"""
 Copyright 2019 Johns Hopkins University  (Author: Jesus Villalba)
 Apache 2.0  (http://www.apache.org/licenses/LICENSE-2.0)
"""

import torch
import torch.nn as nn

class TransformerConv2dSubsampler(nn.Module):
    """Convolutional 2D subsampling (to 1/4 length) Tor transformer

    Attributes:
      in_feats: input feature dimension
      out_feats: Transformer d_model
      hid_act: activation layer object
      pos_enc: positional encoder layer
      time_dim: indicates which is the time dimension in the input tensor
    """

    def __init__(self, in_feats, out_feats, hid_act, pos_enc, time_dim=1):
        super().__init__()
        self.time_dim = time_dim
        self.conv = nn.Sequential(
            nn.Conv2d(1, out_feats, 3, 2, padding=(0,1)),
            hid_act,
            nn.Conv2d(out_feats, out_feats, 3, 2, padding=(0,1)),
            hid_act
        )
        self.out = nn.Sequential(
            nn.Linear(out_feats * (((in_feats - 1) // 2 - 1) // 2), out_feats),
            pos_enc)


    def forward(self, x, mask):
        """Forward function.

        Args:
          x: input tensor with size=(batch, time, num_feats)
          mask: mask to indicate valid time steps for x (batch, time1, time2)

        Returns:
           Tensor with output features
           Tensor with subsampled mask
        """
        if self.time_dim == 1:
            x = x.transpose(1,2)

        x = x.unsqueeze(1)  # (b, c, f, t)
        x = self.conv(x)
        b, c, f, t = x.size()
        x = self.out(x.contiguous().view(b, c * f, t).transpose(1,2))
        if mask is None:
            return x, None
        return x, mask[:, :, :-2:2][:, :, :-2:2]
