Make NewCommonAttributes public

README.md

@@ -2,7 +2,7 @@
 
 An extensible Atom feed generator library that aims to be very close to RFC4287.
 It diligently checks for compliance with the standard and provides functions for
-easy creation and extension of elements.
+easy creation, extension and deletion of elements.
 
 ## Installation
 
@@ -14,9 +14,11 @@ go get git.streifling.com/jason/atom@latest
 
 ## Usage
 
-This library provides convenient functions to safely create and extend elements
-and attributes of Atom feeds. It also provides checks for all constructs'
-adherence to RFC4287.
+### Basic Feed
+
+This library provides convenient functions to safely create, extend and delete
+elements and attributes of Atom feeds. It also provides checks for all
+constructs' adherence to RFC4287.
 
 ```go
 package main
@@ -30,8 +32,6 @@ import (
 
 func main() {
     feed := atom.NewFeed("Example Feed")
- feed.Title = atom.NewText("text", "Example Feed")
- feed.ID = atom.NewID(atom.NewURN())
     if err := feed.Check(); err != nil {
         log.Fatalln(err)
     }
@@ -44,14 +44,13 @@ func main() {
     feed.AddAuthor(author)
 
     entry := atom.NewEntry("First Entry")
- entry.ID = atom.NewID(atom.NewURN())
     entry.Content = atom.NewContent(atom.InlineText, "text", "This is the content of the first entry.")
     if err := entry.Check(); err != nil {
         log.Fatalln(err)
     }
     feed.AddEntry(entry)
 
- feedString, err := feed.ToXML("utf-8")
+    feedString, err := feed.ToXML("UTF-8")
     if err != nil {
         log.Fatalln(err)
     }
@@ -71,6 +70,7 @@ import (
     "time"
 
     "git.streifling.com/jason/atom"
+    "github.com/google/uuid"
 )
 
 func main() {
@@ -81,7 +81,7 @@ func main() {
             Type: "text",
             Text: "Example Feed",
         },
-  ID:      &atom.ID{URI: "urn:uuid:60a76c80-d399-11d9-b93C-0003939e0af6"},
+        ID:      &atom.ID{URI: fmt.Sprint("urn:uuid:", uuid.New())},
         Updated: &atom.Date{DateTime: atom.DateTime(now)},
         Authors: []*atom.Person{
             {
@@ -95,7 +95,7 @@ func main() {
                     Type: "text",
                     Text: "First Entry",
                 },
-    ID:      &atom.ID{URI: "urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a"},
+                ID:      &atom.ID{URI: fmt.Sprint("urn:uuid:", uuid.New())},
                 Updated: &atom.Date{DateTime: atom.DateTime(now)},
                 Content: &atom.InlineTextContent{
                     Type: "text",
@@ -105,7 +105,7 @@ func main() {
         },
     }
 
- feedString, err := feed.ToXML("utf-8")
+    feedString, err := feed.ToXML("UTF-8")
     if err != nil {
         log.Fatalln(err)
     }
@@ -116,7 +116,7 @@ func main() {
 The output of both ways of using it is an RFC4287 compliant Atom feed.
 
 ```xml
-<?xml version="1.0" encoding="utf-8"?>
+<?xml version="1.0" encoding="UTF-8"?>
 <feed xmlns="http://www.w3.org/2005/Atom">
     <author>
         <name>John Doe</name>
@@ -133,3 +133,109 @@ The output of both ways of using it is an RFC4287 compliant Atom feed.
     </entry>
 </feed>
 ```
+
+### Compliance and Error Checking
+
+All Atom constructs have their own ```construct.Check()``` method. It checks for
+compliance with RFC4287 and errors. This allows one to be certain that only
+compliant constructs are added to their respective parent construct. It also
+means that not the entire feed has to be checked every time a new construct is
+added. Instead one only checks the current construct and all of its
+sub-constructs with a single ```construct.Check()``` call.
+
+```go
+package main
+
+import (
+    "fmt"
+    "log"
+
+    "git.streifling.com/jason/atom"
+)
+
+func main() {
+    feed := atom.NewFeed("Example Feed")
+    if err := feed.Check(); err != nil {
+        log.Fatalln(err)
+    }
+
+    entry := atom.NewEntry("One")
+    entry.Content = atom.NewContent(atom.InlineText, "text", "Entry One")
+    entry.AddAuthor(atom.NewPerson("John Doe"))
+    if err := entry.Check(); err != nil {
+        log.Fatalln(err)
+    }
+    feed.AddEntry(entry)
+
+    feedString, err := feed.ToXML("UTF-8")
+    if err != nil {
+        log.Fatalln(err)
+    }
+    fmt.Println(feedString)
+}
+```
+
+### Adding and Deleting Items
+
+To add elements to any slice one calls the appropriate
+```someone.AddSomething(toBeAdded)``` method. It returns the item's index
+number. To delete the item one calls the Delete method.
+
+```go
+package main
+
+import (
+    "fmt"
+    "log"
+
+    "git.streifling.com/jason/atom"
+)
+
+func main() {
+    feed := atom.NewFeed("Feed")
+    if err := feed.Check(); err != nil {
+        log.Fatalln(err)
+    }
+
+    e1 := atom.NewEntry("One")
+    e1.Content = atom.NewContent(atom.InlineText, "text", "Entry One")
+    if err := e1.Check(); err != nil {
+        log.Fatalln(err)
+    }
+    i1 := feed.AddEntry(e1)
+
+    e2 := atom.NewEntry("Two")
+    e2.Content = atom.NewContent(atom.InlineText, "text", "Entry Two")
+    if err := e2.Check(); err != nil {
+        log.Fatalln(err)
+    }
+    feed.AddEntry(e2)
+
+    if err := feed.DeleteEntry(i1); err != nil {
+        log.Fatalln(err)
+    }
+
+    feedString, err := feed.ToXML("UTF-8")
+    if err != nil {
+        log.Fatalln(err)
+    }
+    fmt.Println(feedString)
+}
+```
+
+The output of this example looks like this. It only shows the second entry.
