src/2.1_lrmc_bilevel.py

# lrmc_bilevel.py
# Top-1 LRMC ablation: one-cluster pooling vs. plain GCN on Planetoid (e.g., Cora)
# Requires: torch, torch_geometric, torch_scatter, torch_sparse
#
# Usage examples:
#   python lrmc_bilevel.py --dataset Cora --seeds /path/to/lrmc_seeds.json --variant baseline
#   python lrmc_bilevel.py --dataset Cora --seeds /path/to/lrmc_seeds.json --variant pool
#
# Notes:
#  - We read your LRMC JSON, pick the single cluster with the highest 'score',
#    assign it to cluster id 0, and make all other nodes singletons (1..K-1).
#  - For --variant pool: Node-GCN -> pool (means) -> Cluster-GCN -> broadcast + skip -> Node-GCN -> classifier
#  - For --variant baseline: Standard 2-layer GCN.
#  - Keep flags like --self_loop_scale and --use_a2 if you want A+λI / A^2 augmentation.

import argparse, json
from pathlib import Path
from typing import List, Tuple, Optional

import torch
import torch.nn as nn
import torch.nn.functional as F

from torch import Tensor
from torch_scatter import scatter_add, scatter_mean
from torch_sparse import coalesce, spspmm

from torch_geometric.datasets import Planetoid
from torch_geometric.nn import GCNConv


# ---------------------------
# Utilities: edges and seeds
# ---------------------------

def add_scaled_self_loops(edge_index: Tensor,
                          edge_weight: Optional[Tensor],
                          num_nodes: int,
                          scale: float = 1.0) -> Tuple[Tensor, Tensor]:
    """Add self-loops with chosen weight (scale). If scale=0, return unchanged (and create weights if None)."""
    if scale == 0.0:
        if edge_weight is None:
            edge_weight = torch.ones(edge_index.size(1), device=edge_index.device)
        return edge_index, edge_weight
    device = edge_index.device
    self_loops = torch.arange(num_nodes, device=device)
    self_index = torch.stack([self_loops, self_loops], dim=0)
    self_weight = torch.full((num_nodes,), float(scale), device=device)
    base_w = edge_weight if edge_weight is not None else torch.ones(edge_index.size(1), device=device)
    ei = torch.cat([edge_index, self_index], dim=1)
    ew = torch.cat([base_w, self_weight], dim=0)
    ei, ew = coalesce(ei, ew, num_nodes, num_nodes, op='add')
    return ei, ew


def adjacency_power(edge_index: Tensor, num_nodes: int, k: int = 2) -> Tensor:
    """
    Compute (binary) k-th power adjacency using sparse matmul (torch_sparse.spspmm).
    Here we use k=2. Returns coalesced edge_index without weights.
    """
    row, col = edge_index
    val = torch.ones(row.numel(), device=edge_index.device)
    Ai, Av = edge_index, val
    # A^2
    Ri, Rv = spspmm(Ai, Av, Ai, Av, num_nodes, num_nodes, num_nodes)
    mask = Ri[0] != Ri[1]  # drop diagonal; add custom self-loops later if desired
    Ri = Ri[:, mask]
    Ri, _ = coalesce(Ri, torch.ones(Ri.size(1), device=edge_index.device), num_nodes, num_nodes, op='add')
    return Ri


def build_cluster_graph(edge_index: Tensor,
                        num_nodes: int,
                        node2cluster: Tensor,
                        weight_per_edge: Optional[Tensor] = None,
                        num_clusters: Optional[int] = None
                        ) -> Tuple[Tensor, Tensor, int]:
    """
    Build cluster graph A_c = S^T A S with summed multiplicities as weights.
    node2cluster: [N] long tensor mapping each node -> cluster id.
    """
    K = int(node2cluster.max().item()) + 1 if num_clusters is None else num_clusters
    src, dst = edge_index
    csrc = node2cluster[src]
    cdst = node2cluster[dst]
    edge_c = torch.stack([csrc, cdst], dim=0)
    w = weight_per_edge if weight_per_edge is not None else torch.ones(edge_c.size(1), device=edge_c.device)
    edge_c, w = coalesce(edge_c, w, K, K, op='add')  # sum multiplicities
    return edge_c, w, K


