MergeAlgorithm.java

/*
 * Copyright (C) 2009, Christian Halstrick <christian.halstrick@sap.com> and others
 *
 * This program and the accompanying materials are made available under the
 * terms of the Eclipse Distribution License v. 1.0 which is available at
 * https://www.eclipse.org/org/documents/edl-v10.php.
 *
 * SPDX-License-Identifier: BSD-3-Clause
 */

package org.eclipse.jgit.merge;

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

import org.eclipse.jgit.diff.DiffAlgorithm;
import org.eclipse.jgit.diff.Edit;
import org.eclipse.jgit.diff.EditList;
import org.eclipse.jgit.diff.HistogramDiff;
import org.eclipse.jgit.diff.Sequence;
import org.eclipse.jgit.diff.SequenceComparator;
import org.eclipse.jgit.merge.MergeChunk.ConflictState;

/**
 * Provides the merge algorithm which does a three-way merge on content provided
 * as RawText. By default {@link org.eclipse.jgit.diff.HistogramDiff} is used as
 * diff algorithm.
 */
public final class MergeAlgorithm {
    private final DiffAlgorithm diffAlg;

    /**
     * Creates a new MergeAlgorithm which uses
     * {@link org.eclipse.jgit.diff.HistogramDiff} as diff algorithm
     */
    public MergeAlgorithm() {
        this(new HistogramDiff());
    }

    /**
     * Creates a new MergeAlgorithm
     *
     * @param diff
     *            the diff algorithm used by this merge
     */
    public MergeAlgorithm(DiffAlgorithm diff) {
        this.diffAlg = diff;
    }

    // An special edit which acts as a sentinel value by marking the end the
    // list of edits
    private static final Edit END_EDIT = new Edit(Integer.MAX_VALUE,
            Integer.MAX_VALUE);

    @SuppressWarnings("ReferenceEquality")
    private static boolean isEndEdit(Edit edit) {
        return edit == END_EDIT;
    }

    /**
     * Does the three way merge between a common base and two sequences.
     *
     * @param cmp comparison method for this execution.
     * @param base the common base sequence
     * @param ours the first sequence to be merged
     * @param theirs the second sequence to be merged
     * @return the resulting content
     */
    public <S extends Sequence> MergeResult<S> merge(
            SequenceComparator<S> cmp, S base, S ours, S theirs) {
        List<S> sequences = new ArrayList<>(3);
        sequences.add(base);
        sequences.add(ours);
        sequences.add(theirs);
        MergeResult<S> result = new MergeResult<>(sequences);

        if (ours.size() == 0) {
            if (theirs.size() != 0) {
                EditList theirsEdits = diffAlg.diff(cmp, base, theirs);
                if (!theirsEdits.isEmpty()) {
                    // we deleted, they modified -> Let their complete content
                    // conflict with empty text
                    result.add(1, 0, 0, ConflictState.FIRST_CONFLICTING_RANGE);
                    result.add(2, 0, theirs.size(),
                            ConflictState.NEXT_CONFLICTING_RANGE);
                } else
                    // we deleted, they didn't modify -> Let our deletion win
                    result.add(1, 0, 0, ConflictState.NO_CONFLICT);
            } else
                // we and they deleted -> return a single chunk of nothing
                result.add(1, 0, 0, ConflictState.NO_CONFLICT);
            return result;
        } else if (theirs.size() == 0) {
            EditList oursEdits = diffAlg.diff(cmp, base, ours);
            if (!oursEdits.isEmpty()) {
                // we modified, they deleted -> Let our complete content
                // conflict with empty text
                result.add(1, 0, ours.size(),
                        ConflictState.FIRST_CONFLICTING_RANGE);
                result.add(2, 0, 0, ConflictState.NEXT_CONFLICTING_RANGE);
            } else
                // they deleted, we didn't modify -> Let their deletion win
                result.add(2, 0, 0, ConflictState.NO_CONFLICT);
            return result;
        }

