Cache ItemSet#leading_item_sets return values - #28

lib/propane/parser/item_set.rb

@@ -99,21 +99,24 @@ class Propane
       # @return [Set<ItemSet>]
       #   Set of all ItemSets that lead up to this ItemSet.
       def leading_item_sets
-        result = Set.new
-        eval_sets = Set[self]
-        evaled = Set.new
-        while eval_sets.size > 0
-          eval_set = eval_sets.first
-          eval_sets.delete(eval_set)
-          evaled << eval_set
-          eval_set.in_sets.each do |in_set|
-            result << in_set
-            unless evaled.include?(in_set)
-              eval_sets << in_set
+        @_leading_item_sets ||=
+          begin
+            result = Set.new
+            eval_sets = Set[self]
+            evaled = Set.new
+            while eval_sets.size > 0
+              eval_set = eval_sets.first
+              eval_sets.delete(eval_set)
+              evaled << eval_set
+              eval_set.in_sets.each do |in_set|
+                result << in_set
+                unless evaled.include?(in_set)
+                  eval_sets << in_set
+                end
+              end
             end
+            result
           end
-        end
-        result
       end
 
       # Represent the ItemSet as a String.