"""
BilinearLatentDecoder for grn_att_only: Q@K^T bilinear decoder.
Replaces LatentEmbedder/LatentDecoder from grn_ccfm.
"""

import torch
import torch.nn as nn


class BilinearLatentDecoder(nn.Module):
    """Q@K^T bilinear decoder: (B, G, d_model) -> (B, G, G)"""

    def __init__(self, d_model: int = 128, head_dim: int = 128):
        super().__init__()
        self.q_proj = nn.Linear(d_model, head_dim)
        self.k_proj = nn.Linear(d_model, head_dim)
        self.scale = head_dim ** -0.5
        self.out_gate = nn.Parameter(torch.tensor([0.1]))  # small init: allow gradient flow

    def forward(self, x: torch.Tensor) -> torch.Tensor:
        """x: (B, G, d_model) -> (B, G, G)"""
        Q = self.q_proj(x)   # (B, G, head_dim)
        K = self.k_proj(x)   # (B, G, head_dim)
        return torch.bmm(Q, K.transpose(1, 2)) * self.scale * self.out_gate