start project

app/services/referrer.py

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+"""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