# SPDX-FileCopyrightText: Copyright (c) 2025 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
# SPDX-License-Identifier: Apache-2.0
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

"""
Imaginaire4 Attention Subpackage:
Unified implementation for all Attention implementations.

Flash Attention v2 (flash2) Backend: intermediate API stubs
Always safe to import (as long as torch is available.)
"""

from torch import Tensor

from cosmos_predict2._src.imaginaire.attention.masks import CausalType


def flash2_attention(
    query: Tensor,
    key: Tensor,
    value: Tensor,
    is_causal: bool = False,
    causal_type: CausalType | None = None,
    scale: float | None = None,
    cumulative_seqlen_Q: Tensor | None = None,
    cumulative_seqlen_KV: Tensor | None = None,
    max_seqlen_Q: int | None = None,
    max_seqlen_KV: int | None = None,
    return_lse: bool = False,
    backend_kwargs: dict | None = None,
) -> Tensor | tuple[Tensor, Tensor]:
    raise RuntimeError(
        "Tried to run Flash Attention v2, but it is not supported / available. "
        "Try running with debug logs enabled to see why."
    )