# SPDX-License-Identifier: Apache-2.0
# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
"""TurboQuant attention backend for vLLM.

Prefill: Standard scaled dot-product attention on uncompressed K/V,
         then quantize K and store K+V into combined cache slot.
Decode:  Compute TQ attention scores from compressed cache,
         unpack FP16 values, softmax + weighted sum.

Cache layout (no leading 2 dimension):
  (num_blocks, block_size, num_kv_heads, slot_size)
  where slot_size = key_packed_size + value_fp16_size

Per-head per-position slot layout:
  [key_packed (kps bytes) | value_fp16 (D*2 bytes)]
  For turboquant_k3v4_nc head_dim=256: [100 bytes key | 512 bytes value] = 612
"""

import functools
import math
import os
from dataclasses import dataclass
from typing import ClassVar

import torch
import torch.nn.functional as F

from vllm.config import get_current_vllm_config
from vllm.triton_utils import triton
from vllm.utils.torch_utils import aux_stream
from vllm.v1.attention.ops.triton_turboquant_decode import (
    _tq_full_dequant_kv,
    _use_fp8_e4b15,
    triton_turboquant_decode_attention,
)
from vllm.v1.attention.ops.triton_turboquant_store import triton_turboquant_store

# CUDA stream overlap: disabled by default — degrades TTFT under concurrent
# load (489ms vs 338ms). Enable via TQ_STREAM_OVERLAP=1 for experimentation.
_USE_STREAM_OVERLAP = os.environ.get("TQ_STREAM_OVERLAP", "0") == "1"

# Continuation prefill: for small continuation chunks (q_len ≤ threshold),
# use the TQ decode kernel directly instead of full-dequant + flash_attn.
# do_kv_cache_update already stored all tokens to TQ cache, so the decode
# kernel can read them efficiently. This avoids O(cached_len) dequant work
# per continuation, eliminating the O(N²/chunk_size) collapse at long context.
_CONTINUATION_DECODE_THRESHOLD = 128

from vllm.config.cache import CacheDType
from vllm.v1.attention.backends.fa_utils import (
    is_flash_attn_varlen_func_available,
)

_HAS_FLASH_ATTN = is_flash_attn_varlen_func_available()
if _HAS_FLASH_ATTN:
    from vllm.v1.attention.backends.fa_utils import flash_attn_varlen_func
from vllm.v1.attention.backend import (
    AttentionBackend,
    AttentionCGSupport,
    AttentionImpl,
    AttentionLayer,
    AttentionMetadata,
    AttentionMetadataBuilder,
    AttentionType,
    CommonAttentionMetadata,
    MultipleOf,
)
from vllm.v1.attention.backends.utils import split_decodes_and_prefills


@functools.cache
def _build_hadamard(d: int, device_str: str) -> torch.Tensor:
    """Orthonormal Hadamard matrix (Sylvester construction), built on CPU.

    Precomputed D×D matrix enables matmul-based WHT — single cuBLAS GEMM
    instead of log2(D) butterfly kernel launches. 64KB for D=128.
    """
    H = torch.tensor([[1.0]])
    while H.shape[0] < d:
        H = torch.cat([torch.cat([H, H], 1), torch.cat([H, -H], 1)], 0)
    return (H / math.sqrt(d)).to(torch.device(device_str))


class TurboQuantAttentionBackend(AttentionBackend):
    """Attention backend using TurboQuant KV-cache compression."""

    accept_output_buffer: bool = True
    forward_includes_kv_cache_update: bool = False

    supported_dtypes: ClassVar[list[torch.dtype]] = [
        torch.float16,
        torch.bfloat16,
    ]
    supported_kv_cache_dtypes: ClassVar[list[CacheDType]] = [
        "turboquant_k8v4",
        "turboquant_4bit_nc",
        "turboquant_k3v4_nc",
        "turboquant_3bit_nc",
    ]

    @staticmethod
    def get_name() -> str:
        return "TURBOQUANT"

    @staticmethod
    def get_supported_kernel_block_sizes() -> list[int | MultipleOf]:
        return [16, 32, 64, 128]

