Experimental parser: add backtracking

tools/lexer_generator/dot_utilities.py

 # Copyright 2014 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 55 matching lines @@
       for child_ix in child_ixs:
         edges.append(edge_template % (node_ix[0], child_ix))
       return node_ix[0]
     raise Exception
 
   process(term)
   return 'digraph { %s %s }' % ('\n'.join(nodes), '\n'.join(edges))
 
 def automaton_to_dot(automaton, merge = False):
   terminal_set = automaton.terminal_set()
-  to_skip = set([])
+  to_skip = set()
   # get a replacement action for a state's omega transition
   def analyse_omega_chain(state):
     omega_chain = list(state.omega_chain_iter())
     if len(omega_chain) == 1:
       omega_state = omega_chain[0][0]
       # Handle match nodes
       if omega_chain[0][1] == 0:
         if omega_state in terminal_set:
           to_skip.add(omega_state)  # don't draw omega transition state
           terminal_set.add(state)
           return omega_state.action()
     elif len(omega_chain) == 2:
       first_action = omega_chain[0][0].action().name()
       if ((first_action == 'store_token' or
            first_action == 'store_harmony_token') and
           omega_chain[1][0].action().name() == 'no_op'):
         if (omega_chain[0][0].action().term().args() ==
             omega_chain[1][0].action().term().args()):
           return Action(Term('goto', omega_chain[0][0].node_number()), 0)
         else:
           to_skip.add(omega_chain[0][0])  # don't draw first transition
           return Action(Term('chain',
                         omega_chain[0][0].action().term(),
                         Term('goto', omega_chain[1][0].node_number())), 0)
     return Action.empty_action()
   # draw state
-  skipped = set([])
-  drawn = set([])
+  skipped = set()
+  drawn = set()
   def draw(node, (node_content, edge_content)):
     if node in to_skip:  # skip match nodes
       skipped.add(node)
       return (node_content, edge_content)
     drawn.add(node)
     omega_action = analyse_omega_chain(node) if merge else Action.empty_action()
     node_action = node.action()
     if node_action.name() == 'no_op':
       node_action = Action.empty_action()
     if node_action or omega_action:
@@ skipping 41 matching lines @@
   node [shape = doublecircle, style=unfilled]; %s
   node [shape = circle];
   %s
   %s
 }
   ''' % (start_shape,
          start_number,
          " ".join(terminals),
          "\n".join(edge_content),
          "\n".join(node_content))