Index issue when merging non-unique attributes' elements

[EMFCompare2.git] / plugins / org.eclipse.emf.compare / src / org / eclipse / emf / compare / merge / AttributeChangeMerger.java

/*******************************************************************************
 * Copyright (c) 2012, 2015 Obeo and others.
 * All rights reserved. This program and the accompanying materials
 * are made available under the terms of the Eclipse Public License v1.0
 * which accompanies this distribution, and is available at
 * http://www.eclipse.org/legal/epl-v10.html
 * 
 * Contributors:
 *     Obeo - initial API and implementation
 *     Philip Langer - [446947, 458147] Support for three-way text merging 
 *******************************************************************************/
package org.eclipse.emf.compare.merge;

import static org.eclipse.emf.compare.utils.ReferenceUtil.safeEGet;
import static org.eclipse.emf.compare.utils.ReferenceUtil.safeEIsSet;
import static org.eclipse.emf.compare.utils.ReferenceUtil.safeESet;

import java.util.ArrayList;
import java.util.Collections;
import java.util.Iterator;
import java.util.List;
import java.util.Set;

import org.eclipse.emf.common.util.EList;
import org.eclipse.emf.compare.AttributeChange;
import org.eclipse.emf.compare.Comparison;
import org.eclipse.emf.compare.Diff;
import org.eclipse.emf.compare.DifferenceKind;
import org.eclipse.emf.compare.DifferenceSource;
import org.eclipse.emf.compare.DifferenceState;
import org.eclipse.emf.compare.Match;
import org.eclipse.emf.compare.internal.ThreeWayTextDiff;
import org.eclipse.emf.compare.internal.utils.DiffUtil;
import org.eclipse.emf.compare.utils.IEqualityHelper;
import org.eclipse.emf.ecore.EAttribute;
import org.eclipse.emf.ecore.EEnum;
import org.eclipse.emf.ecore.EEnumLiteral;
import org.eclipse.emf.ecore.ENamedElement;
import org.eclipse.emf.ecore.EObject;
import org.eclipse.emf.ecore.EStructuralFeature;
import org.eclipse.emf.ecore.impl.DynamicEObjectImpl;

/**
 * This specific implementation of {@link AbstractMerger} will be used to merge attribute changes.
 * 
 * @author <a href="mailto:laurent.goubet@obeo.fr">Laurent Goubet</a>
 */
public class AttributeChangeMerger extends AbstractMerger {
        /**
         * {@inheritDoc}
         * 
         * @see org.eclipse.emf.compare.merge.IMerger#isMergerFor(org.eclipse.emf.compare.Diff)
         */
        public boolean isMergerFor(Diff target) {
                return target instanceof AttributeChange;
        }

        /**
         * {@inheritDoc}
         * 
         * @see org.eclipse.emf.compare.merge.AbstractMerger#accept(org.eclipse.emf.compare.Diff, boolean)
         */
        @Override
        protected void accept(final Diff diff, boolean rightToLeft) {
                AttributeChange attributeChange = (AttributeChange)diff;
                switch (diff.getKind()) {
                        case ADD:
                                // Create the same element in right
                                addInTarget(attributeChange, rightToLeft);
                                break;
                        case DELETE:
                                // Delete that same element from right
                                removeFromTarget(attributeChange, rightToLeft);
                                break;
                        case MOVE:
                                moveElement(attributeChange, rightToLeft);
                                break;
                        case CHANGE:
                                changeValue(attributeChange, rightToLeft);
                                break;
                        default:
                                break;
                }
        }

        /**
         * {@inheritDoc}
         * 
         * @see org.eclipse.emf.compare.merge.AbstractMerger#reject(org.eclipse.emf.compare.Diff, boolean)
         */
        @Override
        protected void reject(Diff diff, boolean rightToLeft) {
                AttributeChange attributeChange = (AttributeChange)diff;
                switch (diff.getKind()) {
                        case ADD:
                                // We have a ADD on right. we need to revert this addition
                                removeFromTarget(attributeChange, rightToLeft);
                                break;
                        case DELETE:
                                // DELETE in the right. We need to re-create this element
                                addInTarget(attributeChange, rightToLeft);
                                break;
                        case MOVE:
                                moveElement(attributeChange, rightToLeft);
                                break;
                        case CHANGE:
                                changeValue(attributeChange, rightToLeft);
                                break;
                        default:
                                break;
                }
        }

