From 7f6f9f768282917ca472717eb3d54bd04d915609 Mon Sep 17 00:00:00 2001
From: "Gustavo \"Guz\" L de Mello" <contact@guz.one>
Date: Thu, 22 May 2025 11:08:18 -0300
Subject: [PATCH] feat(attr,ipub): Element interface and ElementKind to prepare
 unmarshalling of un-structured childre

---
 ipub/element/attr/errors.go | 36 +++++++++++++++++++++
 ipub/element/element.go     | 62 +++++++++++++++++++++++++++++++++++++
 ipub/element/sections.go    | 17 ++++++++++
 3 files changed, 115 insertions(+)
 create mode 100644 ipub/element/attr/errors.go
 create mode 100644 ipub/element/element.go

diff --git a/ipub/element/attr/errors.go b/ipub/element/attr/errors.go
new file mode 100644
index 0000000..4612549
--- /dev/null
+++ b/ipub/element/attr/errors.go
@@ -0,0 +1,36 @@
+package attr
+
+import (
+	"encoding/xml"
+	"fmt"
+)
+
+type ErrInvalidName struct {
+	Actual   xml.Name
+	Expected xml.Name
+}
+
+var _ error = ErrInvalidName{}
+
+func (err ErrInvalidName) Error() string {
+	return fmt.Sprintf("attribute %q has invalid name, expected %q", FmtXMLName(err.Actual), FmtXMLName(err.Expected))
+}
+
+type ErrInvalidValue struct {
+	Attr    xml.Attr
+	Message string
+}
+
+var _ error = ErrInvalidValue{}
+
+func (err ErrInvalidValue) Error() string {
+	return fmt.Sprintf("attribute %q's value %q is invalid: %s", FmtXMLName(err.Attr.Name), err.Attr.Value, err.Message)
+}
+
+func FmtXMLName(n xml.Name) string {
+	s := n.Local
+	if n.Space != "" {
+		s = fmt.Sprintf("%s:%s", n.Space, n.Local)
+	}
+	return s
+}
diff --git a/ipub/element/element.go b/ipub/element/element.go
new file mode 100644
index 0000000..3eb09b5
--- /dev/null
+++ b/ipub/element/element.go
@@ -0,0 +1,62 @@
+package element
+
+import (
+	"encoding/xml"
+	"errors"
+	"fmt"
+	"strings"
+
+	"forge.capytal.company/capytalcode/project-comicverse/ipub/element/attr"
+)
+
+type Element interface {
+	Kind() ElementKind
+}
+
+type ElementKind string
+
+func NewElementKind(n string, s Element) ElementKind {
+	k := ElementKind(n)
+
+	if _, ok := elementKindList[k]; ok {
+		panic(fmt.Sprintf("element kind %q already registered", n))
+	}
+
+	elementKindList[k] = s
+	return k
+}
+
+func (k ElementKind) MarshalXMLAttr(n xml.Name) (xml.Attr, error) {
+	if n != elementKindAttrName {
+		return xml.Attr{}, attr.ErrInvalidName{Actual: n, Expected: elementKindAttrName}
+	}
+	return xml.Attr{Name: elementKindAttrName, Value: k.String()}, nil
+}
+
+func (k *ElementKind) UnmarshalXMLAttr(a xml.Attr) error {
+	ak := ElementKind(a.Value)
+
+	if _, ok := elementKindList[ak]; !ok {
+		v := make([]string, 0, len(elementKindList))
+		for k := range elementKindList {
+			v = append(v, k.String())
+		}
+		return attr.ErrInvalidValue{
+			Attr:    a,
+			Message: fmt.Sprintf("must be a registered element (%q)", strings.Join(v, `", "`)),
+		}
+	}
+
+	*k = ak
+
+	return nil
+}
+
+func (k ElementKind) String() string {
+	return string(k)
+}
+
+var (
+	elementKindList     = make(map[ElementKind]Element)
+	elementKindAttrName = xml.Name{Local: "data-ipub-element"}
+)
diff --git a/ipub/element/sections.go b/ipub/element/sections.go
index 0e2a2b3..ae94204 100644
--- a/ipub/element/sections.go
+++ b/ipub/element/sections.go
@@ -7,11 +7,23 @@ type Section struct {
 	Body    Body     `xml:"body"`
 }
 
+var KindSection = NewElementKind("section", Section{})
+
+func (Section) Kind() ElementKind {
+	return KindSection
+}
+
 type Body struct {
 	XMLName xml.Name `xml:"body"`
 	Test    string   `xml:"test,attr"`
 }
 
+var KindBody = NewElementKind("body", Body{})
+
+func (Body) Kind() ElementKind {
+	return KindBody
+}
+
 type Paragraph struct {
 	XMLName     xml.Name    `xml:"p"`
 	DataElement ElementKind `xml:"data-ipub-element,attr"`
@@ -20,3 +32,8 @@ type Paragraph struct {
 	Text string `xml:",chardata"`
 }
 
+var KindParagraph = NewElementKind("paragraph", Paragraph{})
+
+func (Paragraph) Kind() ElementKind {
+	return KindParagraph
+}