internal: add referrer classification

Signed-off-by: NotAShelf <raf@notashelf.dev>
Change-Id: Ibcaa09bfd4767876ea1cdd5b61c53b476a6a6964

internal/normalize/referrer.go

@@ -0,0 +1,97 @@
+package normalize
+
+import (
+	"net/url"
+	"strings"
+
+	"golang.org/x/net/publicsuffix"
+)
+
+// Returns true for localhost, loopback IPs, and private IPs.
+func isInternalHost(hostname string) bool {
+	if hostname == "" {
+		return false
+	}
+
+	// Localhost variants
+	if hostname == "localhost" ||
+		strings.HasPrefix(hostname, "localhost.") ||
+		strings.HasPrefix(hostname, "127.") ||
+		hostname == "::1" {
+		return true
+	}
+
+	// Private IPv4 ranges (RFC1918)
+	if strings.HasPrefix(hostname, "10.") ||
+		strings.HasPrefix(hostname, "192.168.") ||
+		strings.HasPrefix(hostname, "172.16.") ||
+		strings.HasPrefix(hostname, "172.17.") ||
+		strings.HasPrefix(hostname, "172.18.") ||
+		strings.HasPrefix(hostname, "172.19.") ||
+		strings.HasPrefix(hostname, "172.20.") ||
+		strings.HasPrefix(hostname, "172.21.") ||
+		strings.HasPrefix(hostname, "172.22.") ||
+		strings.HasPrefix(hostname, "172.23.") ||
+		strings.HasPrefix(hostname, "172.24.") ||
+		strings.HasPrefix(hostname, "172.25.") ||
+		strings.HasPrefix(hostname, "172.26.") ||
+		strings.HasPrefix(hostname, "172.27.") ||
+		strings.HasPrefix(hostname, "172.28.") ||
+		strings.HasPrefix(hostname, "172.29.") ||
+		strings.HasPrefix(hostname, "172.30.") ||
+		strings.HasPrefix(hostname, "172.31.") {
+		return true
+	}
+
+	// IPv6 loopback and local
+	if strings.HasPrefix(hostname, "::1") ||
+		strings.HasPrefix(hostname, "fe80::") ||
+		strings.HasPrefix(hostname, "fc00::") ||
+		strings.HasPrefix(hostname, "fd00::") {
+		return true
+	}
+
+	return false
+}
+
+// Extracts the eTLD+1 domain from a referrer URL.
+// Returns "direct" for empty or same-domain referrers.
+// Returns empty string for invalid URLs.
+func ExtractReferrerDomain(referrer, siteDomain string) string {
+	if referrer == "" {
+		return "direct"
+	}
+
+	u, err := url.Parse(referrer)
+	if err != nil {
+		return ""
+	}
+
+	hostname := strings.ToLower(u.Hostname())
+	hostname = strings.TrimSuffix(hostname, ".") // remove trailing dot
+	if hostname == "" {
+		return ""
+	}
+
+	// Check for internal/localhost traffic
+	if isInternalHost(hostname) {
+		return "internal"
+	}
+
+	// Same domain check
+	siteDomainLower := strings.ToLower(siteDomain)
+	if hostname == siteDomainLower || strings.HasSuffix(hostname, "."+siteDomainLower) {
+		return "direct"
+	}
+
+	// Extract eTLD+1 (effective top-level domain + 1 label); e.g.
+	// - "www.google.co.uk" -> "google.co.uk"
+	// - "news.ycombinator.com" -> "ycombinator.com"
+	eTLDPlus1, err := publicsuffix.EffectiveTLDPlusOne(hostname)
+	if err != nil {
+		// If public suffix lookup fails, use hostname as-is
+		return hostname
+	}
+
+	return eTLDPlus1
+}