# -----
# Seeds
# -----

def _pick_top1_cluster(obj: dict) -> List[int]:
    """
    From LRMC JSON with structure: {"clusters":[{"members":[...], "score":float, ...}, ...]}
    choose the cluster with max (score, size) and return its members.
    """
    clusters = obj.get("clusters", [])
    if not clusters:
        return []
    # choose by highest score, then by size (tiebreaker)
    best = max(clusters, key=lambda c: (float(c.get("score", 0.0)), len(c.get("members", []))))
    return list(best.get("members", []))


def load_top1_assignment(seeds_json: str, n_nodes: int) -> Tuple[Tensor, Tensor]:
    """
    Create a hard assignment for top-1 LRMC cluster:
      - cluster 0 = top-1 LRMC set
      - nodes outside are singletons (1..K-1)
    Returns:
      node2cluster: [N] long
      cluster_scores: [K,1] with 1.0 for top cluster, 0.0 for singletons
    """
    obj = json.loads(Path(seeds_json).read_text())
    C_star = _pick_top1_cluster(obj)
    C_star = torch.tensor(sorted(set(C_star)), dtype=torch.long)

    node2cluster = torch.full((n_nodes,), -1, dtype=torch.long)
    node2cluster[C_star] = 0
    outside = torch.tensor(sorted(set(range(n_nodes)) - set(C_star.tolist())), dtype=torch.long)
    if outside.numel() > 0:
        node2cluster[outside] = torch.arange(1, 1 + outside.numel(), dtype=torch.long)
    assert int(node2cluster.min()) >= 0, "All nodes must be assigned."

    K = 1 + outside.numel()
    cluster_scores = torch.zeros(K, 1, dtype=torch.float32)
    if C_star.numel() > 0:
        cluster_scores[0, 0] = 1.0  # emphasize the supercluster
    return node2cluster, cluster_scores


# --------------------------
# Models (baseline + pooled)
# --------------------------

class GCN2(nn.Module):
    """Plain 2-layer GCN baseline."""
    def __init__(self, in_dim, hid, out_dim):
        super().__init__()
        self.conv1 = GCNConv(in_dim, hid)
        self.conv2 = GCNConv(hid, out_dim)

    def forward(self, x, edge_index):
        x = F.relu(self.conv1(x, edge_index))
        x = F.dropout(x, p=0.5, training=self.training)
        x = self.conv2(x, edge_index)
        return x


class OneClusterPool(nn.Module):
    """
    Node-GCN -> pool to one-cluster + singletons -> Cluster-GCN -> broadcast + skip -> Node-GCN -> classifier
    """
    def __init__(self,
                 in_dim: int,
                 hid: int,
                 out_dim: int,
                 node2cluster: Tensor,
                 edge_index_node: Tensor,
                 num_nodes: int,
                 self_loop_scale: float = 0.0,
                 use_a2_for_clusters: bool = False):
        super().__init__()
        self.n2c = node2cluster.long()
        self.K = int(self.n2c.max().item()) + 1

        # Node graph (A + λI if desired)
        ei_node = edge_index_node
        ei_node, ew_node = add_scaled_self_loops(ei_node, None, num_nodes, scale=self_loop_scale)
        self.register_buffer("edge_index_node", ei_node)
        self.register_buffer("edge_weight_node", ew_node)

        # Cluster graph from A or A^2
        ei_for_c = adjacency_power(edge_index_node, num_nodes, k=2) if use_a2_for_clusters else edge_index_node
        edge_index_c, edge_weight_c, K = build_cluster_graph(ei_for_c, num_nodes, self.n2c)
        self.register_buffer("edge_index_c", edge_index_c)
        self.register_buffer("edge_weight_c", edge_weight_c)
        self.K = K

        # Layers
        self.gcn_node1 = GCNConv(in_dim, hid, add_self_loops=False, normalize=True)
        self.gcn_cluster = GCNConv(hid, hid, add_self_loops=True,  normalize=True)
        self.gcn_node2 = GCNConv(hid * 2, out_dim)  # on concatenated [h_node, h_broadcast]

    def forward(self, x: Tensor, edge_index_node: Tensor) -> Tensor:
        # Node GCN (uses stored weights)
        h1 = F.relu(self.gcn_node1(x, self.edge_index_node, self.edge_weight_node))

        # Pool to clusters: mean per cluster
        z = scatter_mean(h1, self.n2c, dim=0, dim_size=self.K)  # [K, H]

        # Cluster GCN
        z2 = F.relu(self.gcn_cluster(z, self.edge_index_c, self.edge_weight_c))

        # Broadcast back + skip concat
        hb = z2[self.n2c]                    # [N, H]
        hcat = torch.cat([h1, hb], dim=1)    # [N, 2H]

