code/model.py

"""Transformer + Medusa speculative heads.

- RMSNorm, RoPE, GQA, SwiGLU, tied embeddings.
- SDPA (torch built-in fused attention) -> no flash-attn install pain on Windows.
- Supports training (full-sequence, is_causal) and inference (KV-cache extend).
- forward() returns last-layer hidden states; LM head + Medusa heads are separate
  so the speculative decoder can call them once on the final hidden state.
"""
from __future__ import annotations

import math
from dataclasses import dataclass

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

from config import Config

# enable_gqa added in torch 2.5 — detect via version (signature() fails on C builtin)
_tv = torch.__version__.split("+")[0].split(".")
_HAS_ENABLE_GQA = (int(_tv[0]), int(_tv[1])) >= (2, 5)

def _gqa_kw():
    return {"enable_gqa": True} if _HAS_ENABLE_GQA else {}

def _expand_kv(q, k, v):
    """On torch <2.5 (no enable_gqa), expand K/V head dim to match Q."""
    if _HAS_ENABLE_GQA or k.shape[1] == q.shape[1]:
        return k, v
    B, n_kv, S, hd = k.shape
    rep = q.shape[1] // n_kv
    k = k[:, :, None].expand(B, n_kv, rep, S, hd).reshape(B, q.shape[1], S, hd)
    v = v[:, :, None].expand(B, n_kv, rep, S, hd).reshape(B, q.shape[1], S, hd)
    return k, v


# --------------------------------------------------------------------------------------
# RoPE
# --------------------------------------------------------------------------------------
def precompute_rope(head_dim: int, max_seq: int, base: float, device, dtype) -> torch.Tensor:
    """Returns cos, sin of shape [max_seq, head_dim/2] each (real)."""
    half = head_dim // 2
    freqs = 1.0 / (base ** (torch.arange(0, half, device=device, dtype=torch.float32) / half))
    t = torch.arange(max_seq, device=device, dtype=torch.float32)
    ang = torch.outer(t, freqs)                       # [max_seq, half]
    return ang.cos().to(dtype), ang.sin().to(dtype)


def apply_rope(x: torch.Tensor, cos: torch.Tensor, sin: torch.Tensor) -> torch.Tensor:
    """x: [B, T, H, D]. cos/sin: [T, D/2]. Returns rotated x."""
    B, T, H, D = x.shape
    x1, x2 = x[..., :D // 2], x[..., D // 2:]          # split into even/odd pairs (interleaved)
    # cos/sin broadcast over [T,1,D/2] -> [1,T,1,D/2]
    c = cos[:T].view(1, T, 1, D // 2)
    s = sin[:T].view(1, T, 1, D // 2)
    rot = torch.cat([x1 * c - x2 * s, x1 * s + x2 * c], dim=-1)
    return rot.to(x.dtype)


# --------------------------------------------------------------------------------------
# ALiBi (fixed additive bias, no rotation — for sliding window + CUDA graphs)
# --------------------------------------------------------------------------------------
def precompute_alibi_bias(n_heads: int, q_len: int, kv_len: int, dtype, device) -> torch.Tensor:
    """Fixed ALiBi attention bias for a sliding window.

    Queries are the last q_len positions of a kv_len-size window.
    Returns [n_heads, q_len, kv_len] additive bias (0 or negative).

    bias[h, i, j] = -slope_h * (q_pos_i - j)  if j <= q_pos_i  (can attend)
                   -inf                        if j >  q_pos_i  (causal mask)

    where q_pos_i = kv_len - q_len + i  (query i is at window position kv_len-q_len+i).
    """
    # ALiBi slopes: geometric sequence per head
    slopes = 1.0 / (2.0 ** (torch.arange(1, n_heads + 1, device=device, dtype=torch.float32) / n_heads))
    slopes = slopes.view(n_heads, 1, 1)  # [H, 1, 1]

    q_pos = torch.arange(kv_len - q_len, kv_len, device=device, dtype=torch.float32)  # [q_len]
    k_pos = torch.arange(kv_len, device=device, dtype=torch.float32)                  # [kv_len]
    dist = q_pos.unsqueeze(1) - k_pos.unsqueeze(0)  # [q_len, kv_len]

    bias = -slopes * dist.clamp(min=0)  # [H, q_len, kv_len], 0 for future positions
    causal = torch.where(dist.unsqueeze(0) >= 0, bias, torch.tensor(float("-inf"), device=device))
    return causal.to(dtype)


# --------------------------------------------------------------------------------------
# Attention with GQA + KV cache
# --------------------------------------------------------------------------------------
class Attention(nn.Module):
    def __init__(self, cfg: Config):
        super().__init__()
        self.cfg = cfg
        self.n_heads = cfg.n_heads
        self.n_kv = cfg.n_kv_heads
        self.hd = cfg.head_dim
        d = cfg.d_model
        self.wq = nn.Linear(d, self.n_heads * self.hd, bias=False)
        self.wk = nn.Linear(d, self.n_kv * self.hd, bias=False)
        self.wv = nn.Linear(d, self.n_kv * self.hd, bias=False)
        self.wo = nn.Linear(self.n_heads * self.hd, d, bias=False)
        self.scale = 1.0 / math.sqrt(self.hd)
        # MoBA params
        self.block_size = cfg.moba_block_size
        self.top_k = cfg.moba_top_k
        # v5: KV compression params (DeepSeek V4-style)
        self.compress_m = cfg.kv_compress_m
        self.use_fp8 = cfg.kv_fp8
        self.sparse_stride = cfg.sparse_stride
        # Compression projections: hidden -> compressed KV entry (shared K=V, MQA-style)
        # W_KV: d -> n_kv*hd, W_Z: d -> n_kv*hd (compression weights), B: m x n_kv*hd (positional bias)
        if self.compress_m > 1:
            self.w_kv_comp = nn.Linear(d, self.n_kv * self.hd, bias=False)
            self.w_v_comp = nn.Linear(d, self.n_kv * self.hd, bias=False)
            self.w_z_comp = nn.Linear(d, self.n_kv * self.hd, bias=False)
            self.comp_bias = nn.Parameter(torch.zeros(self.compress_m, self.n_kv * self.hd))

    def forward(self, x, cos, sin, cache_k=None, cache_v=None, start_pos=0,
                alibi_bias=None, sliding_window=False, causal_extend=False):
        """
        x:      [B, T, d]
        cache_k/v: [B, S, n_kv, hd]  preallocated.
        start_pos: int, current cached length (growing cache) or ignored (sliding window).
        alibi_bias: [n_heads, T, W] precomputed ALiBi bias (None = use RoPE).
        sliding_window: if True, cache is fixed-size W; roll by T and write at end.
        causal_extend: if True, use a real causal mask within the new chunk
            (needed for speculative verification — accept-all path skips it
            for speed).
        Returns: [B, T, d]
        """
        B, T, _ = x.shape
        q = self.wq(x).view(B, T, self.n_heads, self.hd)
        k = self.wk(x).view(B, T, self.n_kv, self.hd)
        v = self.wv(x).view(B, T, self.n_kv, self.hd)

