"""undocumented
轻量封装的 Pytorch LSTM 模块.
可在 forward 时传入序列的长度, 自动对padding做合适的处理.
"""

__all__ = [
    "LSTM"
]

import torch
import torch.nn as nn
import torch.nn.utils.rnn as rnn


class LSTM(nn.Module):
    """
    LSTM 模块, 轻量封装的Pytorch LSTM. 在提供seq_len的情况下，将自动使用pack_padded_sequence; 同时默认将forget gate的bias初始化
    为1; 且可以应对DataParallel中LSTM的使用问题。

    """

    def __init__(self, input_size, hidden_size=100, num_layers=1, dropout=0.0, batch_first=True,
                 bidirectional=False, bias=True):
        """
        
        :param input_size:  输入 `x` 的特征维度
        :param hidden_size: 隐状态 `h` 的特征维度.
        :param num_layers: rnn的层数. Default: 1
        :param dropout: 层间dropout概率. Default: 0
        :param bidirectional: 若为 ``True``, 使用双向的RNN. Default: ``False``
        :param batch_first: 若为 ``True``, 输入和输出 ``Tensor`` 形状为
            :(batch, seq, feature). Default: ``False``
        :param bias: 如果为 ``False``, 模型将不会使用bias. Default: ``True``
        """
        super(LSTM, self).__init__()
        self.batch_first = batch_first
        self.lstm = nn.LSTM(input_size, hidden_size, num_layers, bias=bias, batch_first=batch_first,
                            dropout=dropout, bidirectional=bidirectional)
        self.init_param()

    def init_param(self):
        for name, param in self.named_parameters():
            if 'bias' in name:
                # based on https://github.com/pytorch/pytorch/issues/750#issuecomment-280671871
                param.data.fill_(0)
                n = param.size(0)
                start, end = n // 4, n // 2
                param.data[start:end].fill_(1)
            else:
                nn.init.xavier_uniform_(param)

    def forward(self, x, seq_len=None, h0=None, c0=None):
        """

        :param x: [batch, seq_len, input_size] 输入序列
        :param seq_len: [batch, ] 序列长度, 若为 ``None``, 所有输入看做一样长. Default: ``None``
        :param h0: [batch, hidden_size] 初始隐状态, 若为 ``None`` , 设为全0向量. Default: ``None``
        :param c0: [batch, hidden_size] 初始Cell状态, 若为 ``None`` , 设为全0向量. Default: ``None``
        :return (output, ht) 或 output: 若 ``get_hidden=True`` [batch, seq_len, hidden_size*num_direction] 输出序列
            和 [batch, hidden_size*num_direction] 最后时刻隐状态.
        """
        batch_size, max_len, _ = x.size()
        if h0 is not None and c0 is not None:
            hx = (h0, c0)
        else:
            hx = None
        if seq_len is not None and not isinstance(x, rnn.PackedSequence):
            sort_lens, sort_idx = torch.sort(seq_len, dim=0, descending=True)
            if self.batch_first:
                x = x[sort_idx]
            else:
                x = x[:, sort_idx]
            x = rnn.pack_padded_sequence(x, sort_lens, batch_first=self.batch_first)
            output, hx = self.lstm(x, hx)  # -> [N,L,C]
            output, _ = rnn.pad_packed_sequence(output, batch_first=self.batch_first, total_length=max_len)
            _, unsort_idx = torch.sort(sort_idx, dim=0, descending=False)
            if self.batch_first:
                output = output[unsort_idx]
            else:
                output = output[:, unsort_idx]
        else:
            output, hx = self.lstm(x, hx)
        return output, hx
