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This library focuses on providing different flavors of eventsourced aggregrates
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/*
 * Copyright 2021-2024 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package dk.cloudcreate.essentials.components.eventsourced.aggregates.decider;

import static dk.cloudcreate.essentials.shared.FailFast.requireNonNull;

/**
 * Variant of the event sourced Decider pattern, which supports building an Aggregate STATE based on previous EVENT's that relate to the
 * aggregate instance, and which can handle COMMAND's, whose side effect is either an ERROR or a List of EVENT's (can be an empty list)
 *
 * @param  The type of Commands that the {@link #handle(Object, Object)} can process
 * @param    The type of Events that can be returned by {@link #handle(Object, Object)} and applied in the {@link #applyEvent(Object, Object)}
 * @param    The type of Error that can be returned by the {@link #handle(Object, Object)} method
 * @param    The type of Aggregate State that this {@link Decider} works with in {@link #handle(Object, Object)}, {@link #initialState()}, {@link #applyEvent(Object, Object)} and {@link #isFinal(Object)}
 */
public interface Decider extends Handler, StateEvolver, InitialStateProvider, IsStateFinalResolver {
    /**
     * Factory method for creating a {@link Decider} instance from instances of the various {@link Decider} related interfaces
     *
     * @param handler                   A {@link Decider} related interface that is responsible for handling COMMAND(s)
     * @param initialStateProvider      A {@link Decider} related interface, which provides the Initial STATE for a given  aggregate.
     * @param stateEvolver              A {@link Decider} related interface, which can apply EVENT<(s) to an aggregate/i>'s STATE instance
     * @param stateIsStateFinalResolver A {@link Decider} related interface that resolves if the aggregate's state is final and no more changes can occur, i.e. no more events can be persisted
     * @param                  The type of Commands that the {@link #handle(Object, Object)} can process
     * @param                    The type of Events that can be returned by {@link #handle(Object, Object)} and applied in the {@link #applyEvent(Object, Object)}
     * @param                    The type of Error that can be returned by the {@link #handle(Object, Object)} method
     * @param                    The type of Aggregate State that this {@link Decider} works with in {@link #handle(Object, Object)}, {@link #initialState()}, {@link #applyEvent(Object, Object)} and {@link #isFinal(Object)}
     * @return a {@link Decider} instance that delegates to the {@link Decider} related instances provided as parameters
     */
    static  Decider decider(Handler handler,
                                                                                        InitialStateProvider initialStateProvider,
                                                                                        StateEvolver stateEvolver,
                                                                                        IsStateFinalResolver stateIsStateFinalResolver) {
        requireNonNull(handler, "No handler provided");
        requireNonNull(initialStateProvider, "No initialStateProvider provided");
        requireNonNull(stateEvolver, "No stateEvolver provided");
        requireNonNull(stateIsStateFinalResolver, "No stateIsStateFinalResolver provided");
        return new Decider<>() {
            @Override
            public STATE applyEvent(EVENT event, STATE state) {
                return stateEvolver.applyEvent(event, state);
            }

            @Override
            public STATE initialState() {
                return initialStateProvider.initialState();
            }

            @Override
            public HandlerResult handle(COMMAND cmd, STATE state) {
                return handler.handle(cmd, state);
            }

            @Override
            public boolean isFinal(STATE state) {
                return stateIsStateFinalResolver.isFinal(state);
            }
        };
    }

    /**
     * Return true from this method IF the aggregate's state is final and no more changes can occur, i.e. no more events can be persisted (i.e. {@link Handler#handle(Object, Object)} will return an ERROR)

     * Note: This method is called isTerminal in the decider pattern

     * See {@link IsStateFinalResolver#isFinal(Object)} for more details.

     * The default implementation in the {@link Decider} always return false
     *
     * @param state the current STATE of the aggregate
     * @return true IF the aggregate's state is final, otherwise false
     */
    @Override
    default boolean isFinal(STATE state) {
        return false;
    }
}