implement a cache within the where filter (#6868)

Merge pull request 6868

lib/jekyll/filters.rb

@@ -167,11 +167,23 @@ module Jekyll
     #
     # Returns the filtered array of objects
     def where(input, property, value)
+      return input if property.nil? || value.nil?
       return input unless input.respond_to?(:select)
-      input = input.values if input.is_a?(Hash)
-      input.select do |object|
-        Array(item_property(object, property)).map!(&:to_s).include?(value.to_s)
-      end || []
+      input    = input.values if input.is_a?(Hash)
+      input_id = input.hash
+
+      # implement a hash based on method parameters to cache the end-result
+      # for given parameters.
+      @where_filter_cache ||= {}
+      @where_filter_cache[input_id] ||= {}
+      @where_filter_cache[input_id][property] ||= {}
+
+      # stash or retrive results to return
+      @where_filter_cache[input_id][property][value] ||= begin
+        input.select do |object|
+          Array(item_property(object, property)).map!(&:to_s).include?(value.to_s)
+        end || []
+      end
     end
 
     # Filters an array of objects against an expression