100 lines
3.0 KiB
Dart
100 lines
3.0 KiB
Dart
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class NoTransitionPossibleException implements Exception {
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@override
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String errMsg() => "The transition graph allows no transition";
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}
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/// A deterministic finite automaton. [T] is the state type while
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/// [I] is the input type.
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/// Edges of the node must be added with [addTransition]. If a trap state
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/// is required, it can be set in the constructor.
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class DeterministicFiniteAutomaton<T, I> {
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/// The current state of the DFA
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T _state;
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/// The edges of the DFA: State x Input -> State
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Map<T, Map<I, T>> _transitions;
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/// Trap state
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T? trapState;
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/// The argument is the initial state
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DeterministicFiniteAutomaton(this._state, { this.trapState }) : _transitions = {};
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T get state => _state;
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void addTransition(T oldState, I input, T newState) {
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assert(oldState != trapState);
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// These are handled implicitly if no transition has been found
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assert(newState != trapState);
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if (!_transitions.containsKey(oldState)) {
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_transitions[oldState] = {};
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}
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_transitions[oldState]![input] = newState;
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}
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/// Transition the DFA based on its current state and the input [input].
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void onInput(I input) {
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final newState = _transitions[_state]?[input];
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if (newState == null) {
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// Go to the trap state if we can
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if (trapState != null) {
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_state = trapState!;
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return;
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} else {
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throw NoTransitionPossibleException();
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}
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}
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_state = newState;
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}
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/// Returns where [input] would take the automaton to. Returns null if no transition
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/// is possible, ignoring trap transitions.
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T? peekTransition(I input) {
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if (!_transitions.containsKey(_state) || !_transitions[_state]!.containsKey(input)) {
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return null;
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}
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return _transitions[_state]![input]!;
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}
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}
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typedef MealyAutomatonCallback<T, I> = void Function(T oldState, I input);
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class MealyAutomaton<T, I> {
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/// The base automaton
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final DeterministicFiniteAutomaton<T, I> _automaton;
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/// Mapping of State x Input -> Output callback
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Map<T, Map<I, MealyAutomatonCallback<T, I>>> _outputs;
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/// Trap state
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MealyAutomatonCallback<T, I>? trapCallback;
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// TODO: Assert that trapState != null implies trapCallback != null.
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MealyAutomaton(T initialState, { T? trapState, this.trapCallback })
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: _outputs = {},
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_automaton = DeterministicFiniteAutomaton(initialState, trapState: trapState);
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T get state => _automaton.state;
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void addTransition(T oldState, I input, T newState, MealyAutomatonCallback<T, I> callback) {
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_automaton.addTransition(oldState, input, newState);
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if (!_outputs.containsKey(oldState)) {
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_outputs[oldState] = {};
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}
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_outputs[oldState]![input] = callback;
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}
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void onInput(I input) {
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final _state = _automaton.state;
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if (_automaton.peekTransition(input) == null && trapCallback == null) {
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throw new NoTransitionPossibleException();
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}
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final callback = _outputs[_state]?[input] ?? trapCallback!;
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_automaton.onInput(input);
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callback(_state, input);
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}
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}
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