cache the dispatch table on the depth first visitor

We know the API for the depth first visitor in advance, so it's OK to
calcuate this cache in advance

2 files changed, 21 insertions, 6 deletions

diff --git a/lib/arel/visitors/depth_first.rb b/lib/arel/visitors/depth_first.rb
index a434f404c7..d7d85cfcc6 100644
--- a/lib/arel/visitors/depth_first.rb
+++ b/lib/arel/visitors/depth_first.rb
@@ -173,6 +173,12 @@ module Arel
       def visit_Hash o
         o.each { |k,v| visit(k); visit(v) }
       end
+
+      DISPATCH = dispatch_cache
+
+      def get_dispatch_cache
+        DISPATCH
+      end
     end
   end
 end
diff --git a/lib/arel/visitors/visitor.rb b/lib/arel/visitors/visitor.rb
index 146ae216f6..2152da9f05 100644
--- a/lib/arel/visitors/visitor.rb
+++ b/lib/arel/visitors/visitor.rb
@@ -2,7 +2,17 @@ module Arel
   module Visitors
     class Visitor
       def initialize
-        @dispatch = Hash.new do |hash, class_name|
+        @dispatch = get_dispatch_cache
+      end
+
+      def accept object
+        visit object
+      end
+
+      private
+
+      def self.dispatch_cache
+        dispatch = Hash.new do |hash, class_name|
           hash[class_name] = "visit_#{(class_name || '').gsub('::', '_')}"
         end
 
@@ -10,16 +20,15 @@ module Arel
         # instance, but we can do that later.
         self.class.private_instance_methods.sort.each do |name|
           next unless name =~ /^visit_(.*)$/
-          @dispatch[$1.gsub('_', '::')] = name
+          dispatch[$1.gsub('_', '::')] = name
         end
+        dispatch
       end
 
-      def accept object
-        visit object
+      def get_dispatch_cache
+        self.class.dispatch_cache
       end
 
-      private
-
       def dispatch
         @dispatch
       end