    @classmethod
    def supports_attn_type(cls, attn_type: str) -> bool:
        return attn_type == AttentionType.DECODER

    @classmethod
    def supports_per_head_quant_scales(cls) -> bool:
        return False

    @staticmethod
    def get_impl_cls() -> type["TurboQuantAttentionImpl"]:
        return TurboQuantAttentionImpl

    @staticmethod
    def get_builder_cls() -> type["TurboQuantMetadataBuilder"]:
        return TurboQuantMetadataBuilder

    @staticmethod
    def get_kv_cache_shape(
        num_blocks: int,
        block_size: int,
        num_kv_heads: int,
        head_size: int,
        cache_dtype_str: str = "turboquant_4bit_nc",
    ) -> tuple[int, ...]:
        """Combined K+V cache shape — no leading 2 dimension.

        Standard attention backends use (2, num_blocks, block_size, num_kv_heads,
        head_dim) with a leading 2 to separate K and V. TurboQuant packs K+V
        into a single interleaved slot per head per position, so the cache is:

            (num_blocks, block_size, num_kv_heads, slot_size_aligned)

        Each slot = [key_packed | value_packed | padding].
        This is safe because TQ has its own get_kv_cache_shape override and
        never shares cache tensors with other backends. Layers that fall back
        to native dtype via kv_cache_dtype_skip_layers get their own
        standard-shaped cache allocation.

        head_size is the model's real head_dim. slot_size_aligned is computed
        from the TQ config to ensure correct cache allocation for all head dims.
        """
        from vllm.model_executor.layers.quantization.turboquant.config import (
            TurboQuantConfig,
        )

        tq_config = TurboQuantConfig.from_cache_dtype(cache_dtype_str, head_size)
        return (num_blocks, block_size, num_kv_heads, tq_config.slot_size_aligned)

    @classmethod
    def supports_kv_cache_dtype(cls, kv_cache_dtype: CacheDType | None) -> bool:
        if kv_cache_dtype is None:
            return False
        return kv_cache_dtype.startswith("turboquant_")

    @classmethod
    def supports_head_size(cls, head_size: int) -> bool:
        # head_size from spec is effective_head_size (padded_slot//2),
        # not the model's actual head_dim. Accept any positive value.
        return head_size > 0


@dataclass
class TurboQuantMetadata(AttentionMetadata):
    """Metadata for TurboQuant attention."""

    seq_lens: torch.Tensor  # (num_reqs,) — total context length per request
    slot_mapping: torch.Tensor  # (num_tokens,) — cache slot for each token
    block_table: torch.Tensor  # (num_reqs, max_num_blocks)
    query_start_loc: torch.Tensor  # (num_reqs + 1,) — cu_seqlens for queries
    num_actual_tokens: int = 0  # actual tokens (excluding padding)
    max_query_len: int = 0  # longest query in batch
    max_seq_len: int = 0  # longest context in batch
    is_prefill: bool = False
    num_decodes: int = 0  # number of decode requests (first in batch)
    num_decode_tokens: int = 0  # tokens from decode requests


class TurboQuantMetadataBuilder(AttentionMetadataBuilder[TurboQuantMetadata]):
    """Builds TurboQuantMetadata from scheduler output."""

    _cudagraph_support: ClassVar[AttentionCGSupport] = AttentionCGSupport.UNIFORM_BATCH

    def __init__(self, kv_cache_spec, layer_names, vllm_config, device):
        super().__init__(kv_cache_spec, layer_names, vllm_config, device)
        self._init_reorder_batch_threshold(1, supports_spec_as_decode=False)

    def build_for_cudagraph_capture(
        self, common_attn_metadata: CommonAttentionMetadata
    ) -> TurboQuantMetadata:
        attn_metadata = self.build(0, common_attn_metadata)
        # Set seq_lens to 1 so CUDA graph capture is fast
        # (real seq_lens are filled at replay time).
        attn_metadata.seq_lens.fill_(1)
        return attn_metadata

    def build(self, common_prefix_len, common_attn_metadata, fast_build=False):
        """Build TurboQuantMetadata from common attention metadata."""
        cam = common_attn_metadata