        /**
         * This will be called when we need to create an element in the target side.
         * <p>
         * All necessary sanity checks have been made to ensure that the current operation is one that should
         * create an object in its side or add an objet to an attribute. In other words, either :
         * <ul>
         * <li>We are copying from right to left and
         * <ul>
         * <li>we are copying an addition to the right side (we need to create the same object in the left), or
         * </li>
         * <li>we are copying a deletion from the left side (we need to revert the deletion).</li>
         * </ul>
         * </li>
         * <li>We are copying from left to right and
         * <ul>
         * <li>we are copying a deletion from the right side (we need to revert the deletion), or</li>
         * <li>we are copying an addition to the left side (we need to create the same object in the right).</li>
         * </ul>
         * </li>
         * </ul>
         * </p>
         * 
         * @param diff
         *            The diff we are currently merging.
         * @param rightToLeft
         *            Tells us whether we are to add an object on the left or right side.
         */
        @SuppressWarnings("unchecked")
        protected void addInTarget(AttributeChange diff, boolean rightToLeft) {
                final Match match = diff.getMatch();
                final EObject targetContainer = getTargetContainer(diff, rightToLeft);

                if (targetContainer == null) {
                        throw new IllegalStateException(
                                        "Couldn't add in target because its parent hasn't been merged yet: " + diff); //$NON-NLS-1$
                }
                final Comparison comparison = match.getComparison();
                final EStructuralFeature attribute = diff.getAttribute();
                final Object expectedValue = diff.getValue();
                // We have the container, attribute and value. We need to know the insertion index.
                if (attribute.isMany()) {
                        final int insertionIndex = findInsertionIndex(comparison, diff, rightToLeft);

                        final List<Object> targetList = (List<Object>)safeEGet(targetContainer, attribute);
                        addAt(targetList, expectedValue, insertionIndex);
                } else {
                        safeESet(targetContainer, attribute, expectedValue);
                }
        }

        /**
         * This will be called when we need to remove an element from the target side.
         * <p>
         * All necessary sanity checks have been made to ensure that the current operation is one that should
         * delete an object. In other words, we are :
         * <ul>
         * <li>Copying from right to left and either
         * <ul>
         * <li>we are copying a deletion from the right side (we need to remove the same object in the left) or,
         * </li>
         * <li>we are copying an addition to the left side (we need to revert the addition).</li>
         * </ul>
         * </li>
         * <li>Copying from left to right and either
         * <ul>
         * <li>we are copying an addition to the right side (we need to revert the addition), or.</li>
         * <li>we are copying a deletion from the left side (we need to remove the same object in the right).</li>
         * </ul>
         * </li>
         * </ul>
         * </p>
         * 
         * @param diff
         *            The diff we are currently merging.
         * @param rightToLeft
         *            Tells us whether we are to add an object on the left or right side.
         */
        @SuppressWarnings("unchecked")
        protected void removeFromTarget(AttributeChange diff, boolean rightToLeft) {
                final EObject currentContainer = getTargetContainer(diff, rightToLeft);

                if (currentContainer != null) {
                        final Object expectedValue = diff.getValue();
                        final EStructuralFeature attribute = diff.getAttribute();
                        // We have the container, attribute and value to remove.
                        if (attribute.isMany()) {
                                if (attribute.isUnique()) {
                                        final List<Object> targetList = (List<Object>)safeEGet(currentContainer, attribute);
                                        targetList.remove(expectedValue);
                                } else {
                                        removeNonUniqueFromTarget(diff, rightToLeft);
                                }
                        } else {
                                currentContainer.eUnset(attribute);
                        }
                }
        }

        @SuppressWarnings("unchecked")
        private void removeNonUniqueFromTarget(AttributeChange diff, boolean rightToLeft) {
                Comparison comparison = diff.getMatch().getComparison();
                IEqualityHelper equalityHelper = comparison.getEqualityHelper();
                EAttribute attribute = diff.getAttribute();

                EObject sourceContainer;
                EObject targetContainer;
                if (rightToLeft) {
                        sourceContainer = diff.getMatch().getRight();
                        targetContainer = diff.getMatch().getLeft();
                } else {
                        sourceContainer = diff.getMatch().getLeft();
                        targetContainer = diff.getMatch().getRight();
                }