+
+```xml
+<?xml version="1.0" encoding="UTF-8"?>
+<feed xmlns="http://www.w3.org/2005/Atom">
+    <id>urn:uuid:ae89d343-b535-447d-ac14-4b80d3e02a2f</id>
+    <title type="text">Example Feed</title>
+    <updated>2024-10-20T14:57:02+02:00</updated>
+    <entry>
+        <content type="text">Entry Two</content>
+        <id>urn:uuid:620c6f73-ee1d-4c1e-be98-b0b1ad7a053f</id>
+        <title type="text">Two</title>
+        <updated>2024-10-20T14:57:02+02:00</updated>
+    </entry>
+</feed>
+```

atom.go

@@ -1,6 +1,7 @@
 package atom
 
 import (
+	"encoding/xml"
 	"fmt"
 	"html"
 	"mime"
@@ -11,6 +12,32 @@ import (
 	"golang.org/x/text/language"
 )
 
+type Countable interface {
+	*xml.Attr | *Person | *Category | *Link | *ExtensionElement | *Entry
+}
+
+// addToSlice adds a Countable to to a *[]Countable. It returns an int.
+func addToSlice[C Countable](slice *[]C, countable C) int {
+	if *slice == nil {
+		*slice = make([]C, 0)
+	}
+
+	*slice = append(*slice, countable)
+	return len(*slice) - 1
+}
+
+// deleteFromSlice deletes the Countable with the index from the *[]Countable.
+// It return an error.
+func deleteFromSlice[C Countable](slice *[]C, index int) error {
+	length := len(*slice)
+	if index > length {
+		return fmt.Errorf("id %v out of range %v", index, length)
+	}
+
+	*slice = append((*slice)[:index], (*slice)[index+1:]...)
+	return nil
+}
+
 // isValidIRI checks whether an IRI is valid or not. It returns a bool.
 // https://www.w3.org/2011/04/XMLSchema/TypeLibrary-IRI-RFC3987.xsd
 func isValidIRI(iri string) bool {

category.go

@@ -10,22 +10,17 @@ type Category struct {
 	*CommonAttributes
 	Term   string `xml:"term,attr"`
 	Scheme string `xml:"scheme,attr,omitempty"` // IRI
-	Label  string `xml:"label,attr,omitempty"`
+	Label  string `xml:"label,attr,omitempty"`  // Must be unescaped
 }
 
 // NewCategory creates a new Category. It returns a *Category.
 func NewCategory(term string) *Category {
 	return &Category{
-		CommonAttributes: newCommonAttributes(),
+		CommonAttributes: NewCommonAttributes(),
 		Term:             term,
 	}
 }
 
-// SetLabel sets the Label attribute of the Category.
-func (c *Category) SetLabel(label string) {
-	c.Label = Unescape(label)
-}
-
 // Check checks the Category for incompatibilities with RFC4287. It returns an
 // error.
 func (c *Category) Check() error {

commonAttributes.go

@@ -13,18 +13,23 @@ type CommonAttributes struct {
 
 // NewCommonAttributes creates a new set of CommonAttributes. It returns a
 // *CommonAttributes.
-func newCommonAttributes() *CommonAttributes {
+func NewCommonAttributes() *CommonAttributes {
 	return new(CommonAttributes)
 }
 
-// AddAttribute adds the Attribute to the CommonAttributes.
-func (c *CommonAttributes) AddAttribute(name, value string) {
-	if c.UndefinedAttributes == nil {
-		c.UndefinedAttributes = make([]*xml.Attr, 1)
-		c.UndefinedAttributes[0] = &xml.Attr{Name: xml.Name{Local: name}, Value: value}
-	} else {
-		c.UndefinedAttributes = append(c.UndefinedAttributes, &xml.Attr{Name: xml.Name{Local: name}, Value: value})
+// AddAttribute adds the attribute to the CommonAttributes. It returns its index
+// as an int.
+func (c *CommonAttributes) AddAttribute(name, value string) int {
+	return addToSlice(&c.UndefinedAttributes, &xml.Attr{Name: xml.Name{Local: name}, Value: value})
 }
+
+// DeleteAttribute deletes the attribute at index from the CommonAttributes. It
+// return an error.
+func (c *CommonAttributes) DeleteAttribute(index int) error {
+	if err := deleteFromSlice(&c.UndefinedAttributes, index); err != nil {
+		return fmt.Errorf("error deleting undefined attribute %v from common attributes %v: %v", index, c, err)
+	}
+	return nil
 }
 
 // Check checks the CommonAttributes for incompatibilities with RFC4287. It

date.go

@@ -19,7 +19,7 @@ func DateTime(t time.Time) string {
 // NewDate creates a new Date. It returns a *Date.
 func NewDate(t time.Time) *Date {
 	return &Date{
-		CommonAttributes: newCommonAttributes(),
+		CommonAttributes: NewCommonAttributes(),
 		DateTime:         DateTime(t),
 	}
 }

entry.go

@@ -39,16 +39,16 @@ func (e *Entry) checkAuthors(authorInFeed bool) error {
 	if e.Authors == nil {
 		if !authorInFeed {
 			if e.Source == nil {
-				return fmt.Errorf("no authors set in entry %v", e)
+				return fmt.Errorf("no authors set in entry %v", e.ID.URI)
 			}
 			if e.Source.Authors == nil {
-				return fmt.Errorf("no authors set in entry %v", e)
+				return fmt.Errorf("no authors set in entry %v", e.ID.URI)
 			}
 		}
 	} else {
 		for i, a := range e.Authors {
 			if err := a.Check(); err != nil {
-				return fmt.Errorf("author element %v of entry %v: %v", i, e, err)
+				return fmt.Errorf("author element %v of entry %v: %v", i, e.ID.URI, err)
 			}
 		}
 	}
@@ -59,73 +59,149 @@ func (e *Entry) checkAuthors(authorInFeed bool) error {
 // NewEntry creates a new Entry. It returns a *Entry.
 func NewEntry(title string) *Entry {
 	return &Entry{
-		CommonAttributes: newCommonAttributes(),
+		CommonAttributes: NewCommonAttributes(),
 		ID:               NewID(NewURN()),
 		Title:            NewText("text", title),
 		Updated:          NewDate(time.Now()),
 	}
 }
 
-// AddAuthor adds the Person as an author to the Entry.
