Automatically group files using the series meta

Signed-off-by: Danila Fedorin <danila.fedorin@gmail.com>

agda.rb

@@ -280,7 +280,7 @@ class FileGroup
     end
   end
 
-  def cross_link_files(file_documents)
+  def cross_link_files
     # Now, we have a complete list of all the IDs visible in scope.
     # Redirect relevant links to these IDs. This achieves within-post
     # links.
@@ -301,7 +301,6 @@ class FileGroup
           id = target[:id]
 
           relpath = Pathname.new(other_file).dirname.relative_path_from(Pathname.new(file).dirname)
-          puts relpath
           node['href'] = "#{relpath}##{id}"
         else
           # No definitions in any blog page. For now, just delete the anchor.
@@ -313,15 +312,32 @@ class FileGroup
 end
 
 agda_context = AgdaContext.new
+
 file_documents = {}
-files.each do |file|
+series_groups = files.group_by do |file|
   file_documents[file] = document = Nokogiri::HTML.parse(File.open(file))
+  document.css("meta[name=blog-series]")&.attribute("content")&.to_s
 end
-handler = FileGroup.new agda_context
-file_documents.each do |file, document|
-  handler.process_source_file file, document
+
+# For the 'nil' group, process individually.
+if files_with_no_series = series_groups.delete(nil)
+  files_with_no_series.each do |file|
+    file_group = FileGroup.new agda_context
+    file_group.process_source_file file, file_documents[file]
+    file_group.cross_link_files
+  end
 end
-handler.cross_link_files(file_documents)
+
+# For groups, process them together to allow cross-linking
+series_groups.each do |_, files_in_series|
+  file_group = FileGroup.new agda_context
+  files_in_series.each do |file|
+    file_group.process_source_file file, file_documents[file]
+  end
+  file_group.cross_link_files
+end
+
+# Having modified all the HTML files, save them.
 file_documents.each do |file, document|
   File.write(file, document.to_html(encoding: 'UTF-8'))
 end