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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 __future__ import annotations
import sys
from itertools import chain
from typing import Callable, Generic, Optional, Sequence, TypeVar, Hashable
if sys.version_info >= (3, 10):
from typing import TypeAlias
else:
from typing_extensions import TypeAlias
_NO_STATE = "<no state>"
State = TypeVar("State", bound=Hashable)
Input = TypeVar("Input", bound=Hashable)
Output = TypeVar("Output", bound=Hashable)
class NoTransition(Exception, Generic[State, Input]):
"""
A finite state machine in C{state} has no transition for C{symbol}.
@ivar state: See C{state} init parameter.
@ivar symbol: See C{symbol} init parameter.
"""
def __init__(self, state: State, symbol: Input):
"""
Construct a L{NoTransition}.
@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.
"""
self.state = state
self.symbol = symbol
super(Exception, self).__init__(
"no transition for {} in {}".format(symbol, state)
)
class Automaton(Generic[State, Input, Output]):
"""
A declaration of a finite state machine.
Note that this is not the machine itself; it is immutable.
"""
def __init__(self, initial: State | None = None) -> None:
"""
Initialize the set of transitions and the initial state.
"""
if initial is None:
initial = _NO_STATE # type:ignore[assignment]
assert initial is not None
self._initialState: State = initial
self._transitions: set[tuple[State, Input, State, Sequence[Output]]] = set()
self._unhandledTransition: Optional[tuple[State, Sequence[Output]]] = None
@property
def initialState(self) -> State:
"""
Return this automaton's initial state.
"""
return self._initialState
@initialState.setter
def initialState(self, state: State) -> None:
"""
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: State,
inputSymbol: Input,
outState: State,
outputSymbols: tuple[Output, ...],
):
"""
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 {} to {} via {}".format(
inState, anOutState, inputSymbol
)
)
self._transitions.add((inState, inputSymbol, outState, tuple(outputSymbols)))
def unhandledTransition(
self, outState: State, outputSymbols: Sequence[Output]
) -> None:
"""
All unhandled transitions will be handled by transitioning to the given
error state and error-handling output symbols.
"""
self._unhandledTransition = (outState, tuple(outputSymbols))
def allTransitions(self) -> frozenset[tuple[State, Input, State, Sequence[Output]]]:
"""
All transitions.
"""
return frozenset(self._transitions)
def inputAlphabet(self) -> set[Input]:
"""
The full set of symbols acceptable to this automaton.
"""
return {
inputSymbol
for (inState, inputSymbol, outState, outputSymbol) in self._transitions
}
def outputAlphabet(self) -> set[Output]:
"""
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) -> frozenset[State]:
"""
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: State, inputSymbol: Input
) -> tuple[State, Sequence[Output]]:
"""
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))
if self._unhandledTransition is None:
raise NoTransition(state=inState, symbol=inputSymbol)
return self._unhandledTransition
OutputTracer = Callable[[Output], None]
Tracer: TypeAlias = "Callable[[State, Input, State], OutputTracer[Output] | None]"
class Transitioner(Generic[State, Input, Output]):
"""
The combination of a current state and an L{Automaton}.
"""
def __init__(self, automaton: Automaton[State, Input, Output], initialState: State):
self._automaton: Automaton[State, Input, Output] = automaton
self._state: State = initialState
self._tracer: Tracer[State, Input, Output] | None = None
def setTrace(self, tracer: Tracer[State, Input, Output] | None) -> None:
self._tracer = tracer
def transition(
self, inputSymbol: Input
) -> tuple[Sequence[Output], OutputTracer[Output] | None]:
"""
Transition between states, returning any outputs.
"""
outState, outputSymbols = self._automaton.outputForInput(
self._state, inputSymbol
)
outTracer = None
if self._tracer:
outTracer = self._tracer(self._state, inputSymbol, outState)
self._state = outState
return (outputSymbols, outTracer)
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