# -*- coding: utf-8 -*-

import torch
from typing import Callable, Iterable, Sequence, Union


def checkpoint(
    func: Callable[..., Union[torch.Tensor, Sequence[torch.Tensor]]],
    inputs: Sequence[torch.Tensor],
    params: Iterable[torch.Tensor],
    flag: bool,
    use_deepspeed: bool = False
):
    # Evaluate a function without caching intermediate activations, allowing for
    # reduced memory at the expense of extra compute in the backward pass.
    # :param func: the function to evaluate.
    # :param inputs: the argument sequence to pass to `func`.
    # :param params: a sequence of parameters `func` depends on but does not
    #                explicitly take as arguments.
    # :param flag: if False, disable gradient checkpointing.
    # :param use_deepspeed: if True, use deepspeed
    if flag:
        args = tuple(inputs) + tuple(params)
        return CheckpointFunction.apply(func, len(inputs), *args)
    else:
        return func(*inputs)


class CheckpointFunction(torch.autograd.Function):
    @staticmethod
    @torch.cuda.amp.custom_fwd
    def forward(ctx, run_function, length, *args):
        ctx.run_function = run_function
        ctx.input_tensors = list(args[:length])
        ctx.input_params = list(args[length:])

        with torch.no_grad():
            output_tensors = ctx.run_function(*ctx.input_tensors)
        return output_tensors

    @staticmethod
    @torch.cuda.amp.custom_bwd
    def backward(ctx, *output_grads):
        ctx.input_tensors = [x.detach().requires_grad_(True) for x in ctx.input_tensors]
        with torch.enable_grad():
            # Fixes a bug where the first op in run_function modifies the
            # Tensor storage in place, which is not allowed for detach()'d
            # Tensors.
            shallow_copies = [x.view_as(x) for x in ctx.input_tensors]
            output_tensors = ctx.run_function(*shallow_copies)
        input_grads = torch.autograd.grad(
            output_tensors,
            ctx.input_tensors + ctx.input_params,
            output_grads,
            allow_unused=True,
        )
        del ctx.input_tensors
        del ctx.input_params
        del output_tensors
        return (None, None) + input_grads