src/models/mdgt/dynamic_graph.py

from __future__ import annotations

"""
Dynamic k-NN Graph Construction (DGCNN-style).

Core idea — "dynamic" graph:
    At every layer the graph is rebuilt based on the *current* feature space,
    NOT just the initial (x, y) coordinates.

    Layer 1:  neighbours = physically close minutiae (embedding ≈ projected coords)
    Layer L:  neighbours = *semantically* similar minutiae (learned features)
              → two minutiae far apart spatially can become neighbours if their
                 learned representations are similar.

    Graph⁽ˡ⁾: N(i) = KNN(hᵢ⁽ˡ⁾, {hⱼ⁽ˡ⁾}ⱼ₌₁ᴺ, k)

This enables the model to discover semantic similarity beyond spatial proximity
as depth increases — a key advantage over static graph approaches.

Exported API:
    knn(x, k, mask, metric)       → (B, N, k) neighbour indices
    gather_neighbours(x, idx)     → (B, N, k, D) gathered features
    graph_divergence(idx_a, idx_b, mask) → (B,) mean Jaccard distance per sample
"""

import torch


def knn(
    x: torch.Tensor,
    k: int,
    mask: torch.Tensor | None = None,
    metric: str = "euclidean",
) -> torch.Tensor:
    """Compute k-nearest-neighbour indices in feature space.

    Args:
        x:      (B, N, D) point features — the current layer's representation.
        k:      number of neighbours (including self).
        mask:   (B, N) bool — True for real minutiae, False for padding.
                Padded points are pushed infinitely far away so they are
                never chosen as neighbours.
        metric: ``"euclidean"`` or ``"cosine"`` distance.

    Returns:
        idx:    (B, N, k) indices of the k nearest neighbours per node.
    """
    # Push padded positions to infinity so they're never nearest
    if mask is not None:
        large = torch.finfo(x.dtype).max / 2
        x = x.masked_fill(~mask.unsqueeze(-1), large)

    if metric == "cosine":
        x_norm = torch.nn.functional.normalize(x, dim=-1)
        sim = torch.bmm(x_norm, x_norm.transpose(1, 2))   # (B, N, N)
        _, idx = sim.topk(k, dim=-1, largest=True)
    else:
        # Squared Euclidean: ||a−b||² = ||a||² + ||b||² − 2⟨a,b⟩
        inner = torch.bmm(x, x.transpose(1, 2))            # (B, N, N)
        xx = (x * x).sum(dim=-1, keepdim=True)              # (B, N, 1)
        dist = xx + xx.transpose(1, 2) - 2.0 * inner       # (B, N, N)
        _, idx = dist.topk(k, dim=-1, largest=False)        # smallest dist
    return idx


def graph_divergence(
    idx_a: torch.Tensor,
    idx_b: torch.Tensor,
    mask: torch.Tensor | None = None,
) -> torch.Tensor:
    """Measure how much the k-NN graph changed between two layers.

    For each node i, computes Jaccard distance between its neighbour sets:

        divergence(i) = 1 − |N_a(i) ∩ N_b(i)| / |N_a(i) ∪ N_b(i)|

    Returns the mean divergence per sample in the batch.

    Args:
        idx_a: (B, N, k) — neighbour indices from layer l.
        idx_b: (B, N, k) — neighbour indices from layer l+1.
        mask:  (B, N) bool — True for real minutiae. Padded nodes excluded.

    Returns:
        div:   (B,) — mean Jaccard distance per sample (0 = identical, 1 = disjoint).
    """
    B, N, k = idx_a.shape

    # Convert neighbour indices to one-hot sets for intersection/union
    # (B, N, k) → (B, N, N) binary adjacency via scatter
    def _to_adj(idx: torch.Tensor) -> torch.Tensor:
        adj = torch.zeros(B, N, N, device=idx.device, dtype=torch.float32)
        src = torch.ones_like(idx, dtype=torch.float32)
        adj.scatter_(2, idx, src)
        return adj

    adj_a = _to_adj(idx_a)  # (B, N, N)
    adj_b = _to_adj(idx_b)

    intersection = (adj_a * adj_b).sum(dim=-1)           # (B, N)
    union = ((adj_a + adj_b) > 0).float().sum(dim=-1)    # (B, N)

    jaccard = intersection / union.clamp(min=1.0)        # (B, N)
    divergence = 1.0 - jaccard                           # (B, N)

    if mask is not None:
        divergence = divergence * mask.float()
        div_per_sample = divergence.sum(dim=-1) / mask.float().sum(dim=-1).clamp(min=1.0)
    else:
        div_per_sample = divergence.mean(dim=-1)

    return div_per_sample  # (B,)


def gather_neighbours(
    x: torch.Tensor,
    idx: torch.Tensor,
) -> torch.Tensor:
    """Gather features of k neighbours for every node.

    Args:
        x:   (B, N, D) — node features (any dimension).
        idx: (B, N, k) — neighbour indices from :func:`knn`.

    Returns:
        out: (B, N, k, D) — neighbour features per node.
    """
    B, N, D = x.shape
    k = idx.shape[-1]
    idx_exp = idx.unsqueeze(-1).expand(B, N, k, D)      # (B, N, k, D)
    x_exp = x.unsqueeze(1).expand(B, N, N, D)           # (B, N, N, D)
    return torch.gather(x_exp, 2, idx_exp)               # (B, N, k, D)