import torch 
import torch.nn as nn
import torch.nn.functional as F
import numpy as np
class dot_attention(nn.Module):
    """ 点积注意力机制"""

    def __init__(self, attention_dropout=0.0):
        super(dot_attention, self).__init__()
        self.dropout = nn.Dropout(attention_dropout)
        self.softmax = nn.Softmax(dim=2)

    def forward(self, q, k, v, scale=None, attn_mask=None):
        """
        前向传播
        :param q:
        :param k:
        :param v:
        :param scale:
        :param attn_mask:
        :return: 上下文张量和attention张量。
        """
        attention = torch.bmm(q, k.transpose(1, 2))
        if scale:
            attention = attention * scale        # 是否设置缩放
        if attn_mask:
            attention = attention.masked_fill(attn_mask, -np.inf)     # 给需要mask的地方设置一个负无穷。
        # 计算softmax
        attention = self.softmax(attention)
        # 添加dropout
        attention = self.dropout(attention)
        # 和v做点积。
        context = torch.bmm(attention, v)
        return context, attention



class MultiHeadAttention(nn.Module):
    """ 多头自注意力"""
    def __init__(self, model_dim, num_heads, dropout=0.0):
        super(MultiHeadAttention, self).__init__()

        self.dim_per_head = model_dim//num_heads   # 每个头的维度
        self.num_heads = num_heads
        self.linear_k = nn.Linear(model_dim, self.dim_per_head * num_heads)
        self.linear_v = nn.Linear(model_dim, self.dim_per_head * num_heads)
        self.linear_q = nn.Linear(model_dim, self.dim_per_head * num_heads)

        self.dot_product_attention = dot_attention(dropout)

        self.linear_final = nn.Linear(model_dim, model_dim)
        self.dropout = nn.Dropout(dropout)
        self.layer_norm = nn.LayerNorm(model_dim)         # LayerNorm 归一化。

    def forward(self, key, value, query, attn_mask=None):
        # 残差连接
        residual = query

        dim_per_head = self.dim_per_head
        num_heads = self.num_heads
        batch_size = key.size(0)

        # 线性映射
        key = self.linear_k(key)
        value = self.linear_v(value)
        query = self.linear_q(query)

        # 按照头进行分割
        key = key.view(batch_size * num_heads, -1, dim_per_head)
        value = value.view(batch_size * num_heads, -1, dim_per_head)
        query = query.view(batch_size * num_heads, -1, dim_per_head)

        if attn_mask:
            attn_mask = attn_mask.repeat(num_heads, 1, 1)

        # 缩放点击注意力机制
        scale = (key.size(-1) // num_heads) ** -0.5
        context, attention = self.dot_product_attention(query, key, value, scale, attn_mask)

        # 进行头合并 concat heads
        context = context.view(batch_size, -1, dim_per_head * num_heads)

        # 进行线性映射
        output = self.linear_final(context)

        # dropout
        output = self.dropout(output)

        # 添加残差层和正则化层。
        output = self.layer_norm(residual + output)

        return output, attention
