feat: mail service

authsession/internal/service/shared/preferred_language.go

@@ -0,0 +1,27 @@
+package shared
+
+import "golang.org/x/text/language"
+
+const defaultPreferredLanguage = "en"
+
+// ResolvePreferredLanguage returns the first canonical BCP 47 language tag
+// accepted from value, or the stable "en" fallback when the input is absent,
+// malformed, or too unspecific for auth registration purposes.
+func ResolvePreferredLanguage(value string) string {
+	tags, _, err := language.ParseAcceptLanguage(value)
+	if err != nil {
+		return defaultPreferredLanguage
+	}
+
+	for _, tag := range tags {
+		canonical := tag.String()
+		switch canonical {
+		case "", "und", "mul":
+			continue
+		default:
+			return canonical
+		}
+	}
+
+	return defaultPreferredLanguage
+}

authsession/internal/service/shared/preferred_language_test.go

@@ -0,0 +1,51 @@
+package shared
+
+import "testing"
+
+func TestResolvePreferredLanguage(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name  string
+		value string
+		want  string
+	}{
+		{
+			name:  "canonical valid tag",
+			value: "fr-FR, en;q=0.8",
+			want:  "fr-FR",
+		},
+		{
+			name:  "quality ordering",
+			value: "en-US;q=0.9, fr",
+			want:  "fr",
+		},
+		{
+			name:  "wildcard falls back",
+			value: "*",
+			want:  "en",
+		},
+		{
+			name:  "malformed falls back",
+			value: "fr-FR, @@",
+			want:  "en",
+		},
+		{
+			name:  "missing falls back",
+			value: "",
+			want:  "en",
+		},
+	}
+
+	for _, tt := range tests {
+		tt := tt
+
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			if got := ResolvePreferredLanguage(tt.value); got != tt.want {
+				t.Fatalf("ResolvePreferredLanguage(%q) = %q, want %q", tt.value, got, tt.want)
+			}
+		})
+	}
+}