-func (e *Entry) AddAuthor(p *Person) {
-	if e.Authors == nil {
-		e.Authors = make([]*Person, 1)
-		e.Authors[0] = p
+// AddAuthor adds the Person as an author to the Entry. It returns its index as
+// an int.
+func (e *Entry) AddAuthor(p *Person) int {
+	if e.Updated == nil {
+		e.Updated = NewDate(time.Now())
 	} else {
-		e.Authors = append(e.Authors, p)
-	}
-
 		e.Updated.DateTime = DateTime(time.Now())
 	}
 
-// AddCategory adds the Category to the Entry.
-func (e *Entry) AddCategory(c *Category) {
-	if e.Categories == nil {
-		e.Categories = make([]*Category, 1)
-		e.Categories[0] = c
-	} else {
-		e.Categories = append(e.Categories, c)
+	return addToSlice(&e.Authors, p)
 }
 
+// DeleteAuthor deletes the Person at index from the Entry. It return an error.
+func (e *Entry) DeleteAuthor(index int) error {
+	if err := deleteFromSlice(&e.Authors, index); err != nil {
+		return fmt.Errorf("error deleting author %v from entry %v: %v", index, e.ID.URI, err)
+	}
+
+	if e.Updated == nil {
+		e.Updated = NewDate(time.Now())
+	} else {
 		e.Updated.DateTime = DateTime(time.Now())
 	}
 
-// AddContributor adds the Person as a contributor to the Entry.
-func (e *Entry) AddContributor(c *Person) {
-	if e.Contributors == nil {
-		e.Contributors = make([]*Person, 1)
-		e.Contributors[0] = c
-	} else {
-		e.Contributors = append(e.Contributors, c)
+	return nil
 }
 
+// AddCategory adds the Category to the Entry. It returns ts index as an int.
+func (e *Entry) AddCategory(c *Category) int {
+	if e.Updated == nil {
+		e.Updated = NewDate(time.Now())
+	} else {
 		e.Updated.DateTime = DateTime(time.Now())
 	}
 
-// AddLink adds the Link to the Entry.
-func (e *Entry) AddLink(l *Link) {
-	if e.Links == nil {
-		e.Links = make([]*Link, 1)
-		e.Links[0] = l
-	} else {
-		e.Links = append(e.Links, l)
+	return addToSlice(&e.Categories, c)
 }
 
+// DeleteCategory deletes the Category at index from the Entry. It return an
+// error.
+func (e *Entry) DeleteCategory(index int) error {
+	if err := deleteFromSlice(&e.Categories, index); err != nil {
+		return fmt.Errorf("error deleting category %v from entry %v: %v", index, e.ID.URI, err)
+	}
+
+	if e.Updated == nil {
+		e.Updated = NewDate(time.Now())
+	} else {
 		e.Updated.DateTime = DateTime(time.Now())
 	}
 
-// AddExtension adds the ExtensionElement to the Entry.
-func (e *Entry) AddExtension(x *ExtensionElement) {
-	if e.Extensions == nil {
-		e.Extensions = make([]*ExtensionElement, 1)
-		e.Extensions[0] = x
-	} else {
-		e.Extensions = append(e.Extensions, x)
+	return nil
 }
 
+// AddContributor adds the Person as a contributor to the Entry. It returns its
+// index as an int.
+func (e *Entry) AddContributor(c *Person) int {
+	if e.Updated == nil {
+		e.Updated = NewDate(time.Now())
+	} else {
 		e.Updated.DateTime = DateTime(time.Now())
 	}
 
+	return addToSlice(&e.Contributors, c)
+}
+
+// DeleteContributor deletes the Person at index from the Entry. It return an
+// error.
+func (e *Entry) DeleteContributor(index int) error {
+	if err := deleteFromSlice(&e.Contributors, index); err != nil {
+		return fmt.Errorf("error deleting contributor %v from entry %v: %v", index, e.ID.URI, err)
+	}
+
+	if e.Updated == nil {
+		e.Updated = NewDate(time.Now())
+	} else {
+		e.Updated.DateTime = DateTime(time.Now())
+	}
+
+	return nil
+}
+
+// AddLink adds the Link to the Entry. It returns its index as an int.
+func (e *Entry) AddLink(l *Link) int {
+	if e.Updated == nil {
+		e.Updated = NewDate(time.Now())
+	} else {
+		e.Updated.DateTime = DateTime(time.Now())
+	}
+
+	return addToSlice(&e.Links, l)
+}
+
+// DeleteLink deletes the Link at index from the Entry. It return an error.
+func (e *Entry) DeleteLink(index int) error {
+	if err := deleteFromSlice(&e.Links, index); err != nil {
+		return fmt.Errorf("error deleting link %v from entry %v: %v", index, e.ID.URI, err)
+	}
+
+	if e.Updated == nil {
+		e.Updated = NewDate(time.Now())
+	} else {
+		e.Updated.DateTime = DateTime(time.Now())
+	}
+
+	return nil
+}
+
+// AddExtension adds the ExtensionElement to the Entry. It returns its index as
+// an int.