        # With reorder_batch_threshold=1, the model runner guarantees
        # decodes come first in the batch. split_decodes_and_prefills
        # finds the boundary (operates on CPU tensors — no GPU sync).
        num_decodes, num_prefills, num_decode_tokens, _ = split_decodes_and_prefills(
            cam, decode_threshold=self.reorder_batch_threshold
        )

        return TurboQuantMetadata(
            seq_lens=cam.seq_lens,
            slot_mapping=cam.slot_mapping,
            block_table=cam.block_table_tensor,
            query_start_loc=cam.query_start_loc,
            num_actual_tokens=cam.num_actual_tokens,
            max_query_len=cam.max_query_len,
            max_seq_len=cam.max_seq_len,
            is_prefill=(cam.max_query_len > 1),
            num_decodes=num_decodes,
            num_decode_tokens=num_decode_tokens,
        )


class TurboQuantAttentionImpl(AttentionImpl["TurboQuantMetadata"]):
    """TurboQuant attention implementation.

    Vectorized PyTorch: batch quantize/store, vectorized bit-unpack
    decode with einsum scores and value gather.
    """

    supports_quant_query_input: bool = False

    def __init__(
        self,
        num_heads: int,
        head_size: int,
        scale: float,
        num_kv_heads: int | None = None,
        alibi_slopes: list[float] | None = None,
        sliding_window: int | None = None,
        kv_cache_dtype: str = "auto",
        logits_soft_cap: float | None = None,
        attn_type: str = AttentionType.DECODER,
        kv_sharing_target_layer_name: str | None = None,
        **kwargs,
    ):
        self.num_heads = num_heads
        self.head_size = head_size
        self.scale = scale
        self.num_kv_heads = num_kv_heads
        self.num_kv_groups = num_heads // num_kv_heads
        self.kv_cache_dtype = kv_cache_dtype

        from vllm.model_executor.layers.quantization.turboquant.config import (
            TurboQuantConfig,
        )

        self.tq_config = TurboQuantConfig.from_cache_dtype(kv_cache_dtype, head_size)

        # Pre-compute kernel constants from config (avoid repeated arithmetic)
        cfg = self.tq_config
        self._mse_bytes = (
            math.ceil(head_size * cfg.key_mse_bits / 8)
            if not cfg.key_fp8
            else head_size
        )
        self._val_data_bytes = math.ceil(head_size * cfg.effective_value_quant_bits / 8)
        self._n_centroids = cfg.n_centroids if not cfg.key_fp8 else 1

        # Fixed NUM_KV_SPLITS (grid dims must be constant for cudagraph,
        # and benchmarks show no regression vs dynamic in eager mode).
        vllm_config = get_current_vllm_config()
        self.max_num_kv_splits = (
            vllm_config.attention_config.tq_max_kv_splits_for_cuda_graph
        )

    def _ensure_on_device(self, layer, device):
        """One-time migration of TQ buffers to the correct device."""
        if layer._tq_signs.device != device:
            layer._tq_signs = layer._tq_signs.to(device)
            layer._tq_centroids = layer._tq_centroids.to(device)
        if not hasattr(layer, "_tq_cached"):
            D = layer._tq_signs.shape[0]
            signs = layer._tq_signs.float()

            # WHT rotation: orthonormal + self-inverse, enabling future
            # in-kernel butterfly fusion and trivial inverse for continuation.
            H = _build_hadamard(D, str(device))
            layer._tq_PiT = (signs.unsqueeze(1) * H).contiguous()
            layer._tq_Pi = layer._tq_PiT.T.contiguous()

            c = layer._tq_centroids.float()
            # Precompute midpoints for threshold-based quantization
            c_sorted, _ = c.sort()
            layer._tq_midpoints = (c_sorted[:-1] + c_sorted[1:]) / 2
            # Decode buffers are lazily allocated on first decode call.
            # With fixed NUM_KV_SPLITS (cudagraph mode), the first warmup
            # allocates them and subsequent captures reuse via buf_holder.
            layer._tq_mid_o_buf = None
            layer._tq_output_buf = None
            layer._tq_lse_buf = None
            layer._tq_cached = True

    def do_kv_cache_update(
        self,
        layer: torch.nn.Module,
        key: torch.Tensor,
        value: torch.Tensor,
        kv_cache: torch.Tensor,
        slot_mapping: torch.Tensor,
    ) -> None:
        """Store compressed K/V into the combined TQ cache.

