Add page id feature

WebsiteGenerator/Content/Element.swift

@@ -255,6 +255,20 @@ struct Element {
 
 extension Element {
 
+    /**
+     The localized html file name for a language, including a leading slash.
+     */
+    static func htmlPagePathAddition(for language: String) -> String {
+        "/" + htmlPageName(for: language)
+    }
+
+    /**
+     The localized html file name for a language, without the leading slash.
+     */
+    static func htmlPageName(for language: String) -> String {
+        "\(language).html"
+    }
+
     var containsElements: Bool {
         !elements.isEmpty
     }
@@ -286,7 +300,30 @@ extension Element {
      The url of the top-level section of the element.
      */
     func sectionUrl(for language: String) -> String {
-        path.components(separatedBy: "/").first! + "/\(language).html"
+        path.components(separatedBy: "/").first! + Element.htmlPagePathAddition(for: language)
+    }
+
+    /**
+     Create a relative link to another page in the tree.
+     - Parameter pageUrl: The full page url of the target page, including localization
+     - Returns: The relative url from a localized page of the element to the target page.
+     */
+    func relativePathToOtherSiteElement(pageUrl: String) -> String {
+        // Note: The element `path` is missing the last component
+        // i.e. travel/alps instead of travel/alps/en.html
+        let ownParts = path.components(separatedBy: "/")
+        let pageParts = pageUrl.components(separatedBy: "/")
+
+        // Find the common elements of the path, which can be discarded
+        var index = 0
+        while pageParts[index] == ownParts[index] {
+            index += 1
+        }
+        // The relative path needs to go down to the first common folder,
+        // before going up to the target page
+        let allParts = [String](repeating: "..", count: ownParts.count-index)
+                        + pageParts.dropFirst(index)
+        return allParts.joined(separator: "/")
     }
     
     /**
@@ -381,7 +418,10 @@ extension Element {
      Returns the full path (relative to the site root for a page of the element in the given language.
      */
     func localizedPath(for language: String) -> String {
-        path != "" ? "\(path)/\(language).html" : "\(language).html"
+        guard path != "" else {
+            return Element.htmlPageName(for: language)
+        }
+        return path + Element.htmlPagePathAddition(for: language)
     }
 
     /**