src/models/concept_head.py

from __future__ import annotations

import torch
import torch.nn as nn

# Shared concept name registry
_CONCEPT_NAMES = [
    "orientation_coherence",
    "ridge_valley_clarity",
    "continuity",
    "noise_level",
    "contrast_uniformity",
    "minutiae_reliability",
]


class ConceptHead(nn.Module):
    """Predict concept activations in [0, 1] from globally-pooled features.

    Legacy architecture — inputs a [B, D] vector (global-average-pooled backbone
    output).  Kept for backward compatibility with checkpoints v16–v26.

    For new experiments use SpatialConceptHead which operates on the full
    14×14 spatial token map and better captures spatial quality concepts such
    as orientation_coherence, continuity, and minutiae_reliability.
    """

    CONCEPT_NAMES = _CONCEPT_NAMES
    uses_spatial: bool = False

    def __init__(self, in_dim: int, k: int = 6, hidden_dim: int = 256):
        super().__init__()
        self.mlp = nn.Sequential(
            nn.Linear(in_dim, hidden_dim),
            nn.LayerNorm(hidden_dim),
            nn.GELU(),
            nn.Linear(hidden_dim, k),
            nn.Sigmoid(),
        )

    def forward(self, features: torch.Tensor) -> torch.Tensor:
        """Args:
            features: [B, D] globally-pooled backbone features.
        Returns:
            concepts: [B, k] each ∈ (0, 1).
        """
        return self.mlp(features)


class SpatialConceptHead(nn.Module):
    """Predict concept activations from spatial token features [B, N, D].

    Architecture
    ------------
    Shared trunk : Linear(D → hidden_dim) → LayerNorm → GELU  →  [B, N, hidden_dim]
    Per-concept  : Linear(hidden_dim → 1) → mean over N        →  scalar
    Activation   : Sigmoid                                      →  (0, 1)

    Using spatial tokens (instead of the globally-pooled vector) lets each
    concept attend to different image regions:

    - orientation_coherence  : local ridge flow consistency across patches
    - continuity             : ridge break locations
    - minutiae_reliability   : bifurcation / ridge-ending regions

    Separate per-concept projection weights reduce cross-concept entanglement
    compared to a single shared MLP that outputs all k values simultaneously.
    The shared trunk amortises the cost of the first linear projection across
    all 196 tokens.
    """

    CONCEPT_NAMES = _CONCEPT_NAMES
    uses_spatial: bool = True

    def __init__(self, in_dim: int, k: int = 6, hidden_dim: int = 128):
        super().__init__()
        self.trunk = nn.Sequential(
            nn.Linear(in_dim, hidden_dim),
            nn.LayerNorm(hidden_dim),
            nn.GELU(),
        )
        # k independent projections — each learns which spatial regions matter
        # for its concept (reduces entanglement vs. a single shared Linear→k)
        self.concept_projs = nn.ModuleList([
            nn.Linear(hidden_dim, 1) for _ in range(k)
        ])
        self.k = k

    def forward(self, spatial: torch.Tensor) -> torch.Tensor:
        """Args:
            spatial: [B, N, D] spatial token features from backbone.forward_spatial().
                     N = 196 (14×14 patches for 224-px input), D = 320 for TinyViT-5M.
        Returns:
            concepts: [B, k] each ∈ (0, 1), high = better quality for that concept.
        """
        h = self.trunk(spatial)  # [B, N, hidden_dim]
        # Each concept proj: [B, N, 1] → mean over N → [B, 1]
        concepts = torch.cat(
            [proj(h).mean(dim=1) for proj in self.concept_projs],  # k × [B, 1]
            dim=1,
        )  # [B, k]
        return torch.sigmoid(concepts)