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# SPDX-License-Identifier: Apache-2.0
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# Licensed under the Apache License, Version 2.0 (the "License");
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#     http://www.apache.org/licenses/LICENSE-2.0
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import copy
from typing import Callable, Optional, Union

import torch
from torch import Tensor
from torch.nn import Dropout, LayerNorm, Linear, Module, ModuleList, MultiheadAttention
from torch.nn import functional as F


class TransformerDecoder(Module):
    r"""TransformerDecoder is a stack of N decoder layers

    Parameters
    ----------:
        decoder_layer: torch.nn.Module
            Layer used for the doceder
        num_layers: int
            Number of sub-decoder-layers in the decoder.
        norm: str
            Layer normalization component.
    """

    __constants__ = ["norm"]

    def __init__(self, decoder_layer, num_layers, norm=None):
        super().__init__()
        torch._C._log_api_usage_once(f"torch.nn.modules.{self.__class__.__name__}")
        self.layers = _get_clones(decoder_layer, num_layers)
        self.num_layers = num_layers
        self.norm = norm

    def forward(
        self,
        tgt: Tensor,
        tgt_mask: Optional[Tensor] = None,
        tgt_key_padding_mask: Optional[Tensor] = None,
        tgt_is_causal: Optional[bool] = None,
    ) -> Tensor:
        """Pass the inputs (and mask) through the decoder layer in turn."""
        output = tgt

        tgt_is_causal = True

        for mod in self.layers:
            output = mod(
                output,
                tgt_mask=tgt_mask,
                tgt_key_padding_mask=tgt_key_padding_mask,
                tgt_is_causal=tgt_is_causal,
            )

        if self.norm is not None:
            output = self.norm(output)

        return output


class DecoderOnlyLayer(Module):
    r"""

    Parameters
    ----------
        d_model: int
            Number of expected features in the input.
        nhead: int
            Number of heads in the multiheadattention models.
        dim_feedforward: int
            Dimension of the feedforward network model, by default 2048.
        dropout: float
            The dropout value, by default 0.1.
        activation: str
            The activation function of the intermediate layer, by default 'relu'.
        layer_norm_eps: float
            The eps value in layer normalization components, by default 1e-5.
        batch_first: Bool
            If ``True``, then the input and output tensors are provided
            as (batch, seq, feature), by default ``False`` (seq, batch, feature).
        norm_first: Bool
            If ``True``, layer norm is done prior to self attention, multihead
            attention and feedforward operations, respectively. Otherwise it's done after,
            by default ``False`` (after).
        bias: If set to ``False``, ``Linear`` and ``LayerNorm`` layers will not learn an additive
            bias. Default: ``True``.

    """

    __constants__ = ["norm_first"]

    def __init__(
        self,
        d_model: int,
        nhead: int,
        dim_feedforward: int = 2048,
        dropout: float = 0.1,
        activation: Union[str, Callable[[Tensor], Tensor]] = F.relu,
        layer_norm_eps: float = 1e-5,
        batch_first: bool = False,
        norm_first: bool = False,
        bias: bool = True,
        device=None,
        dtype=None,
    ) -> None:
        factory_kwargs = {"device": device, "dtype": dtype}
        super().__init__()
        self.self_attn = MultiheadAttention(
            d_model,
            nhead,
            dropout=dropout,
            batch_first=batch_first,
            bias=bias,
            **factory_kwargs,
        )
        self.multihead_attn = MultiheadAttention(
            d_model,
            nhead,
            dropout=dropout,
            batch_first=batch_first,
            bias=bias,
            **factory_kwargs,
        )
        # Implementation of Feedforward model
        self.linear1 = Linear(d_model, dim_feedforward, bias=bias, **factory_kwargs)
        self.dropout = Dropout(dropout)
        self.linear2 = Linear(dim_feedforward, d_model, bias=bias, **factory_kwargs)

        self.norm_first = norm_first
        self.norm1 = LayerNorm(d_model, eps=layer_norm_eps, bias=bias, **factory_kwargs)
        self.norm2 = LayerNorm(d_model, eps=layer_norm_eps, bias=bias, **factory_kwargs)
        self.norm3 = LayerNorm(d_model, eps=layer_norm_eps, bias=bias, **factory_kwargs)
        self.dropout1 = Dropout(dropout)
        self.dropout2 = Dropout(dropout)
        self.dropout3 = Dropout(dropout)

