Add hasAlternateDuplicateLinks check to entry

entry.go

@@ -104,6 +104,9 @@ func (e *Entry) Check() error {
 			return fmt.Errorf("link element %v of entry %v: %v", i, e.ID.URI, err)
 		}
 	}
+	if hasAlternateDuplicateLinks(e.Links) {
+		return fmt.Errorf("links with with a rel attribute value of \"alternate\" and duplicate type and hreflang attribute values found in entry %v", e.ID.URI)
+	}
 
 	if e.Published != nil {
 		if err := e.Published.Check(); err != nil {

feed.go

@@ -26,25 +26,6 @@ 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)
@@ -189,7 +170,7 @@ func (f *Feed) Check() error {
 		}
 	}
 	if hasAlternateDuplicateLinks(f.Links) {
-		return errors.New("links with with a rel attribute value of \"alternate\" and duplicate type and hreflang attribute values found")
+		return fmt.Errorf("links with with a rel attribute value of \"alternate\" and duplicate type and hreflang attribute values found in feed %v", f.ID.URI)
 	}
 
 	if f.Logo != nil {

link.go

@@ -37,3 +37,22 @@ func (l *Link) Check() error {
 
 	return nil
 }
+
+// atom:feed/entry 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
+}