forked from OSchip/llvm-project
332 lines
13 KiB
Python
332 lines
13 KiB
Python
#!/usr/bin/env python
|
|
|
|
"""This module implements a Finite State Machine (FSM). In addition to state
|
|
this FSM also maintains a user defined "memory". So this FSM can be used as a
|
|
Push-down Automata (PDA) since a PDA is a FSM + memory.
|
|
|
|
The following describes how the FSM works, but you will probably also need to
|
|
see the example function to understand how the FSM is used in practice.
|
|
|
|
You define an FSM by building tables of transitions. For a given input symbol
|
|
the process() method uses these tables to decide what action to call and what
|
|
the next state will be. The FSM has a table of transitions that associate:
|
|
|
|
(input_symbol, current_state) --> (action, next_state)
|
|
|
|
Where "action" is a function you define. The symbols and states can be any
|
|
objects. You use the add_transition() and add_transition_list() methods to add
|
|
to the transition table. The FSM also has a table of transitions that
|
|
associate:
|
|
|
|
(current_state) --> (action, next_state)
|
|
|
|
You use the add_transition_any() method to add to this transition table. The
|
|
FSM also has one default transition that is not associated with any specific
|
|
input_symbol or state. You use the set_default_transition() method to set the
|
|
default transition.
|
|
|
|
When an action function is called it is passed a reference to the FSM. The
|
|
action function may then access attributes of the FSM such as input_symbol,
|
|
current_state, or "memory". The "memory" attribute can be any object that you
|
|
want to pass along to the action functions. It is not used by the FSM itself.
|
|
For parsing you would typically pass a list to be used as a stack.
|
|
|
|
The processing sequence is as follows. The process() method is given an
|
|
input_symbol to process. The FSM will search the table of transitions that
|
|
associate:
|
|
|
|
(input_symbol, current_state) --> (action, next_state)
|
|
|
|
If the pair (input_symbol, current_state) is found then process() will call the
|
|
associated action function and then set the current state to the next_state.
|
|
|
|
If the FSM cannot find a match for (input_symbol, current_state) it will then
|
|
search the table of transitions that associate:
|
|
|
|
(current_state) --> (action, next_state)
|
|
|
|
If the current_state is found then the process() method will call the
|
|
associated action function and then set the current state to the next_state.
|
|
Notice that this table lacks an input_symbol. It lets you define transitions
|
|
for a current_state and ANY input_symbol. Hence, it is called the "any" table.
|
|
Remember, it is always checked after first searching the table for a specific
|
|
(input_symbol, current_state).
|
|
|
|
For the case where the FSM did not match either of the previous two cases the
|
|
FSM will try to use the default transition. If the default transition is
|
|
defined then the process() method will call the associated action function and
|
|
then set the current state to the next_state. This lets you define a default
|
|
transition as a catch-all case. You can think of it as an exception handler.
|
|
There can be only one default transition.
|
|
|
|
Finally, if none of the previous cases are defined for an input_symbol and
|
|
current_state then the FSM will raise an exception. This may be desirable, but
|
|
you can always prevent this just by defining a default transition.
|
|
|
|
Noah Spurrier 20020822
|
|
"""
|
|
|
|
class ExceptionFSM(Exception):
|
|
|
|
"""This is the FSM Exception class."""
|
|
|
|
def __init__(self, value):
|
|
self.value = value
|
|
|
|
def __str__(self):
|
|
return `self.value`
|
|
|
|
class FSM:
|
|
|
|
"""This is a Finite State Machine (FSM).
|
|
"""
|
|
|
|
def __init__(self, initial_state, memory=None):
|
|
|
|
"""This creates the FSM. You set the initial state here. The "memory"
|
|
attribute is any object that you want to pass along to the action
|
|
functions. It is not used by the FSM. For parsing you would typically
|
|
pass a list to be used as a stack. """
|
|
|
|
# Map (input_symbol, current_state) --> (action, next_state).
|
|
self.state_transitions = {}
|
|
# Map (current_state) --> (action, next_state).
|
|
self.state_transitions_any = {}
|
|
self.default_transition = None
|
|
|
|
self.input_symbol = None
|
|
self.initial_state = initial_state
|
|
self.current_state = self.initial_state
|
|
self.next_state = None
|
|
self.action = None
|
|
self.memory = memory
|
|
|
|
def reset (self):
|
|
|
|
"""This sets the current_state to the initial_state and sets
|
|
input_symbol to None. The initial state was set by the constructor
|
|
__init__(). """
|
|
|
|
self.current_state = self.initial_state
|
|
self.input_symbol = None
|
|
|
|
def add_transition (self, input_symbol, state, action=None, next_state=None):
|
|
|
|
"""This adds a transition that associates:
|
|
|
|
(input_symbol, current_state) --> (action, next_state)
|
|
|
|
The action may be set to None in which case the process() method will
|
|
ignore the action and only set the next_state. The next_state may be
|
|
set to None in which case the current state will be unchanged.
