scripts/split_prune.py

"""
SPLIT: Salience-guided Partitioning towards Local coverage for Importance-aware
Token dropping.  Faithful, training-free reimplementation of Algorithm 1 from
SPLIT-VLM (ICML 2026, OpenReview Elm4TdaXi0).

Given the per-layer ViT hidden states of the vision patch tokens and the final
patch-token embeddings, SPLIT selects a subset of `budget` tokens by:
  1. temporal-shift importance   I(x) = mean_l ||h_l - h_{l-1}|| / ||h_l||   (Eq.5-6)
  2. adaptive region budget       B_k = B/K + B * I(X_k)/sum_j I(X_j)          (Eq.7)
  3. diversity-score selection     D(i) = lambda*std_j(S_ij) - mean_j(S_ij)     (Eq.8-10)
     select top-B_k tokens by D within each region.

All operations are batch-size-1 (one image) and vectorised in torch.
"""
from __future__ import annotations
import math
import torch


def temporal_shift_importance(hidden_states, layers=None):
    """
    hidden_states: list/tuple of length L+1, each [N, d]  (patch tokens only,
                   CLS already removed), h[0] = embeddings, h[l] = after layer l.
    layers: iterable of layer indices l>=1 to use (default: all).
    returns I: [N] importance per token (Eq. 6).
    """
    L = len(hidden_states) - 1
    if layers is None:
        layers = range(1, L + 1)
    layers = [l for l in layers if 1 <= l <= L]
    acc = None
    for l in layers:
        h = hidden_states[l]
        hprev = hidden_states[l - 1]
        num = torch.linalg.vector_norm(h - hprev, dim=-1)          # ||h_l - h_{l-1}||
        den = torch.linalg.vector_norm(h, dim=-1).clamp_min(1e-6)  # ||h_l||
        delta = num / den
        acc = delta if acc is None else acc + delta
    return acc / len(layers)


def region_ids_grid(num_tokens, grid_hw, region_grid):
    """
    Map each patch token to a region id for a square grid of patches.
    grid_hw = (H, W) patch grid (e.g. 24,24); region_grid = (rh, rw) e.g. (4,4)=16 regions.
    returns LongTensor [N] with region id in [0, rh*rw).
    """
    H, W = grid_hw
    rh, rw = region_grid
    assert H * W == num_tokens, f"{H}*{W} != {num_tokens}"
    idx = torch.arange(num_tokens)
    row = idx // W
    col = idx % W
    # region row/col via proportional binning (handles non-divisible grids)
    r_row = (row * rh // H).clamp(max=rh - 1)
    r_col = (col * rw // W).clamp(max=rw - 1)
    return (r_row * rw + r_col).long()


def allocate_region_budgets(importance, region_ids, K, budget):
    """
    Eq. 7 hybrid allocation, then integer rounding that sums exactly to `budget`
    and never exceeds region population.
    importance: [N]; region_ids: [N]; K regions; budget total tokens to keep.
    returns dict region_id -> int budget.
    """
    device = importance.device
    reg_imp = torch.zeros(K, device=device)
    reg_cnt = torch.zeros(K, device=device)
    reg_imp.scatter_add_(0, region_ids, importance)
    reg_cnt.scatter_add_(0, region_ids, torch.ones_like(importance))
    # region-level importance = mean token importance within region
    mean_imp = torch.where(reg_cnt > 0, reg_imp / reg_cnt.clamp_min(1), torch.zeros_like(reg_imp))
    total_imp = mean_imp.sum().clamp_min(1e-9)
    # B_k = B/K + B * I(X_k)/sum_j I(X_j)   (Eq. 7).  As written this sums to 2*B
    # (both terms individually sum to B), so we renormalise the hybrid allocation
    # to distribute exactly the total budget B (i.e. a 50/50 uniform+importance mix).
    b_float = budget / K + budget * (mean_imp / total_imp)
    # zero out empty regions
    b_float = torch.where(reg_cnt > 0, b_float, torch.zeros_like(b_float))
    b_float = b_float * (budget / b_float.sum().clamp_min(1e-9))
    # cap by population (redistribute happens via largest-remainder below)
    b_float = torch.minimum(b_float, reg_cnt)
    # round preserving the exact total via largest-remainder
    floor = torch.floor(b_float)
    rem = b_float - floor
    alloc = floor.clone()
    deficit = int(round(budget - float(alloc.sum().item())))
    if deficit > 0:
        # give +1 to regions with largest remainder that still have capacity
        cap = (reg_cnt - alloc)
        order = torch.argsort(rem * (cap > 0), descending=True)
        i = 0
        while deficit > 0 and i < len(order):
            k = order[i].item()
            if cap[k] > 0:
                alloc[k] += 1
                cap[k] -= 1
                deficit -= 1
            i += 1
            if i >= len(order) and deficit > 0:
                # loop again over any region with capacity
                cap = (reg_cnt - alloc)
                order = torch.argsort(cap, descending=True)
                i = 0
                if float(cap.max()) <= 0:
                    break
    elif deficit < 0:
        order = torch.argsort(rem)  # smallest remainder first -> remove
        i = 0
        while deficit < 0 and i < len(order):
            k = order[i].item()
            if alloc[k] > 0:
                alloc[k] -= 1
                deficit += 1
            i += 1
    return {k: int(alloc[k].item()) for k in range(K)}


