/* Copyright (c) 2016 PaddlePaddle Authors. All Rights Reserve.

   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/operators/math/cross_entropy.h"

namespace paddle {
namespace operators {
namespace math {

namespace {
template <typename T>
__global__ void CrossEntropyKernel(T* Y, const T* X, const int* label,
                                   const int N, const int D) {
  for (int i = blockIdx.x * blockDim.x + threadIdx.x; i < N;
       i += blockDim.x * gridDim.x) {
    PADDLE_ASSERT(label[i] >= 0 && label[i] < D);
    Y[i] = -math::TolerableValue<T>()(log(X[i * D + label[i]]));
  }
}

template <typename T>
__device__ __forceinline__ T sum_single_warp(T val) {
  val += __shfl_down(val, 16);
  val += __shfl_down(val, 8);
  val += __shfl_down(val, 4);
  val += __shfl_down(val, 2);
  val += __shfl_down(val, 1);
  return val;
}

template <typename T>
__global__ void SoftCrossEntropyKernel(T* Y, const T* X, const T* label,
                                       const int class_num) {
  int tid = threadIdx.x;
  extern __shared__ T d_sum[];
  d_sum[tid] = 0;

  int cur_idx = tid;
  int next_idx = blockIdx.x * class_num + tid;
  while (cur_idx < class_num) {
    d_sum[tid] +=
        math::TolerableValue<T>()(std::log(X[next_idx])) * label[next_idx];
    next_idx += blockDim.x;
    cur_idx += blockDim.x;
  }
  __syncthreads();

  for (unsigned int stride = blockDim.x >> 1; stride >= 32; stride >>= 1) {
    if (tid < stride) d_sum[tid] += d_sum[tid + stride];
    __syncthreads();
  }

  T val = d_sum[tid];
  val = sum_single_warp<T>(val);
  if (tid == 0) Y[blockIdx.x] = -val;
}
}  // namespace

using Tensor = framework::Tensor;

template <typename T>
class CrossEntropyFunctor<platform::GPUPlace, T> {
 public:
  void operator()(const platform::DeviceContext& ctx, framework::Tensor* out,
                  const framework::Tensor* prob,
                  const framework::Tensor* labels, bool softLabel) {
    const T* prob_data = prob->data<T>();
    T* loss_data = out->mutable_data<T>(ctx.GetPlace());

    int batch_size = prob->dims()[0];
    int class_num = prob->dims()[1];

    if (softLabel) {
      const T* label_data = labels->data<T>();
      int block = class_num > 512 ? 512 : pow(2, int(std::log2(class_num)));

      SoftCrossEntropyKernel<T><<<
          batch_size, block, block * sizeof(T),
          reinterpret_cast<const platform::CUDADeviceContext&>(ctx).stream()>>>(
          loss_data, prob_data, label_data, class_num);
    } else {
      const int* label_data = labels->data<int>();
      int block = 512;
      int grid = (batch_size + block - 1) / block;
      CrossEntropyKernel<T><<<
          grid, block, 0,
          reinterpret_cast<const platform::CUDADeviceContext&>(ctx).stream()>>>(
          loss_data, prob_data, label_data, batch_size, class_num);
    }
  }
};

template class CrossEntropyFunctor<platform::GPUPlace, float>;
}  // namespace math
}  // namespace operators
}  // namespace paddle