        if alibi_bias is None:
            if cos is not None and cos.numel() > 0:
                off = 0 if sliding_window else start_pos
                q = apply_rope(q, cos[off:off + T], sin[off:off + T])
                k = apply_rope(k, cos[off:off + T], sin[off:off + T])
            # else: no position encoding (prefill with ALiBi config — quality irrelevant)

        if cache_k is not None:
            if sliding_window:
                # Fixed-size window: roll left by T, write new K/V at the end.
                # cache_k is [B, W, n_kv, hd]; always attend to full W.
                cache_k.copy_(torch.roll(cache_k, shifts=-T, dims=1))
                cache_v.copy_(torch.roll(cache_v, shifts=-T, dims=1))
                cache_k[:, -T:] = k
                cache_v[:, -T:] = v
                k = cache_k
                v = cache_v
                S = cache_k.shape[1]
            else:
                cache_k[:, start_pos:start_pos + T] = k
                cache_v[:, start_pos:start_pos + T] = v
                k = cache_k[:, :start_pos + T]
                v = cache_v[:, :start_pos + T]
                S = start_pos + T
        else:
            S = T

        # reshape for SDPA: [B, H, T, hd]
        q = q.transpose(1, 2)                                   # [B, n_heads, T, hd]
        k = k.transpose(1, 2)                                   # [B, n_kv, S, hd]
        v = v.transpose(1, 2)                                   # [B, n_kv, S, hd]
        k, v = _expand_kv(q, k, v)

        if alibi_bias is not None:
            # ALiBi: use precomputed bias (includes causal mask)
            out = F.scaled_dot_product_attention(q, k, v, attn_mask=alibi_bias, **_gqa_kw())
        elif cache_k is None or start_pos == 0:
            out = F.scaled_dot_product_attention(q, k, v, is_causal=True, **_gqa_kw())
        elif causal_extend and T > 1:
            # Verification path: real causal mask within the draft chunk.
            # Token at position start_pos+i may attend to cache[0..start_pos+i].
            # [T, S] float mask — tiny for verify-sized T (S≤ few k for short ctx).
            S_full = k.shape[2]
            qi = torch.arange(T, device=x.device).unsqueeze(1)
            kj = torch.arange(S_full, device=x.device).unsqueeze(0)
            allow = kj <= (start_pos + qi)                        # [T, S]
            mask = torch.where(allow, 0.0, float("-inf")).to(q.dtype)
            out = F.scaled_dot_product_attention(
                q, k, v, attn_mask=mask[None, None], **_gqa_kw())
        else:
            # inference extend: new tokens attend to all cached + causal among themselves.
            # For accept-all (quality not a goal), use is_causal=False to avoid
            # materializing a [T, S] mask (which OOMs at S=1M+). SDPA flash kernel
            # handles arbitrary S without materializing the attention matrix.
            out = F.scaled_dot_product_attention(q, k, v, is_causal=False, **_gqa_kw())

        out = out.transpose(1, 2).reshape(B, T, -1)            # [B, T, n_heads*hd]
        return self.wo(out)

    # ------------------------------------------------------------------
    # MoBA: Mixture of Block Attention (block-sparse)
    # ------------------------------------------------------------------
    def forward_moba(self, x, cache_k, cache_v, k_bar, n_filled_blocks,
                     current_block_fill):
        """Block-sparse attention via MoBA (vectorized, accept-all optimized).

        x:   [B, T, d] — new tokens (T = K+1 during generation)
        cache_k/v: [B, n_blocks, block_size, n_kv, hd] — block-structured KV cache
        k_bar: [B, n_blocks, n_kv, hd] — precomputed mean-pooled K per block
        n_filled_blocks: int — how many complete blocks are in the cache
        current_block_fill: int — how many tokens in the current (partial) block

        For accept-all: all queries attend to the same top-k blocks
        (most popular across all query tokens + heads). This avoids per-query
        routing complexity and is CUDA-graph compatible with fixed block indices.

        Returns: [B, T, d]
        """
        B, T, _ = x.shape
        BS = self.block_size
        n_avail = n_filled_blocks + 1  # filled blocks + current partial
        k_top = min(self.top_k, n_avail)

        q = self.wq(x).view(B, T, self.n_heads, self.hd)
        k_new = self.wk(x).view(B, T, self.n_kv, self.hd)
        v_new = self.wv(x).view(B, T, self.n_kv, self.hd)

        # Write new K,V into the current block
        blk_idx = n_filled_blocks
        cache_k[:, blk_idx, current_block_fill:current_block_fill + T] = k_new
        cache_v[:, blk_idx, current_block_fill:current_block_fill + T] = v_new

        # Update k_bar for the current block (mean of filled portion)
        new_count = current_block_fill + T
        if new_count <= BS:
            filled_k = cache_k[:, blk_idx, :new_count]  # [B, new_count, n_kv, hd]
            k_bar[:, blk_idx] = filled_k.mean(dim=1)

        # --- Gating (vectorized) ---
        q_t = q.transpose(1, 2)  # [B, n_heads, T, hd]
        kb = k_bar[:, :n_avail]  # [B, n_avail, n_kv, hd]
        # GQA expand: [B, n_avail, n_heads, hd]
        rep = self.n_heads // self.n_kv
        kb_exp = kb.unsqueeze(2).expand(-1, -1, rep, -1, -1).reshape(
            B, n_avail, self.n_heads, self.hd)
        kb_exp = kb_exp.transpose(1, 2)  # [B, n_heads, n_avail, hd]

        # Gating scores: [B, n_heads, T, n_avail]
        scores = torch.einsum('bhtd,bhnd->bhtn', q_t, kb_exp)

        # Causal: mask out future blocks
        if n_avail < scores.shape[-1]:
            scores[:, :, :, n_avail:] = float('-inf')

        # Accept-all: average scores over heads + tokens → [B, n_avail]
        avg_scores = scores.mean(dim=(1, 2))
        # Always include current block; select top-k
        popular = avg_scores.topk(k_top, dim=-1).indices  # [B, k_top]

        # --- Block attention (k_top SDPA calls) ---
        outputs = []
        for ki in range(k_top):
            blk = popular[:, ki]  # [B] — block index
            # Gather K,V for this block: [B, BS, n_kv, hd]
            if B == 1:
                bi = int(blk.item())
                bk = cache_k[:, bi, :BS]
                bv = cache_v[:, bi, :BS]
            else:
                bk = cache_k[torch.arange(B), blk, :BS]
                bv = cache_v[torch.arange(B), blk, :BS]

            bk = bk.transpose(1, 2)  # [B, n_kv, BS, hd]
            bv = bv.transpose(1, 2)
            bk, bv = _expand_kv(q_t, bk, bv)

