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// 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,
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// 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 <iostream>
#include <memory>

#include "arrow/array/builder_base.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/compute/util.h"
#include "arrow/status.h"
#include "arrow/type.h"
#include "arrow/type_fwd.h"
#include "arrow/type_traits.h"
#include "arrow/util/bit_util.h"
#include "arrow/util/checked_cast.h"
#include "arrow/util/logging.h"

namespace arrow::compute::internal {
namespace {

// We reuse the kernel exec function of a scalar binary function to compute pairwise
// results. For example, for pairwise_diff, we reuse subtract's kernel exec.
struct PairwiseState : KernelState {
  PairwiseState(const PairwiseOptions& options, ArrayKernelExec scalar_exec)
      : periods(options.periods), scalar_exec(scalar_exec) {}

  int64_t periods;
  ArrayKernelExec scalar_exec;
};

/// A generic pairwise implementation that can be reused by different ops.
Status PairwiseExecImpl(KernelContext* ctx, const ArraySpan& input,
                        const ArrayKernelExec& scalar_exec, int64_t periods,
                        ArrayData* result) {
  // We only compute values in the region where the input-with-offset overlaps
  // the original input. The margin where these do not overlap gets filled with null.
  const auto margin_length = std::min(abs(periods), input.length);
  const auto computed_length = input.length - margin_length;
  const auto computed_start = periods > 0 ? margin_length : 0;
  const auto left_start = computed_start;
  const auto right_start = margin_length - computed_start;
  // prepare null bitmap
  int64_t null_count = margin_length;
  for (int64_t i = computed_start; i < computed_start + computed_length; i++) {
    if (input.IsValid(i) && input.IsValid(i - periods)) {
      bit_util::SetBit(result->buffers[0]->mutable_data(), i);
    } else {
      ++null_count;
    }
  }
  result->null_count = null_count;
  // prepare input span
  ArraySpan left(input);
  left.SetSlice(left_start, computed_length);
  ArraySpan right(input);
  right.SetSlice(right_start, computed_length);
  // prepare output span
  ArraySpan output_span;
  output_span.SetMembers(*result);
  output_span.offset = computed_start;
  output_span.length = computed_length;
  ExecResult output{output_span};
  // execute scalar function
  RETURN_NOT_OK(scalar_exec(ctx, ExecSpan({left, right}, computed_length), &output));

  return Status::OK();
}

Status PairwiseExec(KernelContext* ctx, const ExecSpan& batch, ExecResult* out) {
  const auto& state = checked_cast<const PairwiseState&>(*ctx->state());
  auto input = batch[0].array;

  // The scalar diff kernel will only write into the non-null output area.
  // We must therefore pre-initialize the output, otherwise the left or right
  // margin would be left uninitialized.
  ARROW_ASSIGN_OR_RAISE(auto builder,
                        MakeBuilder(out->type()->GetSharedPtr(), ctx->memory_pool()));
  // Append nulls rather than empty values, so as to allocate a null bitmap.
  RETURN_NOT_OK(builder->AppendNulls(out->length()));
  std::shared_ptr<ArrayData> out_data;
  RETURN_NOT_OK(builder->FinishInternal(&out_data));
  out_data->null_count = kUnknownNullCount;
  out->value = std::move(out_data);

  return PairwiseExecImpl(ctx, batch[0].array, state.scalar_exec, state.periods,
                          out->array_data_mutable());
}

const FunctionDoc pairwise_diff_doc(
    "Compute first order difference of an array",
    ("Computes the first order difference of an array, It internally calls \n"
     "the scalar function \"subtract\" to compute \n differences, so its \n"
     "behavior and supported types are the same as \n"
     "\"subtract\". The period can be specified in :struct:`PairwiseOptions`.\n"
     "\n"
     "Results will wrap around on integer overflow. Use function \n"
     "\"pairwise_diff_checked\" if you want overflow to return an error."),
    {"input"}, "PairwiseOptions");

const FunctionDoc pairwise_diff_checked_doc(
    "Compute first order difference of an array",
    ("Computes the first order difference of an array, It internally calls \n"
     "the scalar function \"subtract_checked\" (or the checked variant) to compute \n"
     "differences, so its behavior and supported types are the same as \n"
     "\"subtract_checked\". The period can be specified in :struct:`PairwiseOptions`.\n"
     "\n"
     "This function returns an error on overflow. For a variant that doesn't \n"
     "fail on overflow, use function \"pairwise_diff\"."),
    {"input"}, "PairwiseOptions");

const PairwiseOptions* GetDefaultPairwiseOptions() {
  static const auto kDefaultPairwiseOptions = PairwiseOptions::Defaults();
  return &kDefaultPairwiseOptions;
}

void RegisterPairwiseDiffKernels(std::string_view func_name,
                                 std::string_view base_func_name, const FunctionDoc& doc,
                                 FunctionRegistry* registry) {
  VectorKernel kernel;
  kernel.can_execute_chunkwise = false;
  kernel.null_handling = NullHandling::COMPUTED_PREALLOCATE;
  kernel.mem_allocation = MemAllocation::NO_PREALLOCATE;
  kernel.init = OptionsWrapper<PairwiseOptions>::Init;
  auto func = std::make_shared<VectorFunction>(std::string(func_name), Arity::Unary(),
                                               doc, GetDefaultPairwiseOptions());

  auto base_func_result = registry->GetFunction(std::string(base_func_name));
  DCHECK_OK(base_func_result.status());
  const auto& base_func = checked_cast<const ScalarFunction&>(**base_func_result);
  DCHECK_EQ(base_func.arity().num_args, 2);

  for (const auto& base_func_kernel : base_func.kernels()) {
    const auto& base_func_kernel_sig = base_func_kernel->signature;
    if (!base_func_kernel_sig->in_types()[0].Equals(
            base_func_kernel_sig->in_types()[1])) {
      continue;
    }
    OutputType out_type(base_func_kernel_sig->out_type());
    // Need to wrap base output resolver
    if (out_type.kind() == OutputType::COMPUTED) {
      out_type =
          OutputType([base_resolver = base_func_kernel_sig->out_type().resolver()](
                         KernelContext* ctx, const std::vector<TypeHolder>& input_types) {
            return base_resolver(ctx, {input_types[0], input_types[0]});
          });
    }

    kernel.signature =
        KernelSignature::Make({base_func_kernel_sig->in_types()[0]}, out_type);
    kernel.exec = PairwiseExec;
    kernel.init = [scalar_exec = base_func_kernel->exec](KernelContext* ctx,
                                                         const KernelInitArgs& args) {
      return std::make_unique<PairwiseState>(
          checked_cast<const PairwiseOptions&>(*args.options), scalar_exec);
    };
    DCHECK_OK(func->AddKernel(kernel));
  }

  DCHECK_OK(registry->AddFunction(std::move(func)));
}

}  // namespace

void RegisterVectorPairwise(FunctionRegistry* registry) {
  RegisterPairwiseDiffKernels("pairwise_diff", "subtract", pairwise_diff_doc, registry);
  RegisterPairwiseDiffKernels("pairwise_diff_checked", "subtract_checked",
                              pairwise_diff_checked_doc, registry);
}

}  // namespace arrow::compute::internal
