fix generator hang when state transition cycle is present - close #20

lib/propane/parser.rb

@@ -134,7 +134,7 @@ class Propane
       # possible following tokens, we will only look in the item sets leading
       # up to this one. This restriction gives us a more precise lookahead set,
       # and allows us to parse LALR grammars.
-      item_sets = item_set.leading_item_sets
+      item_sets = Set[item_set] + item_set.leading_item_sets
       reduce_rules.reduce({}) do |reduce_actions, reduce_rule|
         lookahead_tokens_for_rule = build_lookahead_tokens_to_reduce(reduce_rule, item_sets)
         lookahead_tokens_for_rule.each do |lookahead_token|

lib/propane/parser/item_set.rb

@@ -87,14 +87,24 @@ class Propane
 
       # Set of ItemSets that lead to this ItemSet.
       #
-      # This set includes this ItemSet.
-      #
       # @return [Set<ItemSet>]
       #   Set of all ItemSets that lead up to this ItemSet.
       def leading_item_sets
-        @in_sets.reduce(Set[self]) do |result, item_set|
-          result + item_set.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
+            end
+          end
         end
+        result
       end
 
       # Represent the ItemSet as a String.

spec/propane_spec.rb

@@ -851,6 +851,20 @@ EOF
           "R",
         ])
       end
+
+      it "handles when an item set leads to itself" do
+        write_grammar <<EOF
+token one;
+token two;
+
+Start -> Opt one Start;
+Start -> ;
+
+Opt -> two;
+Opt -> ;
+EOF
+        build_parser(language: language)
+      end
     end
   end
 end