package com.gabrielbauman.gist;

import java.util.Arrays;

/**
 * A very simple (but useful) method of getting basic finite state machine behaviour using a Java enum.
 *
 * <code>
 *     EnumStateMachine state = NEUTRAL;
 *     state = state.transitionTo(FIRST);
 *     state.transitionTo(REVERSE); // IllegalStateException
 * </code>
 */
public enum EnumStateMachine {

    FIRST,
    NEUTRAL,
    REVERSE;

    static {
        NEUTRAL.allowTransitionTo(FIRST, REVERSE);
        FIRST.allowTransitionTo(NEUTRAL);
        REVERSE.allowTransitionTo(NEUTRAL);
    }

    // Everything below this line is boilerplate. It's too bad we don't have "abstract base enums" in Java.

    private EnumStateMachine[] possibleTransitions;

    private void allowTransitionTo(EnumStateMachine... allowableStates) {
        possibleTransitions = allowableStates;
    }

    public boolean canTransitionTo(EnumStateMachine anotherState) {
        return Arrays.stream(possibleTransitions).anyMatch(anotherState::equals);
    }

    public EnumStateMachine transitionTo(EnumStateMachine newState) {
        if (!canTransitionTo(newState))
            throw new IllegalStateException(String.format("Illegal transition: %s -> %s", this, newState));
        return newState;
    }

}