                List<Object> sourceList = (List<Object>)safeEGet(sourceContainer, attribute);
                List<Object> targetList = (List<Object>)safeEGet(targetContainer, attribute);
                Object valueToRemove = diff.getValue();

                List<Object> lcs = DiffUtil.longestCommonSubsequence(comparison, sourceList, targetList);

                // The current index, in the target list, of that value
                int currentIndexInTarget = -1;
                Iterator<Object> lcsIteratorForTargetLookup = lcs.iterator();
                Object currentLCS = null;
                if (lcsIteratorForTargetLookup.hasNext()) {
                        currentLCS = lcsIteratorForTargetLookup.next();
                }
                for (int j = 0; j < targetList.size() && currentIndexInTarget == -1; j++) {
                        if (currentLCS == null) {
                                // we've iterated on our whole LCS.
                                // first instance of our target is the one to move.
                                if (equalityHelper.matchingAttributeValues(targetList.get(j), valueToRemove)) {
                                        currentIndexInTarget = j;
                                }
                        } else if (equalityHelper.matchingAttributeValues(targetList.get(j), currentLCS)) {
                                // this matches our current lcs item. continue to next one if any
                                if (lcsIteratorForTargetLookup.hasNext()) {
                                        currentLCS = lcsIteratorForTargetLookup.next();
                                } else {
                                        currentLCS = null;
                                }
                        } else {
                                // This item is not in our LCS. Is it the one we're looking for?
                                if (equalityHelper.matchingAttributeValues(targetList.get(j), valueToRemove)) {
                                        currentIndexInTarget = j;
                                }
                        }
                }
                // value might not exist in target list if it has already been removed or if this was a pseudo
                // conflict deletion
                if (currentIndexInTarget >= 0) {
                        targetList.remove(currentIndexInTarget);
                }
        }

        /**
         * This will be called when trying to copy a "MOVE" diff.
         * 
         * @param diff
         *            The diff we are currently merging.
         * @param rightToLeft
         *            Whether we should move the value in the left or right side.
         */
        protected void moveElement(AttributeChange diff, boolean rightToLeft) {
                final EObject expectedContainer = getTargetContainer(diff, rightToLeft);

                if (expectedContainer == null) {
                        throw new IllegalStateException(
                                        "Couldn't move element because its target parent hasn't been merged yet: " + diff); //$NON-NLS-1$
                }
                final Comparison comparison = diff.getMatch().getComparison();
                final Object expectedValue = diff.getValue();

                // We now know the target container, target attribute and target value.
                doMove(diff, comparison, expectedContainer, expectedValue, rightToLeft);
        }

        /**
         * This will do the actual work of moving the element into its attribute. All sanity checks were made in
         * {@link #moveElement(boolean)} and no more verification will be made here.
         * 
         * @param diff
         *            The diff we are currently merging.
         * @param comparison
         *            Comparison holding this Diff.
         * @param expectedContainer
         *            The container in which we are reorganizing an attribute.
         * @param expectedValue
         *            The value that is to be moved within its attribute.
         * @param rightToLeft
         *            Whether we should move the value in the left or right side.
         */
        protected void doMove(AttributeChange diff, Comparison comparison, EObject expectedContainer,
                        Object expectedValue, boolean rightToLeft) {
                final EStructuralFeature attribute = diff.getAttribute();
                if (attribute.isMany()) {
                        if (!attribute.isUnique()) {
                                doMoveNonUniqueAttribute(diff, comparison, rightToLeft);
                        } else {
                                doMoveUniqueAttribute(diff, comparison, expectedContainer, expectedValue, rightToLeft);
                        }
                } else {
                        // This will never happen with the default diff engine, but may still be done from extenders
                        safeESet(expectedContainer, attribute, expectedValue);
                }
        }

        @SuppressWarnings("unchecked")
        private void doMoveUniqueAttribute(AttributeChange diff, Comparison comparison, EObject expectedContainer,
                        Object expectedValue, boolean rightToLeft) {
                final EStructuralFeature attribute = diff.getAttribute();

