src/cross_features/ttp.py

"""TTP — Token-Trajectory Persistence (cross-layer per-token).

For each token t in the active sequence, its hidden-state trajectory
through the encoder is the polyline
    v^t = (h_0^t, h_1^t, ..., h_L^t)   in   R^{(L+1) x D}.

We extract two complementary signal blocks (~21 scalars total):

  (A) Cloud-topology block (14 scalars).
      Build the T x T distance matrix D_{ij} = mean_l (1 - cos(h_l^i, h_l^j))
      (per-layer cosine distance averaged across all L+1 layers).
      Run Ripser up to H1 on D and summarise both diagrams by 7 scalars:
      bar count, sum/max persistence, top-2 persistence, entropy, mean
      bar midpoint.

  (B) Per-token shape block (7 scalars).
      For each token compute:
        - trajectory length     L^t = sum_l ||h_{l+1}^t - h_l^t||
        - step-to-step cosine   s_l^t = cos(h_l^t, h_{l+1}^t) (mean over l)
        - net displacement      N^t = ||h_L^t - h_0^t||
        - curvature             k^t = mean_l (1 - cos(h_{l+1}-h_l, h_l-h_{l-1}))
      Then aggregate across tokens: mean(L), std(L), mean(s), std(s),
      mean(N), mean(k), std(k).

Admissible: pure hidden-state geometry. No W_cls, no logits, no softmax.
Orthogonal axis to ZAP (which uses attention edges, not hidden states).
"""
from __future__ import annotations

import numpy as np
from ripser import ripser


def _pd_stats(pd: np.ndarray) -> np.ndarray:
    """7-scalar persistence-diagram summary."""
    if len(pd) == 0:
        return np.zeros(7, dtype=np.float32)
    finite = pd[np.isfinite(pd[:, 1])]
    if len(finite) == 0:
        return np.zeros(7, dtype=np.float32)
    lengths = finite[:, 1] - finite[:, 0]
    if len(lengths) == 0:
        return np.zeros(7, dtype=np.float32)
    sort_l = np.sort(lengths)[::-1]
    n = float(len(lengths))
    s = float(lengths.sum())
    top1 = float(sort_l[0]) if len(sort_l) >= 1 else 0.0
    top2 = float(sort_l[1]) if len(sort_l) >= 2 else 0.0
    p = lengths / max(s, 1e-12)
    ent = float(-(p[p > 0] * np.log(p[p > 0])).sum())
    midpoints = (finite[:, 0] + finite[:, 1]) / 2.0
    mid_mean = float(midpoints.mean())
    return np.array([n, s, top1, top2, ent, float(sort_l[-1]) if len(sort_l) else 0.0, mid_mean],
                    dtype=np.float32)


COLUMNS = (
    # H_0 block
    [f"ttp_h0_{s}" for s in ("count", "sum", "max", "top2", "ent", "min", "mid")]
    + [f"ttp_h1_{s}" for s in ("count", "sum", "max", "top2", "ent", "min", "mid")]
    # Shape block
    + ["ttp_len_mean", "ttp_len_std",
       "ttp_stepcos_mean", "ttp_stepcos_std",
       "ttp_netdisp_mean",
       "ttp_curv_mean", "ttp_curv_std"]
)
DIM = len(COLUMNS)


def extract_ttp(model, input_ids, attention_mask, cache, pred_label=None):
    """Return (features (B, DIM), columns).

    Reads cache.hidden_states: list of (L+1) tensors each (B, T, D).
    """
    B = input_ids.shape[0]
    L_total = len(cache.hidden_states)
    feats = np.zeros((B, DIM), dtype=np.float32)

    for b in range(B):
        T_b = int(attention_mask[b].sum().item())
        T_max = cache.hidden_states[0].shape[1]
        T = max(min(T_b, T_max), 4)

        # Stack: (L+1, T, D)
        hs = np.stack(
            [cache.hidden_states[l][b, :T].detach().float().cpu().numpy()
             for l in range(L_total)],
            axis=0,
        )

        # (A) Cloud-topology: T x T distance matrix
        # per-layer cosine distance, averaged across layers
        norms = np.linalg.norm(hs, axis=-1, keepdims=True)         # (L+1, T, 1)
        unit = hs / np.maximum(norms, 1e-9)                          # (L+1, T, D)
        # per-layer sim: (L+1, T, T)
        sim = np.einsum("ltd,lsd->lts", unit, unit)
        dist = (1.0 - sim).mean(axis=0)                              # (T, T)
        dist = 0.5 * (dist + dist.T)
        np.fill_diagonal(dist, 0.0)
        dist = np.clip(dist, 0.0, None)
        try:
            res = ripser(dist, distance_matrix=True, maxdim=1)
            pds = res["dgms"]
        except Exception:
            pds = [np.zeros((0, 2), dtype=np.float32),
                   np.zeros((0, 2), dtype=np.float32)]
        h0_feats = _pd_stats(pds[0])
        h1_feats = _pd_stats(pds[1]) if len(pds) > 1 else np.zeros(7, dtype=np.float32)

        # (B) Per-token shape statistics
        # deltas: (L, T, D)
        deltas = hs[1:] - hs[:-1]
        delta_norms = np.linalg.norm(deltas, axis=-1)                # (L, T)
        traj_len = delta_norms.sum(axis=0)                            # (T,)
        net_disp = np.linalg.norm(hs[-1] - hs[0], axis=-1)            # (T,)
        # step-to-step cosine between consecutive hidden states (not deltas)
        h_norm = np.linalg.norm(hs, axis=-1, keepdims=True)
        h_unit = hs / np.maximum(h_norm, 1e-9)
        stepcos = (h_unit[:-1] * h_unit[1:]).sum(axis=-1)             # (L, T)
        # curvature: angle between consecutive deltas
        if deltas.shape[0] >= 2:
            d_norm = np.linalg.norm(deltas, axis=-1, keepdims=True)
            d_unit = deltas / np.maximum(d_norm, 1e-9)
            curv = 1.0 - (d_unit[:-1] * d_unit[1:]).sum(axis=-1)      # (L-1, T)
            curv_mean = float(curv.mean())
            curv_std = float(curv.std())
        else:
            curv_mean = curv_std = 0.0

        shape_feats = np.array([
            float(traj_len.mean()), float(traj_len.std()),
            float(stepcos.mean()), float(stepcos.std()),
            float(net_disp.mean()),
            curv_mean, curv_std,
        ], dtype=np.float32)

        feats[b, :7] = h0_feats
        feats[b, 7:14] = h1_feats
        feats[b, 14:] = shape_feats

    return feats, COLUMNS