-35786: [C++] Add pairwise_diff function

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

bkietz commented on code in PR #35787:
URL: https://github.com/apache/arrow/pull/35787#discussion_r1242346402


##########
cpp/src/arrow/compute/api_vector.h:
##########
@@ -603,6 +614,24 @@ Result<Datum> CumulativeSum(
     const CumulativeSumOptions& options = CumulativeSumOptions::Defaults(),
     ExecContext* ctx = NULLPTR);
 
+/// \brief Return the first order difference of an array.
+///
+/// Computes the first order difference of an array, i.e. output[i] = input[i] - input[i -
+/// p] if i >= p, otherwise output[i] = null, where p is the period. For example, with p =
+/// 1, Diff([1, 4, 9, 10, 15]) = [null, 3, 5, 1, 5]. With p = 2, Diff([1, 4, 9, 10, 15]) =
+/// [null, null, 8, 6, 6]

Review Comment:
   Looks like you had formatting here which got munged, guessing:
   ```suggestion
   /// Computes the first order difference of an array, i.e.
   ///   output[i] = input[i] - input[i - p]  if i >= p
   ///   output[i] = null                     otherwise
   /// where p is the period. For example, with p = 1,
   ///   Diff([1, 4, 9, 10, 15]) = [null, 3, 5, 1, 5].
   /// With p = 2,
   ///   Diff([1, 4, 9, 10, 15]) = [null, null, 8, 6, 6]
   ```



##########
docs/source/cpp/compute.rst:
##########
@@ -1835,3 +1835,25 @@ replaced, based on the remaining inputs.
   results in a corresponding null in the output.
 
   Also see: :ref:`if_else <cpp-compute-scalar-selections>`.
+
+Pairwise functions
+~~~~~~~~~~~~~~~~~~~~
+Pairwise functions are unary vector functions that performs a binary operation on 

Review Comment:
   ```suggestion
   Pairwise functions are unary vector functions that perform a binary operation on 
   ```



##########
cpp/src/arrow/compute/kernels/vector_pairwise.cc:
##########
@@ -0,0 +1,272 @@
+// 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.
+
+// Vector kernels for pairwise computation
+
+#include <memory>
+#include "arrow/builder.h"
+#include "arrow/compute/api_vector.h"
+#include "arrow/compute/exec.h"
+#include "arrow/compute/function.h"
+#include "arrow/compute/kernel.h"
+#include "arrow/compute/kernels/base_arithmetic_internal.h"
+#include "arrow/compute/kernels/codegen_internal.h"
+#include "arrow/compute/registry.h"
+#include "arrow/status.h"
+#include "arrow/type.h"
+#include "arrow/type_fwd.h"
+#include "arrow/type_traits.h"
+#include "arrow/util/checked_cast.h"
+#include "arrow/util/logging.h"
+#include "arrow/visit_type_inline.h"
+
+namespace arrow::compute::internal {
+
+template <typename InputType>
+Result<std::shared_ptr<DataType>> GetDiffOutputType(
+    const std::shared_ptr<arrow::DataType>& type) {
+  std::shared_ptr<DataType> output_type;
+  if constexpr (is_timestamp_type<InputType>::value) {  // timestamp -> duration with same
+                                                        // time unit
+    const auto* real_type = checked_cast<const TimestampType*>(type.get());
+    return std::make_shared<DurationType>(real_type->unit());
+  } else if constexpr (is_time_type<InputType>::value) {  // time -> duration with same
+                                                          // time unit
+    const auto* real_type = checked_cast<const InputType*>(type.get());
+    return std::make_shared<DurationType>(real_type->unit());
+  } else if constexpr (is_date_type<InputType>::value) {  // date -> duration
+    if constexpr (InputType::type_id == Type::DATE32) {   // date32 -> second
+      return duration(TimeUnit::SECOND);
+    } else {  // date64 -> millisecond
+      return duration(TimeUnit::MILLI);
+    }
+  } else if constexpr (is_decimal_type<InputType>::value) {  // decimal -> decimal with
+                                                             // precision + 1
+    const auto* real_type = checked_cast<const InputType*>(type.get());
+    if constexpr (InputType::type_id == Type::DECIMAL128) {
+      return Decimal128Type::Make(real_type->precision() + 1, real_type->scale());
+    } else {
+      return Decimal256Type::Make(real_type->precision() + 1, real_type->scale());
+    }
+  } else {
+    return type;
+  }
+}
+
+/// A generic pairwise implementation that can be reused by different Ops.
+template <typename InputType, typename OutputType, typename Op>
+Status PairwiseKernelImpl(const ArraySpan& input, int64_t periods,
+                          const std::shared_ptr<DataType>& output_type,
+                          std::shared_ptr<ArrayData>* result) {
+  typename TypeTraits<OutputType>::BuilderType builder(output_type,
+                                                       default_memory_pool());
+  RETURN_NOT_OK(builder.Reserve(input.length));
+
+  Status status;
+  auto valid_func = [&](typename GetViewType<InputType>::T left,
+                        typename GetViewType<InputType>::T right) {
+    auto result = Op::template Call<typename GetOutputType<OutputType>::T>(
+        nullptr, left, right, &status);
+    builder.UnsafeAppend(result);
+  };
+  auto null_func = [&]() { builder.UnsafeAppendNull(); };
+
+  if (periods > 0) {
+    periods = std::min(periods, input.length);
+    RETURN_NOT_OK(builder.AppendNulls(periods));
+    ArraySpan left(input);
+    left.SetSlice(periods, input.length - periods);
+    ArraySpan right(input);
+    right.SetSlice(0, input.length - periods);
+    VisitTwoArrayValuesInline<InputType, InputType>(left, right, valid_func, null_func);
+    RETURN_NOT_OK(status);
+  } else {
+    periods = std::max(periods, -input.length);
+    ArraySpan left(input);
+    left.SetSlice(0, input.length + periods);
+    ArraySpan right(input);
+    right.SetSlice(-periods, input.length + periods);
+    VisitTwoArrayValuesInline<InputType, InputType>(left, right, valid_func, null_func);
+    RETURN_NOT_OK(status);
+    RETURN_NOT_OK(builder.AppendNulls(-periods));
+  }
+  RETURN_NOT_OK(builder.FinishInternal(result));
+  return Status::OK();
+}
+
+template <typename InputType, typename OutputType, typename Op>
+Status PairwiseDiffKernel(KernelContext* ctx, const ExecSpan& batch, ExecResult* out) {

Review Comment:
   Nit:
   ```suggestion
   Status PairwiseDiffExec(KernelContext* ctx, const ExecSpan& batch, ExecResult* out) {
   ```



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