                // Element to move cannot be part of the LCS... or there would not be a MOVE diff
                int insertionIndex = findInsertionIndex(comparison, diff, rightToLeft);
                /*
                 * However, it could still have been located "before" its new index, in which case we need to take it
                 * into account.
                 */
                final List<Object> targetList = (List<Object>)safeEGet(expectedContainer, attribute);
                final int currentIndex = targetList.indexOf(expectedValue);
                if (insertionIndex > currentIndex) {
                        insertionIndex--;
                }

                if (targetList instanceof EList<?>) {
                        if (insertionIndex < 0 || insertionIndex >= targetList.size()) {
                                ((EList<Object>)targetList).move(targetList.size() - 1, expectedValue);
                        } else {
                                ((EList<Object>)targetList).move(insertionIndex, expectedValue);
                        }
                } else {
                        targetList.remove(expectedValue);
                        if (insertionIndex < 0 || insertionIndex > targetList.size()) {
                                targetList.add(expectedValue);
                        } else {
                                targetList.add(insertionIndex, expectedValue);
                        }
                }
        }

        @SuppressWarnings("unchecked")
        private void doMoveNonUniqueAttribute(AttributeChange diff, Comparison comparison, boolean rightToLeft) {
                IEqualityHelper equalityHelper = comparison.getEqualityHelper();
                EAttribute attribute = diff.getAttribute();

                EObject sourceContainer;
                EObject targetContainer;
                if (rightToLeft) {
                        sourceContainer = diff.getMatch().getRight();
                        targetContainer = diff.getMatch().getLeft();
                } else {
                        sourceContainer = diff.getMatch().getLeft();
                        targetContainer = diff.getMatch().getRight();
                }

                List<Object> sourceList = (List<Object>)safeEGet(sourceContainer, attribute);
                List<Object> targetList = (List<Object>)safeEGet(targetContainer, attribute);
                // copy this one : we'll have to modify target while iterating over this view
                List<Object> copyTarget = new ArrayList<>(targetList);
                Object valueToMove = diff.getValue();

                Set<Object> ignoredElements = DiffUtil.computeIgnoredElements(comparison, equalityHelper, targetList,
                                diff, rightToLeft);
                // We're "moving" an element, so it is present on both sides and should not be part of the LCS
                // computation since it is our move target. However, if we also have a diff on that same value, we do
                // not have to ignore it.
                Iterator<Diff> siblingDiffs = diff.getMatch().getDifferences().stream()
                                .filter(AttributeChange.class::isInstance)
                                .filter(d -> d.getState() == DifferenceState.UNRESOLVED && equalityHelper
                                                .matchingAttributeValues(valueToMove, ((AttributeChange)d).getValue()))
                                .iterator();
                boolean ignoreValue = true;
                while (siblingDiffs.hasNext()) {
                        Diff sibling = siblingDiffs.next();
                        if (sibling.getKind() == DifferenceKind.ADD) {
                                // We have another duplicate of that value on this side that corresponds to none on the other
                                if (sibling.getSource() == DifferenceSource.LEFT && rightToLeft) {
                                        ignoreValue = false;
                                } else if (sibling.getSource() == DifferenceSource.RIGHT && !rightToLeft) {
                                        ignoreValue = false;
                                }
                        } else if (sibling.getKind() == DifferenceKind.DELETE) {
                                // reverse the above
                                if (sibling.getSource() == DifferenceSource.LEFT && !rightToLeft) {
                                        ignoreValue = false;
                                } else if (sibling.getSource() == DifferenceSource.RIGHT && rightToLeft) {
                                        ignoreValue = false;
                                }
                        }
                }
                if (ignoreValue) {
                        if (ignoredElements.isEmpty()) {
                                ignoredElements = Collections.singleton(valueToMove);
                        } else {
                                ignoredElements.add(valueToMove);
                        }
                }
                List<Object> lcs = DiffUtil.longestCommonSubsequence(comparison, ignoredElements, sourceList,
                                copyTarget);