+func (e *Entry) AddExtension(x *ExtensionElement) int {
+	if e.Updated == nil {
+		e.Updated = NewDate(time.Now())
+	} else {
+		e.Updated.DateTime = DateTime(time.Now())
+	}
+
+	return addToSlice(&e.Extensions, x)
+}
+
+// DeleteExtension deletes the Extension at index from the Entry. It return an
+// error.
+func (e *Entry) DeleteExtension(index int) error {
+	if err := deleteFromSlice(&e.Extensions, index); err != nil {
+		return fmt.Errorf("error deleting extension %v from entry %v: %v", index, e.ID.URI, err)
+	}
+
+	if e.Updated == nil {
+		e.Updated = NewDate(time.Now())
+	} else {
+		e.Updated.DateTime = DateTime(time.Now())
+	}
+
+	return nil
+}
+
 // Check checks the Entry for incompatibilities with RFC4287. It returns an
 // error.
 func (e *Entry) Check() error {
@@ -138,64 +214,64 @@ func (e *Entry) Check() error {
 	}
 
 	if err := e.checkAuthors(true); err != nil {
-		return fmt.Errorf("entry %v: %v", e, err)
+		return fmt.Errorf("entry %v: %v", e.ID.URI, err)
 	}
 
 	for i, c := range e.Categories {
 		if err := c.Check(); err != nil {
-			return fmt.Errorf("category element %v of entry %v: %v", i, e, err)
+			return fmt.Errorf("category element %v of entry %v: %v", i, e.ID.URI, err)
 		}
 	}
 
 	if e.Content != nil {
 		if err := e.Content.Check(); err != nil {
-			return fmt.Errorf("content element of entry %v: %v", e, err)
+			return fmt.Errorf("content element of entry %v: %v", e.ID.URI, err)
 		}
 	} else {
 		// atom:entry elements that contain no child atom:content element MUST
 		// contain at least one atom:link element with a rel attribute value of
 		// "alternate".
 		if !alternateRelExists(e.Links) {
-			return fmt.Errorf("no content element of entry %v and no link element with rel \"alternate\"", e)
+			return fmt.Errorf("no content element of entry %v and no link element with rel \"alternate\"", e.ID.URI)
 		}
 	}
 
 	for i, c := range e.Contributors {
 		if err := c.Check(); err != nil {
-			return fmt.Errorf("contributor element %v of entry %v: %v", i, e, err)
+			return fmt.Errorf("contributor element %v of entry %v: %v", i, e.ID.URI, err)
 		}
 	}
 
 	for i, l := range e.Links {
 		if err := l.Check(); err != nil {
-			return fmt.Errorf("link element %v of entry %v: %v", i, e, err)
+			return fmt.Errorf("link element %v of entry %v: %v", i, e.ID.URI, err)
 		}
 	}
 	if hasAlternateDuplicateLinks(e.Links) {
-		return fmt.Errorf("links with a rel attribute value of \"alternate\" and duplicate type and hreflang attribute values found in entry %v", e)
+		return fmt.Errorf("links 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 {
-			return fmt.Errorf("published element of entry %v: %v", e, err)
+			return fmt.Errorf("published element of entry %v: %v", e.ID.URI, err)
 		}
 	}
 
 	if e.Rights != nil {
 		if err := e.Rights.Check(); err != nil {
-			return fmt.Errorf("rights element of entry %v: %v", e, err)
+			return fmt.Errorf("rights element of entry %v: %v", e.ID.URI, err)
 		}
 	}
 
 	if e.Source != nil {
 		if err := e.Source.Check(); err != nil {
-			return fmt.Errorf("source element of entry %v: %v", e, err)
+			return fmt.Errorf("source element of entry %v: %v", e.ID.URI, err)
 		}
 	}
 
 	if e.Summary != nil {
 		if err := e.Summary.Check(); err != nil {
-			return fmt.Errorf("summary element of entry %v: %v", e, err)
+			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
@@ -203,7 +279,7 @@ func (e *Entry) Check() error {
 		// 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)
+			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
@@ -211,29 +287,29 @@ func (e *Entry) Check() error {
 		// does not end with "/xml" or "+xml".
 		mediaType := e.Content.getType()
 		if isValidMediaType(mediaType) && !isXMLMediaType(mediaType) && !strings.HasPrefix(mediaType, "text/") {
-			return fmt.Errorf("no summary element of entry %v but media type not xml", e)
+			return fmt.Errorf("no summary element of entry %v but media type not xml", e.ID.URI)
 		}
 	}
 
 	if e.Title == nil {
-		return fmt.Errorf("no title element of entry %v", e)
+		return fmt.Errorf("no title element of entry %v", e.ID.URI)
 	} else {
 		if err := e.Title.Check(); err != nil {
-			return fmt.Errorf("title element of entry %v: %v", e, err)
+			return fmt.Errorf("title element of entry %v: %v", e.ID.URI, err)
 		}
 	}
 
 	if e.Updated == nil {
-		return fmt.Errorf("no updated element of entry %v", e)
+		return fmt.Errorf("no updated element of entry %v", e.ID.URI)
 	} else {
 		if err := e.Updated.Check(); err != nil {
-			return fmt.Errorf("updated element of entry %v: %v", e, err)
+			return fmt.Errorf("updated element of entry %v: %v", e.ID.URI, err)
 		}
 	}
 
 	for i, x := range e.Extensions {
 		if err := x.Check(); err != nil {
-			return fmt.Errorf("extension element %v of entry %v: %v", i, e, err)
+			return fmt.Errorf("extension element %v of entry %v: %v", i, e.ID.URI, err)
 		}
 	}
 
@@ -244,7 +320,7 @@ func (e *Entry) Check() error {
 func (e *Entry) ToXML(encoding string) (string, error) {
 	xml, err := xml.MarshalIndent(e, "", "  ")
 	if err != nil {
-		return "", fmt.Errorf("error xml encoding entry: %v", err)
+		return "", fmt.Errorf("error xml encoding entry %v: %v", e.ID.URI, err)
 	}
 
 	return fmt.Sprintln(`<?xml version="1.0" encoding="`+encoding+`"?>`) + string(xml), nil

feed.go

@@ -28,85 +28,202 @@ type Feed struct {
 // NewFeed creates a new Feed. It returns a *Feed.
