import torch
from torch.nn.modules.batchnorm import _BatchNorm
from torch.nn import functional as F

import syncbn
from .optimized_sync_batchnorm_kernel import SyncBatchnormFunction


class SyncBatchNorm(_BatchNorm):
    """
    synchronized batch normalization module extented from `torch.nn.BatchNormNd`
    with the added stats reduction across multiple processes.
    :class:`apex.parallel.SyncBatchNorm` is designed to work with
    `DistributedDataParallel`.

    When running in training mode, the layer reduces stats across all processes
    to increase the effective batchsize for normalization layer. This is useful
    in applications where batch size is small on a given process that would
    diminish converged accuracy of the model. The model uses collective
    communication package from `torch.distributed`.

    When running in evaluation mode, the layer falls back to
    `torch.nn.functional.batch_norm`

    Args:
        num_features: :math:`C` from an expected input of size
            :math:`(N, C, L)` or :math:`L` from input of size :math:`(N, L)`
        eps: a value added to the denominator for numerical stability.
            Default: 1e-5
        momentum: the value used for the running_mean and running_var
            computation. Can be set to ``None`` for cumulative moving average
            (i.e. simple average). Default: 0.1
        affine: a boolean value that when set to ``True``, this module has
            learnable affine parameters. Default: ``True``
        track_running_stats: a boolean value that when set to ``True``, this
            module tracks the running mean and variance, and when set to ``False``,
            this module does not track such statistics and always uses batch
            statistics in both training and eval modes. Default: ``True``
        process_group: pass in a process group within which the stats of the
            mini-batch is being synchronized. ``None`` for using default process
            group
        channel_last: a boolean value that when set to ``True``, this module
            take the last dimension of the input tensor to be the channel
            dimension. Default: False

    Examples::
        >>> # channel first tensor
        >>> sbn = apex.parallel.SyncBatchNorm(100).cuda()
        >>> inp = torch.randn(10, 100, 14, 14).cuda()
        >>> out = sbn(inp)
        >>> inp = torch.randn(3, 100, 20).cuda()
        >>> out = sbn(inp)
        >>> # channel last tensor
        >>> sbn = apex.parallel.SyncBatchNorm(100, channel_last=True).cuda()
        >>> inp = torch.randn(10, 14, 14, 100).cuda()
    """

    def __init__(self, num_features, eps=1e-5, momentum=0.1, affine=True, track_running_stats=True, process_group=None, channel_last=False):
        super(SyncBatchNorm, self).__init__(num_features, eps=eps, momentum=momentum, affine=affine, track_running_stats=track_running_stats)
        self.process_group = process_group
        self.channel_last = channel_last

    def _specify_process_group(self, process_group):
        self.process_group = process_group

    def _specify_channel_last(self, channel_last):
        self.channel_last = channel_last

    def forward(self, input):
        if not self.training and self.track_running_stats and not self.channel_last:
            # fall back to pytorch implementation for inference
            return F.batch_norm(input, self.running_mean, self.running_var, self.weight, self.bias, False, 0.0, self.eps)
        else:
            exponential_average_factor = 0.0
            if self.training and self.track_running_stats:
                self.num_batches_tracked += 1
                if self.momentum is None:
                    exponential_average_factor = 1.0 / float(self.num_batches_tracked)
                else:
                    exponential_average_factor = self.momentum
            return SyncBatchnormFunction.apply(input, self.weight, self.bias, self.running_mean, self.running_var, self.eps, self.training or not self.track_running_stats, exponential_average_factor, self.process_group, self.channel_last)
