"""Length-framed SHA-512 KDF (docs/protocol.md)."""

from __future__ import annotations

import hashlib

__all__ = ["kdf"]


def kdf(*data: str | bytes, length: int = 32) -> bytes:
    """kdf(info, *items, length<=64) -> SHA-512 over length-framed items."""
    if not 1 <= length <= 64:
        raise ValueError("length must be in 1..64")
    items = [d.encode() if isinstance(d, str) else bytes(d) for d in data]
    h = hashlib.sha512(len(items).to_bytes(8, "little"))
    for item in items:
        h.update(len(item).to_bytes(8, "little"))
        h.update(item)
    return h.digest()[:length]