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fsm.py
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fsm.py
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# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
# http://www.apache.org/licenses/LICENSE-2.0
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
"""
Basic FSM for building CLI parsers for prompt expansions, etc.
"""
ANY = -1
class FSMError(Exception):
pass
class FSM:
ANY = ANY
def __init__(self, initial_state=0):
self._transitions = {} # Map (input_symbol, state) to (action, next_state).
self.default_transition = None
self.RESET = initial_state
self.initial_state = self.RESET
self._reset()
def _reset(self):
self.stack = []
self.current_state = self.initial_state
def reset(self):
self._reset()
def push(self, obj):
self.stack.append(obj)
def pop(self):
return self.stack.pop()
def add_default_transition(self, action, next_state):
if action is None and next_state is None:
self.default_transition = None
else:
self.default_transition = (action, next_state)
def add_transition(self, input_symbol, state, action, next_state):
self._transitions[(input_symbol, state)] = (action, next_state)
def add_transitions(self, symbols, state, action, next_state):
for c in symbols:
self.add_transition(c, state, action, next_state)
def get_transition(self, input_symbol, state):
try:
return self._transitions[(input_symbol, state)]
except KeyError:
try:
return self._transitions[(ANY, state)]
except KeyError:
# no expression matched, so check for default
if self.default_transition is not None:
return self.default_transition
else:
raise FSMError('Transition {!r} is undefined.'.format(input_symbol))
def process(self, input_symbol):
action, next_state = self.get_transition(input_symbol, self.current_state)
if action is not None:
action(input_symbol, self)
if next_state is not None:
self.current_state = next_state
def process_string(self, s):
for c in s:
self.process(c)
if __name__ == "__main__":
pass
# vim:ts=4:sw=4:softtabstop=4:smarttab:expandtab