        EditList oursEdits = diffAlg.diff(cmp, base, ours);
        Iterator<Edit> baseToOurs = oursEdits.iterator();
        EditList theirsEdits = diffAlg.diff(cmp, base, theirs);
        Iterator<Edit> baseToTheirs = theirsEdits.iterator();
        int current = 0; // points to the next line (first line is 0) of base
                         // which was not handled yet
        Edit oursEdit = nextEdit(baseToOurs);
        Edit theirsEdit = nextEdit(baseToTheirs);

        // iterate over all edits from base to ours and from base to theirs
        // leave the loop when there are no edits more for ours or for theirs
        // (or both)
        while (!isEndEdit(theirsEdit) || !isEndEdit(oursEdit)) {
            if (oursEdit.getEndA() < theirsEdit.getBeginA()) {
                // something was changed in ours not overlapping with any change
                // from theirs. First add the common part in front of the edit
                // then the edit.
                if (current != oursEdit.getBeginA()) {
                    result.add(0, current, oursEdit.getBeginA(),
                            ConflictState.NO_CONFLICT);
                }
                result.add(1, oursEdit.getBeginB(), oursEdit.getEndB(),
                        ConflictState.NO_CONFLICT);
                current = oursEdit.getEndA();
                oursEdit = nextEdit(baseToOurs);
            } else if (theirsEdit.getEndA() < oursEdit.getBeginA()) {
                // something was changed in theirs not overlapping with any
                // from ours. First add the common part in front of the edit
                // then the edit.
                if (current != theirsEdit.getBeginA()) {
                    result.add(0, current, theirsEdit.getBeginA(),
                            ConflictState.NO_CONFLICT);
                }
                result.add(2, theirsEdit.getBeginB(), theirsEdit.getEndB(),
                        ConflictState.NO_CONFLICT);
                current = theirsEdit.getEndA();
                theirsEdit = nextEdit(baseToTheirs);
            } else {
                // here we found a real overlapping modification

                // if there is a common part in front of the conflict add it
                if (oursEdit.getBeginA() != current
                        && theirsEdit.getBeginA() != current) {
                    result.add(0, current, Math.min(oursEdit.getBeginA(),
                            theirsEdit.getBeginA()), ConflictState.NO_CONFLICT);
                }

                // set some initial values for the ranges in A and B which we
                // want to handle
                int oursBeginB = oursEdit.getBeginB();
                int theirsBeginB = theirsEdit.getBeginB();
                // harmonize the start of the ranges in A and B
                if (oursEdit.getBeginA() < theirsEdit.getBeginA()) {
                    theirsBeginB -= theirsEdit.getBeginA()
                            - oursEdit.getBeginA();
                } else {
                    oursBeginB -= oursEdit.getBeginA() - theirsEdit.getBeginA();
                }

                // combine edits:
                // Maybe an Edit on one side corresponds to multiple Edits on
                // the other side. Then we have to combine the Edits of the
                // other side - so in the end we can merge together two single
                // edits.
                //
                // It is important to notice that this combining will extend the
                // ranges of our conflict always downwards (towards the end of
                // the content). The starts of the conflicting ranges in ours
                // and theirs are not touched here.
                //
                // This combining is an iterative process: after we have
                // combined some edits we have to do the check again. The
                // combined edits could now correspond to multiple edits on the
                // other side.
                //
                // Example: when this combining algorithm works on the following
                // edits
                // oursEdits=((0-5,0-5),(6-8,6-8),(10-11,10-11)) and
                // theirsEdits=((0-1,0-1),(2-3,2-3),(5-7,5-7))
                // it will merge them into
                // oursEdits=((0-8,0-8),(10-11,10-11)) and
                // theirsEdits=((0-7,0-7))
                //
                // Since the only interesting thing to us is how in ours and
                // theirs the end of the conflicting range is changing we let
                // oursEdit and theirsEdit point to the last conflicting edit
                Edit nextOursEdit = nextEdit(baseToOurs);
                Edit nextTheirsEdit = nextEdit(baseToTheirs);
                for (;;) {
                    if (oursEdit.getEndA() >= nextTheirsEdit.getBeginA()) {
                        theirsEdit = nextTheirsEdit;
                        nextTheirsEdit = nextEdit(baseToTheirs);
                    } else if (theirsEdit.getEndA() >= nextOursEdit.getBeginA()) {
                        oursEdit = nextOursEdit;
                        nextOursEdit = nextEdit(baseToOurs);
                    } else {
                        break;
                    }
                }

