// Copyright (c) 2022 PaddlePaddle Authors. All Rights Reserved.
//
// Licensed 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 "paddle/phi/kernels/take_along_axis_grad_kernel.h"

#include "paddle/phi/backends/cpu/cpu_context.h"
#include "paddle/phi/common/place.h"
#include "paddle/phi/core/kernel_registry.h"
#include "paddle/phi/kernels/funcs/gather_scatter_functor.h"
#include "paddle/phi/kernels/funcs/math_function.h"

namespace phi {

template <typename T, typename Context>
void TakeAlongAxisGradKernel(const Context& dev_ctx,
                             const DenseTensor& x,
                             const DenseTensor& index,
                             const DenseTensor& out_grad,
                             int axis,
                             DenseTensor* x_grad) {
  PADDLE_ENFORCE_EQ(
      dev_ctx.GetPlace().GetType() == phi::AllocationType::CPU,
      true,
      errors::PreconditionNotMet("This kernel only runs on CPU."));

  // We need to know the shape of input matrix to determine the shape of grad
  // matrix of input.
  x_grad->Resize(x.dims());
  dev_ctx.template Alloc<T>(x_grad);

  // Set to zero tensor.
  phi::funcs::SetConstant<Context, T> functor;
  functor(dev_ctx, x_grad, static_cast<T>(0));

  const auto& index_type = index.dtype();
  if (index_type == phi::DataType::INT32) {
    phi::funcs::cpu_scatter_add_kernel<T, int32_t>(
        *x_grad,
        axis,
        index,
        out_grad,
        dev_ctx);  // the gradient of gather is scatter
  } else if (index_type == phi::DataType::INT64) {
    phi::funcs::cpu_scatter_add_kernel<T, int64_t>(
        *x_grad, axis, index, out_grad, dev_ctx);
  }
}

}  // namespace phi

PD_REGISTER_KERNEL(take_along_axis_grad,
                   CPU,
                   ALL_LAYOUT,
                   phi::TakeAlongAxisGradKernel,
                   float,
                   double,
                   int,
                   uint8_t,
                   int64_t) {}