                // We need to find the current index, in the source list, of the value to move
                int currentIndexInSource = -1;
                Iterator<Object> lcsIteratorForSourceLookup = lcs.iterator();
                Object currentLCS = null;
                if (lcsIteratorForSourceLookup.hasNext()) {
                        currentLCS = lcsIteratorForSourceLookup.next();
                }
                for (int j = 0; j < sourceList.size() && currentIndexInSource == -1; j++) {
                        if (currentLCS == null) {
                                // we've iterated on our whole LCS.
                                // first instance of our target is the one to move.
                                if (equalityHelper.matchingAttributeValues(sourceList.get(j), valueToMove)) {
                                        currentIndexInSource = j;
                                }
                        } else if (equalityHelper.matchingAttributeValues(sourceList.get(j), currentLCS)) {
                                // this matches our current lcs item. continue to next one if any
                                if (lcsIteratorForSourceLookup.hasNext()) {
                                        currentLCS = lcsIteratorForSourceLookup.next();
                                } else {
                                        currentLCS = null;
                                }
                        } else {
                                // This item is not in our LCS. Is it the one we're looking for?
                                if (equalityHelper.matchingAttributeValues(sourceList.get(j), valueToMove)) {
                                        currentIndexInSource = j;
                                }
                        }
                }
                // The current index, in the target list, of that value
                int currentIndexInTarget = -1;
                Iterator<Object> lcsIteratorForTargetLookup = lcs.iterator();
                currentLCS = null;
                if (lcsIteratorForTargetLookup.hasNext()) {
                        currentLCS = lcsIteratorForTargetLookup.next();
                }
                for (int j = 0; j < targetList.size() && currentIndexInTarget == -1; j++) {
                        if (currentLCS == null) {
                                // we've iterated on our whole LCS.
                                // first instance of our target is the one to move.
                                if (equalityHelper.matchingAttributeValues(targetList.get(j), valueToMove)) {
                                        currentIndexInTarget = j;
                                }
                        } else if (equalityHelper.matchingAttributeValues(targetList.get(j), currentLCS)) {
                                // this matches our current lcs item. continue to next one if any
                                if (lcsIteratorForTargetLookup.hasNext()) {
                                        currentLCS = lcsIteratorForTargetLookup.next();
                                } else {
                                        currentLCS = null;
                                }
                        } else {
                                // This item is not in our LCS. Is it the one we're looking for?
                                if (equalityHelper.matchingAttributeValues(targetList.get(j), valueToMove)) {
                                        currentIndexInTarget = j;
                                }
                        }
                }

                // Then the index, in the target, at which this value needs to be moved
                int insertionIndex = DiffUtil.findInsertionIndexForElementAt(comparison, sourceList, copyTarget, lcs,
                                currentIndexInSource);
                /*
                 * However, it could still have been located "before" its new index, in which case we need to take it
                 * into account.
                 */
                if (insertionIndex > currentIndexInTarget) {
                        insertionIndex--;
                }

                if (targetList instanceof EList<?>) {
                        if (insertionIndex < 0 || insertionIndex >= targetList.size()) {
                                ((EList<Object>)targetList).move(targetList.size() - 1, currentIndexInTarget);
                        } else {
                                ((EList<Object>)targetList).move(insertionIndex, currentIndexInTarget);
                        }
                } else {
                        targetList.remove(currentIndexInTarget);
                        if (insertionIndex < 0 || insertionIndex > targetList.size()) {
                                targetList.add(valueToMove);
                        } else {
                                targetList.add(insertionIndex, valueToMove);
                        }
                }
        }

        /**
         * This will be called by the merge operations in order to reset an attribute to its original value, be
         * that the left or right side.
         * <p>
         * Should never be called on multi-valued attributes.
         * </p>
         * 
         * @param diff
         *            The diff we are currently merging.
         * @param rightToLeft
         *            Tells us the direction of this merge operation.
         * @deprecated this has been refactored into {@link #changeValue(AttributeChange, boolean)}.
         */
        @Deprecated
        protected void resetInTarget(AttributeChange diff, boolean rightToLeft) {
                final EStructuralFeature attribute = diff.getAttribute();
                final EObject targetContainer = getTargetContainer(diff, rightToLeft);
                final EObject sourceContainer = getSourceContainer(diff, rightToLeft);

                if (sourceContainer == null || !safeEIsSet(targetContainer, attribute)
                                || !safeEIsSet(sourceContainer, attribute)) {
                        targetContainer.eUnset(attribute);
                } else {
                        final Object expectedValue = safeEGet(sourceContainer, attribute);
                        safeESet(targetContainer, attribute, expectedValue);
                }
        }

        /**
         * This will be called by the merge operations in order to change single-valued attributes.
         * 
         * @param diff
         *            The diff we are currently merging.
         * @param rightToLeft
         *            Direction of the merge.
         */
        protected void changeValue(AttributeChange diff, boolean rightToLeft) {
                final EObject targetContainer = getTargetContainer(diff, rightToLeft);
                if (targetContainer == null) {
                        // We're merging the unset of an attribute value under a containment deletion.
                        // There's actually no action to take in such a case.
                        return;
                }

                final EStructuralFeature attribute = diff.getAttribute();
                final Object targetValue = getTargetValue(diff, rightToLeft);

                if (isUnset(diff, targetValue)) {
                        targetContainer.eUnset(attribute);
                } else {
                        safeESet(targetContainer, attribute, targetValue);
                }
        }