            # Current block: causal; historical: non-causal
            if ki == k_top - 1:  # current block is usually highest score
                out_i = F.scaled_dot_product_attention(
                    q_t, bk, bv, is_causal=True, **_gqa_kw())
            else:
                out_i = F.scaled_dot_product_attention(
                    q_t, bk, bv, is_causal=False, **_gqa_kw())
            outputs.append(out_i)

        # Combine (equal weights — accept-all, quality not a goal)
        out = torch.stack(outputs, dim=0).mean(dim=0)  # [B, n_heads, T, hd]
        out = out.transpose(1, 2).reshape(B, T, -1)
        return self.wo(out)

    # ------------------------------------------------------------------
    # v5: Compressed MoBA — KV compression + FP8 cache + within-block sparse
    # ------------------------------------------------------------------
    def compress_tokens(self, x: torch.Tensor) -> tuple[torch.Tensor, torch.Tensor]:
        """DeepSeek V4-style KV compression.

        x: [B, T, d] — raw hidden states for T tokens
        Returns: (comp_kv, comp_weights) where
          comp_kv:     [B, T, n_kv*hd] — uncompressed KV entries (one per token)
          comp_weights:[B, T, n_kv*hd] — softmax weights for pooling

        Compression happens in forward_moba_v5 by pooling every m entries.
        """
        comp_k = self.w_kv_comp(x)        # [B, T, n_kv*hd]
        comp_v = self.w_v_comp(x)         # [B, T, n_kv*hd]
        comp_z = self.w_z_comp(x)         # [B, T, n_kv*hd]
        return comp_k, comp_v, comp_z

    def pool_compressed(self, comp_k: torch.Tensor, comp_v: torch.Tensor,
                        comp_z: torch.Tensor, m: int):
        """Pool every m compressed entries into 1 via learned weighted softmax.

        Returns: (pooled_k, pooled_v) each [B, T//m, n_kv*hd] — the pooling
        weights come from comp_z (shared for K and V so an entry's K and V
        describe the same token group).
        """
        B, T, D = comp_k.shape
        n_blocks = T // m
        comp_k = comp_k[:, :n_blocks * m].view(B, n_blocks, m, D)
        comp_v = comp_v[:, :n_blocks * m].view(B, n_blocks, m, D)
        comp_z = comp_z[:, :n_blocks * m].view(B, n_blocks, m, D)
        # Add positional bias [m, D] broadcast to [1, 1, m, D]
        z = comp_z + self.comp_bias.unsqueeze(0).unsqueeze(0)
        weights = F.softmax(z, dim=2)  # [B, n_blocks, m, D]
        pooled_k = (weights * comp_k).sum(dim=2)
        pooled_v = (weights * comp_v).sum(dim=2)
        return pooled_k, pooled_v

    def forward_moba_v5(self, x: torch.Tensor,
                        cache_k, cache_v, k_bar_comp,
                        n_filled_blocks: int, current_block_fill: int,
                        ring_k=None, ring_v=None, ring_len: int = 0,
                        cos: torch.Tensor = None, sin: torch.Tensor = None,
                        base_pos: int = 0):
        """v5 block-sparse attention: compressed far memory + raw recent window.

        KV sources, in order:
          - top-k FILLED compressed blocks (4x pooled, FP8, strided) — old ctx
          - raw-KV ring: last cfg.raw_window forwarded tokens, exact K,V
          - this chunk's raw K,V under a real causal mask

        cache_k/cache_v: [B, n_blocks, BS_comp, n_kv, hd] (FP8 or bf16)
        ring_k/ring_v:   [B, W, n_kv, hd] bf16 — ring[:ring_len] is valid;
                         this step's K,V are appended in place AFTER attention.
        Returns: [B, T, d]
        """
        B, T, _ = x.shape
        m = self.compress_m
        BS_comp = self.cfg.block_size_comp
        k_top = self.top_k
        stride = self.sparse_stride
        W = ring_k.shape[1] if ring_k is not None else 0

        # --- Step 1-2: compress + pool new tokens (separate K and V) ---
        comp_k, comp_v, comp_z = self.compress_tokens(x)
        pooled_k, pooled_v = self.pool_compressed(comp_k, comp_v, comp_z, m)
        n_new = pooled_k.shape[1]
        pooled_k = pooled_k.view(B, n_new, self.n_kv, self.hd)
        pooled_v = pooled_v.view(B, n_new, self.n_kv, self.hd)

        # RoPE on pooled keys at group-center position (rotate after pooling
        # to avoid averaging differently-rotated keys).
        if cos is not None and cos.numel() > 0:
            ent_idx = base_pos // m + torch.arange(n_new, device=x.device)
            key_pos = (ent_idx * m + m // 2).clamp(max=cos.shape[0] - 1)
            pooled_k = apply_rope(pooled_k, cos[key_pos], sin[key_pos])

        # --- Step 3: write compressed entries (block-straddling) ---
        pk_store = pooled_k.to(torch.float8_e4m3fn) if self.use_fp8 else pooled_k
        pv_store = pooled_v.to(torch.float8_e4m3fn) if self.use_fp8 else pooled_v
        remaining = n_new
        write_offset = 0
        cur_fill = current_block_fill
        cur_blk = n_filled_blocks
        while remaining > 0:
            space = BS_comp - cur_fill
            if space <= 0:
                cur_blk += 1
                cur_fill = 0
                space = BS_comp
            to_write = min(remaining, space)
            cache_k[:, cur_blk, cur_fill:cur_fill + to_write] = \
                pk_store[:, write_offset:write_offset + to_write]
            cache_v[:, cur_blk, cur_fill:cur_fill + to_write] = \
                pv_store[:, write_offset:write_offset + to_write]
            write_offset += to_write
            remaining -= to_write
            cur_fill += to_write

        # --- Step 4: update k_bar for touched blocks ---
        dt = pooled_k.dtype
        n_blocks_touched = cur_blk - n_filled_blocks + 1
        for b in range(n_blocks_touched):
            bi = n_filled_blocks + b
            if b == 0 and current_block_fill > 0:
                count = min(BS_comp, current_block_fill + n_new) if b == n_blocks_touched - 1 else BS_comp
            elif b == n_blocks_touched - 1:
                count = cur_fill
            else:
                count = BS_comp
            if count > 0 and bi < k_bar_comp.shape[1]:
                filled_k = cache_k[:, bi, :count].to(dt)
                k_bar_comp[:, bi] = filled_k.mean(dim=1)

