def read_in_chunks(file_object, chunk_size=1024 * 40):

            """Lazy function (generator) to read a file piece by piece.

            Default chunk size: 40k."""

            while True:

                data = file_object.read(chunk_size)

                if not data:

                    break

                yield data        

        archive = tempfile.TemporaryFile()

        repo = HgModel().get_repo(repo_name).repo

        fname = '%s-%s%s' % (repo_name, revision, fileformat)

        archival.archive(repo, archive, revision, archive_specs[fileformat][1],

                         prefix='%s-%s' % (repo_name, revision))

        response.content_type = archive_specs[fileformat][0]

        response.content_disposition = 'attachment; filename=%s' % fname

        archive.seek(0)

        return read_in_chunks(archive)

    def diff(self, repo_name, f_path):

        hg_model = HgModel()

        diff1 = request.GET.get('diff1')

        diff2 = request.GET.get('diff2')

        c.no_changes = diff1 == diff2

        c.f_path = f_path

        c.repo = hg_model.get_repo(c.repo_name)

        try:

            if diff1 not in ['', None, 'None', '0' * 12]:

                c.changeset_1 = c.repo.get_changeset(diff1)

                node1 = c.changeset_1.get_node(f_path)

            else:

                c.changeset_1 = EmptyChangeset()

                node1 = FileNode('.', '')

            if diff2 not in ['', None, 'None', '0' * 12]:

                c.changeset_2 = c.repo.get_changeset(diff2)

                node2 = c.changeset_2.get_node(f_path)

            else:

                c.changeset_2 = EmptyChangeset()

                node2 = FileNode('.', '') 

        except RepositoryError:

            return redirect(url('files_home',

                                repo_name=c.repo_name, f_path=f_path))

        c.diff1 = 'r%s:%s' % (c.changeset_1.revision, c.changeset_1.raw_id)

        c.diff2 = 'r%s:%s' % (c.changeset_2.revision, c.changeset_2.raw_id)

        f_udiff = differ.get_udiff(node1, node2)

        diff = differ.DiffProcessor(f_udiff)

            diff_name = '%s_vs_%s.diff' % (diff1, diff2)

            response.content_type = 'text/plain'

            response.content_disposition = 'attachment; filename=%s' \

                                                    % diff_name             

            return diff.raw_diff()

            c.cur_diff = '<pre class="raw">%s</pre>' % h.escape(diff.raw_diff())

            c.cur_diff = diff.as_html()

        return render('files/file_diff.html')

    def _get_history(self, repo, node, f_path):

        from vcs.nodes import NodeKind

        if not node.kind is NodeKind.FILE:

            return []

        changesets = node.history

        hist_l = []

        for chs in changesets:

            n_desc = 'r%s:%s' % (chs.revision, chs._short)

            hist_l.append((chs._short, n_desc,))

        return hist_l