vlib: add an encoding.xml module with parser, validation, entity encoding, unit tests (#19708)

vlib/encoding/xml/validation.v

@@ -0,0 +1,96 @@
+module xml
+
+fn (node XMLNode) validate(elements map[string]DTDElement, entities map[string]string) !XMLNode {
+	mut children := []XMLNodeContents{cap: node.children.len}
+
+	valid_elements := elements[node.name].definition
+	mut validate_node_children := node.name in elements
+
+	// Check if the node will match everything
+	if valid_elements.len == 1 && valid_elements[0] == '#PCDATA' {
+		validate_node_children = false
+	}
+
+	for child in node.children {
+		match child {
+			XMLNode {
+				if validate_node_children {
+					name := child.name
+					if name !in valid_elements {
+						return error('Invalid child element ${name} for ${node.name}')
+					}
+				}
+				children << child.validate(elements, entities)!
+			}
+			string {
+				children << unescape_text(child, entities: entities)!
+			}
+			else {
+				// Ignore other nodes
+				children << child
+			}
+		}
+	}
+
+	return XMLNode{
+		name: node.name
+		attributes: node.attributes
+		children: children
+	}
+}
+
+// validate checks the document is well-formed and valid. It returns a new
+// document with the parsed entities expanded when validation is successful.
+// Otherwise it returns an error.
+pub fn (doc XMLDocument) validate() !XMLDocument {
+	// The document is well-formed because we were able to parse it properly.
+	match doc.doctype.dtd {
+		DocumentTypeDefinition {
+			// Store the element and entity definitions
+			mut elements := map[string]DTDElement{}
+			mut entities := default_entities.clone()
+			mut reverse_entities := default_entities_reverse.clone()
+
+			for item in doc.doctype.dtd.list {
+				match item {
+					DTDElement {
+						name := item.name
+						if name in elements {
+							return error('Duplicate element definition for ${name}')
+						}
+						elements[name] = item
+					}
+					DTDEntity {
+						name := item.name
+						if name in entities {
+							return error('Duplicate entity definition for ${name}')
+						}
+						entities[name] = item.value
+						reverse_entities[item.value] = name
+					}
+				}
+			}
+
+			// Now validate the document against the elements and entities.
+			new_root := doc.root.validate(elements, entities)!
+
+			// Check the DOCTYPE name matches the root name
+			if doc.doctype.name.len > 0 && doc.doctype.name != new_root.name {
+				return error('Root element ${new_root.name} does not match DOCTYPE ${doc.doctype.name}')
+			}
+
+			return XMLDocument{
+				version: doc.version
+				encoding: doc.encoding
+				doctype: doc.doctype
+				comments: doc.comments
+				root: new_root
+				parsed_reverse_entities: reverse_entities
+			}
+		}
+		string {
+			// TODO: Validate the document against the DTD string.
+			return doc
+		}
+	}
+}