import copy
import torch
import torch.nn as nn
import torch.nn.functional as F


def masked_softmax(vector, mask, dim=-1, memory_efficient=False, mask_fill_value=-1e32):
    """A masked softmax module to correctly implement attention in Pytorch.
    Implementation adapted from: https://github.com/allenai/allennlp/blob/master/allennlp/nn/util.py
    ``torch.nn.functional.softmax(vector)`` does not work if some elements of ``vector`` should be
    masked.  This performs a softmax on just the non-masked portions of ``vector``.  Passing
    ``None`` in for the mask is also acceptable; you'll just get a regular softmax.
    ``vector`` can have an arbitrary number of dimensions; the only requirement is that ``mask`` is
    broadcastable to ``vector's`` shape.  If ``mask`` has fewer dimensions than ``vector``, we will
    unsqueeze on dimension 1 until they match.  If you need a different unsqueezing of your mask,
    do it yourself before passing the mask into this function.
    If ``memory_efficient`` is set to true, we will simply use a very large negative number for those
    masked positions so that the probabilities of those positions would be approximately 0.
    This is not accurate in math, but works for most cases and consumes less memory.
    In the case that the input vector is completely masked and ``memory_efficient`` is false, this function
    returns an array of ``0.0``. This behavior may cause ``NaN`` if this is used as the last layer of
    a model that uses categorical cross-entropy loss. Instead, if ``memory_efficient`` is true, this function
    will treat every element as equal, and do softmax over equal numbers.
    
    Args:
        vector (torch.tensor): The tensor to softmax.
        mask (torch.tensor): The tensor to indicate which indices are to be masked and not included in the softmax operation.
        dim (int, optional): The dimension to softmax over.
                            Defaults to -1.
        memory_efficient (bool, optional): Whether to use a less precise, but more memory efficient implementation of masked softmax.
                                            Defaults to False.
        mask_fill_value ([type], optional): The value to fill masked values with if `memory_efficient` is `True`.
                                            Defaults to -1e32.
    
    Returns:
        torch.tensor: The masked softmaxed output
    """
    if mask is None:
        result = F.softmax(vector, dim=dim)
    else:
        mask = mask.float()
        while mask.dim() < vector.dim():
            mask = mask.unsqueeze(1)
        if not memory_efficient:
            # To limit numerical errors from large vector elements outside the mask, we zero these out.
            result = F.softmax(vector * mask, dim=dim)
            result = result * mask
            result = result / (result.sum(dim=dim, keepdim=True) + 1e-13)
        else:
            masked_vector = vector.masked_fill((1 - mask).byte(), mask_fill_value)
            result = F.softmax(masked_vector, dim=dim)
    return result


def _get_clones(module, N):
    return nn.ModuleList([copy.deepcopy(module) for i in range(N)])


def _get_activation_fn(activation):
    if activation == "relu":
        return F.relu
    elif activation == "gelu":
        return F.gelu
    else:
        raise RuntimeError("activation should be relu/gelu, not %s." % activation)


class TransformerEncoder(nn.Module):
    """TransformerEncoder is a stack of N encoder layers

    Args:
        encoder_layer: an instance of the TransformerEncoderLayer() class (required).
        num_layers: the number of sub-encoder-layers in the encoder (required).
        norm: the layer normalization component (optional).

    Examples::
        >>> encoder_layer = nn.TransformerEncoderLayer(d_model=512, nhead=8)
        >>> transformer_encoder = nn.TransformerEncoder(encoder_layer, num_layers=6)
        >>> src = torch.rand(10, 32, 512)
        >>> out = transformer_encoder(src)
    """

    def __init__(self, cfg, drop_out=0.1, activation='relu', norm=None):
        super(TransformerEncoder, self).__init__()
        num_features = cfg.MODEL.FEAT_DIM
        dim_feedforward = cfg.MODEL.TRANSFORMER_DIM
        num_heads = cfg.MODEL.ATTENTION_HEADS
        num_layers = cfg.MODEL.ATTENTION_LAYERS
        encoder_layer = TransformerEncoderLayer(num_features, num_heads, dim_feedforward, drop_out, activation)
        self.layers = _get_clones(encoder_layer, num_layers)
        self.num_layers = num_layers
        self.norm = norm

    def forward(self, src, mask=None, key_padding_mask=None):
        """Pass the input through the encoder layers in turn.

