Experimental parser: add backtracking

tools/lexer_generator/dfa.py

 # Copyright 2013 the V8 project authors. All rights reserved.
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 #       notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 #       copyright notice, this list of conditions and the following
 #       disclaimer in the documentation and/or other materials provided
@@ skipping 140 matching lines @@
     return iter(self.__disjoint_keys)
 
   def node_count(self):
     return self.__node_count
 
   def start_state(self):
     return self.__start
 
   def terminal_set(self):
     return set(self.__terminal_set)
+
+  def build_incoming_transitions_map(self):
+    incoming_transitions = {}
+    def f(state, visitor_state):
+      for key, transition_state in state.key_state_iter():
+        if not transition_state in incoming_transitions:
+          incoming_transitions[transition_state] = []
+        incoming_transitions[transition_state].append((key, state))
+    self.visit_all_states(f)
+    return incoming_transitions