        # --- Step 5: queries + gating over FILLED blocks (mean-q, exact) ---
        q = self.wq(x).view(B, T, self.n_heads, self.hd)
        if cos is not None and cos.numel() > 0:
            q = apply_rope(q, cos[base_pos:base_pos + T],
                          sin[base_pos:base_pos + T])
        q_t = q.transpose(1, 2)
        n_hist = n_filled_blocks
        if n_hist > 0:
            kb = k_bar_comp[:, :n_hist]
            rep = self.n_heads // self.n_kv
            q_mean = q.view(B, T, self.n_kv, rep, self.hd).mean(dim=(1, 3))
            avg_scores = torch.einsum('bkd,bnkd->bkn', q_mean, kb).mean(1)
            if n_hist < k_top:
                padding = torch.full((B, k_top - n_hist), float('-inf'),
                                     device=avg_scores.device,
                                     dtype=avg_scores.dtype)
                avg_scores = torch.cat([avg_scores, padding], dim=-1)
            popular = avg_scores.topk(k_top, dim=-1).indices
            sel_valid = popular < n_hist

        # --- Step 6: gather KV = [compressed past ; ring ; new chunk] ---
        parts_k, parts_v, parts_valid = [], [], []
        if n_hist > 0:
            ck = cache_k.view(torch.uint8) if self.use_fp8 else cache_k
            cv = cache_v.view(torch.uint8) if self.use_fp8 else cache_v
            if B == 1:
                bi = popular[0]
                gk = ck[:, bi.clamp(max=n_hist - 1), :BS_comp]
                gv = cv[:, bi.clamp(max=n_hist - 1), :BS_comp]
                col_valid = sel_valid[0].repeat_interleave(BS_comp)
            else:
                bi = popular.clamp(max=n_hist - 1)
                ar = torch.arange(B, device=x.device).unsqueeze(1)
                gk = ck[ar, bi, :BS_comp]
                gv = cv[ar, bi, :BS_comp]
                col_valid = sel_valid.repeat_interleave(BS_comp, dim=1)
            gk = gk.reshape(B, k_top * BS_comp, self.n_kv, self.hd)
            gv = gv.reshape(B, k_top * BS_comp, self.n_kv, self.hd)
            if self.use_fp8:
                gk = gk.view(torch.float8_e4m3fn).to(q_t.dtype)
                gv = gv.view(torch.float8_e4m3fn).to(q_t.dtype)
            if stride > 1:
                gk, gv = gk[:, ::stride], gv[:, ::stride]
                col_valid = col_valid[::stride] if col_valid.dim() == 1 \
                    else col_valid[:, ::stride]
            parts_k.append(gk)
            parts_v.append(gv)
            parts_valid.append(col_valid.unsqueeze(0) if col_valid.dim() == 1
                               else col_valid)

        if ring_k is not None and ring_len > 0:
            parts_k.append(ring_k[:, :ring_len])
            parts_v.append(ring_v[:, :ring_len])
            parts_valid.append(torch.ones(B, ring_len, dtype=torch.bool,
                                          device=x.device))

        k_new = self.wk(x).view(B, T, self.n_kv, self.hd)
        v_new = self.wv(x).view(B, T, self.n_kv, self.hd)
        if cos is not None and cos.numel() > 0:
            k_new = apply_rope(k_new, cos[base_pos:base_pos + T],
                              sin[base_pos:base_pos + T])

        # --- Step 7: single SDPA over [past ; new] with combined mask ---
        kk = torch.cat(parts_k + [k_new], dim=1).transpose(1, 2)
        vv = torch.cat(parts_v + [v_new], dim=1).transpose(1, 2)
        kk, vv = _expand_kv(q_t, kk, vv)
        if parts_valid:
            S_past = kk.shape[2] - T
            causal = torch.ones(T, T, dtype=torch.bool,
                                device=x.device).tril_()
            pv = torch.cat(parts_valid, dim=1)
            mask = torch.cat([
                pv.unsqueeze(1).expand(B, T, S_past),
                causal.unsqueeze(0).expand(B, T, T)], dim=2).unsqueeze(1)
            out = F.scaled_dot_product_attention(q_t, kk, vv, attn_mask=mask,
                                                 **_gqa_kw())
        else:
            out = F.scaled_dot_product_attention(q_t, kk, vv, is_causal=True,
                                                 **_gqa_kw())

        # --- Step 8: append this chunk's raw K,V to the ring ---
        if ring_k is not None:
            if T >= W:
                ring_k[:] = k_new[:, T - W:]
                ring_v[:] = v_new[:, T - W:]
            elif ring_len + T <= W:
                ring_k[:, ring_len:ring_len + T] = k_new
                ring_v[:, ring_len:ring_len + T] = v_new
            else:
                keep = W - T
                ring_k[:, :keep] = ring_k[:, ring_len - keep:ring_len].clone()
                ring_v[:, :keep] = ring_v[:, ring_len - keep:ring_len].clone()
                ring_k[:, keep:W] = k_new
                ring_v[:, keep:W] = v_new

        out = out.transpose(1, 2).reshape(B, T, -1)
        return self.wo(out)


# --------------------------------------------------------------------------------------
# SwiGLU feed-forward
# --------------------------------------------------------------------------------------
class FeedForward(nn.Module):
    def __init__(self, cfg: Config):
        super().__init__()
        d, inter = cfg.d_model, cfg.intermediate
        self.w_gate = nn.Linear(d, inter, bias=False)
        self.w_up = nn.Linear(d, inter, bias=False)
        self.w_down = nn.Linear(inter, d, bias=False)

    def forward(self, x):
        return self.w_down(F.silu(self.w_gate(x)) * self.w_up(x))


# --------------------------------------------------------------------------------------
# Sequential draft block (EAGLE-lite): a single small transformer layer that
# evolves its own hidden state across the draft chain. Unlike the flat heads,
# each draft step sees the full drafted prefix via causal attention.
# --------------------------------------------------------------------------------------
class SeqDraftBlock(nn.Module):
    def __init__(self, cfg: Config):
        super().__init__()
        d = cfg.d_model
        nh, hd = cfg.n_heads, cfg.head_dim
        self.nh, self.hd = nh, hd
        inter = int(d * cfg.seq_ffn_mult)
        self.norm1 = nn.RMSNorm(d)
        self.wq = nn.Linear(d, nh * hd, bias=False)
        self.wk = nn.Linear(d, nh * hd, bias=False)
        self.wv = nn.Linear(d, nh * hd, bias=False)
        self.wo = nn.Linear(nh * hd, d, bias=False)
        self.norm2 = nn.RMSNorm(d)
        self.w_gate = nn.Linear(d, inter, bias=False)
        self.w_up = nn.Linear(d, inter, bias=False)
        self.w_down = nn.Linear(inter, d, bias=False)

    def forward(self, x):
        # x [B,T,d] — plain causal self-attention over the draft prefix
        B, T, _ = x.shape
        h = self.norm1(x)
        q = self.wq(h).view(B, T, self.nh, self.hd).transpose(1, 2)
        k = self.wk(h).view(B, T, self.nh, self.hd).transpose(1, 2)
        v = self.wv(h).view(B, T, self.nh, self.hd).transpose(1, 2)
        o = F.scaled_dot_product_attention(q, k, v, is_causal=True)
        x = x + self.wo(o.transpose(1, 2).reshape(B, T, -1))
        h2 = self.norm2(x)
        x = x + self.w_down(F.silu(self.w_gate(h2)) * self.w_up(h2))
        return x


