content: /*
   * Copyright (C) 2020 ActiveJ LLC.
   *
   * 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
   *
   * http://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 io.activej.ot.reducers;
  
  import io.activej.ot.system.OTSystem;
  import org.jetbrains.annotations.Nullable;
  
  import java.util.ArrayList;
  import java.util.List;
  import java.util.function.BiFunction;
  import java.util.function.BinaryOperator;
  
  import static io.activej.common.collection.CollectionUtils.concat;
  
  public interface DiffsReducer<A, D> {
  	A initialValue();
  
  	A accumulate(A accumulatedDiffs, List<? extends D> diffs);
  
  	A combine(A existing, A additional);
  
  	static <A, D> DiffsReducer<A, D> of(@Nullable A initialValue, BiFunction<A, List<? extends D>, A> reduceFunction) {
  		return of(initialValue, reduceFunction, (existing, additional) -> existing);
  	}
  
  	static <A, D> DiffsReducer<A, D> of(@Nullable A initialValue, BiFunction<A, List<? extends D>, A> reduceFunction, BinaryOperator<A> combiner) {
  		return new DiffsReducer<>() {
  			@Override
  			public @Nullable A initialValue() {
  				return initialValue;
  			}
  
  			@Override
  			public A accumulate(A accumulatedDiffs, List<? extends D> diffs) {
  				return reduceFunction.apply(accumulatedDiffs, diffs);
  			}
  
  			@Override
  			public A combine(A existing, A additional) {
  				return combiner.apply(existing, additional);
  			}
  		};
  	}
  
  	static <D> DiffsReducer<Void, D> toVoid() {
  		return of(null, ($, lists) -> null);
  	}
  
  	static <D> DiffsReducer<List<D>, D> toList() {
  		return of(new ArrayList<>(), (accumulatedDiffs, diffs) ->
  			concat(diffs, accumulatedDiffs));
  	}
  
  	static <D> DiffsReducer<List<D>, D> toSquashedList(OTSystem<D> system) {
  		return of(new ArrayList<>(), (accumulatedDiffs, diffs) ->
  			system.squash(concat(diffs, accumulatedDiffs)));
  	}
  
  }
  