        /**
         * Returns the target value, that is, the value to be set when merging the given {@code diff} in the
         * direction indicated by {@code rightToLeft}.
         * 
         * @param diff
         *            The diff we are currently merging.
         * @param rightToLeft
         *            Direction of the merge.
         * @return The target value to be set when merging.
         */
        private Object getTargetValue(AttributeChange diff, boolean rightToLeft) {
                final EStructuralFeature attribute = diff.getAttribute();
                final EObject targetContainer = getTargetContainer(diff, rightToLeft);
                final EObject sourceContainer = getSourceContainer(diff, rightToLeft);
                final Object sourceValue = safeEGet(sourceContainer, attribute);

                final Object targetValue;
                if (isEnumChangeOfDynamicEObject(diff, targetContainer)) {
                        final EEnum eEnum = getAttributeTypeEnumFromDynamicObject(targetContainer, attribute);
                        targetValue = eEnum.getEEnumLiteral(((ENamedElement)sourceValue).getName());
                } else if (requireThreeWayTextMerge(diff)) {
                        targetValue = performThreeWayTextMerge(diff, rightToLeft);
                } else {
                        targetValue = sourceValue;
                }

                return targetValue;
        }

        /**
         * Specifies whether the given {@code diff} concerns a change of a dynamic EObject at an attribute with an
         * EEnum type.
         * 
         * @param diff
         *            The diff to check.
         * @param targetContainer
         *            The target container.
         * @return <code>true</code> if is a change of a dynamic EObject with an EEnum-typed attribute,
         *         <code>false</code> otherwise.
         */
        private boolean isEnumChangeOfDynamicEObject(AttributeChange diff, EObject targetContainer) {
                return diff.getAttribute().getEType() instanceof EEnum
                                && targetContainer instanceof DynamicEObjectImpl;
        }

        /**
         * Returns the EEnum that is the attribute type of the given {@code attribute} used in the given container
         * object {@code dynamicObject}.
         * 
         * @param dynamicObject
         *            The container object.
         * @param attribute
         *            The attribute.
         * @return The EEnum acting as type of {@code attribute}.
         */
        private EEnum getAttributeTypeEnumFromDynamicObject(EObject dynamicObject, EStructuralFeature attribute) {
                return (EEnum)((EEnumLiteral)safeEGet(dynamicObject, attribute)).eContainer();
        }

        /**
         * Returns the source container, that is the original container holding the original value before the
         * given {@code diff} is applied. This is different depending on direction indicated by
         * {@code rightToLeft}.
         * 
         * @param diff
         *            The diff we are currently merging.
         * @param rightToLeft
         *            Direction of the merge.
         * @return The source container.
         */
        private EObject getSourceContainer(AttributeChange diff, boolean rightToLeft) {
                final EObject sourceContainer;
                final Match match = diff.getMatch();
                if (match.getComparison().isThreeWay() && isRejectingChange(diff, rightToLeft)) {
                        sourceContainer = match.getOrigin();
                } else if (rightToLeft) {
                        sourceContainer = match.getRight();
                } else {
                        sourceContainer = match.getLeft();
                }
                return sourceContainer;
        }

        /**
         * Returns the target container, that is the container holding the value to be updated when merging the
         * given {@code diff} in the direction indicated by {@code rightToLeft}.
         * 
         * @param diff
         *            The diff we are currently merging.
         * @param rightToLeft
         *            Direction of the merge.
         * @return The target container to be updated when merging.
         */
        private EObject getTargetContainer(AttributeChange diff, boolean rightToLeft) {
                final Match match = diff.getMatch();
                if (rightToLeft) {
                        return match.getLeft();
                } else {
                        return match.getRight();
                }
        }

        /**
         * Specifies whether the given {@code diff} unsets the attribute value when updating the attribute with
         * the given {@code targetValue}.
         * 
         * @param diff
         *            The difference to check.
         * @param targetValue
         *            The value to be set.
         * @return <code>true</code> if setting {@code targetValue} is an unset, <code>false</code> otherwise.
         */
        private boolean isUnset(AttributeChange diff, Object targetValue) {
                final Object defaultValue = diff.getAttribute().getDefaultValue();
                return targetValue == null || targetValue.equals(defaultValue)
                                || (defaultValue == null && "".equals(targetValue)); //$NON-NLS-1$
        }

