diff --git a/src/library/title.cr b/src/library/title.cr
index 8260be9..2e9db26 100644
--- a/src/library/title.cr
+++ b/src/library/title.cr
@@ -70,11 +70,12 @@ class Title
     end
   end
 
-  def examine : Bool
+  def examine(cache = {} of String => String) : Bool
     return false unless Dir.exists? @dir # no title, should be removed
-    contents_signature = Dir.contents_signature @dir
+    contents_signature = Dir.contents_signature @dir, cache
     # not changed, preserve
     return true if @contents_signature == contents_signature
+    puts "Contents changed in #{@dir}"
 
     # fix title
     @contents_signature = contents_signature
@@ -95,7 +96,7 @@ class Title
     previous_titles_size = @title_ids.size
     @title_ids.select! do |title_id|
       title = Library.default.get_title! title_id
-      title.examine
+      title.examine cache
     end
     remained_title_dirs = @title_ids.map do |id|
       title = Library.default.get_title! id
diff --git a/src/util/signature.cr b/src/util/signature.cr
index f2bf103..e56aa06 100644
--- a/src/util/signature.cr
+++ b/src/util/signature.cr
@@ -54,7 +54,8 @@ class Dir
   # Rescan conditions:
   #   - When a file added, moved, removed, renamed (including which in nested
   #       directories)
-  def self.contents_signature(dirname) : String
+  def self.contents_signature(dirname, cache = {} of String => String) : String
+    return cache[dirname] if cache[dirname]?
     signatures = [] of String
     self.open dirname do |dir|
       dir.entries.sort.each do |fn|
@@ -69,6 +70,8 @@ class Dir
         end
       end
     end
-    Digest::SHA1.hexdigest(signatures.sort.join)
+    hash = Digest::SHA1.hexdigest(signatures.sort.join)
+    cache[dirname] = hash
+    hash
   end
 end