Generate only items which are not #fresh?.

bin/ace

@@ -71,6 +71,5 @@ end
 puts
 
 Ace::Item.all_instances.each do |item|
-  puts "~ Generating #{item.output_path}"
   item.save!
 end

lib/ace.rb

@@ -143,12 +143,29 @@ module Ace
       end
     end
 
-    def save!
-      content = self.render # so filters can influence output_path
+    def compare_mtime(one, others)
+      File.exist?(one) && File.mtime(one) > others.map { |post| File.mtime(post) }.max
+    end
 
-      FileUtils.mkdir_p File.dirname(self.output_path)
-      File.open(self.output_path, "w") do |file|
-        file.puts(content)
+    def source_files
+      [self.original_path]
+    end
+
+    def fresh?
+      @fresh ||= compare_mtime(self.output_path, self.source_files)
+    end
+
+    def save!
+      if self.fresh?
+        puts "~ [IGNORE] #{self.output_path}"
+      else
+        puts "~ [UPDATE] #{self.output_path}"
+        content = self.render # so filters can influence output_path
+
+        FileUtils.mkdir_p File.dirname(self.output_path)
+        File.open(self.output_path, "w") do |file|
+          file.puts(content)
+        end
       end
     end
   end