from __future__ import division
from __future__ import unicode_literals

import torch


def get_param_buffer_for_ema(model, update_buffer=False, required_buffers=['running_mean', 'running_var']):
    params = model.parameters()
    all_param_buffer = [p for p in params if p.requires_grad]
    if update_buffer:
        named_buffers = model.named_buffers()
        for key, value in named_buffers:
            for buffer_name in required_buffers:
                if buffer_name in key:
                    all_param_buffer.append(value)
                    break
    return all_param_buffer


class ExponentialMovingAverage:
    """
    Maintains (exponential) moving average of a set of parameters.
    """
    def __init__(self, parameters, decay, use_num_updates=True):
        """
        Args:
          parameters: Iterable of `torch.nn.Parameter`; usually the result of `model.parameters()`.
          decay: The exponential decay.
          use_num_updates: Whether to use number of updates when computing averages.
        """
        if decay < 0.0 or decay > 1.0:
            raise ValueError('Decay must be between 0 and 1')
        self.decay = decay
        self.num_updates = 0 if use_num_updates else None
        self.shadow_params = [p.clone().detach() for p in parameters]
        self.collected_params = []

    def update(self, parameters):
        """
        Update currently maintained parameters.
        Call this every time the parameters are updated, such as the result of the `optimizer.step()` call.
        Args:
          parameters: Iterable of `torch.nn.Parameter`; usually the same set of parameters used to initialize this object.
        """
        decay = self.decay
        if self.num_updates is not None:
            self.num_updates += 1
            decay = min(decay,
                        (1 + self.num_updates) / (10 + self.num_updates))
        one_minus_decay = 1.0 - decay
        with torch.no_grad():
            for s_param, param in zip(self.shadow_params, parameters):
                s_param.sub_(one_minus_decay * (s_param - param))

    def copy_to(self, parameters):
        """
        Copy current parameters into given collection of parameters.
        Args:
          parameters: Iterable of `torch.nn.Parameter`; the parameters to be
            updated with the stored moving averages.
        """
        for s_param, param in zip(self.shadow_params, parameters):
            param.data.copy_(s_param.data)

    def store(self, parameters):
        """
        Save the current parameters for restoring later.
        Args:
          parameters: Iterable of `torch.nn.Parameter`; the parameters to be temporarily stored.
        """
        self.collected_params = [param.clone() for param in parameters]

    def restore(self, parameters):
        """
        Restore the parameters stored with the `store` method.
        Useful to validate the model with EMA parameters without affecting the
        original optimization process. Store the parameters before the
        `copy_to` method. After validation (or model saving), use this to
        restore the former parameters.
        Args:
          parameters: Iterable of `torch.nn.Parameter`; the parameters to be
            updated with the stored parameters.
        """
        for c_param, param in zip(self.collected_params, parameters):
            param.data.copy_(c_param.data)
        del (self.collected_params)
