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from __future__ import annotations

from dataclasses import dataclass
from enum import Enum

import torch


class GdnCheckpointLayout(str, Enum):
    """Backend-native checkpoint layout returned by GDN chunk prefill."""

    NONE = "none"
    FLA = "fla"
    FLASHINFER = "flashinfer"


@dataclass(frozen=True)
class GdnChunkPrefillResult:
    """Structured result for GDN chunk prefill.

    Args:
        out: GDN output tensor.
        final_state: Final recurrent state, when requested.
        h: Optional backend-native intermediate recurrent checkpoints.
        h_cu_starts: Optional cumulative checkpoint starts for FlashInfer layout.
        h_layout: Layout of ``h``.
    """

    out: torch.Tensor
    final_state: torch.Tensor | None
    h: torch.Tensor | None = None
    h_cu_starts: torch.Tensor | None = None
    h_layout: GdnCheckpointLayout = GdnCheckpointLayout.NONE