# Copyright The PyTorch Lightning team.
#
# 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.
from unittest.mock import MagicMock, Mock

import pytest
import torch
from torch.nn.parallel import DistributedDataParallel

from lightning_lite.strategies import DDPStrategy
from lightning_lite.strategies.ddp import _DDPBackwardSyncControl


@pytest.mark.parametrize(
    ["process_group_backend", "device_str", "expected_process_group_backend"],
    [
        pytest.param("foo", "cpu", "foo"),
        pytest.param("foo", "cuda:0", "foo"),
        pytest.param(None, "cuda:0", "nccl"),
        pytest.param(None, "cpu", "gloo"),
    ],
)
def test_ddp_process_group_backend(process_group_backend, device_str, expected_process_group_backend):
    """Test settings for process group backend."""

    class MockDDPStrategy(DDPStrategy):
        def __init__(self, root_device, process_group_backend):
            self._root_device = root_device
            super().__init__(process_group_backend=process_group_backend)

        @property
        def root_device(self):
            return self._root_device

    strategy = MockDDPStrategy(process_group_backend=process_group_backend, root_device=torch.device(device_str))
    assert strategy._get_process_group_backend() == expected_process_group_backend


def test_ddp_no_backward_sync():
    """Test that the backward sync control calls `.no_sync()`, and only on a DDP-wrapped module."""
    strategy = DDPStrategy()
    assert isinstance(strategy._backward_sync_control, _DDPBackwardSyncControl)

    with pytest.raises(
        TypeError, match="is only possible if the module passed to .* is wrapped in `DistributedDataParallel`"
    ):
        with strategy._backward_sync_control.no_backward_sync(Mock()):
            pass

    module = MagicMock(spec=DistributedDataParallel)
    with strategy._backward_sync_control.no_backward_sync(module):
        pass

    module.no_sync.assert_called_once()