import torch


def detach_variable(inputs):
    if isinstance(inputs, tuple):
        out = []
        for inp in inputs:
            x = inp.detach()
            x.requires_grad = inp.requires_grad
            out.append(x)
        return tuple(out)
    else:
        raise RuntimeError(
            "Only tuple of tensors is supported. Got Unsupported input type: ", type(inputs).__name__)


class CheckpointFunction(torch.autograd.Function):

    @staticmethod
    def forward(ctx, run_function, *args):
        ctx.run_function = run_function
        ctx.save_for_backward(*args)
        with torch.no_grad():
            outputs = run_function(*args)
        return outputs

    @staticmethod
    def backward(ctx, *args):
        if not torch.autograd._is_checkpoint_valid():
            raise RuntimeError("Checkpointing is not compatible with .grad(), please use .backward() if possible")
        inputs = ctx.saved_tensors
        detached_inputs = detach_variable(inputs)
        with torch.enable_grad():
            outputs = ctx.run_function(*detached_inputs)

        if isinstance(outputs, torch.Tensor):
            outputs = (outputs,)
        torch.autograd.backward(outputs, args)
        return (None,) + tuple(inp.grad for inp in detached_inputs)


def checkpoint(function, *args):
    r"""Checkpoint a model or part of the model

    Checkpointing works by trading compute for memory. Rather than storing all
    intermediate activations of the entire computation graph for computing
    backward, the checkpointed part does **not** save intermediate activations,
    and instead recomputes them in backward pass. It can be applied on any part
    of a model.

    Specifically, in the forward pass, :attr:`function` will run in
    :func:`torch.no_grad` manner, i.e., not storing the intermediate
    activations. Instead, the forward pass saves the inputs tuple and the
    :attr:`function` parameter. In the backwards pass, the saved inputs and
    :attr:`function` is retreived, and the forward pass is computed on
    :attr:`function` again, now tracking the intermediate activations, and then
    the gradients are calculated using these activation values.

    .. warning::
        Checkpointing doesn't work with :func:`torch.autograd.grad`, but only
        with :func:`torch.autograd.backward`.

    .. warning::
        If :attr:`function` invocation during backward does anything different
        than the one during forward, e.g., due to some global variable, the
        checkpointed version won't be equivalent, and unfortunately it can't be
        detected.

    Args:
        function: describes what to run in the forward pass of the model or
            part of the model. It should also know how to handle the inputs
            passed as the tuple. For example, in LSTM, if user passes
            ``(activation, hidden)``, :attr:`function` should correctly use the
            first input as ``activation`` and the second input as ``hidden``
        args: tuple containing inputs to the :attr:`function`

    Returns:
        Output of running :attr`function` on *:attr:`args`
    """
    return CheckpointFunction.apply(function, *args)


def checkpoint_sequential(functions, segments, *inputs):
    r"""A helper function for checkpointing sequential models.

    Sequential models execute a list of modules/functions in order
    (sequentially). Therefore, we can divide such a model in various segments
    and checkpoint each segment. All segments except the last will run in
    :func:`torch.no_grad` manner, i.e., not storing the intermediate
    activations. The inputs of each checkpointed segment will be saved for
    re-running the segment in the backward pass.

    See :func:`~torch.utils.checkpoint.checkpoint` on how checkpointing works.

    .. warning::
        Checkpointing doesn't work with :func:`torch.autograd.grad`, but only
        with :func:`torch.autograd.backward`.

    Args:
        functions: A :class:`torch.nn.Sequential` or the list of modules or
            functions (comprising the model) to run sequentially.
        segments: Number of chunks to create in the model
        inputs: tuple of Tensors that are inputs to :attr:`functions`

    Returns:
        Output of running :attr:`functions` sequentially on *:attr:`inputs`

    Example:
        >>> model = nn.Sequential(...)
        >>> input_var = checkpoint_sequential(model, chunks, input_var)
    """

    def run_function(start, end, functions):
        def forward(*inputs):
            input = inputs[0]
            for j in range(start, end + 1):
                input = functions[j](input)
            return input
        return forward

    if isinstance(functions, torch.nn.Sequential):
        functions = list(functions.children())

    segment_size = len(functions) // segments
    # the last chunk has to be non-volatile
    end = -1
    for start in range(0, segment_size * (segments - 1), segment_size):
        end = start + segment_size - 1
        inputs = checkpoint(run_function(start, end, functions), *inputs)
        if not isinstance(inputs, tuple):
            inputs = (inputs,)
    return run_function(end + 1, len(functions) - 1, functions)(*inputs)