                // harmonize the end of the ranges in A and B
                int oursEndB = oursEdit.getEndB();
                int theirsEndB = theirsEdit.getEndB();
                if (oursEdit.getEndA() < theirsEdit.getEndA()) {
                    oursEndB += theirsEdit.getEndA() - oursEdit.getEndA();
                } else {
                    theirsEndB += oursEdit.getEndA() - theirsEdit.getEndA();
                }

                // A conflicting region is found. Strip off common lines in
                // in the beginning and the end of the conflicting region

                // Determine the minimum length of the conflicting areas in OURS
                // and THEIRS. Also determine how much bigger the conflicting
                // area in THEIRS is compared to OURS. All that is needed to
                // limit the search for common areas at the beginning or end
                // (the common areas cannot be bigger then the smaller
                // conflicting area. The delta is needed to know whether the
                // complete conflicting area is common in OURS and THEIRS.
                int minBSize = oursEndB - oursBeginB;
                int BSizeDelta = minBSize - (theirsEndB - theirsBeginB);
                if (BSizeDelta > 0)
                    minBSize -= BSizeDelta;

                int commonPrefix = 0;
                while (commonPrefix < minBSize
                        && cmp.equals(ours, oursBeginB + commonPrefix, theirs,
                                theirsBeginB + commonPrefix))
                    commonPrefix++;
                minBSize -= commonPrefix;
                int commonSuffix = 0;
                while (commonSuffix < minBSize
                        && cmp.equals(ours, oursEndB - commonSuffix - 1, theirs,
                                theirsEndB - commonSuffix - 1))
                    commonSuffix++;
                minBSize -= commonSuffix;

                // Add the common lines at start of conflict
                if (commonPrefix > 0)
                    result.add(1, oursBeginB, oursBeginB + commonPrefix,
                            ConflictState.NO_CONFLICT);

                // Add the conflict (Only if there is a conflict left to report)
                if (minBSize > 0 || BSizeDelta != 0) {
                    result.add(1, oursBeginB + commonPrefix, oursEndB
                            - commonSuffix,
                            ConflictState.FIRST_CONFLICTING_RANGE);
                    result.add(2, theirsBeginB + commonPrefix, theirsEndB
                            - commonSuffix,
                            ConflictState.NEXT_CONFLICTING_RANGE);
                }

                // Add the common lines at end of conflict
                if (commonSuffix > 0)
                    result.add(1, oursEndB - commonSuffix, oursEndB,
                            ConflictState.NO_CONFLICT);

                current = Math.max(oursEdit.getEndA(), theirsEdit.getEndA());
                oursEdit = nextOursEdit;
                theirsEdit = nextTheirsEdit;
            }
        }
        // maybe we have a common part behind the last edit: copy it to the
        // result
        if (current < base.size()) {
            result.add(0, current, base.size(), ConflictState.NO_CONFLICT);
        }
        return result;
    }

    /**
     * Helper method which returns the next Edit for an Iterator over Edits.
     * When there are no more edits left this method will return the constant
     * END_EDIT.
     *
     * @param it
     *            the iterator for which the next edit should be returned
     * @return the next edit from the iterator or END_EDIT if there no more
     *         edits
     */
    private static Edit nextEdit(Iterator<Edit> it) {
        return (it.hasNext() ? it.next() : END_EDIT);
    }
}