internal: add referrer classification

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

internal/normalize/referrer_registry.go

@@ -0,0 +1,48 @@
+package normalize
+
+import "sync"
+
+// Bounded set of observed referrer domains.
+type ReferrerRegistry struct {
+	mu            sync.RWMutex
+	sources       map[string]struct{}
+	maxSources    int
+	overflowCount int
+}
+
+func NewReferrerRegistry(maxSources int) *ReferrerRegistry {
+	return &ReferrerRegistry{
+		sources:    make(map[string]struct{}, maxSources),
+		maxSources: maxSources,
+	}
+}
+
+// Attempt to add a referrer source to the registry. Returns the source to use ("other" if rejected).
+func (r *ReferrerRegistry) Add(source string) string {
+	if source == "direct" || source == "internal" {
+		return source
+	}
+
+	r.mu.Lock()
+	defer r.mu.Unlock()
+
+	// Already exists
+	if _, exists := r.sources[source]; exists {
+		return source
+	}
+
+	// Check limit
+	if len(r.sources) >= r.maxSources {
+		r.overflowCount++
+		return "other"
+	}
+
+	r.sources[source] = struct{}{}
+	return source
+}
+
+func (r *ReferrerRegistry) OverflowCount() int {
+	r.mu.RLock()
+	defer r.mu.RUnlock()
+	return r.overflowCount
+}