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Filename : _core.py
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# -*- 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)