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JaVers - object auditing and diff framework for Java
There is a newer version: 7.6.1
package org.javers.guava;

import com.google.common.collect.Multimap;
import org.javers.common.exception.JaversException;
import org.javers.core.diff.NodePair;
import org.javers.core.diff.appenders.CorePropertyChangeAppender;
import org.javers.core.diff.changetype.map.EntryAdded;
import org.javers.core.diff.changetype.map.EntryChange;
import org.javers.core.diff.changetype.map.EntryRemoved;
import org.javers.core.diff.changetype.map.MapChange;
import org.javers.core.metamodel.object.DehydrateMapFunction;
import org.javers.core.metamodel.object.GlobalIdFactory;
import org.javers.core.metamodel.object.OwnerContext;
import org.javers.core.metamodel.object.PropertyOwnerContext;
import org.javers.core.metamodel.type.*;

import java.util.ArrayList;
import java.util.Collection;
import java.util.List;
import java.util.Objects;

import static java.util.Collections.EMPTY_LIST;
import static org.javers.common.exception.JaversExceptionCode.VALUE_OBJECT_IS_NOT_SUPPORTED_AS_MAP_KEY;

/**
 * Compares Guava Multimaps.
 * 

 *
 * It's automatically registered, if Guava is detected on a classpath.
 *
 * @author akrystian
 */
class MultimapChangeAppender extends CorePropertyChangeAppender{

    private final TypeMapper typeMapper;
    private final GlobalIdFactory globalIdFactory;

    MultimapChangeAppender(TypeMapper typeMapper, GlobalIdFactory globalIdFactory){
        this.typeMapper = typeMapper;
        this.globalIdFactory = globalIdFactory;
    }

    @Override
    public boolean supports(JaversType propertyType) {
        if (!(propertyType instanceof MultimapType)){
            return false;
        }
        MapContentType mapContentType = typeMapper.getMapContentType((KeyValueType) propertyType);
        if (mapContentType.getKeyType() instanceof ValueObjectType){
            throw new JaversException(VALUE_OBJECT_IS_NOT_SUPPORTED_AS_MAP_KEY, propertyType);
        }
        return true;
    }

    @Override
    public MapChange calculateChanges(NodePair pair, JaversProperty property){
        Multimap left =  (Multimap)pair.getLeftPropertyValue(property);
        Multimap right = (Multimap)pair.getRightPropertyValue(property);

        MultimapType multimapType = ((JaversProperty) property).getType();
        OwnerContext owner = new PropertyOwnerContext(pair.getGlobalId(), property.getName());

        List entryChanges = calculateChanges(multimapType, left, right, owner);
        if (!entryChanges.isEmpty()){
            renderNotParametrizedWarningIfNeeded(multimapType.getKeyType(), "key", "Multimap", property);
            renderNotParametrizedWarningIfNeeded(multimapType.getValueType(), "value", "Multimap", property);
            return new MapChange(pair.getGlobalId(), property.getName(), entryChanges);
        }else{
            return null;
        }
    }

    private List calculateChanges(MultimapType multimapType, Multimap left, Multimap right, OwnerContext owner){
        DehydrateMapFunction dehydrateFunction = getDehydrateMapFunction(multimapType);

        Multimap leftMultimap = multimapType.map(left, dehydrateFunction, owner);
        Multimap rightMultimap = multimapType.map(right, dehydrateFunction, owner);

        List changes = new ArrayList<>();

        for (Object commonKey : Multimaps.commonKeys(leftMultimap, rightMultimap)){
            Collection leftValues = leftMultimap.get(commonKey);
            Collection rightValues = rightMultimap.get(commonKey);

            Collection difference = difference(leftValues, rightValues);
            difference.addAll(difference(rightValues, leftValues));
            if (difference.size() > 0){
                calculateValueChanges(changes, commonKey, leftValues, rightValues);
            }
        }
        calculateKeyChanges(leftMultimap, rightMultimap, changes);
        return changes;
    }

    /**
     * Difference that handle properly collections with duplicates.
     */
    private static Collection difference(Collection first, Collection second){
        if (first == null) {
            return EMPTY_LIST;
        }

        if (second == null) {
            return first;
        }

        Collection difference = new ArrayList<>(first);
        for (Object current : second){
            difference.remove(current);
        }
        return difference;
    }

    private void calculateKeyChanges(Multimap leftMultimap, Multimap rightMultimap, List changes){
        for (Object addedKey : Multimaps.keysDifference(rightMultimap, leftMultimap)){
            Collection difference = difference(rightMultimap.get(addedKey), leftMultimap.get(addedKey));
            for (Object addedValue : difference){
                changes.add(new EntryAdded(addedKey, addedValue));
            }
        }
        for (Object removedKey : Multimaps.keysDifference(leftMultimap, rightMultimap)){
            for (Object removedValue : difference(leftMultimap.get(removedKey), rightMultimap.get(removedKey))){
                changes.add(new EntryRemoved(removedKey, removedValue));
            }
        }
    }

    private void calculateValueChanges(List changes, Object commonKey, Collection leftVal, Collection rightVal){
        if (Objects.equals(leftVal, rightVal)){
            return;
        }
        final Collection valuesRemoved = difference(leftVal, rightVal);
        if(valuesRemoved.size() > 0){
            for (Object addedValue : valuesRemoved){
                changes.add( new EntryRemoved(commonKey, addedValue));
            }
        }
        final Collection valuesAdded = difference(rightVal, leftVal);
        if(valuesAdded.size() > 0){
            for (Object addedValue : valuesAdded){
                changes.add( new EntryAdded(commonKey, addedValue));
            }
        }
    }

    private DehydrateMapFunction getDehydrateMapFunction(MultimapType multimapType){
        JaversType keyType = typeMapper.getJaversType(multimapType.getKeyType());
        JaversType valueType = typeMapper.getJaversType(multimapType.getValueType());

        MapContentType multiMapContentType = new MapContentType(keyType, valueType);
        return new DehydrateMapFunction(globalIdFactory, multiMapContentType);
    }
}