        # Legacy string support for activation function.
        if isinstance(activation, str):
            self.activation = _get_activation_fn(activation)
        else:
            self.activation = activation

    def __setstate__(self, state):
        if "activation" not in state:
            state["activation"] = F.relu
        super().__setstate__(state)

    def forward(
        self,
        tgt: Tensor,
        tgt_mask: Optional[Tensor] = None,
        tgt_key_padding_mask: Optional[Tensor] = None,
        tgt_is_causal: bool = False,
    ) -> Tensor:
        r"""Pass the inputs (and mask) through the decoder layer."""

        # see Fig. 1 of https://arxiv.org/pdf/2002.04745v1.pdf

        x = tgt
        if self.norm_first:
            x = x + self._sa_block(
                self.norm1(x), tgt_mask, tgt_key_padding_mask, tgt_is_causal
            )
            x = x + self._mha_block(
                self.norm2(x), tgt_mask, tgt_key_padding_mask, tgt_is_causal
            )
            x = x + self._ff_block(self.norm3(x))
        else:
            x = self.norm1(
                x + self._sa_block(x, tgt_mask, tgt_key_padding_mask, tgt_is_causal)
            )
            x = self.norm2(
                x + self._mha_block(x, tgt_mask, tgt_key_padding_mask, tgt_is_causal)
            )
            x = self.norm3(x + self._ff_block(x))

        return x

    # self-attention block
    def _sa_block(
        self,
        x: Tensor,
        attn_mask: Optional[Tensor],
        key_padding_mask: Optional[Tensor],
        is_causal: bool = False,
    ) -> Tensor:
        x = self.self_attn(
            x,
            x,
            x,
            attn_mask=attn_mask,
            key_padding_mask=key_padding_mask,
            is_causal=is_causal,
            need_weights=False,
        )[0]
        return self.dropout1(x)

    # multihead attention block
    def _mha_block(
        self,
        x: Tensor,
        attn_mask: Optional[Tensor],
        key_padding_mask: Optional[Tensor],
        is_causal: bool = False,
    ) -> Tensor:
        x = self.multihead_attn(
            x,
            x,
            x,
            attn_mask=attn_mask,
            key_padding_mask=key_padding_mask,
            is_causal=is_causal,
            need_weights=False,
        )[0]
        return self.dropout2(x)

    # feed forward block
    def _ff_block(self, x: Tensor) -> Tensor:
        x = self.linear2(self.dropout(self.activation(self.linear1(x))))
        return self.dropout3(x)


def _get_clones(module, N):
    # FIXME: copy.deepcopy() is not defined on nn.module
    return ModuleList([copy.deepcopy(module) for i in range(N)])


def _get_activation_fn(activation: str) -> Callable[[Tensor], Tensor]:
    if activation == "relu":
        return F.relu
    elif activation == "gelu":
        return F.gelu

    raise RuntimeError(f"activation should be relu/gelu, not {activation}")


def _detect_is_causal_mask(
    mask: Optional[Tensor],
    is_causal: Optional[bool] = None,
    size: Optional[int] = None,
) -> bool:
    """Return whether the given attention mask is causal."""
    # Prevent type refinement
    make_causal = is_causal is True

    if is_causal is None and mask is not None:
        sz = size if size is not None else mask.size(-2)
        # ruff: noqa: F821
        causal_comparison = _generate_square_subsequent_mask(
            sz, device=mask.device, dtype=mask.dtype
        )

        # Do not use `torch.equal` so we handle batched masks by
        # broadcasting the comparison.
        if mask.size() == causal_comparison.size():
            make_causal = bool((mask == causal_comparison).all())
        else:
            make_causal = False

    return make_causal


def _get_seq_len(src: Tensor, batch_first: bool) -> Optional[int]:
    if src.is_nested:
        return None
    else:
        src_size = src.size()
        if len(src_size) == 2:
            # unbatched: S, E
            return src_size[0]
        else:
            # batched: B, S, E if batch_first else S, B, E
            seq_len_pos = 1 if batch_first else 0
            return src_size[seq_len_pos]