        Called as a separate custom op (unified_kv_cache_update) BEFORE
        the attention forward, matching FlashAttention's split pattern.
        slot_mapping is already sliced to num_actual_tokens by the caller.

        With stream overlap enabled, the store runs on a secondary CUDA
        stream so it can overlap with the next layer's forward pass.
        """
        N = slot_mapping.shape[0]
        if N <= 0:
            return

        device = key.device
        self._ensure_on_device(layer, device)

        k = key[:N].view(N, self.num_kv_heads, self.head_size)
        v = value[:N].view(N, self.num_kv_heads, self.head_size)
        # Use stream overlap only when not capturing CUDA graphs
        stream = aux_stream() if _USE_STREAM_OVERLAP else None
        use_overlap = (
            stream is not None and not torch.cuda.is_current_stream_capturing()
        )

        if use_overlap:
            # Wait for any previous store to finish before starting new one
            torch.cuda.current_stream(device).wait_stream(stream)

            # Launch store on secondary stream
            with torch.cuda.stream(stream):
                self._store_kv(k, v, kv_cache, slot_mapping, layer._tq_centroids, layer)
        else:
            self._store_kv(k, v, kv_cache, slot_mapping, layer._tq_centroids, layer)

    def forward(
        self,
        layer: AttentionLayer,
        query: torch.Tensor,
        key: torch.Tensor,
        value: torch.Tensor,
        kv_cache: torch.Tensor,
        attn_metadata: "TurboQuantMetadata",
        output: torch.Tensor | None = None,
        output_scale: torch.Tensor | None = None,
        output_block_scale: torch.Tensor | None = None,
    ) -> torch.Tensor:
        num_tokens = query.shape[0]

        if output is None:
            output = torch.zeros(
                num_tokens,
                self.num_heads * self.head_size,
                dtype=query.dtype,
                device=query.device,
            )

        if attn_metadata is None:
            return output.fill_(0)

        # Slice to actual tokens
        N = attn_metadata.num_actual_tokens
        if N <= 0:
            return output.fill_(0)

        q = query[:N].view(N, self.num_heads, self.head_size)

        # Get TQ buffers, ensure on device (one-time migration)
        device = q.device
        self._ensure_on_device(layer, device)
        Pi = layer._tq_Pi
        PiT = layer._tq_PiT
        centroids = layer._tq_centroids

        # Ensure any async store has completed before decode reads cache
        if (
            _USE_STREAM_OVERLAP
            and not attn_metadata.is_prefill
            and not torch.cuda.is_current_stream_capturing()
        ):
            stream = aux_stream()
            if stream is not None:
                torch.cuda.current_stream(device).wait_stream(stream)

        # Compute attention (KV cache was already updated by do_kv_cache_update)
        # With reorder_batch_threshold=1, decodes come first in the batch.
        # num_decodes/num_decode_tokens from metadata give the split point.
        num_decodes = attn_metadata.num_decodes
        num_decode_tokens = attn_metadata.num_decode_tokens

        if not attn_metadata.is_prefill:
            # Pure decode batch — fast path
            attn_out = self._decode_attention(
                q, kv_cache, attn_metadata, Pi, centroids, PiT, layer
            )
        elif num_decodes == 0:
            # Pure prefill batch
            k = key[:N].view(N, self.num_kv_heads, self.head_size)
            v = value[:N].view(N, self.num_kv_heads, self.head_size)
            attn_out = self._prefill_attention(
                q, k, v, kv_cache, attn_metadata, Pi, centroids, PiT
            )
        else:
            # Mixed batch: decodes first (guaranteed by reorder_batch).
            attn_out = torch.zeros(
                N, self.num_heads, self.head_size, device=device, dtype=q.dtype
            )