|
|
|
|
You can also set transitions for a list of symbols by using
|
|
add_transition_list(). """
|
|
|
|
if next_state is None:
|
|
next_state = state
|
|
self.state_transitions[(input_symbol, state)] = (action, next_state)
|
|
|
|
def add_transition_list (self, list_input_symbols, state, action=None, next_state=None):
|
|
|
|
"""This adds the same transition for a list of input symbols.
|
|
You can pass a list or a string. Note that it is handy to use
|
|
string.digits, string.whitespace, string.letters, etc. to add
|
|
transitions that match character classes.
|
|
|
|
The action may be set to None in which case the process() method will
|
|
ignore the action and only set the next_state. The next_state may be
|
|
set to None in which case the current state will be unchanged. """
|
|
|
|
if next_state is None:
|
|
next_state = state
|
|
for input_symbol in list_input_symbols:
|
|
self.add_transition (input_symbol, state, action, next_state)
|
|
|
|
def add_transition_any (self, state, action=None, next_state=None):
|
|
|
|
"""This adds a transition that associates:
|
|
|
|
(current_state) --> (action, next_state)
|
|
|
|
That is, any input symbol will match the current state.
|
|
The process() method checks the "any" state associations after it first
|
|
checks for an exact match of (input_symbol, current_state).
|
|
|
|
The action may be set to None in which case the process() method will
|
|
ignore the action and only set the next_state. The next_state may be
|
|
set to None in which case the current state will be unchanged. """
|
|
|
|
if next_state is None:
|
|
next_state = state
|
|
self.state_transitions_any [state] = (action, next_state)
|
|
|
|
def set_default_transition (self, action, next_state):
|
|
|
|
"""This sets the default transition. This defines an action and
|
|
next_state if the FSM cannot find the input symbol and the current
|
|
state in the transition list and if the FSM cannot find the
|
|
current_state in the transition_any list. This is useful as a final
|
|
fall-through state for catching errors and undefined states.
|
|
|
|
The default transition can be removed by setting the attribute
|
|
default_transition to None. """
|
|
|
|
self.default_transition = (action, next_state)
|
|
|
|
def get_transition (self, input_symbol, state):
|
|
|
|
"""This returns (action, next state) given an input_symbol and state.
|
|
This does not modify the FSM state, so calling this method has no side
|
|
effects. Normally you do not call this method directly. It is called by
|
|
process().
|
|
|
|
The sequence of steps to check for a defined transition goes from the
|
|
most specific to the least specific.
|
|
|
|
1. Check state_transitions[] that match exactly the tuple,
|
|
(input_symbol, state)
|
|
|
|
2. Check state_transitions_any[] that match (state)
|
|
In other words, match a specific state and ANY input_symbol.
|
|
|
|
3. Check if the default_transition is defined.
|
|
This catches any input_symbol and any state.
|
|
This is a handler for errors, undefined states, or defaults.
|
|
|
|
4. No transition was defined. If we get here then raise an exception.
|
|
"""
|
|
|
|
if self.state_transitions.has_key((input_symbol, state)):
|
|
return self.state_transitions[(input_symbol, state)]
|
|
elif self.state_transitions_any.has_key (state):
|
|
return self.state_transitions_any[state]
|
|
elif self.default_transition is not None:
|
|
return self.default_transition
|
|
else:
|
|
raise ExceptionFSM ('Transition is undefined: (%s, %s).' %
|
|
(str(input_symbol), str(state)) )
|
|
|
|
def process (self, input_symbol):
|
|
|
|
"""This is the main method that you call to process input. This may
|
|
cause the FSM to change state and call an action. This method calls
|
|
get_transition() to find the action and next_state associated with the
|
|
input_symbol and current_state. If the action is None then the action
|
|
is not called and only the current state is changed. This method
|
|
processes one complete input symbol. You can process a list of symbols
|
|
(or a string) by calling process_list(). """
|
|
|
|
self.input_symbol = input_symbol
|
|
(self.action, self.next_state) = self.get_transition (self.input_symbol, self.current_state)
|
|
if self.action is not None:
|
|
self.action (self)
|
|
self.current_state = self.next_state
|
|
self.next_state = None
|
|
|
|
def process_list (self, input_symbols):
|
|
|
|
"""This takes a list and sends each element to process(). The list may
|
|
be a string or any iterable object. """
|
|
|
|
for s in input_symbols:
|
|
self.process (s)