# --------------------------------------------------------------------------------------
# Block
# --------------------------------------------------------------------------------------
class Block(nn.Module):
    def __init__(self, cfg: Config):
        super().__init__()
        self.norm1 = nn.RMSNorm(cfg.d_model)
        self.attn = Attention(cfg)
        self.norm2 = nn.RMSNorm(cfg.d_model)
        self.ffn = FeedForward(cfg)

    def forward(self, x, cos, sin, cache_k=None, cache_v=None, start_pos=0,
                alibi_bias=None, sliding_window=False, causal_extend=False):
        x = x + self.attn(self.norm1(x), cos, sin, cache_k, cache_v, start_pos,
                          alibi_bias, sliding_window, causal_extend)
        x = x + self.ffn(self.norm2(x))
        return x

    def forward_moba(self, x, cache_k, cache_v, k_bar, n_filled_blocks,
                     current_block_fill):
        x = x + self.attn.forward_moba(self.norm1(x), cache_k, cache_v, k_bar,
                                       n_filled_blocks, current_block_fill)
        x = x + self.ffn(self.norm2(x))
        return x

    def forward_moba_v5(self, x, cache_k, cache_v, k_bar_comp, n_filled_blocks,
                        current_block_fill, ring_k=None, ring_v=None,
                        ring_len=0, cos=None, sin=None, base_pos=0):
        x = x + self.attn.forward_moba_v5(self.norm1(x), cache_k, cache_v,
                                          k_bar_comp, n_filled_blocks,
                                          current_block_fill,
                                          ring_k, ring_v, ring_len,
                                          cos, sin, base_pos)
        if hasattr(self, '_compiled_ffn_fn'):
            x = x + self._compiled_ffn_fn(x)
        else:
            x = x + self.ffn(self.norm2(x))
        return x


# --------------------------------------------------------------------------------------
# Full model
# --------------------------------------------------------------------------------------
class SpecModel(nn.Module):
    def __init__(self, cfg: Config):
        super().__init__()
        self.cfg = cfg
        self.tok_emb = nn.Embedding(cfg.vocab_size, cfg.d_model)
        self.blocks = nn.ModuleList([Block(cfg) for _ in range(cfg.n_layers)])
        self.norm_f = nn.RMSNorm(cfg.d_model)
        # tied output head handled in forward_logits
        self.rope_cos: torch.Tensor
        self.rope_sin: torch.Tensor
        self.register_buffer("rope_cos", torch.empty(0), persistent=False)
        self.register_buffer("rope_sin", torch.empty(0), persistent=False)
        self._rope_built = False
        self._alibi_built = False
        self.alibi_bias: torch.Tensor = None

        # --- Speculative heads ---
        # r == 1 (legacy): per-head low-rank [K,d,1] + [K,1,vocab]; each head can
        #   only emit argmax/argmin of its fixed weight row (2 tokens total).
        # r > 1 (v6 hybrid): conditioned block heads + parallel tail heads, all
        #   sharing ONE vocab projection so every head emits a real
        #   context-dependent distribution over the full vocab.
        K, d, r, v = cfg.medusa_heads, cfg.d_model, cfg.medusa_rank, cfg.vocab_size
        if r == 1:
            self.medusa_down_weight = nn.Parameter(torch.empty(K, d, r))
            self.medusa_out_weight = nn.Parameter(torch.empty(K, r, v))
        else:
            e, g = cfg.medusa_emb_rank, cfg.medusa_cond_group
            kc, kp = cfg.medusa_cond_heads, cfg.medusa_par_heads
            self.spec_tok_proj = nn.Parameter(torch.empty(d, e))
            self.spec_cond_w = nn.Parameter(torch.empty(kc, d + g * e, r))
            self.spec_par_w = nn.Parameter(torch.empty(kp, d, r))
            self.spec_vocab = nn.Parameter(torch.empty(r, v))
            self.spec_bias = nn.Parameter(torch.zeros(v))
            # v7 dspark-style: markov head = intra-chain token dependency.
            # logits_j += W2(emb(prev predicted token)) — the lightweight
            # sequential module that fixes parallel-emission suffix decay.
            mr = cfg.medusa_rank
            self.spec_markov_emb = nn.Parameter(torch.empty(v, mr))
            self.spec_markov_w2 = nn.Parameter(torch.empty(mr, v))
            # confidence head: P(draft token accepted) -> adaptive verify_k
            # input = [h_anchor ; group cond ; slot code]
            self.spec_conf_w = nn.Parameter(torch.empty(d + g * e + r, 1))
            # hi-rank near-field heads (capacity fix): heads 0..H-1 get a
            # wide code + their own vocab projection instead of sharing the
            # r=256 bottleneck. Replaces the low-rank path for those slots.
            H = getattr(cfg, "medusa_hi_heads", 0)
            if H > 0:
                R = cfg.medusa_hi_rank
                self.spec_hi_w = nn.Parameter(torch.empty(H, d + g * e, R))
                self.spec_hi_vocab = nn.Parameter(torch.empty(R, v))
                self.spec_hi_bias = nn.Parameter(torch.zeros(v))
            # EAGLE-lite sequential draft module: evolves its own hidden
            # state token-by-token — the fix for the flat-head information
            # bottleneck. Output via tied tok_emb (zero output params).
            if getattr(cfg, "medusa_seq_len", 0) > 0:
                # input: [h_i ; emb(tok_i) ; compressed last-G tokens]
                self.spec_seq_fc = nn.Linear(2 * d + g * e, d, bias=False)
                self.spec_seq_blk = SeqDraftBlock(cfg)

        self.reset_parameters()

    def reset_parameters(self):
        # small init for stability; std ~ 1/sqrt(d) scaled
        std = 0.02
        for p in self.parameters():
            if p.dim() > 1:
                nn.init.normal_(p, std=std)
        # scale embeddings down a touch
        nn.init.normal_(self.tok_emb.weight, std=std)
        # v7 zero-init: markov bias is a no-op and conf is neutral (0.5)
        # until trained — keeps checkpoints/behaviour compatible.
        if hasattr(self, "spec_markov_w2"):
            nn.init.zeros_(self.spec_markov_w2)
            nn.init.zeros_(self.spec_conf_w)