            # --- Decode portion (first num_decodes requests) ---
            # Use full-batch max_seq_len as safe upper bound (no GPU sync).
            decode_meta = TurboQuantMetadata(
                seq_lens=attn_metadata.seq_lens[:num_decodes],
                slot_mapping=attn_metadata.slot_mapping[:num_decode_tokens],
                block_table=attn_metadata.block_table[:num_decodes],
                query_start_loc=attn_metadata.query_start_loc[: num_decodes + 1],
                num_actual_tokens=num_decode_tokens,
                max_query_len=1,
                max_seq_len=attn_metadata.max_seq_len,
                is_prefill=False,
            )
            attn_out[:num_decode_tokens] = self._decode_attention(
                q[:num_decode_tokens], kv_cache, decode_meta, Pi, centroids, PiT, layer
            )

            # --- Prefill portion (remaining requests) ---
            # CRITICAL: use prefill-specific max_seq_len so flash_attn's
            # fast path (max_query_len == max_seq_len) triggers for
            # first-chunk prefills. Using full-batch max_seq_len breaks
            # this because decode requests inflate max_seq_len.
            prefill_seq_lens = attn_metadata.seq_lens[num_decodes:]
            prefill_max_seq = prefill_seq_lens.max().item()
            prefill_qsl = (
                attn_metadata.query_start_loc[num_decodes:] - num_decode_tokens
            )
            prefill_meta = TurboQuantMetadata(
                seq_lens=prefill_seq_lens,
                slot_mapping=attn_metadata.slot_mapping[num_decode_tokens:N],
                block_table=attn_metadata.block_table[num_decodes:],
                query_start_loc=prefill_qsl,
                num_actual_tokens=N - num_decode_tokens,
                max_query_len=attn_metadata.max_query_len,
                max_seq_len=prefill_max_seq,
                is_prefill=True,
            )
            k = key[:N].view(N, self.num_kv_heads, self.head_size)
            v = value[:N].view(N, self.num_kv_heads, self.head_size)
            attn_out[num_decode_tokens:] = self._prefill_attention(
                q[num_decode_tokens:],
                k[num_decode_tokens:],
                v[num_decode_tokens:],
                kv_cache,
                prefill_meta,
                Pi,
                centroids,
                PiT,
            )

        # Write into output buffer: attn_out is (N, Hq, D)
        # output may be 2D (N, Hq*D) or 3D (N, Hq, D)
        if output.ndim == 3:
            output[:N] = attn_out.to(output.dtype)
        else:
            output[:N] = attn_out.reshape(N, -1).to(output.dtype)
        return output

    # ------------------------------------------------------------------ #
    #  Store K/V into combined cache (vectorized)                         #
    # ------------------------------------------------------------------ #
    def _store_kv(
        self,
        key: torch.Tensor,  # (N, Hk, D)
        value: torch.Tensor,  # (N, Hk, D)
        kv_cache: torch.Tensor,  # (num_blocks, block_size, Hk, slot_size)
        slot_mapping: torch.Tensor,
        centroids: torch.Tensor,
        layer: "AttentionLayer",
    ):
        """Quantize + store via fused Triton kernel."""
        triton_turboquant_store(
            key,
            value,
            kv_cache,
            slot_mapping,
            layer._tq_PiT,
            centroids,
            layer._tq_midpoints,
            mse_bits=self.tq_config.key_mse_bits,
            key_packed_size=self.tq_config.key_packed_size,
            value_quant_bits=self.tq_config.effective_value_quant_bits,
            key_fp8=self.tq_config.key_fp8,
        )

    # ------------------------------------------------------------------ #
    #  Prefill: SDPA on raw Q/K/V with causal mask                        #
    # ------------------------------------------------------------------ #
    def _prefill_attention(
        self,
        query: torch.Tensor,  # (N, Hq, D)
        key: torch.Tensor,  # (N, Hk, D)
        value: torch.Tensor,  # (N, Hk, D)
        kv_cache: torch.Tensor,  # (num_blocks, block_size, Hk, slot_size)
        attn_metadata: TurboQuantMetadata,
        Pi: torch.Tensor,
        centroids: torch.Tensor,
        PiT: torch.Tensor | None = None,
    ) -> torch.Tensor:
        N, Hq, D = query.shape