|
|
|
|
##############################################################################
|
|
# The following is an example that demonstrates the use of the FSM class to
|
|
# process an RPN expression. Run this module from the command line. You will
|
|
# get a prompt > for input. Enter an RPN Expression. Numbers may be integers.
|
|
# Operators are * / + - Use the = sign to evaluate and print the expression.
|
|
# For example:
|
|
#
|
|
# 167 3 2 2 * * * 1 - =
|
|
#
|
|
# will print:
|
|
#
|
|
# 2003
|
|
##############################################################################
|
|
|
|
import sys, os, traceback, optparse, time, string
|
|
|
|
#
|
|
# These define the actions.
|
|
# Note that "memory" is a list being used as a stack.
|
|
#
|
|
|
|
def BeginBuildNumber (fsm):
|
|
fsm.memory.append (fsm.input_symbol)
|
|
|
|
def BuildNumber (fsm):
|
|
s = fsm.memory.pop ()
|
|
s = s + fsm.input_symbol
|
|
fsm.memory.append (s)
|
|
|
|
def EndBuildNumber (fsm):
|
|
s = fsm.memory.pop ()
|
|
fsm.memory.append (int(s))
|
|
|
|
def DoOperator (fsm):
|
|
ar = fsm.memory.pop()
|
|
al = fsm.memory.pop()
|
|
if fsm.input_symbol == '+':
|
|
fsm.memory.append (al + ar)
|
|
elif fsm.input_symbol == '-':
|
|
fsm.memory.append (al - ar)
|
|
elif fsm.input_symbol == '*':
|
|
fsm.memory.append (al * ar)
|
|
elif fsm.input_symbol == '/':
|
|
fsm.memory.append (al / ar)
|
|
|
|
def DoEqual (fsm):
|
|
print str(fsm.memory.pop())
|
|
|
|
def Error (fsm):
|
|
print 'That does not compute.'
|
|
print str(fsm.input_symbol)
|
|
|
|
def main():
|
|
|
|
"""This is where the example starts and the FSM state transitions are
|
|
defined. Note that states are strings (such as 'INIT'). This is not
|
|
necessary, but it makes the example easier to read. """
|
|
|
|
f = FSM ('INIT', []) # "memory" will be used as a stack.
|
|
f.set_default_transition (Error, 'INIT')
|
|
f.add_transition_any ('INIT', None, 'INIT')
|
|
f.add_transition ('=', 'INIT', DoEqual, 'INIT')
|
|
f.add_transition_list (string.digits, 'INIT', BeginBuildNumber, 'BUILDING_NUMBER')
|
|
f.add_transition_list (string.digits, 'BUILDING_NUMBER', BuildNumber, 'BUILDING_NUMBER')
|
|
f.add_transition_list (string.whitespace, 'BUILDING_NUMBER', EndBuildNumber, 'INIT')
|
|
f.add_transition_list ('+-*/', 'INIT', DoOperator, 'INIT')
|
|
|
|
print
|
|
print 'Enter an RPN Expression.'
|
|
print 'Numbers may be integers. Operators are * / + -'
|
|
print 'Use the = sign to evaluate and print the expression.'
|
|
print 'For example: '
|
|
print ' 167 3 2 2 * * * 1 - ='
|
|
inputstr = raw_input ('> ')
|
|
f.process_list(inputstr)
|
|
|
|
if __name__ == '__main__':
|
|
try:
|
|
start_time = time.time()
|
|
parser = optparse.OptionParser(formatter=optparse.TitledHelpFormatter(), usage=globals()['__doc__'], version='$Id: FSM.py 490 2007-12-07 15:46:24Z noah $')
|
|
parser.add_option ('-v', '--verbose', action='store_true', default=False, help='verbose output')
|
|
(options, args) = parser.parse_args()
|
|
if options.verbose: print time.asctime()
|
|
main()
|
|
if options.verbose: print time.asctime()
|
|
if options.verbose: print 'TOTAL TIME IN MINUTES:',
|
|
if options.verbose: print (time.time() - start_time) / 60.0
|
|
sys.exit(0)
|
|
except KeyboardInterrupt, e: # Ctrl-C
|
|
raise e
|
|
except SystemExit, e: # sys.exit()
|
|
raise e
|
|
except Exception, e:
|
|
print 'ERROR, UNEXPECTED EXCEPTION'
|
|
print str(e)
|
|
traceback.print_exc()
|
|
os._exit(1)
|