"""Attention implementation identifiers and validation helpers."""


STANDARD_ATTENTION = "standard"
DIFF_ATTENTION = "diff"
DEFAULT_ATTENTION_TYPE = STANDARD_ATTENTION
SUPPORTED_ATTENTION_TYPES = (
    STANDARD_ATTENTION,
    DIFF_ATTENTION,
)


def resolve_attention_type(attention_type: str | None = None) -> str:
    """Resolve an attention selector, defaulting old configs to standard."""
    resolved = DEFAULT_ATTENTION_TYPE if attention_type is None else attention_type
    if not isinstance(resolved, str):
        raise ValueError(
            "attention_type must be one of "
            f"{SUPPORTED_ATTENTION_TYPES}, got {resolved!r}"
        )
    if resolved not in SUPPORTED_ATTENTION_TYPES:
        raise ValueError(
            f"Unsupported attention_type={resolved!r}; "
            f"expected one of {SUPPORTED_ATTENTION_TYPES}."
        )
    return resolved