        /**
         * Specifies whether a three-way text merge is required for applying the given {@code diff} in the
         * direction indicated in {@code rightToLeft}.
         * <p>
         * Three-way text merging is required when accepting or rejecting the changes of a
         * {@link #isStringAttribute(EAttribute) String attributes} in a three-way merge scenario.
         * </p>
         * 
         * @param diff
         *            The diff to be applied.
         * @return <code>true</code> if three-way text merging is required, <code>false</code> otherwise.
         */
        private boolean requireThreeWayTextMerge(AttributeChange diff) {
                return diff.getMatch().getComparison().isThreeWay() && isStringAttribute(diff.getAttribute());
        }

        /**
         * Specifies whether the given {@code attribute} is an attribute of type String.
         * 
         * @param attribute
         *            The attribute to be checked.
         * @return <code>true</code> if it is a String attribute, <code>false</code> otherwise.
         */
        private boolean isStringAttribute(EAttribute attribute) {
                return attribute.getEAttributeType().getInstanceClass() == String.class;
        }

        /**
         * Specifies whether applying the given {@code diff} in the direction indicated in {@code rightToLeft}
         * means accepting the change as opposed to rejecting the change.
         * 
         * @param diff
         *            The diff to be checked.
         * @param rightToLeft
         *            The direction of applying {@code diff}.
         * @return <code>true</code> if it means accepting the change, <code>false</code> otherwise.
         */
        private boolean isAcceptingChange(AttributeChange diff, boolean rightToLeft) {
                return (DifferenceSource.LEFT.equals(diff.getSource()) && !rightToLeft)
                                || (DifferenceSource.RIGHT.equals(diff.getSource()) && rightToLeft);
        }

        /**
         * Specifies whether applying the given {@code diff} in the direction indicated in {@code rightToLeft}
         * means rejecting the change as opposed to accepting the change.
         * 
         * @param diff
         *            The diff to be checked.
         * @param rightToLeft
         *            The direction of applying {@code diff}.
         * @return <code>true</code> if it means rejecting the change, <code>false</code> otherwise.
         */
        private boolean isRejectingChange(AttributeChange diff, boolean rightToLeft) {
                return !isAcceptingChange(diff, rightToLeft);
        }

        /**
         * Performs a three-way text merge for the given {@code diff} and returns the merged text.
         * <p>
         * Depending on whether the given {@code diff} is an accept or reject in the context of the merge
         * direction indicated by {@code rightToLeft}, this method will perform different strategies of merging.
         * </p>
         * 
         * @param diff
         *            The diff for which a three-way text diff is to be performed.
         * @param rightToLeft
         *            The direction of applying the {@code diff}.
         * @return The merged text.
         */
        private String performThreeWayTextMerge(AttributeChange diff, boolean rightToLeft) {
                if (isAcceptingChange(diff, rightToLeft)) {
                        return performAcceptingThreeWayTextMerge(diff);
                } else {
                        return performRejectingThreeWayTextMerge(diff, rightToLeft);
                }
        }

        /**
         * Performs a three-way text merge accepting the given {@code diff} and returns the merged text.
         * <p>
         * This method must only be called for {@link #isStringAttribute(EAttribute) String attributes}. As the
         * three-way text merging is symmetric, the result is equal irrespectively of the direction of merging as
         * long as applying the change {@link #isAcceptingChange(AttributeChange, boolean) means accepting it} as
         * opposed to rejecting it.
         * </p>
         * 
         * @param diff
         *            The diff for which a three-way text diff is to be performed.
         * @return The merged text.
         */
        private String performAcceptingThreeWayTextMerge(AttributeChange diff) {
                final Match match = diff.getMatch();
                final EAttribute attribute = diff.getAttribute();
                final String originValue = (String)safeEGet(match.getOrigin(), attribute);
                final String leftValue = (String)safeEGet(match.getLeft(), attribute);
                final String rightValue = (String)safeEGet(match.getRight(), attribute);