 func NewFeed(title string) *Feed {
 	return &Feed{
-		CommonAttributes: newCommonAttributes(),
+		CommonAttributes: NewCommonAttributes(),
 		ID:               NewID(NewURN()),
 		Title:            NewText("text", title),
 		Updated:          NewDate(time.Now()),
 	}
 }
 
-// AddAuthor adds the Person as an author to the Feed.
-func (f *Feed) AddAuthor(p *Person) {
-	if f.Authors == nil {
-		f.Authors = make([]*Person, 1)
-		f.Authors[0] = p
+// AddAuthor adds the Person as an author to the Feed. It returns its index as
+// an int.
+func (f *Feed) AddAuthor(p *Person) int {
+	if f.Updated == nil {
+		f.Updated = NewDate(time.Now())
 	} else {
-		f.Authors = append(f.Authors, p)
-	}
-
 		f.Updated.DateTime = DateTime(time.Now())
 	}
 
-// AddCategory adds the Category to the Feed.
-func (f *Feed) AddCategory(c *Category) {
-	if f.Categories == nil {
-		f.Categories = make([]*Category, 1)
-		f.Categories[0] = c
-	} else {
-		f.Categories = append(f.Categories, c)
+	return addToSlice(&f.Authors, p)
 }
 
+// DeleteAuthor deletes the Person at index from the Feed. It return an error.
+func (f *Feed) DeleteAuthor(index int) error {
+	if err := deleteFromSlice(&f.Authors, index); err != nil {
+		return fmt.Errorf("error deleting author %v from entry %v: %v", index, f.ID.URI, err)
+	}
+
+	if f.Updated == nil {
+		f.Updated = NewDate(time.Now())
+	} else {
 		f.Updated.DateTime = DateTime(time.Now())
 	}
 
-// AddContributor adds the Person as a contributor to the Feed.
-func (f *Feed) AddContributor(c *Person) {
-	if f.Contributors == nil {
-		f.Contributors = make([]*Person, 1)
-		f.Contributors[0] = c
-	} else {
-		f.Contributors = append(f.Contributors, c)
+	return nil
 }
 
+// AddCategory adds the Category to the Feed. It returns its index as an int.
+func (f *Feed) AddCategory(c *Category) int {
+	if f.Updated == nil {
+		f.Updated = NewDate(time.Now())
+	} else {
 		f.Updated.DateTime = DateTime(time.Now())
 	}
 
+	return addToSlice(&f.Categories, c)
+}
+
+// DeleteCategory deletes the Category at index from the Feed. It return an
+// error.
+func (f *Feed) DeleteCategory(index int) error {
+	if err := deleteFromSlice(&f.Categories, index); err != nil {
+		return fmt.Errorf("error deleting category %v from entry %v: %v", index, f.ID.URI, err)
+	}
+
+	if f.Updated == nil {
+		f.Updated = NewDate(time.Now())
+	} else {
+		f.Updated.DateTime = DateTime(time.Now())
+	}
+
+	return nil
+}
+
+// AddContributor adds the Person as a contributor to the Feed. It returns its
+// index as an int.
+func (f *Feed) AddContributor(c *Person) int {
+	if f.Updated == nil {
+		f.Updated = NewDate(time.Now())
+	} else {
+		f.Updated.DateTime = DateTime(time.Now())
+	}
+
+	return addToSlice(&f.Contributors, c)
+}
+
+// DeleteContributor deletes the Person at index from the Feed. It return an
+// error.
+func (f *Feed) DeleteContributor(index int) error {
+	if err := deleteFromSlice(&f.Contributors, index); err != nil {
+		return fmt.Errorf("error deleting contributor %v from entry %v: %v", index, f.ID.URI, err)
+	}
+
+	if f.Updated == nil {
+		f.Updated = NewDate(time.Now())
+	} else {
+		f.Updated.DateTime = DateTime(time.Now())
+	}
+
+	return nil
+}
+
 // AddLink adds the Link to the Feed. There should be one Link with Rel "self".
-func (f *Feed) AddLink(l *Link) {
-	if f.Links == nil {
-		f.Links = make([]*Link, 1)
-		f.Links[0] = l
+// It returns its index as an int.
+func (f *Feed) AddLink(l *Link) int {
+	if f.Updated == nil {
+		f.Updated = NewDate(time.Now())
 	} else {
-		f.Links = append(f.Links, l)
-	}
-
 		f.Updated.DateTime = DateTime(time.Now())
 	}
 
-// AddExtension adds the Extension to the Feed.
-func (f *Feed) AddExtension(e *ExtensionElement) {
-	if f.Extensions == nil {
-		f.Extensions = make([]*ExtensionElement, 1)
-		f.Extensions[0] = e
-	} else {
-		f.Extensions = append(f.Extensions, e)
+	return addToSlice(&f.Links, l)
 }
 
+// DeleteLink deletes the Link at index from the Feed. It return an error.
+func (f *Feed) DeleteLink(index int) error {
+	if err := deleteFromSlice(&f.Links, index); err != nil {
+		return fmt.Errorf("error deleting link %v from entry %v: %v", index, f.ID.URI, err)
+	}
+
+	if f.Updated == nil {
+		f.Updated = NewDate(time.Now())
+	} else {
 		f.Updated.DateTime = DateTime(time.Now())
 	}
 
-// AddEntry adds the Entry to the Feed.
