Cost Function as a DFA

API Reference

class masa.common.ltl.DFACostFn(dfa: DFA)

Bases: DFA, Callable[[Iterable[str]], float]

DFA-backed MASA cost function.

This wrapper interprets accepting automaton states as constraint violations (or terminal “bad” states): a transition that lands in an accepting state yields cost 1.0 and otherwise 0.0.

Important

• The internal DFA state is advanced by calling __call__().

• Use cost() for counterfactual evaluation from an explicit DFA state without mutating internal state.

• DFA.step() is intentionally disabled to avoid ambiguous state updates via the inherited DFA interface.

Variables:

dfa – The wrapped DFA instance whose internal state is advanced when the cost function is called.

Creates a DFA cost function wrapper.

Parameters:

dfa – The DFA to wrap. The wrapper keeps a reference to this DFA and uses its internal state for sequential evaluation.

add_edge(parent: int, child: int, condition: Formula)

Disables edge modification after wrapping.

Raises:

RuntimeError – Always raised. Build the DFA fully before wrapping it as a cost function to avoid unintended side effects.

reset()

Resets the internal DFA state.

Notes

This delegates to the wrapped DFA’s DFA.reset().

step(labels: Iterable[str]) → Tuple[bool, int]

Disables stepping via the DFA interface.

Raises:

RuntimeError – Always raised. Use __call__() to advance the internal DFA and return the cost signal.

cost(state: int, labels: Iterable[str]) → float

Computes the one-step cost from an explicit DFA state without mutation.

Parameters:

• state – The DFA state to evaluate from (does not need to equal the internal automaton state).

• labels – Iterable of atomic proposition names for the current step.

Returns:

1.0 iff the next state reached from state under labels is accepting; otherwise 0.0.

Notes

This is intended for counterfactual evaluation and does not change the wrapped DFA’s internal state.

property automaton_state

Returns the current state of the wrapped DFA.

Returns:

The wrapped DFA’s current automaton state (DFA.state).

ltl.dfa_to_costfn()

Wraps a DFA as a DFACostFn via a deep copy.

Parameters:

dfa – The DFA to wrap.

Returns:

A DFACostFn wrapper around a deep-copied DFA.

Notes

The deep copy prevents unexpected side effects if the caller later mutates the original DFA (e.g., by adding edges).