import hashlib


def header_hex_to_block_hash(header_hex: str) -> str:
    """Block hash from a raw Electrum header: sha256d, byte-reversed, hex.
    Verified against a real mainnet block (blockchain.transaction.get's own
    reported blockhash) during development."""
    header_bytes = bytes.fromhex(header_hex)
    digest = hashlib.sha256(hashlib.sha256(header_bytes).digest()).digest()
    return digest[::-1].hex()


def draw_winner(participants: list[str], block_hash_hex: str) -> str:
    """v1 draw algorithm (flowchart.mmd, node R): seed = block hash as an integer,
    index = seed mod participant_count, winner = participants[index]. Anyone can
    recompute and verify it from public data. Deliberately simple/replaceable."""
    if not participants:
        raise ValueError("no participants to draw from")
    seed = int(block_hash_hex, 16)
    index = seed % len(participants)
    return participants[index]