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# 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.
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# http://www.apache.org/licenses/LICENSE-2.0
#
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"""
Imaginaire4 Attention Subpackage:
Unified implementation for all Attention implementations.

Flash Attention v2 (flash2) Backend
"""

import torch

from cosmos_predict2._src.imaginaire.attention.utils import safe_log as log

# We lock to safe releases of Flash 2
# We will have a separate backend identifier for 2025 releases with CuTeDSL
# kernels.
FLASH_ATTENTION_V2_MIN_VERSION = [2, 7, 0]
FLASH_ATTENTION_V2_MAX_VERSION = [2, 7, 4]


def flash2_supported() -> bool:
    """
    Returns whether Flash Attention is supported in this environment.
    Requirements are:
        * Presence of CUDA Runtime (via PyTorch)
        * Presence of Flash Attention, meeting minimum version requirements

    This check guards imports / dependencies on the Flash Attention package.
    """
    if not torch.cuda.is_available():
        log.debug("Flash Attention v2 is not supported because PyTorch did not detect CUDA runtime.")
        return False

    try:
        import flash_attn

    except ImportError:
        log.debug("Flash Attention v2 is not supported because the Python package was not found.")
        return False
    except Exception as e:
        log.debug(f"Flash Attention v2 is not supported because importing the Python package failed: {e}")
        return False

    flash2_version_str = None
    if not hasattr(flash_attn, "__version__"):
        from importlib.metadata import version

        flash2_version_str = version("flash_attn")
    else:
        flash2_version_str = flash_attn.__version__

    flash2_version_split = flash2_version_str.split(".")
    if len(flash2_version_split) < 3:
        log.debug(f"Unable to parse Flash Attention v2 version {flash2_version_str}.")
        return False

    try:
        flash2_version = [int(x) for x in flash2_version_split[:3]]

    except ValueError:
        log.debug(f"Unable to parse Flash Attention v2 version as an int list: {flash2_version_str}.")
        return False

    if flash2_version > FLASH_ATTENTION_V2_MAX_VERSION or flash2_version < FLASH_ATTENTION_V2_MIN_VERSION:
        log.debug(
            "Flash Attention v2 build is not supported; this backend only supports versions "
            f"{FLASH_ATTENTION_V2_MIN_VERSION} through {FLASH_ATTENTION_V2_MAX_VERSION}, got "
            f"{flash2_version}."
        )
        return False

    return True


FLASH2_SUPPORTED = flash2_supported()

if FLASH2_SUPPORTED:
    from cosmos_predict2._src.imaginaire.attention.flash2.functions import flash2_attention

else:
    from cosmos_predict2._src.imaginaire.attention.flash2.stubs import flash2_attention

__all__ = ["flash2_attention", "FLASH2_SUPPORTED"]