import torch
from torch.nn import (
    Module,
    ModuleList,
    Linear,
    Dropout,
    LayerNorm,
    Identity,
    Parameter,
    init,
)
import torch.nn.functional as F
from stochastic_depth import DropPath
from collections import OrderedDict

class Attention(Module):
    """
    Obtained from timm: github.com:rwightman/pytorch-image-models
    """

    def __init__(self, dim, num_heads=8, attention_dropout=0.1, projection_dropout=0.1):
        super().__init__()
        self.num_heads = num_heads
        head_dim = dim // self.num_heads
        self.scale = head_dim ** -0.5

        self.qkv = Linear(dim, dim * 3, bias=False)
        self.attn_drop = Dropout(attention_dropout)
        self.proj = Linear(dim, dim)
        self.proj_drop = Dropout(projection_dropout)

    def forward(self, x):
        B, N, C = x.shape
        qkv = (
            self.qkv(x)
            .reshape(B, N, 3, self.num_heads, C // self.num_heads)
            .permute(2, 0, 3, 1, 4)
        )
        q, k, v = qkv[0], qkv[1], qkv[2]

        attn = (q @ k.transpose(-2, -1)) * self.scale
        attn = attn.softmax(dim=-1)
        attn = self.attn_drop(attn)

        x = (attn @ v).transpose(1, 2).reshape(B, N, C)
        x = self.proj(x)
        x = self.proj_drop(x)
        return x


class MaskedAttention(Module):
    def __init__(self, dim, num_heads=8, attention_dropout=0.1, projection_dropout=0.1):
        super().__init__()
        self.num_heads = num_heads
        head_dim = dim // self.num_heads
        self.scale = head_dim ** -0.5

        self.qkv = Linear(dim, dim * 3, bias=False)
        self.attn_drop = Dropout(attention_dropout)
        self.proj = Linear(dim, dim)
        self.proj_drop = Dropout(projection_dropout)

    def forward(self, x, mask=None):
        B, N, C = x.shape
        qkv = (
            self.qkv(x)
            .reshape(B, N, 3, self.num_heads, C // self.num_heads)
            .permute(2, 0, 3, 1, 4)
        )
        q, k, v = qkv[0], qkv[1], qkv[2]

        attn = (q @ k.transpose(-2, -1)) * self.scale

        if mask is not None:
            mask_value = -torch.finfo(attn.dtype).max
            assert mask.shape[-1] == attn.shape[-1], "mask has incorrect dimensions"
            mask = mask[:, None, :] * mask[:, :, None]
            mask = mask.unsqueeze(1).repeat(1, self.num_heads, 1, 1)
            attn.masked_fill_(~mask, mask_value)

        attn = attn.softmax(dim=-1)
        attn = self.attn_drop(attn)

        x = (attn @ v).transpose(1, 2).reshape(B, N, C)
        x = self.proj(x)
        x = self.proj_drop(x)
        return x


class TransformerEncoderLayer(Module):
    """
    Inspired by torch.nn.TransformerEncoderLayer and timm.
    """

    def __init__(
        self,
        d_model,
        nhead,
        dim_feedforward=2048,
        dropout=0.1,
        attention_dropout=0.1,
        drop_path_rate=0.1,
    ):
        super(TransformerEncoderLayer, self).__init__()
        self.pre_norm = LayerNorm(d_model)
        self.self_attn = Attention(
            dim=d_model,
            num_heads=nhead,
            attention_dropout=attention_dropout,
            projection_dropout=dropout,
        )

        self.linear1 = Linear(d_model, dim_feedforward)
        self.dropout1 = Dropout(dropout)
        self.norm1 = LayerNorm(d_model)
        self.linear2 = Linear(dim_feedforward, d_model)
        self.dropout2 = Dropout(dropout)

        self.drop_path = DropPath(drop_path_rate) if drop_path_rate > 0 else Identity()

        self.activation = F.gelu

    def forward(self, src: torch.Tensor, *args, **kwargs) -> torch.Tensor:
        src = src + self.drop_path(self.self_attn(self.pre_norm(src)))
        src = self.norm1(src)
        src2 = self.linear2(self.dropout1(self.activation(self.linear1(src))))
        src = src + self.drop_path(self.dropout2(src2))
        return src


