#define TORCH_ASSERT_ONLY_METHOD_OPERATORS
#include <ATen/core/Tensor.h>
#include <ATen/Dispatch.h>
#include <ATen/Parallel.h>
#include <ATen/cpu/vec/vec.h>
#include <ATen/native/UnfoldBackward.h>
#include <ATen/native/cpu/Loops.h>
#include <c10/util/irange.h>

#if (defined(_WIN32) || defined(_WIN64))
#define RESTRICT __restrict
#else
#define RESTRICT __restrict__
#endif

// Note on naming: it is unconventional.
// grad_in does not mean that it is a gradient wrt to input,
// grad_in/grad_out is just an input/output of unfold_backward kernel.
//
// unfold_backward, the algorithm.
//
// Consider out = in.unfold(dim, size, step), then
// out.shape[dim] == (in.shape[dim] - size) / step + 1,
// out.shape[-1] == size.
// out.dims() == in.dims() + 1
//
// unfold_backward receives grad_in and returns grad_out such that
// grad_in.shape == out.shape,
// grad_out.shape = in.shape.
//
// unfold_backward considers the following two cases:
// case1. step >= size.
// case2. step < size.
//
// case1. step >= size.
// In this case the iteration takes over grad_in and performs the following copy:
// grad_out[..., i_out_dim,...] = grad_in[..., i_in_dim,..., i_in_last_dim],
// where i_out_dim = i_in_dim * step + i_in_last_dim.
//
// case2. step < size.
// In this case the iteration takes over grad_out,
// where grad_out[...,i_out_dim,...] accumulates all values
// grad_in[...,i_in_dim,...,i_in_last_dim], where
// i_in_dim is in [left_idx_fold, right_idx_fold],
// i_in_last_dim = i_out_dim - i_in_dim * step,
// left_idx_fold = (i_out_dim - size) / step
//  if i_out_dim in [left_idx_fold * step, left_idx_fold * step + size)
//  else (i_out_dim - size) / step + 1,
// right_idx_fold = i_out_dim / step.
//
// Simply put, given i_out_dim, we find which folds of grad_in
// intersect with i_out_dim, these are precisely [left_idx_fold, right_idx_fold],
// and then the corresponding value of grad_in[...,i_in_dim,...,i_in_last_dim]
// gets added up to grad_out[...,i_out_dim,...].

namespace at::native {

namespace {

template <typename scalar_t>
void _unfold_backward_internal_kernel(
  TensorIterator& iter,
  int64_t size,
  int64_t step,
  int64_t grad_in_dim_stride,
  int64_t grad_in_last_dim_stride,
  int64_t grad_in_dim_size,
  int64_t grad_out_dim_stride
) {
  if (iter.numel() == 0) {
    return;
  }

  auto loop = [&](char** data, const int64_t* strides, int64_t nelems) {
    auto* RESTRICT grad_out_ptr = data[0];
    auto* RESTRICT grad_in_ptr = data[1];
    auto* RESTRICT idx_dim_ptr = data[2];

    for ([[maybe_unused]] const auto elem : c10::irange(nelems)) {
      auto* RESTRICT grad_out_data = reinterpret_cast<scalar_t*>(grad_out_ptr);
      auto* RESTRICT grad_in_data = reinterpret_cast<scalar_t*>(grad_in_ptr);

      auto idx_dim = *reinterpret_cast<int64_t*>(idx_dim_ptr);

      // left_fold potentially intersecting with idx_dim
      // is either (idx_dim - size) / step or the next integer.
      int64_t left_fold_idx = (idx_dim > size) ? (idx_dim - size) / step : 0;
      if (!(left_fold_idx * step <= idx_dim && idx_dim < left_fold_idx * step + size)) {
        ++left_fold_idx;
      }

      auto right_fold_idx = idx_dim / step;
      right_fold_idx = (right_fold_idx >= grad_in_dim_size)
        ? (grad_in_dim_size - 1) : right_fold_idx;

      for (auto fold_idx = left_fold_idx; fold_idx <= right_fold_idx; ++fold_idx) {
        auto idx_last_dim = idx_dim - fold_idx * step;
        *grad_out_data += grad_in_data[fold_idx * grad_in_dim_stride
                                    + idx_last_dim * grad_in_last_dim_stride];
      }

      grad_out_ptr += strides[0];
      grad_in_ptr += strides[1];
      idx_dim_ptr += strides[2];
    }
  };

  iter.for_each(loop);
}

void unfold_backward_cpu_kernel(
  Tensor& grad_out,
  const Tensor& grad_in,
  int64_t dim,
  int64_t size,
  int64_t step
) {
  dim = maybe_wrap_dim(dim, grad_out.dim());
  // last dim stores the folds
  auto last_dim = maybe_wrap_dim(-1, grad_in.dim());

  auto grad_in_dim_stride = ensure_nonempty_stride(grad_in, dim);
  auto grad_in_last_dim_stride = ensure_nonempty_stride(grad_in, last_dim);
  auto grad_in_dim_size = ensure_nonempty_size(grad_in, dim);

  auto grad_out_dim_stride = ensure_nonempty_stride(grad_out, dim);

  TensorIterator iter = _make_unfold_backward_iter_over_grad_out(
      grad_out, grad_in, dim, size, step);

  AT_DISPATCH_ALL_TYPES_AND_COMPLEX_AND3(
    at::ScalarType::Half, at::ScalarType::Bool, at::ScalarType::BFloat16,
    iter.dtype(),
    "unfold_backward_cpu", [&] {
      _unfold_backward_internal_kernel<scalar_t>(
        iter,
        size,
        step,
        grad_in_dim_stride,
        grad_in_last_dim_stride,
        grad_in_dim_size,
        grad_out_dim_stride
      );
    }
  );
}

}

REGISTER_DISPATCH(unfold_backward_stub, &unfold_backward_cpu_kernel)

} // namespace at::native
