import threading
import torch
from torch.cuda._utils import _get_device_index
from torch._utils import ExceptionWrapper


def get_a_var(obj):
    if isinstance(obj, torch.Tensor):
        return obj

    if isinstance(obj, list) or isinstance(obj, tuple):
        for result in map(get_a_var, obj):
            if isinstance(result, torch.Tensor):
                return result
    if isinstance(obj, dict):
        for result in map(get_a_var, obj.items()):
            if isinstance(result, torch.Tensor):
                return result
    return None


def parallel_apply(modules, inputs, kwargs_tup=None, devices=None):
    r"""Applies each `module` in :attr:`modules` in parallel on arguments
    contained in :attr:`inputs` (positional) and :attr:`kwargs_tup` (keyword)
    on each of :attr:`devices`.

    Args:
        modules (Module): modules to be parallelized
        inputs (tensor): inputs to the modules
        devices (list of int or torch.device): CUDA devices

    :attr:`modules`, :attr:`inputs`, :attr:`kwargs_tup` (if given), and
    :attr:`devices` (if given) should all have same length. Moreover, each
    element of :attr:`inputs` can either be a single object as the only argument
    to a module, or a collection of positional arguments.
    """
    assert len(modules) == len(inputs)
    if kwargs_tup is not None:
        assert len(modules) == len(kwargs_tup)
    else:
        kwargs_tup = ({},) * len(modules)
    if devices is not None:
        assert len(modules) == len(devices)
    else:
        devices = [None] * len(modules)
    devices = list(map(lambda x: _get_device_index(x, True), devices))
    lock = threading.Lock()
    results = {}
    grad_enabled = torch.is_grad_enabled()

    def _worker(i, module, input, kwargs, device=None):
        torch.set_grad_enabled(grad_enabled)
        if device is None:
            device = get_a_var(input).get_device()
        try:
            with torch.cuda.device(device):
                # this also avoids accidental slicing of `input` if it is a Tensor
                if not isinstance(input, (list, tuple)):
                    input = (input,)
                output = module(*input, **kwargs)
            with lock:
                results[i] = output
        except Exception:
            with lock:
                results[i] = ExceptionWrapper(
                    where="in replica {} on device {}".format(i, device))

    if len(modules) > 1:
        threads = [threading.Thread(target=_worker,
                                    args=(i, module, input, kwargs, device))
                   for i, (module, input, kwargs, device) in
                   enumerate(zip(modules, inputs, kwargs_tup, devices))]

        for thread in threads:
            thread.start()
        for thread in threads:
            thread.join()
    else:
        _worker(0, modules[0], inputs[0], kwargs_tup[0], devices[0])

    outputs = []
    for i in range(len(inputs)):
        output = results[i]
        if isinstance(output, ExceptionWrapper):
            output.reraise()
        outputs.append(output)
    return outputs
