Check for links with "alternate" ref and duplicate type and hreflang attributes

feed.go

@@ -26,6 +26,25 @@ type Feed struct {
 	Entries      []*Entry            `xml:"entry,omitempty"`
 }
 
+// atom:feed elements MUST NOT contain more than one atom:link
+// element with a rel attribute value of "alternate" that has the
+// same combination of type and hreflang attribute values.
+func hasAlternateDuplicateLinks(l []*Link) bool {
+	linkMap := make(map[string]bool)
+
+	for _, link := range l {
+		if link.Rel == "alternate" {
+			key := fmt.Sprint(link.Type, "|", link.HrefLang)
+			if linkMap[key] {
+				return true
+			}
+			linkMap[key] = true
+		}
+	}
+
+	return false
+}
+
 // NewFeed creates a new feed.
 func NewFeed(title string) (*Feed, error) {
 	text, err := NewText("text", title)
@@ -172,6 +191,9 @@ func (f *Feed) Check() error {
 			}
 		}
 	}
+	if hasAlternateDuplicateLinks(f.Links) {
+		return errors.New("duplicate links with with a rel attribute value of \"alternate\" found")
+	}
 
 	if f.Logo != nil {
 		if err := f.Logo.Check(); err != nil {