#no remote compare do it on the same repository

            else:

                hgrepo = other_repo._repo

            if org_repo.EMPTY_CHANGESET in (org_rev, other_rev):

                # work around unexpected behaviour in Mercurial < 3.4

                ancestor = org_repo.EMPTY_CHANGESET

            else:

                ancestors = hgrepo.revs("ancestor(id(%s), id(%s))", org_rev, other_rev)

                if ancestors:

                    # FIXME: picks arbitrary ancestor - but there is usually only one

                    try:

                        ancestor = hgrepo[ancestors.first()].hex()

                    except AttributeError:

                        # removed in hg 3.2

                        ancestor = hgrepo[ancestors[0]].hex()

            other_revs = hgrepo.revs("ancestors(id(%s)) and not ancestors(id(%s)) and not id(%s)",

                                     other_rev, org_rev, org_rev)

            other_changesets = [other_repo.get_changeset(rev) for rev in other_revs]

            org_revs = hgrepo.revs("ancestors(id(%s)) and not ancestors(id(%s)) and not id(%s)",

                                   org_rev, other_rev, other_rev)

            org_changesets = [org_repo.get_changeset(hgrepo[rev].hex()) for rev in org_revs]

        elif alias == 'git':

            if org_repo != other_repo:

                from dulwich.repo import Repo

                from dulwich.client import SubprocessGitClient

                gitrepo = Repo(org_repo.path)

                SubprocessGitClient(thin_packs=False).fetch(safe_str(other_repo.path), gitrepo)

                gitrepo_remote = Repo(other_repo.path)

                SubprocessGitClient(thin_packs=False).fetch(safe_str(org_repo.path), gitrepo_remote)

                revs = []

                for x in gitrepo_remote.get_walker(include=[other_rev],

                                                   exclude=[org_rev]):

                    revs.append(x.commit.id)

                other_changesets = [other_repo.get_changeset(rev) for rev in reversed(revs)]

                if other_changesets:

                    ancestor = other_changesets[0].parents[0].raw_id

                else:

                    # no changesets from other repo, ancestor is the other_rev

                    ancestor = other_rev

            else:

                so, se = org_repo.run_git_command(

                    ['log', '--reverse', '--pretty=format:%H',

                     '-s', '%s..%s' % (org_rev, other_rev)]

                )

                other_changesets = [org_repo.get_changeset(cs)

                              for cs in re.findall(r'[0-9a-fA-F]{40}', so)]

                so, se = org_repo.run_git_command(

                    ['merge-base', org_rev, other_rev]

                )

                ancestor = re.findall(r'[0-9a-fA-F]{40}', so)[0]

            org_changesets = []

        else:

            raise Exception('Bad alias only git and hg is allowed')

        return other_changesets, org_changesets, ancestor

    @LoginRequired()

    @HasRepoPermissionAnyDecorator('repository.read', 'repository.write',

                                   'repository.admin')

    def index(self, repo_name):

        c.compare_home = True

        org_repo = c.db_repo.repo_name

        other_repo = request.GET.get('other_repo', org_repo)

        c.a_repo = Repository.get_by_repo_name(org_repo)

        c.cs_repo = Repository.get_by_repo_name(other_repo)

        c.a_ref_name = c.cs_ref_name = _('Select changeset')

        return render('compare/compare_diff.html')

    @LoginRequired()

    @HasRepoPermissionAnyDecorator('repository.read', 'repository.write',

                                   'repository.admin')

    def compare(self, repo_name, org_ref_type, org_ref_name, other_ref_type, other_ref_name):

        org_ref_name = org_ref_name.strip()

        other_ref_name = other_ref_name.strip()

        org_repo = c.db_repo.repo_name

        other_repo = request.GET.get('other_repo', org_repo)

        # If merge is True:

        #   Show what org would get if merged with other:

        #   List changesets that are ancestors of other but not of org.

        #   New changesets in org is thus ignored.

        #   Diff will be from common ancestor, and merges of org to other will thus be ignored.

        # If merge is False:

        #   Make a raw diff from org to other, no matter if related or not.

        #   Changesets in one and not in the other will be ignored