                return performThreeWayTextMerge(leftValue, rightValue, originValue);
        }

        /**
         * Performs a three-way text merge rejecting the given {@code diff} and returns the merged text.
         * <p>
         * When rejecting an attribute change, we need to undo its effects on the attribute value, which is in
         * most of the cases done by setting the value to the original value. However, if there is a concurrent
         * attribute change of the same attribute value at the opposite side, it might have been merged to the
         * current side already. Therefore, we need to undo only those differences in the attribute value that
         * come from the current to-be-rejected diff.
         * </p>
         * <p>
         * This is done by applying a normal three-way merge, but instead of the origin value, left value, and
         * right value, we compute the three-way merge as follows: as origin value, we use the value of the
         * {@code diff}, which is the value as it was set on the respective side. As a left value, the value of
         * the current side as it is currently stored in the model; thus, a potential merging of opposite diffs
         * may have changed this value already. And as a right value, we use the actual origin value from the
         * origin model.
         * </p>
         * <p>
         * Since we consider the current value as it is in the model right now (including potential previous
         * merges) and the origin value as the two changed sides, a three-way merge will apply the changes we did
         * through merging and reset all other to the origin value.
         * </p>
         * <p>
         * Note that, if {@code diff} is an unset (that is, the current value is empty or null or default), we
         * just use the original value.
         * </p>
         * 
         * @param diff
         *            The diff for which a three-way text diff is to be performed.
         * @param rightToLeft
         *            The direction of applying the {@code diff}.
         * @return The merged text.
         */
        private String performRejectingThreeWayTextMerge(AttributeChange diff, boolean rightToLeft) {
                final EAttribute attribute = diff.getAttribute();
                final EObject originContainer = diff.getMatch().getOrigin();
                final String originValue = (String)safeEGet(originContainer, attribute);
                final String changedValueFromModel = getChangedValueFromModel(diff);
                final String changedValue = (String)diff.getValue();

                if (isUnset(diff, changedValueFromModel)) {
                        return originValue;
                } else {
                        return performThreeWayTextMerge(changedValueFromModel, originValue, changedValue);
                }
        }

        /**
         * Returns the changed value, as it is right now stored in the model, of the attribute that is affected by
         * the given {@code diff}.
         * 
         * @param diff
         *            The diff to get the changed value for.
         * @return The changed value.
         */
        private String getChangedValueFromModel(AttributeChange diff) {
                final EAttribute attribute = diff.getAttribute();
                final EObject changedContainer;
                switch (diff.getSource()) {
                        case LEFT:
                                changedContainer = diff.getMatch().getLeft();
                                break;
                        case RIGHT:
                                changedContainer = diff.getMatch().getRight();
                                break;
                        default:
                                throw new IllegalArgumentException();
                }
                return (String)safeEGet(changedContainer, attribute);
        }

        /**
         * Performs a three-way text merge for the given {@code origin}, {@code left}, and {@code right} text
         * versions.
         * 
         * @param left
         *            The left version of the String.
         * @param right
         *            The right version of the String.
         * @param origin
         *            The original version of the String.
         * @return The merged version.
         * @since 3.2
         */
        protected String performThreeWayTextMerge(final String left, final String right, final String origin) {
                ThreeWayTextDiff textDiff = new ThreeWayTextDiff(origin, left, right);
                return textDiff.getMerged();
        }

        /**
         * This will be used by the distinct merge actions in order to find the index at which a value should be
         * inserted in its target list. See {@link DiffUtil#findInsertionIndex(Comparison, Diff, boolean)} for
         * more on this.
         * <p>
         * Sub-classes can override this if the insertion order is irrelevant. A return value of {@code -1} will
         * be considered as "no index" and the value will be inserted at the end of its target list.
         * </p>
         * 
         * @param comparison
         *            This will be used in order to retrieve the Match for EObjects when comparing them.
         * @param diff
         *            The diff which merging will trigger the need for an insertion index in its target list.
         * @param rightToLeft
         *            {@code true} if the merging will be done into the left list, so that we should consider the
         *            right model as the source and the left as the target.
         * @return The index at which this {@code diff}'s value should be inserted into the 'target' list, as
         *         inferred from {@code rightToLeft}. {@code -1} if the value should be inserted at the end of its
         *         target list.
         * @see DiffUtil#findInsertionIndex(Comparison, Diff, boolean)
         */
        protected int findInsertionIndex(Comparison comparison, Diff diff, boolean rightToLeft) {
                return DiffUtil.findInsertionIndex(comparison, diff, rightToLeft);
        }
}