-func (f *Feed) AddEntry(e *Entry) {
-	if f.Entries == nil {
-		f.Entries = make([]*Entry, 1)
-		f.Entries[0] = e
-	} else {
-		f.Entries = append(f.Entries, e)
+	return nil
 }
 
+// AddExtension adds the Extension to the Feed. It returns its index as an int.
+func (f *Feed) AddExtension(e *ExtensionElement) int {
+	if f.Updated == nil {
+		f.Updated = NewDate(time.Now())
+	} else {
 		f.Updated.DateTime = DateTime(time.Now())
 	}
 
+	return addToSlice(&f.Extensions, e)
+}
+
+// DeleteExtension deletes the Extension at index from the Feed. It return an
+// error.
+func (f *Feed) DeleteExtension(index int) error {
+	if err := deleteFromSlice(&f.Extensions, index); err != nil {
+		return fmt.Errorf("error deleting extension %v from entry %v: %v", index, f.ID.URI, err)
+	}
+
+	if f.Updated == nil {
+		f.Updated = NewDate(time.Now())
+	} else {
+		f.Updated.DateTime = DateTime(time.Now())
+	}
+
+	return nil
+}
+
+// AddEntry adds the Entry to the Feed. It returns its index as an int.
+func (f *Feed) AddEntry(e *Entry) int {
+	if f.Updated == nil {
+		f.Updated = NewDate(time.Now())
+	} else {
+		f.Updated.DateTime = DateTime(time.Now())
+	}
+
+	return addToSlice(&f.Entries, e)
+}
+
+// DeleteEntry deletes the Entry at index from the Feed. It return an error.
+func (f *Feed) DeleteEntry(index int) error {
+	if err := deleteFromSlice(&f.Entries, index); err != nil {
+		return fmt.Errorf("error deleting entry %v from entry %v: %v", index, f.ID.URI, err)
+	}
+
+	if f.Updated == nil {
+		f.Updated = NewDate(time.Now())
+	} else {
+		f.Updated.DateTime = DateTime(time.Now())
+	}
+
+	return nil
+}
+
+// DeleteEntryByURI deletes the Entry from the Feed. It return an error.
+func (f *Feed) DeleteEntryByURI(uri string) error {
+	if !isValidIRI(uri) {
+		return fmt.Errorf("error deleting entry from feed %v: uri %v invalid", f.ID.URI, uri)
+	}
+
+	index := -1
+	for i, e := range f.Entries {
+		if e.ID.URI == uri {
+			index = i
+			break
+		}
+	}
+	if index < 0 {
+		return fmt.Errorf("error deleting entry from feed %v: id %v not found", f.ID.URI, uri)
+	}
+
+	f.Entries = append(f.Entries[:index], f.Entries[index+1:]...)
+	if f.Updated == nil {
+		f.Updated = NewDate(time.Now())
+	} else {
+		f.Updated.DateTime = DateTime(time.Now())
+	}
+
+	return nil
+}
+
 // Check checks the Feed for incompatibilities with RFC4287. It returns an
 // error.
 func (f *Feed) Check() error {
@@ -124,93 +241,93 @@ func (f *Feed) Check() error {
 	if f.Authors == nil {
 		for _, e := range f.Entries {
 			if err := e.checkAuthors(false); err != nil {
-				return fmt.Errorf("no authors set in feed %v: %v", f, err)
+				return fmt.Errorf("no authors set in feed %v: %v", f.ID.URI, err)
 			}
 		}
 	} else {
 		for i, a := range f.Authors {
 			if err := a.Check(); err != nil {
-				return fmt.Errorf("author element %v of feed %v: %v", i, f, err)
+				return fmt.Errorf("author element %v of feed %v: %v", i, f.ID.URI, err)
 			}
 		}
 	}
 
 	for i, c := range f.Categories {
 		if err := c.Check(); err != nil {
-			return fmt.Errorf("category element %v of feed %v: %v", i, f, err)
+			return fmt.Errorf("category element %v of feed %v: %v", i, f.ID.URI, err)
 		}
 	}
 
 	for i, c := range f.Contributors {
 		if err := c.Check(); err != nil {
-			return fmt.Errorf("contributor element %v of feed %v: %v", i, f, err)
+			return fmt.Errorf("contributor element %v of feed %v: %v", i, f.ID.URI, err)
 		}
 	}
 
 	if f.Generator != nil {
 		if err := f.Generator.Check(); err != nil {
-			return fmt.Errorf("generator element of feed %v: %v", f, err)
+			return fmt.Errorf("generator element of feed %v: %v", f.ID.URI, err)
 		}
 	}
 
 	if f.Icon != nil {
 		if err := f.Icon.Check(); err != nil {
-			return fmt.Errorf("icon element of feed %v: %v", f, err)
+			return fmt.Errorf("icon element of feed %v: %v", f.ID.URI, err)
 		}
 	}
 
 	for i, l := range f.Links {
 		if err := l.Check(); err != nil {
-			return fmt.Errorf("link element %v of feed %v: %v", i, f, err)
+			return fmt.Errorf("link element %v of feed %v: %v", i, f.ID.URI, err)
 		}
 	}
 	if hasAlternateDuplicateLinks(f.Links) {
-		return fmt.Errorf("links with a rel attribute value of \"alternate\" and duplicate type and hreflang attribute values found in feed %v", f)
+		return fmt.Errorf("links 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 {
 		if err := f.Logo.Check(); err != nil {
-			return fmt.Errorf("logo element of feed %v: %v", f, err)
+			return fmt.Errorf("logo element of feed %v: %v", f.ID.URI, err)
 		}
 	}
 
 	if f.Rights != nil {
 		if err := f.Rights.Check(); err != nil {
-			return fmt.Errorf("rights element of feed %v: %v", f, err)
+			return fmt.Errorf("rights element of feed %v: %v", f.ID.URI, err)
 		}
 	}
 
 	if f.Subtitle != nil {
 		if err := f.Subtitle.Check(); err != nil {
