// 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/where_index_kernel.h"

#include "paddle/phi/core/dense_tensor.h"
#include "paddle/phi/core/kernel_registry.h"
#include "paddle/phi/kernels/funcs/for_range.h"
#include "paddle/phi/kernels/funcs/math_function.h"

namespace phi {

template <typename T>
struct WhereIndexFunctor {
  WhereIndexFunctor(
      const T* true_index, int true_num, const T* stride, int rank, T* out)
      : true_index_(true_index),
        true_num_(true_num),
        stride_(stride),
        rank_(rank),
        out_ptr_(out) {}

  HOSTDEVICE void operator()(size_t idx) const {
    T index = true_index_[idx];
    for (int j = 0; j < rank_; j++) {
      out_ptr_[idx * rank_ + j] = index / stride_[j];
      index -= out_ptr_[idx * rank_ + j] * stride_[j];
    }
  }

  const T* true_index_;
  int true_num_;
  const T* stride_;
  int rank_;
  T* out_ptr_;
};

template <typename T, typename Context>
void WhereIndexKernel(const Context& dev_ctx,
                      const DenseTensor& condition,
                      DenseTensor* out) {
  const T* cond_data = condition.data<T>();
  auto numel = condition.numel();
  auto dims = condition.dims();
  const int rank = dims.size();

  std::vector<int64_t> true_index;
  for (auto i = 0; i < numel; i++) {
    if (static_cast<bool>(cond_data[i])) {
      true_index.push_back(i);
    }
  }
  auto true_num = true_index.size();
  out->Resize(phi::make_ddim({static_cast<int64_t>(true_num), rank}));
  auto* out_ptr = dev_ctx.template Alloc<int64_t>(out);

  if (true_num == 0) {
    return;
  }

  std::vector<int64_t> stride(rank);
  stride[rank - 1] = 1;
  for (int i = rank - 2; i >= 0; i--) {
    stride[i] = stride[i + 1] * dims[i + 1];
  }

  WhereIndexFunctor<int64_t> functor(
      true_index.data(), true_num, stride.data(), rank, out_ptr);
  phi::funcs::ForRange<phi::CPUContext> for_range(dev_ctx, true_num);
  for_range(functor);
}

}  // namespace phi

PD_REGISTER_KERNEL(where_index,
                   CPU,
                   ALL_LAYOUT,
                   phi::WhereIndexKernel,
                   int64_t,
                   int,
                   int16_t,
                   bool,
                   float,
                   double) {}