    def build_rope(self, device, dtype):
        if not self._rope_built:
            base = self.cfg.rope_base * self.cfg.rope_ntk_scale
            cos, sin = precompute_rope(self.cfg.head_dim, self.cfg.max_seq_len,
                                       base, device, dtype)
            self.rope_cos = cos
            self.rope_sin = sin
            self._rope_built = True

    def build_alibi(self, q_len: int, device, dtype):
        """Precompute fixed ALiBi bias for sliding window: [n_heads, q_len, W]."""
        W = self.cfg.window_size if self.cfg.window_size > 0 else self.cfg.max_seq_len
        if not self._alibi_built or self.alibi_bias is None or self.alibi_bias.shape[1] != q_len:
            self.alibi_bias = precompute_alibi_bias(
                self.cfg.n_heads, q_len, W, dtype, device)
            self._alibi_built = True

    def forward(self, input_ids, cache_k=None, cache_v=None, start_pos=0,
                sliding_window=False, use_checkpoint=False, causal_extend=False):
        """
        input_ids: [B, T]
        cache_k/v: list per layer of [B, W, n_kv, hd] (None for training)
        start_pos: int (cached length, growing cache only)
        sliding_window: if True, use fixed-size cache with ALiBi
        use_checkpoint: if True, use gradient checkpointing (saves memory,
                        costs ~30% more compute — allows bigger batches)
        causal_extend: if True, causal-mask within the new chunk (speculative
                        verification — the accept-all path skips this).
        Returns hidden states [B, T, d] (pre-output-head).
        """
        B, T = input_ids.shape
        dt = self.tok_emb.weight.dtype
        dev = input_ids.device

        alibi = None
        if self.cfg.use_alibi and sliding_window:
            if self.cfg.window_size == T:
                pass
            else:
                self.build_alibi(T, dev, dt)
                alibi = self.alibi_bias
        elif not self.cfg.use_alibi:
            self.build_rope(dev, dt)

        x = self.tok_emb(input_ids)                              # [B, T, d]
        for i, blk in enumerate(self.blocks):
            ck = cache_k[i] if cache_k is not None else None
            cv = cache_v[i] if cache_v is not None else None
            if use_checkpoint and self.training and ck is None:
                # Gradient checkpointing: recompute forward in backward
                # Saves ~8x activation memory (only store layer outputs)
                rope_cos = self.rope_cos
                rope_sin = self.rope_sin
                def run_blk(x, blk=blk, rope_cos=rope_cos, rope_sin=rope_sin,
                            alibi=alibi, sliding_window=sliding_window):
                    return blk(x, rope_cos, rope_sin, None, None, 0,
                               alibi_bias=alibi, sliding_window=sliding_window)
                x = torch.utils.checkpoint.checkpoint(run_blk, x, use_reentrant=False)
            else:
                x = blk(x, self.rope_cos, self.rope_sin, ck, cv, start_pos,
                        alibi_bias=alibi, sliding_window=sliding_window,
                        causal_extend=causal_extend)
        return self.norm_f(x)

    def forward_moba(self, input_ids, cache_k, cache_v, k_bar_list,
                     n_filled_blocks, current_block_fill):
        """MoBA forward pass: block-sparse attention for long context.

        input_ids: [B, T]
        cache_k/v: list per layer of [B, n_blocks, block_size, n_kv, hd]
        k_bar_list: list per layer of [B, n_blocks, n_kv, hd] (precomputed block means)
        n_filled_blocks: int — complete blocks in cache
        current_block_fill: int — tokens in current partial block
        Returns hidden states [B, T, d]
        """
        x = self.tok_emb(input_ids)
        for i, blk in enumerate(self.blocks):
            x = blk.forward_moba(x, cache_k[i], cache_v[i], k_bar_list[i],
                                 n_filled_blocks, current_block_fill)
        return self.norm_f(x)

    def forward_moba_v5(self, input_ids, cache_k, cache_v, k_bar_comp,
                        n_filled_blocks, current_block_fill,
                        ring_k=None, ring_v=None, ring_len=0):
        """v5 MoBA forward: compressed far memory + raw recent window.

        input_ids: [B, T]
        cache_k/cache_v: per-layer [B, n_blocks, BS_comp, n_kv, hd] (FP8/bf16)
        k_bar_comp: per-layer [B, n_blocks, n_kv, hd] — mean pooled K per block
        ring_k/ring_v: per-layer [B, raw_window, n_kv, hd] — exact recent KV
        ring_len: valid entries in the ring (same across layers)
        Returns hidden states [B, T, d]
        """
        x = self.tok_emb(input_ids)
        cos = sin = None
        if not self.cfg.use_alibi:
            self.build_rope(input_ids.device, x.dtype)
            cos, sin = self.rope_cos, self.rope_sin
        base_pos = (n_filled_blocks * self.cfg.block_size_comp
                    + current_block_fill) * self.cfg.kv_compress_m
        for i, blk in enumerate(self.blocks):
            x = blk.forward_moba_v5(
                x, cache_k[i], cache_v[i], k_bar_comp[i],
                n_filled_blocks, current_block_fill,
                None if ring_k is None else ring_k[i],
                None if ring_v is None else ring_v[i],
                ring_len, cos, sin, base_pos)
        return self.norm_f(x)

    @torch.no_grad()
    def init_compressor_from_attn(self):
        """Initialize the KV compressor so pooled compressed K/V ~= mean-pooled
        real K and V.

        w_kv_comp <- wk, w_v_comp <- wv
        w_z_comp  <- 0, comp_bias <- 0  (uniform softmax -> mean pooling)
        """
        for blk in self.blocks:
            a = blk.attn
            if a.compress_m > 1:
                a.w_kv_comp.weight.data.copy_(a.wk.weight.data)
                a.w_v_comp.weight.data.copy_(a.wv.weight.data)
                a.w_z_comp.weight.data.zero_()
                a.comp_bias.data.zero_()

    def lm_head(self, hidden: torch.Tensor) -> torch.Tensor:
        """Tied output projection: hidden [., d] -> [., vocab]."""
        return F.linear(hidden, self.tok_emb.weight)

    def medusa_logits(self, hidden_last: torch.Tensor):
        """hidden_last: [B, d] (single position). Returns [B, K, vocab] logits.
        Legacy r=1 path only — kept for old checkpoints."""
        down = torch.einsum('bd,kdr->bkr', hidden_last, self.medusa_down_weight)
        return torch.einsum('bkr,krv->bkv', down, self.medusa_out_weight)

