Add html ids to headlines

Sources/Generator/Extensions/String+Extensions.swift

@@ -82,6 +82,10 @@ extension Substring {
     func between(first: String, andLast last: String) -> String {
         dropBeforeFirst(first).dropAfterLast(last)
     }
+
+    func last(after: String) -> String {
+        components(separatedBy: after).last!
+    }
 }
 
 extension String {

Sources/Generator/Generators/PageContentGenerator.swift

@@ -41,8 +41,11 @@ struct PageContentGenerator {
         let htmlModifier = Modifier(target: .html) { html, markdown in
             handleHTML(page: page, language: language, html: html, markdown: markdown)
         }
+        let headlinesModifier = Modifier(target: .headings) { html, markdown in
+            handleHeadlines(page: page, language: language, html: html, markdown: markdown)
+        }
 
-        let parser = MarkdownParser(modifiers: [imageModifier, codeModifier, linkModifier, htmlModifier])
+        let parser = MarkdownParser(modifiers: [imageModifier, codeModifier, linkModifier, htmlModifier, headlinesModifier])
         return (parser.html(from: content), hasCodeContent)
     }
 
@@ -77,6 +80,19 @@ struct PageContentGenerator {
         return html
     }
 
+    private func handleHeadlines(page: Element, language: String, html: String, markdown: Substring) -> String {
+        let id = markdown
+            .last(after: "#")
+            .trimmed
+            .filter { $0.isNumber || $0.isLetter || $0 == " " }
+            .lowercased()
+            .components(separatedBy: " ")
+            .filter { $0 != "" }
+            .joined(separator: "-")
+        let parts =  html.components(separatedBy: ">")
+        return parts[0] + " id=\"\(id)\">" + parts.dropFirst().joined(separator: ">")
+    }
+
     private func processMarkdownImage(markdown: Substring, html: String, page: Element, language: String, largeImageCount: inout Int) -> String {
         // Split the markdown ![alt](file title)
         // There are several known shorthand commands