Add check for summary element of entry

content.go

@@ -1,6 +1,8 @@
 package atomfeed
 
 type Content interface {
+	isContent() bool
+	hasSRC() bool
+	getType() string
 	Check() error
-	IsContent() bool
 }

entry.go

@@ -4,6 +4,8 @@ import (
 	"encoding/xml"
 	"errors"
 	"fmt"
+	"mime"
+	"strings"
 )
 
 // It is advisable that each atom:entry element contain a non-empty atom:title
@@ -14,7 +16,7 @@ type Entry struct {
 	*CommonAttributes
 	Authors      []*Person           `xml:"author,omitempty"`
 	Categories   []*Category         `xml:"category,omitempty"`
-	Content      *Content            `xml:"content,omitempty"`
+	Content      Content             `xml:"content,omitempty"`
 	Contributors []*Person           `xml:"contributors,omitempty"`
 	ID           *ID                 `xml:"id"`
 	Links        []*Link             `xml:"link,omitempty"`
@@ -57,6 +59,10 @@ func alternateRelExists(l []*Link) bool {
 	return false
 }
 
+func isXMLMediaType(mediaType string) bool {
+	return strings.HasSuffix(mediaType, "/xml") || strings.HasSuffix(mediaType, "+xml")
+}
+
 func (e *Entry) AddExtension(name string, value any) {
 	e.Extensions = append(e.Extensions, &ExtensionElement{XMLName: xml.Name{Local: name}, Value: value})
 }
@@ -81,7 +87,7 @@ func (e *Entry) Check() error {
 	}
 
 	if e.Content != nil {
-		if err := (*e.Content).Check(); err != nil {
+		if err := e.Content.Check(); err != nil {
 			return fmt.Errorf("content element of entry %v: %v", e.ID.URI, err)
 		}
 	} else {
@@ -130,6 +136,25 @@ func (e *Entry) Check() error {
 		if err := e.Summary.Check(); err != nil {
 			return fmt.Errorf("summary element of entry %v: %v", e.ID.URI, err)
 		}
+	} else {
+		// atom:entry elements MUST contain an atom:summary element in either
+		// of the following cases:
+		// the atom:entry contains an atom:content that has a "src" attribute
+		// (and is thus empty).
+		if e.Content.hasSRC() {
+			return fmt.Errorf("no summary element of entry %v but content of type out of line content", e.ID.URI)
+		}
+		// the atom:entry contains content that is encoded in Base64; i.e., the
+		// "type" attribute of atom:content is a MIME media type [MIMEREG], but
+		// is not an XML media type [RFC3023], does not begin with "text/", and
+		// does not end with "/xml" or "+xml".
+		mediaType, _, err := mime.ParseMediaType(e.Content.getType())
+		if err != nil {
+			return fmt.Errorf("type attribute of content element of entry %v: %v", e.ID.URI, err)
+		}
+		if !isXMLMediaType(mediaType) && !strings.HasPrefix(mediaType, "text/") {
+			return fmt.Errorf("no summary element of entry %v but media type not xml", e.ID.URI)
+		}
 	}
 
 	if e.Title == nil {

inlineOtherContent.go

@@ -6,6 +6,10 @@ type InlineOtherContent struct {
 	AnyElement []*any    `xml:"anyelement,omitempty"`
 }
 
-func (i *InlineOtherContent) IsContent() bool { return true }
+func (i *InlineOtherContent) isContent() bool { return true }
+
+func (i *InlineOtherContent) hasSRC() bool { return false }
+
+func (i *InlineOtherContent) getType() string { return string(i.Type) }
 
 func (i *InlineOtherContent) Check() error { return nil }

inlineTextContent.go

@@ -8,7 +8,11 @@ type InlineTextContent struct {
 	Texts []string `xml:"texts,omitempty"`
 }
 
-func (i *InlineTextContent) IsContent() bool { return true }
+func (i *InlineTextContent) isContent() bool { return true }
+
+func (i *InlineTextContent) hasSRC() bool { return false }
+
+func (i *InlineTextContent) getType() string { return i.Type }
 
 func (i *InlineTextContent) Check() error {
 	if i.Type != "" && i.Type != "text" && i.Type != "html" {

inlineXHTMLContent.go

@@ -8,7 +8,11 @@ type InlineXHTMLContent struct {
 	XHTMLDiv string `xml:"xhtmldiv"`
 }
 
-func (i *InlineXHTMLContent) IsContent() bool { return true }
+func (i *InlineXHTMLContent) isContent() bool { return true }
+
+func (i *InlineXHTMLContent) hasSRC() bool { return false }
+
+func (i *InlineXHTMLContent) getType() string { return i.Type }
 
 func (i *InlineXHTMLContent) Check() error {
 	if i.Type != "xhtml" {

outOfLineContent.go

@@ -8,7 +8,11 @@ type OutOfLineContent struct {
 	SRC  URI       `xml:"src,attr"`
 }
 
-func (o *OutOfLineContent) IsContent() bool { return true }
+func (o *OutOfLineContent) isContent() bool { return true }
+
+func (o *OutOfLineContent) hasSRC() bool { return true }
+
+func (o *OutOfLineContent) getType() string { return string(o.Type) }
 
 func (o *OutOfLineContent) Check() error {
 	if o.SRC == "" {