        # Final node GCN head -> logits
        out = self.gcn_node2(hcat, edge_index_node)
        return out


# -------------
# Training glue
# -------------

@torch.no_grad()
def accuracy(logits: Tensor, y: Tensor, mask: Tensor) -> float:
    pred = logits[mask].argmax(dim=1)
    return (pred == y[mask]).float().mean().item()


def run_train_eval(model: nn.Module, data, epochs=200, lr=0.01, wd=5e-4):
    opt = torch.optim.Adam(model.parameters(), lr=lr, weight_decay=wd)
    best_val, best_state = 0.0, None
    for ep in range(1, epochs + 1):
        model.train()
        opt.zero_grad(set_to_none=True)
        logits = model(data.x, data.edge_index)
        loss = F.cross_entropy(logits[data.train_mask], data.y[data.train_mask])
        loss.backward(); opt.step()

        # track best on val
        model.eval()
        logits = model(data.x, data.edge_index)
        val_acc = accuracy(logits, data.y, data.val_mask)
        if val_acc > best_val:
            best_val, best_state = val_acc, {k: v.detach().clone() for k, v in model.state_dict().items()}
        if ep % 20 == 0:
            tr = accuracy(logits, data.y, data.train_mask)
            te = accuracy(logits, data.y, data.test_mask)
            print(f"[{ep:04d}] loss={loss.item():.4f}  train={tr:.3f}  val={val_acc:.3f}  test={te:.3f}")

    # test @ best val
    if best_state is not None:
        model.load_state_dict(best_state)
    model.eval()
    logits = model(data.x, data.edge_index)
    return {
        "val": accuracy(logits, data.y, data.val_mask),
        "test": accuracy(logits, data.y, data.test_mask)
    }


# -----------
# Entrypoint
# -----------

def main():
    ap = argparse.ArgumentParser()
    ap.add_argument("--dataset", required=True, choices=["Cora", "Citeseer", "Pubmed"])
    ap.add_argument("--seeds", required=True, help="Path to LRMC seeds JSON (single large graph).")
    ap.add_argument("--variant", choices=["baseline", "pool"], default="pool",
                    help="baseline=plain GCN; pool=top-1 LRMC one-cluster pooling")
    ap.add_argument("--hidden", type=int, default=128)
    ap.add_argument("--epochs", type=int, default=200)
    ap.add_argument("--lr", type=float, default=0.01)
    ap.add_argument("--wd", type=float, default=5e-4)
    ap.add_argument("--dropout", type=float, default=0.5)  # used in baseline only
    ap.add_argument("--self_loop_scale", type=float, default=0.0, help="λ for A+λI on node graph (0 disables)")
    ap.add_argument("--use_a2", action="store_true", help="Use A^2 to build the cluster graph (recommended for pool)")
    ap.add_argument("--seed", type=int, default=42)
    args = ap.parse_args()

    torch.manual_seed(args.seed)

    # Load dataset
    ds = Planetoid(root=f"./data/{args.dataset}", name=args.dataset)
    data = ds[0]
    in_dim, out_dim, n = ds.num_node_features, ds.num_classes, data.num_nodes

    if args.variant == "baseline":
        model = GCN2(in_dim, args.hidden, out_dim)
        # use default add_self_loops=True behavior inside convs
        res = run_train_eval(model, data, epochs=args.epochs, lr=args.lr, wd=args.wd)
        print(f"Baseline GCN: val={res['val']:.4f}  test={res['test']:.4f}")
        return

    # Top-1 LRMC assignment
    node2cluster, _ = load_top1_assignment(args.seeds, n)

    # One-cluster pooled model
    model = OneClusterPool(in_dim=in_dim,
                           hid=args.hidden,
                           out_dim=out_dim,
                           node2cluster=node2cluster,
                           edge_index_node=data.edge_index,
                           num_nodes=n,
                           self_loop_scale=args.self_loop_scale,
                           use_a2_for_clusters=args.use_a2)
    res = run_train_eval(model, data, epochs=args.epochs, lr=args.lr, wd=args.wd)
    print(f"L-RMC (top-1 pool): val={res['val']:.4f}  test={res['test']:.4f}")


if __name__ == "__main__":
    main()