optimization: write static file to the destination only if source file timestamp differs

Also make sure static files get regenerated when they are missing in destination.

This is useful in --server --auto mode when it reduces disk/cpu load and also plays nice with xrefresh-server (which was my main motivation) -> soft CSS refresh works again!

lib/jekyll/static_file.rb

@@ -1,6 +1,8 @@
 module Jekyll
 
   class StaticFile
+    @@mtimes = Hash.new # the cache of last modification times [path] -> mtime
+
     # Initialize a new StaticFile.
     #   +site+ is the Site
     #   +base+ is the String path to the <source>
@@ -15,13 +17,59 @@ module Jekyll
       @name = name
     end
 
-    # Write the static file to the destination directory.
+    # Obtains source file path.
+    #
+    # Returns source file path.
+    def path
+      File.join(@base, @dir, @name)
+    end
+
+    # Obtain destination path.
     #   +dest+ is the String path to the destination dir
     #
-    # Returns nothing
+    # Returns destination file path.
+    def destination(dest)
+      File.join(dest, @dir, @name)
+    end
+
+    # Obtain mtime of the source path.
+    #
+    # Returns last modifiaction time for this file.
+    def mtime
+      File.stat(path).mtime.to_i
+    end
+
+    # Is source path modified?
+    #
+    # Returns true if modified since last write.
+    def modified?
+      @@mtimes[path] != mtime
+    end
+
+    # Write the static file to the destination directory (if modified).
+    #   +dest+ is the String path to the destination dir
+    #
+    # Returns false if the file was not modified since last time (no-op).
     def write(dest)
-      FileUtils.mkdir_p(File.join(dest, @dir))
+      dest_path = destination(dest)
-      FileUtils.cp(File.join(@base, @dir, @name), File.join(dest, @dir, @name))
+      dest_dir = File.join(dest, @dir)
+
+      return false if File.exist? dest_path and !modified?
+      @@mtimes[path] = mtime
+
+      FileUtils.mkdir_p(dest_dir)
+      FileUtils.cp(path, dest_path)
+
+      true
+    end
+
+    # Reset the mtimes cache (for testing purposes).
+    #
+    # Returns nothing.
+    def self.reset_cache
+      @@mtimes = Hash.new
+
+      nil
     end
   end
 

test/test_site.rb

@@ -34,6 +34,64 @@ class TestSite < Test::Unit::TestCase
       assert before_time <= @site.time
     end
 
+    should "write only modified static files" do
+      clear_dest
+      StaticFile.reset_cache
+
+      @site.process
+      some_static_file = @site.static_files[0].path
+      dest = File.expand_path(@site.static_files[0].destination(@site.dest))
+      mtime1 = File.stat(dest).mtime.to_i # first run must generate dest file
+
+      # need to sleep because filesystem timestamps have best resolution in seconds
+      sleep 1
+      @site.process
+      mtime2 = File.stat(dest).mtime.to_i
+      assert_equal mtime1, mtime2
+
+      # simulate file modification by user
+      FileUtils.touch some_static_file
+
+      sleep 1
+      @site.process
+      mtime3 = File.stat(dest).mtime.to_i
+      assert_not_equal mtime2, mtime3 # must be regenerated!
+
+      sleep 1
+      @site.process
+      mtime4 = File.stat(dest).mtime.to_i
+      assert_equal mtime3, mtime4 # no modifications, so must be the same
+    end
+
+    should "write static files if not modified but missing in destination" do
+      clear_dest
+      StaticFile.reset_cache
+
+      @site.process
+      some_static_file = @site.static_files[0].path
+      dest = File.expand_path(@site.static_files[0].destination(@site.dest))
+      mtime1 = File.stat(dest).mtime.to_i # first run must generate dest file
+
+      # need to sleep because filesystem timestamps have best resolution in seconds
+      sleep 1
+      @site.process
+      mtime2 = File.stat(dest).mtime.to_i
+      assert_equal mtime1, mtime2
+
+      # simulate destination file deletion
+      File.unlink dest
+
+      sleep 1
+      @site.process
+      mtime3 = File.stat(dest).mtime.to_i
+      assert_not_equal mtime2, mtime3 # must be regenerated and differ!
+
+      sleep 1
+      @site.process
+      mtime4 = File.stat(dest).mtime.to_i
+      assert_equal mtime3, mtime4 # no modifications, so must be the same
+    end
+
     should "read layouts" do
       @site.read_layouts
       assert_equal ["default", "simple"].sort, @site.layouts.keys.sort