FSM.java

gistfile1.java

import com.google.common.base.Predicate;
import com.google.common.base.Supplier;
import com.google.common.base.Suppliers;
import com.google.common.cache.Cache;
import com.google.common.cache.CacheBuilder;
import com.google.common.cache.CacheLoader;
import com.google.common.collect.ArrayListMultimap;
import com.google.common.collect.Iterables;
import com.google.common.collect.Multimap;

import javax.annotation.Nullable;
import java.util.Collection;
import java.util.NoSuchElementException;

import static java.lang.String.format;

/**
 * @author Joern Barthel <[email protected]>
 */
public class FSM<E extends Enum, S extends Enum> {

    public class Transition<S extends Enum> implements Supplier<Boolean> {

        private final S to;
        private final Supplier<Boolean> guard;

        public Transition(S to) {
            this(to, Suppliers.ofInstance(Boolean.TRUE));
        }

        public <T> Transition(S to, Supplier<Boolean> guard) {

            this.to = to;
            this.guard = guard;

        }

        public Boolean get() {
            return guard.get();
        }

        public S getTo() {
            return to;
        }

    }

    private S state;

    private final Cache<E, Multimap<S, Transition<S>>> transitions = CacheBuilder.newBuilder()
            .build(CacheLoader.from(new Supplier<Multimap<S, Transition<S>>>() {

                public Multimap<S, Transition<S>> get() {
                    return ArrayListMultimap.create();
                }

            }));

    public FSM(S initialState) {
        this.state = initialState;
    }

    public void add(E on, S from, S to) {
        transitions.getUnchecked(on).put(from, new Transition<S>(to));
    }

    public void add(E on, S from, S to, Supplier<Boolean> guard) {
        transitions.getUnchecked(on).put(from, new Transition<S>(to, guard));
    }

    public void add(E on, S from, Transition<S> to) {
        transitions.getUnchecked(on).put(from, to);
    }

    public S event(E e) {

        Collection<Transition<S>> targets = transitions.getUnchecked(e).get(state);

        try {

            this.state = Iterables.find(targets, new Predicate<Supplier<Boolean>>() {

                public boolean apply(@Nullable Supplier<Boolean> input) {
                    return input.get();
                }

            }).getTo();

        } catch (NoSuchElementException ex) {
            throw new IllegalStateException(format("Cannot transition via '%s' from '%s'", e, state));
        }

        return state;

    }

}