-			return fmt.Errorf("subtitle element of feed %v: %v", f, err)
+			return fmt.Errorf("subtitle element of feed %v: %v", f.ID.URI, err)
 		}
 	}
 
 	if f.Title == nil {
-		return fmt.Errorf("no title element of feed %v", f)
+		return fmt.Errorf("no title element of feed %v", f.ID.URI)
 	} else {
 		if err := f.Title.Check(); err != nil {
-			return fmt.Errorf("title element of feed %v: %v", f, err)
+			return fmt.Errorf("title element of feed %v: %v", f.ID.URI, err)
 		}
 	}
 
 	if f.Updated == nil {
-		return fmt.Errorf("no updated element of feed %v", f)
+		return fmt.Errorf("no updated element of feed %v", f.ID.URI)
 	} else {
 		if err := f.Updated.Check(); err != nil {
-			return fmt.Errorf("updated element of feed %v: %v", f, err)
+			return fmt.Errorf("updated element of feed %v: %v", f.ID.URI, err)
 		}
 	}
 
 	for i, x := range f.Extensions {
 		if err := x.Check(); err != nil {
-			return fmt.Errorf("extension element %v of feed %v: %v", i, f, err)
+			return fmt.Errorf("extension element %v of feed %v: %v", i, f.ID.URI, err)
 		}
 	}
 
 	for i, n := range f.Entries {
 		if err := n.Check(); err != nil {
-			return fmt.Errorf("entry element %v of feed %v: %v", i, f, err)
+			return fmt.Errorf("entry element %v of feed %v: %v", i, f.ID.URI, err)
 		}
 	}
 
@@ -221,7 +338,7 @@ func (f *Feed) Check() error {
 func (f *Feed) ToXML(encoding string) (string, error) {
 	xml, err := xml.MarshalIndent(f, "", "  ")
 	if err != nil {
-		return "", fmt.Errorf("error xml encoding feed: %v", err)
+		return "", fmt.Errorf("error xml encoding feed %v: %v", f.ID.URI, err)
 	}
 
 	return fmt.Sprintln(`<?xml version="1.0" encoding="`+encoding+`"?>`) + string(xml), nil

generator.go

@@ -17,7 +17,7 @@ type Generator struct {
 // NewGenerator creates a new Generator. It returns a *Generator.
 func NewGenerator(text string) *Generator {
 	return &Generator{
-		CommonAttributes: newCommonAttributes(),
+		CommonAttributes: NewCommonAttributes(),
 		Text:             html.UnescapeString(text),
 	}
 }

icon.go

@@ -15,7 +15,7 @@ type Icon struct {
 // NewIcon creates a new Icon. It returns a *Icon.
 func NewIcon(uri string) *Icon {
 	return &Icon{
-		CommonAttributes: newCommonAttributes(),
+		CommonAttributes: NewCommonAttributes(),
 		URI:              uri,
 	}
 }

id.go

@@ -15,7 +15,7 @@ type ID struct {
 // NewID creates a new ID. It returns a *ID.
 func NewID(uri string) *ID {
 	return &ID{
-		CommonAttributes: newCommonAttributes(),
+		CommonAttributes: NewCommonAttributes(),
 		URI:              uri,
 	}
 }

inlineOtherContent.go

@@ -19,7 +19,7 @@ func newInlineOtherContent(mediaType string, content any) *InlineOtherContent {
 	mediaType, _, _ = mime.ParseMediaType(mediaType)
 
 	return &InlineOtherContent{
-		CommonAttributes: newCommonAttributes(),
+		CommonAttributes: NewCommonAttributes(),
 		Type:             mediaType,
 		AnyElement:       content,
 	}

inlineTextContent.go

@@ -16,7 +16,7 @@ type InlineTextContent struct {
 // *InlineTextContent.
 func newInlineTextContent(mediaType, text string) *InlineTextContent {
 	return &InlineTextContent{
-		CommonAttributes: newCommonAttributes(),
+		CommonAttributes: NewCommonAttributes(),
 		Type:             mediaType,
 		Text:             text,
 	}

inlineXHTMLContent.go

@@ -16,7 +16,7 @@ type InlineXHTMLContent struct {
 // *InlineXHTMLContent.
 func newInlineXHTMLContent(mediaType string, div *XHTMLDiv) *InlineXHTMLContent {
 	return &InlineXHTMLContent{
-		CommonAttributes: newCommonAttributes(),
+		CommonAttributes: NewCommonAttributes(),
 		Type:             mediaType,
 		XHTMLDiv:         div,
 	}

link.go

@@ -20,7 +20,7 @@ type Link struct {
 // NewLink creates a new Link. It returns a *Link.
 func NewLink(href string) *Link {
 	return &Link{
-		CommonAttributes: newCommonAttributes(),
+		CommonAttributes: NewCommonAttributes(),
 		Href:             href,
 	}
 }

logo.go

@@ -14,7 +14,7 @@ type Logo struct {
 // NewLogo creates a new Logo. It returns a *Logo.
 func NewLogo(uri string) *Logo {
 	return &Logo{
-		CommonAttributes: newCommonAttributes(),
+		CommonAttributes: NewCommonAttributes(),
 		URI:              uri,
 	}
 }

outOfLineContent.go

@@ -19,7 +19,7 @@ func newOutOfLineContent(mediaType, src string) *OutOfLineContent {
 	mediaType, _, _ = mime.ParseMediaType(mediaType)
 
 	return &OutOfLineContent{
-		CommonAttributes: newCommonAttributes(),
+		CommonAttributes: NewCommonAttributes(),
 		Type:             mediaType,
 		SRC:              src,
 	}

person.go

@@ -16,19 +16,24 @@ type Person struct {
 // NewPerson creates a new Person. It returns a *Person.
 func NewPerson(name string) *Person {
 	return &Person{
-		CommonAttributes: newCommonAttributes(),
+		CommonAttributes: NewCommonAttributes(),
 		Name:             name,
 	}
 }
 
-// AddExtension adds the Extension to the Person.