        # Fast path: use flash_attn for first-chunk prefills (all K/V in batch).
        # max_query_len == max_seq_len means no request has prior cached KV.
        # Both are Python ints — no GPU sync.
        if _HAS_FLASH_ATTN and attn_metadata.max_query_len == attn_metadata.max_seq_len:
            output = torch.empty(N, Hq, D, device=query.device, dtype=query.dtype)
            return flash_attn_varlen_func(
                        q=query,
                        k=key,
                        v=value,
                        cu_seqlens_q=attn_metadata.query_start_loc,
                        cu_seqlens_k=attn_metadata.query_start_loc,
                        max_seqlen_q=attn_metadata.max_query_len,
                        max_seqlen_k=attn_metadata.max_query_len,
                        softmax_scale=self.scale,
                        causal=True,
                    )

        # Continuation or no flash_attn: per-request attention.
        # For continuation chunks (seq_len > q_len), we must attend to
        # previously cached K/V from the TQ cache, not just the current
        # chunk's raw K/V.
        Hk = key.shape[1]
        use_gqa = Hk < Hq
        query_start_loc = attn_metadata.query_start_loc
        num_reqs = query_start_loc.shape[0] - 1

        output = torch.zeros(N, Hq, D, device=query.device, dtype=query.dtype)

        # Convert to Python lists once (single CPU-GPU sync) instead of
        # per-request .item() calls that each force a sync.
        qsl = query_start_loc.tolist()
        seq_lens_list = attn_metadata.seq_lens.tolist()

        for i in range(num_reqs):
            q_start = qsl[i]
            q_end = qsl[i + 1]
            q_len = q_end - q_start
            if q_len <= 0:
                continue

            seq_len = seq_lens_list[i]
            q_seq = query[q_start:q_end]  # (q_len, Hq, D)
            k_seq = key[q_start:q_end]  # (q_len, Hk, D)
            v_seq = value[q_start:q_end]  # (q_len, Hk, D)

            if q_len == seq_len:
                # First-chunk prefill: all K/V are in the current batch.
                if _HAS_FLASH_ATTN:
                    out = torch.empty_like(q_seq)
                    cu = torch.tensor(
                        [0, q_len], device=query.device, dtype=torch.int32
                    )
                    out = flash_attn_varlen_func(
                        q=q_seq,
                        k=k_seq,
                        v=v_seq,
                        cu_seqlens_q=cu,
                        cu_seqlens_k=cu,
                        max_seqlen_q=q_len,
                        max_seqlen_k=q_len,
                        softmax_scale=self.scale,
                        causal=True,
                    )
                else:
                    q_t = q_seq.transpose(0, 1).contiguous()
                    k_t = k_seq.transpose(0, 1).contiguous()
                    v_t = v_seq.transpose(0, 1).contiguous()
                    out = F.scaled_dot_product_attention(
                        q_t,
                        k_t,
                        v_t,
                        is_causal=True,
                        scale=self.scale,
                        enable_gqa=use_gqa,
                    ).transpose(0, 1)
                output[q_start:q_end] = out.to(query.dtype)
            else:
                # Continuation chunk: tokens already stored to TQ cache
                # by do_kv_cache_update. Use decode kernel directly to
                # avoid O(cached_len) full-dequant per continuation.
                # For large continuations, fall back to _continuation_prefill.
                cached_len = seq_len - q_len
                if q_len <= _CONTINUATION_DECODE_THRESHOLD:
                    # Fast path: treat each query as a decode request
                    # with incremental seq_lens for causal masking.
                    synth_seq_lens = torch.arange(
                        cached_len + 1,
                        seq_len + 1,
                        device=query.device,
                        dtype=attn_metadata.seq_lens.dtype,
                    )
                    synth_bt = attn_metadata.block_table[i : i + 1].expand(q_len, -1)
                    out = triton_turboquant_decode_attention(
                        query=q_seq,
                        kv_cache=kv_cache,
                        block_table=synth_bt,
                        seq_lens=synth_seq_lens,
                        Pi=Pi,
                        centroids=centroids,
                        scale=self.scale,
                        mse_bits=self.tq_config.key_mse_bits,
                        key_packed_size=self.tq_config.key_packed_size,
                        value_quant_bits=(self.tq_config.effective_value_quant_bits),
                        key_fp8=self.tq_config.key_fp8,
                        norm_correction=self.tq_config.norm_correction,
                        PiT=PiT,
                    )
                else:
                    # Large continuation: dequant cached K/V and use
                    # flash_attn for better throughput.
                    out = self._continuation_prefill(
                        q_seq,
                        k_seq,
                        v_seq,
                        kv_cache,
                        attn_metadata.block_table[i : i + 1],
                        cached_len,
                        seq_len,
                        Pi,
                        centroids,
                    )
                output[q_start:q_end] = out.to(query.dtype)

