if org_rev == other_rev:

            org_changesets = []

            other_changesets = []

            ancestor = org_rev

        elif alias == 'hg':

            #case two independent repos

            if org_repo != other_repo:

                hgrepo = unionrepo.unionrepository(other_repo.baseui,

                                                   other_repo.path,

                                                   org_repo.path)

                # all ancestors of other_rev will be in other_repo and

                # rev numbers from hgrepo can be used in other_repo - org_rev ancestors cannot

            #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(other_repo.path, gitrepo)

                gitrepo_remote = Repo(other_repo.path)

                SubprocessGitClient(thin_packs=False).fetch(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_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

        merge = bool(request.GET.get('merge'))

        # fulldiff disables cut_off_limit

        c.fulldiff = request.GET.get('fulldiff')

        # partial uses compare_cs.html template directly

        partial = request.environ.get('HTTP_X_PARTIAL_XHR')

        # as_form puts hidden input field with changeset revisions

        c.as_form = partial and request.GET.get('as_form')

        # swap url for compare_diff page - never partial and never as_form

        c.swap_url = h.url('compare_url',

            repo_name=other_repo,

            org_ref_type=other_ref_type, org_ref_name=other_ref_name,

            other_repo=org_repo,

            other_ref_type=org_ref_type, other_ref_name=org_ref_name,

            merge=merge or '')

        # set callbacks for generating markup for icons

        c.ignorews_url = _ignorews_url

        c.context_url = _context_url

        ignore_whitespace = request.GET.get('ignorews') == '1'

        line_context = request.GET.get('context', 3)

        org_repo = Repository.get_by_repo_name(org_repo)

        other_repo = Repository.get_by_repo_name(other_repo)

        if org_repo is None:

            msg = 'Could not find org repo %s' % org_repo

            log.error(msg)

            h.flash(msg, category='error')

            return redirect(url('compare_home', repo_name=c.repo_name))

        if other_repo is None:

            msg = 'Could not find other repo %s' % other_repo

            log.error(msg)

            h.flash(msg, category='error')

            return redirect(url('compare_home', repo_name=c.repo_name))

        if org_repo.scm_instance.alias != other_repo.scm_instance.alias:

            msg = 'compare of two different kind of remote repos not available'

            log.error(msg)

            h.flash(msg, category='error')

            return redirect(url('compare_home', repo_name=c.repo_name))

        c.a_rev = self._get_ref_rev(org_repo, org_ref_type, org_ref_name,

            returnempty=True)

        c.cs_rev = self._get_ref_rev(other_repo, other_ref_type, other_ref_name)

        c.compare_home = False

        c.a_repo = org_repo

        c.a_ref_name = org_ref_name

        c.a_ref_type = org_ref_type

        c.cs_repo = other_repo

        c.cs_ref_name = other_ref_name

        c.cs_ref_type = other_ref_type

        c.cs_ranges, c.cs_ranges_org, c.ancestor = self._get_changesets(

            org_repo.scm_instance.alias, org_repo.scm_instance, c.a_rev,

            other_repo.scm_instance, c.cs_rev)

        raw_ids = [x.raw_id for x in c.cs_ranges]

        c.cs_comments = other_repo.get_comments(raw_ids)

        c.statuses = other_repo.statuses(raw_ids)

        revs = [ctx.revision for ctx in reversed(c.cs_ranges)]

        c.jsdata = json.dumps(graph_data(c.cs_repo.scm_instance, revs))

        if partial:

            return render('compare/compare_cs.html')

        if merge and c.ancestor:

            # case we want a simple diff without incoming changesets,

            # previewing what will be merged.

            # Make the diff on the other repo (which is known to have other_rev)

            log.debug('Using ancestor %s as rev1 instead of %s',

                      c.ancestor, c.a_rev)

            rev1 = c.ancestor

            org_repo = other_repo

        else: # comparing tips, not necessarily linearly related

            if merge:

                log.error('Unable to find ancestor revision')

            if org_repo != other_repo:

                # TODO: we could do this by using hg unionrepo

                log.error('cannot compare across repos %s and %s', org_repo, other_repo)

                h.flash(_('Cannot compare repositories without using common ancestor'), category='error')

                raise HTTPBadRequest

            rev1 = c.a_rev

        diff_limit = self.cut_off_limit if not c.fulldiff else None