        Args:
            src: the sequnce to the encoder (required).
            mask: the mask for the src sequence (optional).
            src_key_padding_mask: the mask for the src keys per batch (optional).

        Shape:
            see the docs in Transformer class.
        """
        output = src

        for i in range(self.num_layers):
            output = self.layers[i](output, src_mask=mask,
                                    key_padding_mask=key_padding_mask)

        if self.norm:
            output = self.norm(output)

        return output


class TransformerEncoderLayer(nn.Module):
    """TransformerEncoderLayer is made up of self-attn and feedforward network.
    This standard encoder layer is based on the paper "Attention Is All You Need".
    Ashish Vaswani, Noam Shazeer, Niki Parmar, Jakob Uszkoreit, Llion Jones, Aidan N Gomez,
    Lukasz Kaiser, and Illia Polosukhin. 2017. Attention is all you need. In Advances in
    Neural Information Processing Systems, pages 6000-6010. Users may modify or implement
    in a different way during application.

    Args:
        d_model: the number of expected features in the input (required).
        nhead: the number of heads in the multiheadattention models (required).
        dim_feedforward: the dimension of the feedforward network model (default=2048).
        dropout: the dropout value (default=0.1).
        activation: the activation function of intermediate layer, relu or gelu (default=relu).

    Examples::
        >>> encoder_layer = nn.TransformerEncoderLayer(d_model=512, nhead=8)
        >>> src = torch.rand(10, 32, 512)
        >>> out = encoder_layer(src)
    """

    def __init__(self, d_model, nhead, dim_feedforward=2048, dropout=0.1, activation="relu"):
        super(TransformerEncoderLayer, self).__init__()
        self.self_attn = nn.MultiheadAttention(d_model, nhead, dropout=dropout)
        # Implementation of Feedforward model
        self.linear1 = nn.Linear(d_model, dim_feedforward)
        self.dropout = nn.Dropout(dropout)
        self.linear2 = nn.Linear(dim_feedforward, d_model)

        self.norm1 = nn.LayerNorm(d_model)
        self.norm2 = nn.LayerNorm(d_model)
        self.dropout1 = nn.Dropout(dropout)
        self.dropout2 = nn.Dropout(dropout)

        self.activation = _get_activation_fn(activation)

    def forward(self, src, src_mask=None, key_padding_mask=None):
        """Pass the input through the encoder layer.

        Args:
            src: the sequnce to the encoder layer (required).
            src_mask: the mask for the src sequence (optional).
            src_key_padding_mask: the mask for the src keys per batch (optional).

        Shape:
            see the docs in Transformer class.
        """
        src2 = self.self_attn(src, src, src, attn_mask=src_mask,
                              key_padding_mask=key_padding_mask)[0]
        if key_padding_mask is not None:
            src2 = src2.masked_fill(key_padding_mask.permute(1, 0).unsqueeze(-1), 0)
        src = src + self.dropout1(src2)
        src = self.norm1(src)
        if hasattr(self, "activation"):
            src2 = self.linear2(self.dropout(self.activation(self.linear1(src))))
        else:  # for backward compatibility
            src2 = self.linear2(self.dropout(F.relu(self.linear1(src))))
        src = src + self.dropout2(src2)
        src = self.norm2(src)
        if key_padding_mask is not None:
            src = src.masked_fill(key_padding_mask.permute(1, 0).unsqueeze(-1), 0)
        return src

