// Copyright (c) 2026 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 <ATen/core/Tensor.h>

namespace at {

inline at::Tensor narrow(const at::Tensor& self,
                         int64_t dim,
                         int64_t start,
                         int64_t length) {
  // Bounds checks matching PyTorch behavior
  PD_CHECK(self.dim() > 0, "narrow() cannot be applied to a 0-dim tensor.");
  PD_CHECK(length >= 0, "narrow(): length must be non-negative.");

  // Normalize negative dim
  int64_t ndim = self.dim();
  if (dim < 0) dim += ndim;
  PD_CHECK(dim >= 0 && dim < ndim,
           "start out of range (expected to be in range of [",
           -ndim,
           ", ",
           ndim - 1,
           "], but got ",
           dim,
           ")");

  int64_t cur_size = self.sizes()[dim];

  // Wrap negative start (matching PyTorch: only wrap when start != cur_size)
  if (start < 0) {
    start = start + cur_size;
  }
  PD_CHECK(start <= cur_size - length,
           "start (",
           start,
           ") + length (",
           length,
           ") exceeds dimension size (",
           cur_size,
           ").");

  // Use slice to implement narrow: narrow(dim, start, length) is equivalent
  // to slice(dim, start, start + length)
  return Tensor(paddle::experimental::slice(
      self._PD_GetInner(), {dim}, {start}, {start + length}, {1}, {}));
}

inline at::Tensor narrow_symint(const at::Tensor& self,
                                int64_t dim,
                                c10::SymInt start,
                                c10::SymInt length) {
  return narrow(self, dim, start, length);
}

inline at::Tensor narrow(const at::Tensor& self,
                         int64_t dim,
                         const at::Tensor& start,
                         int64_t length) {
  // Extract scalar value from start tensor
  PD_CHECK(start.numel() == 1,
           "start must be a 0-dim tensor or 1-element tensor");
  int64_t start_val = start.item<int64_t>();
  return narrow(self, dim, start_val, length);
}

inline at::Tensor narrow_symint(const at::Tensor& self,
                                int64_t dim,
                                const at::Tensor& start,
                                c10::SymInt length) {
  return narrow(self, dim, start, length);
}

}  // namespace at

namespace at {

inline at::Tensor Tensor::narrow(int64_t dim,
                                 int64_t start,
                                 int64_t length) const {
  return at::narrow(*this, dim, start, length);
}

inline at::Tensor Tensor::narrow_symint(int64_t dim,
                                        c10::SymInt start,
                                        c10::SymInt length) const {
  return at::narrow_symint(*this, dim, start, length);
}

inline at::Tensor Tensor::narrow(int64_t dim,
                                 const at::Tensor& start,
                                 int64_t length) const {
  return at::narrow(*this, dim, start, length);
}

inline at::Tensor Tensor::narrow_symint(int64_t dim,
                                        const at::Tensor& start,
                                        c10::SymInt length) const {
  return at::narrow_symint(*this, dim, start, length);
}

}  // namespace at