/* Copyright (c) 2016 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. */

#pragma once
#include <memory.h>
#include <cstring>

#include "paddle/fluid/framework/ddim.h"
#include "paddle/fluid/framework/eigen.h"
#include "paddle/fluid/framework/tensor.h"
#include "paddle/fluid/platform/place.h"

namespace paddle {
namespace operators {

using framework::Tensor;

/**
 * A thin wrapper for gathering on cpu tensor
 * Return a new tensor from source tensor, gathered according to index
 * input[src]: type-T source Tensor
 * input[index]: type-int index Tensor (1-D)
 * return: output tensor
 */
template <typename T>
void CPUGather(const platform::DeviceContext& ctx, const Tensor& src,
               const Tensor& index, Tensor* output) {
  PADDLE_ENFORCE(platform::is_cpu_place(ctx.GetPlace()));
  // check index of shape 1-D
  PADDLE_ENFORCE(index.dims().size() == 1);
  int index_size = index.dims()[0];

  auto src_dims = src.dims();
  framework::DDim output_dims(src_dims);
  output_dims[0] = index_size;

  const T* p_src = src.data<T>();
  const int* p_index = index.data<int>();
  T* p_output = output->data<T>();

  // slice size
  int slice_size = 1;
  for (int i = 1; i < src_dims.size(); ++i) slice_size *= src_dims[i];

  const size_t slice_bytes = slice_size * sizeof(T);

  for (int i = 0; i < index_size; ++i) {
    int index_ = p_index[i];
    memcpy(p_output + i * slice_size, p_src + index_ * slice_size, slice_bytes);
  }
}

}  // namespace operators
}  // namespace paddle
