import contextlib
from typing import Generator
import warnings

from torch._C import default_generator
import torch


def set_rng_state(new_state: torch.Tensor) -> None:
    r"""Sets the random number generator state.

    .. note: This function only works for CPU. For CUDA, please use
             torch.manual_seed(seed), which works for both CPU and CUDA.

    Args:
        new_state (torch.ByteTensor): The desired state
    """
    default_generator.set_state(new_state)


def get_rng_state() -> torch.Tensor:
    r"""Returns the random number generator state as a `torch.ByteTensor`."""
    return default_generator.get_state()


def manual_seed(seed) -> torch._C.Generator:
    r"""Sets the seed for generating random numbers. Returns a
    `torch.Generator` object.

    Args:
        seed (int): The desired seed. Value must be within the inclusive range
            `[-0x8000_0000_0000_0000, 0xffff_ffff_ffff_ffff]`. Otherwise, a RuntimeError
            is raised. Negative inputs are remapped to positive values with the formula
            `0xffff_ffff_ffff_ffff + seed`.
    """
    seed = int(seed)
    import torch.cuda

    if not torch.cuda._is_in_bad_fork():
        torch.cuda.manual_seed_all(seed)

    import torch.mps
    if not torch.mps._is_in_bad_fork():
        torch.mps.manual_seed(seed)

    _seed_custom_device(seed)

    return default_generator.manual_seed(seed)


def seed() -> int:
    r"""Sets the seed for generating random numbers to a non-deterministic
    random number. Returns a 64 bit number used to seed the RNG.
    """
    seed = default_generator.seed()
    import torch.cuda

    if not torch.cuda._is_in_bad_fork():
        torch.cuda.manual_seed_all(seed)

    import torch.mps
    if not torch.mps._is_in_bad_fork():
        torch.mps.manual_seed(seed)

    _seed_custom_device(seed)

    return seed


def _seed_custom_device(seed) -> None:
    r"""Sets the seed to generate random numbers for custom device.

    Args:
        seed (int): The desired seed.

    See [Note: support the custom device with privateuse1]
    """
    seed = int(seed)
    custom_backend_name = torch._C._get_privateuse1_backend_name()
    if hasattr(torch, custom_backend_name):
        custom_device_mod = getattr(torch, custom_backend_name)
        _bad_fork_name = "_is_in_bad_fork"
        _seed_all_name = "manual_seed_all"
        if hasattr(custom_device_mod, _bad_fork_name) and hasattr(custom_device_mod, _seed_all_name):
            if not getattr(custom_device_mod, _bad_fork_name)():
                getattr(custom_device_mod, _seed_all_name)(seed)
        else:
            message = f"Set seed for `{custom_backend_name}` device does not take effect, please add API's "
            message += f"`{_bad_fork_name}` and `{_seed_all_name}` to `{custom_backend_name}` device module."
            warnings.warn(message, UserWarning, stacklevel=3)


def initial_seed() -> int:
    r"""Returns the initial seed for generating random numbers as a
    Python `long`.
    """
    return default_generator.initial_seed()


_fork_rng_warned_already = False


@contextlib.contextmanager
def fork_rng(devices=None, enabled=True, _caller="fork_rng", _devices_kw="devices", device_type="cuda") -> Generator:
    """
    Forks the RNG, so that when you return, the RNG is reset
    to the state that it was previously in.

    Args:
        devices (iterable of Device IDs): devices for which to fork
            the RNG. CPU RNG state is always forked. By default, :meth:`fork_rng` operates
            on all devices, but will emit a warning if your machine has a lot
            of devices, since this function will run very slowly in that case.
            If you explicitly specify devices, this warning will be suppressed
        enabled (bool): if ``False``, the RNG is not forked.  This is a convenience
            argument for easily disabling the context manager without having
            to delete it and unindent your Python code under it.
        deivce_type (str): device type str, default is `cuda`. As for custom device,
            see details in [Note: support the custom device with privateuse1]
    """

    device_type = torch.device(device_type).type
    device_mod = getattr(torch, device_type, None)
    if device_mod is None:
        raise RuntimeError(f"torch has no module of `{device_type}`, you should register " +
                           "a module by `torch._register_device_module`.")
    global _fork_rng_warned_already

    # Internal arguments:
    #   _caller: the function which called fork_rng, which the user used
    #   _devices_kw: the devices keyword of _caller

    if not enabled:
        yield
        return

    if devices is None:
        num_devices = device_mod.device_count()
        if num_devices > 1 and not _fork_rng_warned_already:
            message = (f"{device_type.upper()} reports that you have {num_devices} available devices, and "
                       f"you have used {_caller} without explicitly specifying which devices are being used. "
                       f"For safety, we initialize *every* {device_type.upper()} device by default, which can "
                       f"be quite slow if you have a lot of {device_type.upper()}s. If you know that you are only"
                       f" making use of a few {device_type.upper()} devices, set the environment variable "
                       f"{device_type.upper()}_VISIBLE_DEVICES or the '{_devices_kw}' keyword argument of {_caller} "
                       "with the set of devices you are actually using. For example, if you are using CPU only, "
                       "set device.upper()_VISIBLE_DEVICES= or devices=[]; if you are using device 0 only, "
                       f"set {device_type.upper()}_VISIBLE_DEVICES=0 or devices=[0].  To initialize all devices "
                       f"and suppress this warning, set the '{_devices_kw}' keyword argument to "
                       f"`range(torch.{device_type}.device_count())`.")
            warnings.warn(message)
            _fork_rng_warned_already = True
        devices = list(range(num_devices))
    else:
        # Protect against user passing us a generator; we need to traverse this
        # multiple times but a generator will be exhausted upon first traversal
        devices = list(devices)

    cpu_rng_state = torch.get_rng_state()
    device_rng_states = []
    for device in devices:
        device_rng_states.append(device_mod.get_rng_state(device))

    try:
        yield
    finally:
        torch.set_rng_state(cpu_rng_state)
        for device, device_rng_state in zip(devices, device_rng_states):
            device_mod.set_rng_state(device_rng_state, device)
