CasperSecurity
# -*- test-case-name: automat._test.test_core -*-
"""
A core state-machine abstraction.
Perhaps something that could be replaced with or integrated into machinist.
"""
from itertools import chain
_NO_STATE = "<no state>"
class NoTransition(Exception):
"""
A finite state machine in C{state} has no transition for C{symbol}.
@param state: the finite state machine's state at the time of the
illegal transition.
@param symbol: the input symbol for which no transition exists.
"""
def __init__(self, state, symbol):
self.state = state
self.symbol = symbol
super(Exception, self).__init__(
"no transition for {} in {}".format(symbol, state)
)
class Automaton(object):
"""
A declaration of a finite state machine.
Note that this is not the machine itself; it is immutable.
"""
def __init__(self):
"""
Initialize the set of transitions and the initial state.
"""
self._initialState = _NO_STATE
self._transitions = set()
@property
def initialState(self):
"""
Return this automaton's initial state.
"""
return self._initialState
@initialState.setter
def initialState(self, state):
"""
Set this automaton's initial state. Raises a ValueError if
this automaton already has an initial state.
"""
if self._initialState is not _NO_STATE:
raise ValueError(
"initial state already set to {}".format(self._initialState))
self._initialState = state
def addTransition(self, inState, inputSymbol, outState, outputSymbols):
"""
Add the given transition to the outputSymbol. Raise ValueError if
there is already a transition with the same inState and inputSymbol.
"""
# keeping self._transitions in a flat list makes addTransition
# O(n^2), but state machines don't tend to have hundreds of
# transitions.
for (anInState, anInputSymbol, anOutState, _) in self._transitions:
if (anInState == inState and anInputSymbol == inputSymbol):
raise ValueError(
"already have transition from {} via {}".format(inState, inputSymbol))
self._transitions.add(
(inState, inputSymbol, outState, tuple(outputSymbols))
)
def allTransitions(self):
"""
All transitions.
"""
return frozenset(self._transitions)
def inputAlphabet(self):
"""
The full set of symbols acceptable to this automaton.
"""
return {inputSymbol for (inState, inputSymbol, outState,
outputSymbol) in self._transitions}
def outputAlphabet(self):
"""
The full set of symbols which can be produced by this automaton.
"""
return set(
chain.from_iterable(
outputSymbols for
(inState, inputSymbol, outState, outputSymbols)
in self._transitions
)
)
def states(self):
"""
All valid states; "Q" in the mathematical description of a state
machine.
"""
return frozenset(
chain.from_iterable(
(inState, outState)
for
(inState, inputSymbol, outState, outputSymbol)
in self._transitions
)
)
def outputForInput(self, inState, inputSymbol):
"""
A 2-tuple of (outState, outputSymbols) for inputSymbol.
"""
for (anInState, anInputSymbol,
outState, outputSymbols) in self._transitions:
if (inState, inputSymbol) == (anInState, anInputSymbol):
return (outState, list(outputSymbols))
raise NoTransition(state=inState, symbol=inputSymbol)
class Transitioner(object):
"""
The combination of a current state and an L{Automaton}.
"""
def __init__(self, automaton, initialState):
self._automaton = automaton
self._state = initialState
self._tracer = None
def setTrace(self, tracer):
self._tracer = tracer
def transition(self, inputSymbol):
"""
Transition between states, returning any outputs.
"""
outState, outputSymbols = self._automaton.outputForInput(self._state,
inputSymbol)
outTracer = None
if self._tracer:
outTracer = self._tracer(self._state._name(),
inputSymbol._name(),
outState._name())
self._state = outState
return (outputSymbols, outTracer)