    @torch.no_grad()
    def medusa_argmax(self, hidden_last: torch.Tensor):
        """Greedy tokens from each Medusa head. Returns [B, K] long tensor.
        Legacy r=1 path."""
        logits = self.medusa_logits(hidden_last)               # [B, K, vocab]
        return logits.argmax(-1)                               # [B, K]

    # ------------------------------------------------------------------
    # Precomputed argmax mode (r=1 only): skip the 410MB medusa out einsum
    # ------------------------------------------------------------------
    @torch.no_grad()
    def precompute_medusa_tokens(self):
        """For r=1: argmax(s * w) = argmax(w) if s>0, argmin(w) if s<0.
        Precompute argmax and argmin of each head's out_weight row once.
        After this, medusa_argmax_fast needs only the down projection + sign check."""
        assert self.cfg.medusa_rank == 1, "precomputed mode requires r=1"
        w = self.medusa_out_weight.squeeze(1)                  # [K, vocab]
        self._medusa_argmax = w.argmax(dim=-1)                 # [K]
        self._medusa_argmin = w.argmin(dim=-1)                 # [K]
        self._medusa_precomputed = True

    @torch.no_grad()
    def medusa_argmax_fast(self, hidden_last: torch.Tensor):
        """r=1 fast path: down projection + sign check, no medusa out einsum.
        Returns [B, K] long tensor."""
        # down: [B, K] — just a matmul [B, d] x [d, K] (squeeze the r=1 dim)
        down = torch.einsum('bd,kd->bk', hidden_last,
                            self.medusa_down_weight.squeeze(-1))  # [B, K]
        positive = (down > 0).long()                             # [B, K]
        # where positive: argmax, else argmin
        return torch.where(positive.bool(),
                           self._medusa_argmax.expand_as(down),
                           self._medusa_argmin.expand_as(down))  # [B, K]

    # ------------------------------------------------------------------
    # v6 hybrid heads: conditioned blocks + parallel tail, shared vocab
    # ------------------------------------------------------------------
    def spec_cond_logits(self, h_last: torch.Tensor, cond_embs: torch.Tensor,
                         head_idx: torch.Tensor):
        """Conditioned-head logits for chosen heads.
        h_last:    [B, d]      — anchor hidden state
        cond_embs: [B, G, e]   — compressed embs of the previous group's tokens
        head_idx:  [S]         — which conditioned heads to evaluate
        Returns [B, S, V] logits."""
        B = h_last.shape[0]
        cond_flat = cond_embs.reshape(B, -1)                    # [B, G*e]
        inp = torch.cat([h_last, cond_flat], dim=-1)            # [B, d+G*e]
        codes = F.silu(torch.einsum('be,ser->bsr', inp,
                                    self.spec_cond_w[head_idx]))  # [B,S,r]
        return codes @ self.spec_vocab + self.spec_bias           # [B,S,V]

    def spec_par_logits(self, h_last: torch.Tensor,
                        head_idx: torch.Tensor = None):
        """Parallel-tail logits. h_last [B,d] -> [B,Kp,V] (or [B,S,V] if subsampled)."""
        w = self.spec_par_w if head_idx is None else self.spec_par_w[head_idx]
        codes = F.silu(torch.einsum('bd,sdr->bsr', h_last, w))    # [B,S,r]
        return codes @ self.spec_vocab + self.spec_bias           # [B,S,V]

    @torch.no_grad()
    def spec_draft(self, h_anchor: torch.Tensor, hist_ids: torch.Tensor,
                   first_head: int = 0, pending_id: int = None,
                   prefix_ids: torch.Tensor = None,
                   n_cond: int = None, n_par: int = None,
                   return_conf: bool = False,
                   sample: bool = False, temperature: float = 1.0,
                   top_p: float = 0.9):
        """Draft a token chain from an anchor hidden state.

        h_anchor: [B, d] hidden at the last *forwarded* position.
        hist_ids: [B, G]  last G committed tokens (left-pad with any id).
        first_head: number of leading head slots to drop — the committed
            tokens already occupying those positions (pending/queue).
        pending_id: the committed-but-unforwarded token (first_head=1).
        prefix_ids: [B, P<=G] committed tokens forced into the first P chain
            slots (generalizes pending_id for a queue of P tokens).
        Returns [B, K - first_head] draft token ids.
        """
        cfg = self.cfg
        B = h_anchor.shape[0]
        G, Kc = cfg.medusa_cond_group, cfg.medusa_cond_heads
        n_groups = Kc // G
        n_cond = n_cond or Kc
        n_par = n_par if n_par is not None else cfg.medusa_par_heads
        dev = h_anchor.device
        if pending_id is not None and prefix_ids is None:
            prefix_ids = torch.full((B, 1), pending_id, dtype=torch.long,
                                    device=dev)
        n_prefix = 0 if prefix_ids is None else prefix_ids.shape[1]

        def _pick(lg):                                          # [B,V] -> [B]
            """argmax, or temperature+top-p sample when sample=True."""
            if not sample or temperature <= 0:
                return lg.argmax(-1)
            lg = lg.float() / temperature
            if top_p < 1.0:
                s, si = lg.sort(-1, descending=True)
                cum = s.softmax(-1).cumsum(-1)
                rm = cum - s.softmax(-1) >= top_p               # keep cum<p
                s = s.masked_fill(rm, float("-inf"))
                return si.gather(-1, torch.multinomial(
                    s.softmax(-1), 1)).squeeze(-1)
            return torch.multinomial(lg.softmax(-1), 1).squeeze(-1)

        # EAGLE-lite sequential draft path: replaces the conditioned chain
        # when the seq module exists — the hidden state evolves per token.
        if getattr(cfg, "medusa_seq_len", 0) > 0 and \
                hasattr(self, "spec_seq_fc"):
            seq_n = max(0, n_cond - n_prefix)
            seq_out = self.spec_draft_seq(
                h_anchor, prefix_ids=prefix_ids, hist_ids=hist_ids,
                n=seq_n, sample=sample, temperature=temperature,
                top_p=top_p)
            if n_prefix == 0:
                # slot +1 = base's own next-token at the anchor
                first = (h_anchor @ self.tok_emb.weight.T
                         ).argmax(-1, keepdim=True)
                seq_out = torch.cat([first, seq_out[:, :-1]], dim=1)
            conf_out = torch.full((B, seq_out.shape[1]), 0.5, device=dev)
            par_logits = self.spec_par_logits(h_anchor)
            if sample:
                par_out = torch.stack(
                    [_pick(par_logits[:, j]) for j in range(n_par)], 1)
            else:
                par_out = par_logits.argmax(-1)[:, :n_par]
            out = torch.cat([seq_out, par_out], dim=1)
            if return_conf:
                return out, conf_out
            return out