def diversity_scores(token_embeds, lam=0.5, chunk=None):
    """
    Eq. 8-10.  token_embeds: [N, d].  D(i) = lam*sigma_i - mu_i, where S is the
    cosine self-similarity matrix.  Returns D: [N].
    """
    x = torch.nn.functional.normalize(token_embeds.float(), dim=-1)
    N = x.shape[0]
    # S = x x^T  (N x N); compute mu, sigma row-wise (optionally chunked for memory)
    if chunk is None or N <= chunk:
        S = x @ x.t()
        mu = S.mean(dim=1)
        sigma = S.std(dim=1, unbiased=False)
    else:
        mu = torch.empty(N, device=x.device)
        sigma = torch.empty(N, device=x.device)
        for s in range(0, N, chunk):
            e = min(s + chunk, N)
            Sc = x[s:e] @ x.t()
            mu[s:e] = Sc.mean(dim=1)
            sigma[s:e] = Sc.std(dim=1, unbiased=False)
    return lam * sigma - mu


def split_select(hidden_states, token_embeds, budget,
                 grid_hw=(24, 24), region_grid=(4, 4), layers=None, lam=0.5):
    """
    Full SPLIT selection. Returns sorted LongTensor of kept token indices (len=budget).
    hidden_states: per-layer ViT hidden states for the N patch tokens (CLS removed).
    token_embeds:  [N, d] final patch embeddings used for diversity.
    """
    N = token_embeds.shape[0]
    if budget >= N:
        return torch.arange(N, device=token_embeds.device)
    K = region_grid[0] * region_grid[1]
    device = token_embeds.device
    imp = temporal_shift_importance(hidden_states, layers).to(device)      # [N]
    rid = region_ids_grid(N, grid_hw, region_grid).to(device)             # [N]
    budgets = allocate_region_budgets(imp, rid, K, budget)
    D = diversity_scores(token_embeds, lam=lam)                            # [N]
    keep = []
    for k in range(K):
        bk = budgets[k]
        if bk <= 0:
            continue
        mask = (rid == k).nonzero(as_tuple=True)[0]
        if mask.numel() == 0:
            continue
        dk = D[mask]
        topk = torch.topk(dk, min(bk, mask.numel())).indices
        keep.append(mask[topk])
    keep = torch.cat(keep) if keep else torch.arange(min(budget, N), device=device)
    return torch.sort(keep).values


# ---- baselines for comparison (Claim 3 control) ----
def random_select(N, budget, generator=None, device="cpu"):
    if budget >= N:
        return torch.arange(N, device=device)
    perm = torch.randperm(N, generator=generator)[:budget]
    return torch.sort(perm).values.to(device)


def attention_select(cls_attn, budget):
    """FastV/HiRED-style: keep top-`budget` tokens by CLS->patch attention.
    cls_attn: [N] attention from CLS to each patch (already averaged over heads)."""
    N = cls_attn.shape[0]
    if budget >= N:
        return torch.arange(N, device=cls_attn.device)
    idx = torch.topk(cls_attn, budget).indices
    return torch.sort(idx).values