import torch
import torch.nn as nn
from collections import OrderedDict
from copy import deepcopy

import contextlib

class EMA(nn.Module):

    def __init__(self, model, decay):
        super().__init__()
        self.decay = decay

        self.model = model
        self.shadow = deepcopy(self.model)

        for param in self.shadow.parameters():
            param.detach_()

    @torch.no_grad()
    def update(self):
        if not self.training:
            print("EMA update should only be called during training", file=stderr, flush=True)
            return

        model_params = OrderedDict(self.model.named_parameters())
        shadow_params = OrderedDict(self.shadow.named_parameters())

        # check if both model contains the same set of keys
        assert model_params.keys() == shadow_params.keys()

        for name, param in model_params.items():
            # see https://www.tensorflow.org/api_docs/python/tf/train/ExponentialMovingAverage
            # shadow_variable -= (1 - decay) * (shadow_variable - variable)
            if param.requires_grad:
                shadow_params[name].sub_((1. - self.decay) * (shadow_params[name] - param))

        model_buffers = OrderedDict(self.model.named_buffers())
        shadow_buffers = OrderedDict(self.shadow.named_buffers())

        # check if both model contains the same set of keys
        assert model_buffers.keys() == shadow_buffers.keys()

        for name, buffer in model_buffers.items():
            # buffers are copied
            shadow_buffers[name].copy_(buffer)

    #fd A hack to allow non-DDP models to be passed into the Trainer
    def no_sync(self):
        return contextlib.nullcontext()

    def forward(self, *args, **kwargs):
        if self.training:
            return self.model(*args, **kwargs)
        else:
            return self.shadow(*args, **kwargs)

def count_parameters(model):
    return sum(p.numel() for p in model.parameters() if p.requires_grad)