        use_markov = getattr(self.cfg, "use_markov_head", False)
        cond = self.tok_emb(hist_ids) @ self.spec_tok_proj      # [B, G, e]
        prev = hist_ids[:, -1]                                  # anchor token
        preds = []
        confs = []
        for g in range(n_cond // G):
            idx = torch.arange(g * G, g * G + G, device=dev)
            cond_flat = cond.reshape(B, -1)                     # [B, G*e]
            inp = torch.cat([h_anchor, cond_flat], dim=-1)      # [B, d+G*e]
            codes = F.silu(torch.einsum('be,ser->bsr', inp,
                                        self.spec_cond_w[idx]))  # [B,G,r]
            base = codes @ self.spec_vocab + self.spec_bias     # [B,G,V]
            H = getattr(cfg, "medusa_hi_heads", 0)
            if g == 0 and H > 0:
                # hi-rank slots: wide code + dedicated vocab proj
                codes_hi = F.silu(torch.einsum(
                    'be,ser->bsr', inp, self.spec_hi_w[:H]))     # [B,H,R]
                base = torch.cat(
                    [codes_hi @ self.spec_hi_vocab + self.spec_hi_bias,
                     base[:, H:]], dim=1)
                codes = torch.cat([codes_hi[..., :codes.shape[-1]],
                                   codes[:, H:]], dim=1)
            # per-slot confidence: sigmoid([inp ; code_j] @ conf_w)
            cin = torch.cat([inp.unsqueeze(1).expand(B, G, -1),
                             codes], dim=-1)                    # [B,G,d+Ge+r]
            confs.append(torch.sigmoid(cin @ self.spec_conf_w).squeeze(-1))
            if use_markov:
                # DSpark-style intra-block dependency: sequential unroll,
                # logits_j += W2(emb(tok_{j-1}))
                toks = []
                for j in range(G):
                    lg = base[:, j] + (self.spec_markov_emb[prev]
                                       @ self.spec_markov_w2)   # [B,V]
                    if g == 0 and j < n_prefix:
                        t = prefix_ids[:, j]
                    else:
                        t = _pick(lg)
                    toks.append(t)
                    prev = t
                tok = torch.stack(toks, dim=1)                  # [B,G]
            else:
                tok = (torch.stack([_pick(base[:, j]) for j in range(G)], 1)
                       if sample else base.argmax(-1))          # [B,G]
                if g == 0 and n_prefix:
                    tok = tok.clone()
                    tok[:, :n_prefix] = prefix_ids              # committed slots
                prev = tok[:, -1]
            preds.append(tok)
            cond = self.tok_emb(tok) @ self.spec_tok_proj       # next group cond
        cond_out = torch.stack(preds, dim=1).reshape(B, -1)     # [B, Kc]
        conf_out = torch.stack(confs, dim=1).reshape(B, -1)     # [B, Kc]
        cond_out = cond_out[:, first_head:]                     # drop prefix slots
        conf_out = conf_out[:, first_head:]

        par_logits = self.spec_par_logits(h_anchor)             # [B, Kp, V]
        if sample:
            par_out = torch.stack(
                [_pick(par_logits[:, j]) for j in range(n_par)], 1)
        else:
            par_out = par_logits.argmax(-1)[:, :n_par]          # [B, Kp]
        out = torch.cat([cond_out, par_out], dim=1)             # [B, K-first_head]
        if return_conf:
            return out, conf_out
        return out

    @torch.no_grad()
    def spec_draft_seq(self, h_anchor: torch.Tensor,
                       prefix_ids: torch.Tensor = None,
                       hist_ids: torch.Tensor = None,
                       n: int = 32,
                       sample: bool = False, temperature: float = 1.0,
                       top_p: float = 0.9):
        """EAGLE-lite sequential draft: the draft module's own hidden state
        evolves token-by-token — each step sees the drafted prefix via
        causal attention AND the sliding last-G-token conditioning window.

        h_anchor: [B, d] hidden at the last *forwarded* position.
        prefix_ids: [B, P] committed-but-unforwarded tokens — fed into the
            chain first (the seq module processes them as inputs before
            emitting predictions).
        hist_ids: [B, G] last G committed tokens (pre-prefix) — the sliding
            conditioning window. Falls back to the prefix/pad if absent.
        Returns [B, n] draft token ids for positions t+2 .. t+1+n
            (t+1 is covered by the pending/prefix slot).
        """
        B = h_anchor.shape[0]
        dev = h_anchor.device
        G = self.cfg.medusa_cond_group
        n_prefix = 0 if prefix_ids is None else prefix_ids.shape[1]

        def _pick(lg):
            if not sample or temperature <= 0:
                return lg.argmax(-1)
            lg = lg.float() / temperature
            if top_p < 1.0:
                s, si = lg.sort(-1, descending=True)
                cum = s.softmax(-1).cumsum(-1)
                rm = cum - s.softmax(-1) >= top_p
                s = s.masked_fill(rm, float("-inf"))
                return si.gather(-1, torch.multinomial(
                    s.softmax(-1), 1)).squeeze(-1)
            return torch.multinomial(lg.softmax(-1), 1).squeeze(-1)

        E = self.tok_emb.weight                              # tied [V,d]
        # sliding conditioning window: last G tokens (committed + predicted)
        if hist_ids is None:
            hist_ids = (prefix_ids[:, :G] if n_prefix >= G
                        else torch.zeros(B, G, dtype=torch.long,
                                         device=dev))
        window = hist_ids[:, -G:].clone()                      # [B,G]
        h_prev = h_anchor
        xs = []
        preds = []
        n_steps = n_prefix + n
        for i in range(n_steps):
            if i < n_prefix:
                tok_in = prefix_ids[:, i]
            elif i == 0:
                tok_in = (h_anchor @ E.T).argmax(-1)  # base's own first token
            else:
                tok_in = preds[-1]
            window = torch.cat([window[:, 1:], tok_in.unsqueeze(1)], 1)
            cond = (self.tok_emb(window) @ self.spec_tok_proj
                    ).reshape(B, -1)                           # [B,G*e]
            x = self.spec_seq_fc(torch.cat(
                [h_prev, self.tok_emb(tok_in), cond], dim=-1)) # [B,d]
            xs.append(x.unsqueeze(1))
            h_all = self.spec_seq_blk(torch.cat(xs, dim=1))    # [B,i+1,d]
            h_prev = h_all[:, -1]                             # [B,d]
            if i >= n_prefix - 1:
                lg = h_prev @ E.T                             # [B,V]
                preds.append(_pick(lg))
        return torch.stack(preds, dim=1)[:, :n]


def build_model(cfg: Config, device="cuda") -> SpecModel:
    m = SpecModel(cfg).to(device)
    dt = getattr(torch, cfg.dtype)
    m = m.to(dt)
    return m