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Collections and collection utilities for Neo4j.
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/*
 * Copyright (c) "Neo4j"
 * Neo4j Sweden AB [https://neo4j.com]
 *
 * This file is part of Neo4j.
 *
 * Neo4j is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see .
 */
package org.neo4j.collection.diffset;

import java.util.Collections;
import java.util.Iterator;
import java.util.Set;
import java.util.function.Predicate;

/**
 * Given a sequence of add and removal operations, instances of DiffSets track
 * which elements need to actually be added and removed at minimum from some
 * hypothetical target collection such that the result is equivalent to just
 * executing the sequence of additions and removals in order
 *
 * @param  type of elements
 */
public interface DiffSets {

    boolean isAdded(T elem);

    boolean isRemoved(T elem);

    Set getAdded();

    Set getRemoved();

    boolean isEmpty();

    Iterator apply(Iterator source);

    DiffSets filterAdded(Predicate addedFilter);

    final class Empty implements DiffSets {
        @SuppressWarnings("unchecked")
        public static  DiffSets instance() {
            return (DiffSets) INSTANCE;
        }

        @SuppressWarnings("unchecked")
        public static  DiffSets ifNull(DiffSets diffSets) {
            return diffSets == null ? (DiffSets) INSTANCE : diffSets;
        }

        private static final DiffSets INSTANCE = new Empty<>();

        private Empty() {
            // singleton
        }

        @Override
        public boolean isAdded(T elem) {
            return false;
        }

        @Override
        public boolean isRemoved(T elem) {
            return false;
        }

        @Override
        public Set getAdded() {
            return Collections.emptySet();
        }

        @Override
        public Set getRemoved() {
            return Collections.emptySet();
        }

        @Override
        public boolean isEmpty() {
            return true;
        }

        @SuppressWarnings("unchecked")
        @Override
        public Iterator apply(Iterator source) {
            return (Iterator) source;
        }

        @Override
        public DiffSets filterAdded(Predicate addedFilter) {
            return this;
        }
    }
}