        return output

    def _continuation_prefill(
        self,
        query: torch.Tensor,  # (q_len, Hq, D)
        key_chunk: torch.Tensor,  # (q_len, Hk, D)
        val_chunk: torch.Tensor,  # (q_len, Hk, D)
        kv_cache: torch.Tensor,  # (num_blocks, block_size, Hk, slot_size)
        block_table: torch.Tensor,  # (1, max_num_blocks)
        cached_len: int,
        seq_len: int,
        Pi: torch.Tensor,
        centroids: torch.Tensor,
    ) -> torch.Tensor:
        """Handle continuation chunk by dequanting cached K/V from TQ cache.

        Dequants previously cached K/V, concatenates with the current
        chunk's raw K/V, then runs flash_attn with causal masking.
        """
        q_len, Hq, D = query.shape
        Hk = key_chunk.shape[1]
        device = query.device
        block_size = kv_cache.shape[1]
        BLOCK_D = triton.next_power_of_2(D)

        mse_bytes = self._mse_bytes
        val_data_bytes = self._val_data_bytes
        n_centroids = self._n_centroids

        # Dequant cached K/V from TQ cache
        # Allocate slightly over to align to block_size for the grid
        alloc_len = math.ceil(cached_len / block_size) * block_size
        k_cached = torch.zeros(1, Hk, alloc_len, D, dtype=torch.float16, device=device)
        v_cached = torch.zeros(1, Hk, alloc_len, D, dtype=torch.float16, device=device)

        grid = (alloc_len, 1 * Hk)
        _tq_full_dequant_kv[grid](
            kv_cache,
            block_table,
            centroids.float(),
            k_cached,
            v_cached,
            k_cached.stride(0),
            k_cached.stride(1),
            k_cached.stride(2),
            v_cached.stride(0),
            v_cached.stride(1),
            v_cached.stride(2),
            kv_cache.stride(0),
            kv_cache.stride(1),
            kv_cache.stride(2),
            block_table.stride(0),
            HEAD_DIM=D,
            BLOCK_SIZE=block_size,
            NUM_KV_HEADS=Hk,
            MSE_BYTES=mse_bytes,
            KPS=self.tq_config.key_packed_size,
            VQB=self.tq_config.effective_value_quant_bits,
            VAL_DATA_BYTES=val_data_bytes,
            MSE_BITS=self.tq_config.key_mse_bits,
            N_CENTROIDS=n_centroids,
            KEY_FP8=1 if self.tq_config.key_fp8 else 0,
            BLOCK_D=BLOCK_D,
            NORM_CORRECTION=1 if self.tq_config.norm_correction else 0,
            FP8_E4B15=_use_fp8_e4b15(device.index or 0),
            num_warps=4,
        )

        # Inverse-rotate MSE keys back to original space
        if not self.tq_config.key_fp8:
            k_flat = k_cached[0, :, :cached_len, :].reshape(-1, D).float()
            k_flat = k_flat @ Pi.float()
            k_cached_trim = (
                k_flat.to(torch.float16).reshape(Hk, cached_len, D).transpose(0, 1)
            )  # (cached_len, Hk, D)
        else:
            k_cached_trim = (
                k_cached[0, :, :cached_len, :].transpose(0, 1).contiguous()
            )  # (cached_len, Hk, D)

        v_cached_trim = (
            v_cached[0, :, :cached_len, :].transpose(0, 1).contiguous()
        )  # (cached_len, Hk, D)