-func (p *Person) AddExtension(e *ExtensionElement) {
-	if p.Extensions == nil {
-		p.Extensions = make([]*ExtensionElement, 1)
-		p.Extensions[0] = e
-	} else {
-		p.Extensions = append(p.Extensions, e)
+// AddExtension adds the Extension to the Person. It returns its index as an
+// int.
+func (p *Person) AddExtension(e *ExtensionElement) int {
+	return addToSlice(&p.Extensions, e)
 }
+
+// DeleteExtension deletes the Extension at index from the Person. It return an
+// error.
+func (p *Person) DeleteExtension(index int) error {
+	if err := deleteFromSlice(&p.Extensions, index); err != nil {
+		return fmt.Errorf("error deleting extension %v from person %v: %v", index, p, err)
+	}
+	return nil
 }
 
 // Check checks the Person for incompatibilities with RFC4287. It returns an
@@ -50,13 +55,11 @@ func (p *Person) Check() error {
 		}
 	}
 
-	if p.Extensions != nil {
 	for i, e := range p.Extensions {
 		if err := e.Check(); err != nil {
 			return fmt.Errorf("extension element %v of person %v: %v", i, p, err)
 		}
 	}
-	}
 
 	return nil
 }

plainText.go

@@ -16,7 +16,7 @@ func (p *PlainText) isText() bool { return true }
 // newPlainText creates a new PlainText. It returns a *PlainText.
 func newPlainText(textType, content string) *PlainText {
 	return &PlainText{
-		CommonAttributes: newCommonAttributes(),
+		CommonAttributes: NewCommonAttributes(),
 		Type:             textType,
 		Text:             content,
 	}

source.go

@@ -25,7 +25,78 @@ type Source struct {
 
 // NewSource creates a new Source. It returns a *Source.
 func NewSource() *Source {
-	return &Source{CommonAttributes: newCommonAttributes()}
+	return &Source{CommonAttributes: NewCommonAttributes()}
+}
+
+// AddAuthor adds the Person as an author to the Source. It returns its index as
+// an int.
+func (s *Source) AddAuthor(p *Person) int {
+	return addToSlice(&s.Authors, p)
+}
+
+// DeleteAuthor deletes the Person at index from the Source. It return an error.
+func (s *Source) DeleteAuthor(index int) error {
+	if err := deleteFromSlice(&s.Authors, index); err != nil {
+		return fmt.Errorf("error deleting author %v from source %v: %v", index, s, err)
+	}
+	return nil
+}
+
+// AddCategory adds the Category to the Source. It returns its index as an int.
+func (s *Source) AddCategory(c *Category) int {
+	return addToSlice(&s.Categories, c)
+}
+
+// DeleteCategory deletes the Category at index from the Source. It return an
+// error.
+func (s *Source) DeleteCategory(index int) error {
+	if err := deleteFromSlice(&s.Categories, index); err != nil {
+		return fmt.Errorf("error deleting category %v from source %v: %v", index, s, err)
+	}
+	return nil
+}
+
+// AddContributor adds the Person as a contributor to the Source. It returns its
+// index as an int.
+func (s *Source) AddContributor(c *Person) int {
+	return addToSlice(&s.Contributors, c)
+}
+
+// DeleteContributor deletes the Person at index from the Source. It return an
+// error.
+func (s *Source) DeleteContributor(index int) error {
+	if err := deleteFromSlice(&s.Contributors, index); err != nil {
+		return fmt.Errorf("error deleting contributor %v from source %v: %v", index, s, err)
+	}
+	return nil
+}
+
+// AddLink adds the Link to the Source. It returns its index as an int.
+func (s *Source) AddLink(l *Link) int {
+	return addToSlice(&s.Links, l)
+}
+
+// DeleteLink deletes the Link at index from the Source. It return an error.
+func (s *Source) DeleteLink(index int) error {
+	if err := deleteFromSlice(&s.Links, index); err != nil {
+		return fmt.Errorf("error deleting link %v from source %v: %v", index, s, err)
+	}
+	return nil
+}
+
+// AddExtension adds the ExtensionElement to the Source. It returns its index as
+// an int.
+func (s *Source) AddExtension(e *ExtensionElement) int {
+	return addToSlice(&s.Extensions, e)
+}
+
+// DeleteExtension deletes the Extension at index from the Source. It return an
+// error.
+func (s *Source) DeleteExtension(index int) error {
+	if err := deleteFromSlice(&s.Extensions, index); err != nil {
+		return fmt.Errorf("error deleting extension %v from source %v: %v", index, s, err)
+	}
+	return nil
 }
 
 // Check checks the Source for incompatibilities with RFC4287. It returns an

xhtmlText.go

@@ -16,7 +16,7 @@ func (x *XHTMLText) isText() bool { return true }
 // newPlainText creates a new PlainText. It returns a *PlainText.
 func newXHTMLText(textType, content string) *XHTMLText {
 	return &XHTMLText{
-		CommonAttributes: newCommonAttributes(),
+		CommonAttributes: NewCommonAttributes(),
 		Type:             textType,
 		XHTMLDiv:         NewXHTMLDiv(content),
 	}