"""Referrer -> domain bucketing (Chapter 08 rollup, Method A)."""
from __future__ import annotations

from urllib.parse import urlparse


def referrer_domain(referrer: str | None) -> str | None:
    """Bucket a raw referrer URL to its host, stripping a leading 'www.'.

    Full-URL cardinality would make the daily rollup unbounded in row
    count; domain bucketing keeps it small (Ch03's bounded-resource bias).
    Empty/unparsable referrers return None and are excluded from the rollup.
    """
    if not referrer:
        return None
    host = urlparse(referrer).netloc.lower()
    if not host:
        return None
    return host[4:] if host.startswith("www.") else host