class TransformerClassifier(Module):
    def __init__(
        self,
        seq_pool=True,
        embedding_dim=768,
        num_layers=12,
        num_heads=12,
        mlp_ratio=4.0,
        num_classes=1000,
        dropout=0.1,
        attention_dropout=0.1,
        stochastic_depth=0.1,
        positional_embedding="learnable",
        sequence_length=None,
    ):
        super().__init__()
        positional_embedding = (
            positional_embedding
            if positional_embedding in ["sine", "learnable", "none"]
            else "sine"
        )
        dim_feedforward = int(embedding_dim * mlp_ratio)
        self.embedding_dim = embedding_dim
        self.sequence_length = sequence_length
        self.seq_pool = seq_pool
        self.num_tokens = 0

        assert sequence_length is not None or positional_embedding == "none", (
            f"Positional embedding is set to {positional_embedding} and"
            f" the sequence length was not specified."
        )

        if not seq_pool:
            sequence_length += 1
            self.class_emb = Parameter(
                torch.zeros(1, 1, self.embedding_dim), requires_grad=True
            )
            self.num_tokens = 1
        else:
            self.attention_pool = Linear(self.embedding_dim, 1)

        if positional_embedding != "none":
            if positional_embedding == "learnable":
                self.positional_emb = Parameter(
                    torch.zeros(1, sequence_length, embedding_dim), requires_grad=True
                )
                init.trunc_normal_(self.positional_emb, std=0.2)
            else:
                self.positional_emb = Parameter(
                    self.sinusoidal_embedding(sequence_length, embedding_dim),
                    requires_grad=False,
                )
        else:
            self.positional_emb = None

        self.dropout = Dropout(p=dropout)
        self.blocks = torch.nn.Sequential(
            OrderedDict([(f"transformer_block_{i}",
                    TransformerEncoderLayer(
                        d_model=embedding_dim,
                        nhead=num_heads,
                        dim_feedforward=dim_feedforward,
                        dropout=dropout,
                        attention_dropout=attention_dropout,
                        drop_path_rate=0,
                    ))
                    for i in range(num_layers)
        ]))
        self.norm = LayerNorm(embedding_dim)

        self.fc = Linear(embedding_dim, num_classes)
        self.apply(self.init_weight)

    def forward(self, x):
        if self.positional_emb is None and x.size(1) < self.sequence_length:
            x = F.pad(
                x, (0, 0, 0, self.n_channels - x.size(1)), mode="constant", value=0
            )

        if not self.seq_pool:
            cls_token = self.class_emb.expand(x.shape[0], -1, -1)
            x = torch.cat((cls_token, x), dim=1)

        if self.positional_emb is not None:
            x += self.positional_emb

        x = self.dropout(x)

        #         for blk in self.blocks:
        #             x = blk(x)
        x = self.blocks(x)
        x = self.norm(x)

        if self.seq_pool:
            x = torch.matmul(
                F.softmax(self.attention_pool(x), dim=1).transpose(-1, -2), x
            ).squeeze(-2)
        else:
            x = x[:, 0]

        x = self.fc(x)
        return x

    @staticmethod
    def init_weight(m):
        if isinstance(m, Linear):
            init.trunc_normal_(m.weight, std=0.02)
            if isinstance(m, Linear) and m.bias is not None:
                init.constant_(m.bias, 0)
        elif isinstance(m, LayerNorm):
            init.constant_(m.bias, 0)
            init.constant_(m.weight, 1.0)

    @staticmethod
    def sinusoidal_embedding(n_channels, dim):
        pe = torch.FloatTensor(
            [
                [p / (10000 ** (2 * (i // 2) / dim)) for i in range(dim)]
                for p in range(n_channels)
            ]
        )
        pe[:, 0::2] = torch.sin(pe[:, 0::2])
        pe[:, 1::2] = torch.cos(pe[:, 1::2])
        return pe.unsqueeze(0)