        # Concatenate cached + current chunk K/V (match query dtype)
        qdtype = query.dtype
        k_full = torch.cat([k_cached_trim.to(qdtype), key_chunk], dim=0)
        v_full = torch.cat([v_cached_trim.to(qdtype), val_chunk], dim=0)

        # Attention: q_len queries attending to seq_len K/V with causal mask
        if _HAS_FLASH_ATTN:
            output = torch.empty(q_len, Hq, D, device=device, dtype=query.dtype)
            cu_seqlens_q = torch.tensor([0, q_len], device=device, dtype=torch.int32)
            cu_seqlens_k = torch.tensor([0, seq_len], device=device, dtype=torch.int32)
            return flash_attn_varlen_func(
                q=query,
                k=k_full,
                v=v_full,
                cu_seqlens_q=cu_seqlens_q,
                cu_seqlens_k=cu_seqlens_k,
                max_seqlen_q=q_len,
                max_seqlen_k=seq_len,
                softmax_scale=self.scale,
                causal=True,
            )
        else:
            # SDPA fallback: expand KV for GQA, build causal mask
            q_t = query.transpose(0, 1).unsqueeze(0)  # (1, Hq, q_len, D)
            k_t = k_full.transpose(0, 1).unsqueeze(0)  # (1, Hk, seq_len, D)
            v_t = v_full.transpose(0, 1).unsqueeze(0)  # (1, Hk, seq_len, D)
            # Build causal mask: query position p can attend to K position j
            # where j <= cached_len + p (p is 0-indexed within chunk)
            q_pos = torch.arange(q_len, device=device).unsqueeze(1) + cached_len
            k_pos = torch.arange(seq_len, device=device).unsqueeze(0)
            mask = k_pos <= q_pos  # (q_len, seq_len)
            out = F.scaled_dot_product_attention(
                q_t,
                k_t,
                v_t,
                attn_mask=mask,
                scale=self.scale,
                enable_gqa=(Hk < Hq),
            )  # (1, Hq, q_len, D)
            return out[0].transpose(0, 1)  # (q_len, Hq, D)

    # ------------------------------------------------------------------ #
    #  Decode: Triton TQ decode attention                                 #
    # ------------------------------------------------------------------ #
    def _decode_attention(
        self,
        query: torch.Tensor,  # (B, Hq, D)
        kv_cache: torch.Tensor,  # (num_blocks, block_size, Hk, slot_size)
        attn_metadata: TurboQuantMetadata,
        Pi: torch.Tensor,
        centroids: torch.Tensor,
        PiT: torch.Tensor | None = None,
        layer: torch.nn.Module | None = None,
    ) -> torch.Tensor:
        # Grab cached decode buffers from the layer (lazily allocated).
        mid_o_buf = output_buf = lse_buf = None
        if layer is not None:
            mid_o_buf = getattr(layer, "_tq_mid_o_buf", None)
            output_buf = getattr(layer, "_tq_output_buf", None)
            lse_buf = getattr(layer, "_tq_lse_buf", None)

        result = triton_turboquant_decode_attention(
            query=query,
            kv_cache=kv_cache,
            block_table=attn_metadata.block_table,
            seq_lens=attn_metadata.seq_lens,
            Pi=Pi,
            centroids=centroids,
            scale=self.scale,
            mse_bits=self.tq_config.key_mse_bits,
            key_packed_size=self.tq_config.key_packed_size,
            value_quant_bits=self.tq_config.effective_value_quant_bits,
            key_fp8=self.tq_config.key_fp8,
            norm_correction=self.tq_config.norm_correction,
            PiT=PiT,
            mid_o_buf=mid_o_buf,
            output_buf=output_buf,
            lse_buf=lse_buf,
            buf_holder=layer,
            max_num_kv_splits=self.max_num_kv_splits,
        )
        return result
