# Copyright The Lightning AI 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.
import time
from copy import deepcopy
from typing import Callable

import pytest
import torch
import torch.distributed
import torch.nn.functional

from lightning.fabric.fabric import Fabric
from parity_fabric.models import ConvNet
from parity_fabric.utils import (
    cuda_reset,
    get_model_input_dtype,
    is_cuda_memory_close,
    is_state_dict_equal,
    is_timing_close,
    make_deterministic,
)
from tests_fabric.helpers.runif import RunIf


def train_torch(
    move_to_device: Callable,
    precision_context,
    input_dtype=torch.float32,
):
    make_deterministic(warn_only=True)
    memory_stats = {}

    model = ConvNet()
    model = move_to_device(model)
    dataloader = model.get_dataloader()
    optimizer = model.get_optimizer()
    loss_fn = model.get_loss_function()

    memory_stats["start"] = torch.cuda.memory_stats()

    model.train()
    iteration_timings = []
    iterator = iter(dataloader)
    for _ in range(model.num_steps):
        t0 = time.perf_counter()

        inputs, labels = next(iterator)
        inputs, labels = move_to_device(inputs), move_to_device(labels)
        optimizer.zero_grad()
        with precision_context():
            outputs = model(inputs.to(input_dtype))
        loss = loss_fn(outputs.float(), labels)
        loss.backward()
        optimizer.step()

        t1 = time.perf_counter()
        iteration_timings.append(t1 - t0)

    memory_stats["end"] = torch.cuda.memory_stats()

    return model.state_dict(), torch.tensor(iteration_timings), memory_stats


def train_fabric(fabric):
    make_deterministic(warn_only=True)
    memory_stats = {}

    model = ConvNet()
    initial_state_dict = deepcopy(model.state_dict())

    optimizer = model.get_optimizer()
    model, optimizer = fabric.setup(model, optimizer)

    dataloader = model.get_dataloader()
    dataloader = fabric.setup_dataloaders(dataloader)
    loss_fn = model.get_loss_function()

    memory_stats["start"] = torch.cuda.memory_stats()

    model.train()
    iteration_timings = []
    iterator = iter(dataloader)
    for _ in range(model.num_steps):
        t0 = time.perf_counter()

        inputs, labels = next(iterator)
        optimizer.zero_grad()
        outputs = model(inputs)
        loss = loss_fn(outputs, labels)
        fabric.backward(loss)
        optimizer.step()

        t1 = time.perf_counter()
        iteration_timings.append(t1 - t0)

    memory_stats["end"] = torch.cuda.memory_stats()

    # check that the model has changed
    assert not is_state_dict_equal(initial_state_dict, model.state_dict())

    return model.state_dict(), torch.tensor(iteration_timings), memory_stats


@pytest.mark.flaky(reruns=3)
@pytest.mark.usefixtures("reset_deterministic_algorithm", "reset_cudnn_benchmark")
@pytest.mark.parametrize(
    ("precision", "accelerator"),
    [
        (32, "cpu"),
        pytest.param(32, "cuda", marks=RunIf(min_cuda_gpus=1)),
        # pytest.param(16, "cuda", marks=RunIf(min_cuda_gpus=1)),  # TODO: requires GradScaler
        pytest.param("bf16", "cpu", marks=RunIf(skip_windows=True)),
        pytest.param("bf16", "cuda", marks=RunIf(min_cuda_gpus=1, bf16_cuda=True)),
        pytest.param(32, "mps", marks=RunIf(mps=True)),
    ],
)
def test_parity_single_device(precision, accelerator):
    input_dtype = get_model_input_dtype(precision)

    cuda_reset()

    # Train with Fabric
    fabric = Fabric(precision=precision, accelerator=accelerator, devices=1)
    state_dict_fabric, timings_fabric, memory_fabric = train_fabric(fabric)

    cuda_reset()

    # Train with raw PyTorch
    state_dict_torch, timings_torch, memory_torch = train_torch(
        fabric.to_device, precision_context=fabric.autocast, input_dtype=input_dtype
    )

    # Compare the final weights
    assert is_state_dict_equal(state_dict_torch, state_dict_fabric)

    # Compare the time per iteration
    assert is_timing_close(timings_torch, timings_fabric, rtol=1e-2, atol=0.1)

    # Compare memory usage
    if accelerator == "cuda":
        assert is_cuda_memory_close(memory_torch["start"], memory_fabric["start"])
        assert is_cuda_memory_close(memory_torch["end"], memory_fabric["end"])
