import copy
import itertools
import logging
from collections import defaultdict
from enum import Enum
from typing import Any, Callable, Dict, Iterable, List, Optional, Set, Type, Union
import torch
from fvcore.common.param_scheduler import CosineParamScheduler, MultiStepParamScheduler

from detectron2.config import CfgNode

_GradientClipperInput = Union[torch.Tensor, Iterable[torch.Tensor]]
_GradientClipper = Callable[[_GradientClipperInput], None]


class GradientClipType(Enum):
    VALUE = "value"
    NORM = "norm"


def _create_gradient_clipper(cfg: CfgNode) -> _GradientClipper:
    """
    Creates gradient clipping closure to clip by value or by norm,
    according to the provided config.
    """
    cfg = copy.deepcopy(cfg)

    def clip_grad_norm(p: _GradientClipperInput):
        torch.nn.utils.clip_grad_norm_(p, cfg.CLIP_VALUE, cfg.NORM_TYPE)

    def clip_grad_value(p: _GradientClipperInput):
        torch.nn.utils.clip_grad_value_(p, cfg.CLIP_VALUE)

    _GRADIENT_CLIP_TYPE_TO_CLIPPER = {
        GradientClipType.VALUE: clip_grad_value,
        GradientClipType.NORM: clip_grad_norm,
    }
    return _GRADIENT_CLIP_TYPE_TO_CLIPPER[GradientClipType(cfg.CLIP_TYPE)]


def _generate_optimizer_class_with_gradient_clipping(
    optimizer: Type[torch.optim.Optimizer],
    *,
    per_param_clipper: Optional[_GradientClipper] = None,
    global_clipper: Optional[_GradientClipper] = None,
) -> Type[torch.optim.Optimizer]:
    """
    Dynamically creates a new type that inherits the type of a given instance
    and overrides the `step` method to add gradient clipping
    """
    assert (
        per_param_clipper is None or global_clipper is None
    ), "Not allowed to use both per-parameter clipping and global clipping"

    def optimizer_wgc_step(self, closure=None):
        if per_param_clipper is not None:
            for group in self.param_groups:
                for p in group["params"]:
                    per_param_clipper(p)
        else:
            # global clipper for future use with detr
            # (https://github.com/facebookresearch/detr/pull/287)
            all_params = itertools.chain(*[g["params"] for g in self.param_groups])
            global_clipper(all_params)
        super(type(self), self).step(closure)

    OptimizerWithGradientClip = type(
        optimizer.__name__ + "WithGradientClip",
        (optimizer,),
        {"step": optimizer_wgc_step},
    )
    return OptimizerWithGradientClip


def maybe_add_gradient_clipping(
    cfg: CfgNode, optimizer: Type[torch.optim.Optimizer]
) -> Type[torch.optim.Optimizer]:
    """
    If gradient clipping is enabled through config options, wraps the existing
    optimizer type to become a new dynamically created class OptimizerWithGradientClip
    that inherits the given optimizer and overrides the `step` method to
    include gradient clipping.

    Args:
        cfg: CfgNode, configuration options
        optimizer: type. A subclass of torch.optim.Optimizer

    Return:
        type: either the input `optimizer` (if gradient clipping is disabled), or
            a subclass of it with gradient clipping included in the `step` method.
    """
    if not cfg.SOLVER.CLIP_GRADIENTS.ENABLED:
        return optimizer
    if isinstance(optimizer, torch.optim.Optimizer):
        optimizer_type = type(optimizer)
    else:
        assert issubclass(optimizer, torch.optim.Optimizer), optimizer
        optimizer_type = optimizer

    grad_clipper = _create_gradient_clipper(cfg.SOLVER.CLIP_GRADIENTS)
    OptimizerWithGradientClip = _generate_optimizer_class_with_gradient_clipping(
        optimizer_type, per_param_clipper=grad_clipper
    )
    if isinstance(optimizer, torch.optim.Optimizer):
        optimizer.__class__ = OptimizerWithGradientClip  # a bit hacky, not recommended
        return optimizer
    else:
        return OptimizerWithGradientClip