1 """Finite state machine, featuring transition actions."""
   2 
   3 FSMError = 'Invalid input to finite state machine'
   4 
   5 class FSM:
   6 
   7     """Finite state machine, featuring transition actions.
   8 
   9     The class stores a dictionary of (state, input) keys,
  10     and (state, action) values.
  11 
  12     When a (state, input) search is performed:
  13     * an exact match is checked first,
  14     * (state, None) is checked next.
  15 
  16     The action is of the following form:
  17     * function(current_state, input)
  18     """
  19     
  20     def __init__(self):
  21         self.states = {}
  22         self.state = None
  23         self.dbg = None
  24 
  25     def add(self, state, input, newstate, action):
  26         """Add a transition to the FSM."""
  27         self.states[(state, input)] = (newstate, action)
  28 
  29     def execute(self, input):
  30         """Perform a transition and execute action."""
  31         si = (self.state, input)
  32         sn = (self.state, None)
  33         # exact state match?
  34         if si in self.states:
  35             newstate, action = self.states[si]
  36         # no, how about a None (catch-all) match?
  37         elif sn in self.states:
  38             newstate, action = self.states[sn]
  39         if self.dbg != None:
  40             self.dbg.write('State: %s / Input: %s /'
  41                            'Next State: %s / Action: %s\n' %
  42                            (self.state, input, newstate, action))
  43         apply(action, (self.state, input))
  44         self.state = newstate
  45 
  46     def start(self, state):
  47         """Define the start state.
  48 
  49         Actually, this just resets the current state.
  50         """
  51         self.state = state
  52 
  53     def debug(self, out):
  54         """Assign a writable file to log transitions."""
  55         self.dbg = out