content.py

        try:
            return module_dict[classname]
        except AttributeError:
            raise ClassNotFoundError(typename)
    else:
        raise ClassNotFoundError(typename)


def instantiate_flow_page(location, repo, page_desc, commit_sha):
    class_ = get_flow_page_class(repo, page_desc.type, commit_sha)

    return class_(None, location, page_desc)


def _adjust_flow_session_page_data_inner(repo, flow_session,
        course_identifier, flow_desc):
    commit_sha = get_course_commit_sha(
            flow_session.course, flow_session.participation)

    from course.models import FlowPageData

    def remove_page(fpd):
        if fpd.ordinal is not None:
            fpd.ordinal = None
            fpd.save()

    desc_group_ids = []

    ordinal = [0]
    for grp in flow_desc.groups:
        desc_group_ids.append(grp.id)

        shuffle = getattr(grp, "shuffle", False)
        max_page_count = getattr(grp, "max_page_count", None)

        available_page_ids = [page_desc.id for page_desc in grp.pages]

        if max_page_count is None:
            max_page_count = len(available_page_ids)

        group_pages = []

        # {{{ helper functions

        def find_page_desc(page_id):
            new_page_desc = None

            for page_desc in grp.pages:
                if page_desc.id == page_id:
                    new_page_desc = page_desc
                    break

            assert new_page_desc is not None

            return new_page_desc

        def create_fpd(new_page_desc):
            page = instantiate_flow_page(
                    "course '%s', flow '%s', page '%s/%s'"
                    % (course_identifier, flow_session.flow_id,
                        grp.id, new_page_desc.id),
                    repo, new_page_desc, commit_sha)

            return FlowPageData(
                    flow_session=flow_session,
                    ordinal=None,
                    page_type=new_page_desc.type,
                    group_id=grp.id,
                    page_id=new_page_desc.id,
                    data=page.make_page_data())

        def add_page(fpd):
            if fpd.ordinal != ordinal[0]:
                fpd.ordinal = ordinal[0]
                fpd.save()

            ordinal[0] += 1
            available_page_ids.remove(fpd.page_id)
            group_pages.append(fpd)

        # }}}

        if shuffle:
            # maintain order of existing pages as much as possible
            for fpd in (FlowPageData.objects
                    .filter(
                        flow_session=flow_session,
                        group_id=grp.id,
                        ordinal__isnull=False)
                    .order_by("ordinal")):

                if (fpd.page_id in available_page_ids
                        and len(group_pages) < max_page_count):
                    add_page(fpd)
                else:
                    remove_page(fpd)

            assert len(group_pages) <= max_page_count

            from random import choice

            # then add randomly chosen new pages
            while len(group_pages) < max_page_count and available_page_ids:
                new_page_id = choice(available_page_ids)

                new_page_fpds = (FlowPageData.objects
                        .filter(
                            flow_session=flow_session,
                            group_id=grp.id,
                            page_id=new_page_id))

                if new_page_fpds.count():
                    # We already have FlowPageData for this page, revive it
                    new_page_fpd, = new_page_fpds
                    assert new_page_fpd.id == new_page_id
                else:
                    # Make a new FlowPageData instance
                    page_desc = find_page_desc(new_page_id)
                    assert page_desc.id == new_page_id
                    new_page_fpd = create_fpd(page_desc)
                    assert new_page_fpd.page_id == new_page_id

                add_page(new_page_fpd)

        else:
            # reorder pages to order in flow
            id_to_fpd = dict(
                    ((fpd.group_id, fpd.page_id), fpd)
                    for fpd in FlowPageData.objects.filter(
                        flow_session=flow_session,
                        group_id=grp.id))

            for page_desc in grp.pages:
                key = (grp.id, page_desc.id)

                if key in id_to_fpd:
                    fpd = id_to_fpd.pop(key)
                else:
                    fpd = create_fpd(page_desc)

                if len(group_pages) < max_page_count:
                    add_page(fpd)

            for fpd in id_to_fpd.values():
                remove_page(fpd)

    # {{{ remove pages orphaned because of group renames

    for fpd in (
            FlowPageData.objects
            .filter(
                flow_session=flow_session,
                ordinal__isnull=False)
            .exclude(group_id__in=desc_group_ids)
            ):
        remove_page(fpd)

    # }}}

    if flow_session.page_count != ordinal[0]:
        flow_session.page_count = ordinal[0]
        flow_session.save()


def adjust_flow_session_page_data(repo, flow_session,
        course_identifier, flow_desc):
    # The atomicity is not done as a decorator above because we can't import
    # django.db at the module level here. The relate-validate script wants to
    # import this module, and it obviously has no database.

    from django.db import transaction
    with transaction.atomic():
        return _adjust_flow_session_page_data_inner(
                repo, flow_session, course_identifier, flow_desc)


def get_course_commit_sha(course, participation):
    sha = course.active_git_commit_sha

    if participation is not None and participation.preview_git_commit_sha:
        sha = participation.preview_git_commit_sha

    return sha.encode()


def list_flow_ids(repo, commit_sha):
    flow_ids = []
    try:
        flows_tree = get_repo_blob(repo, "flows", commit_sha)
    except ObjectDoesNotExist:
        # That's OK--no flows yet.
        pass
    else:
        for entry in flows_tree.items():
            if entry.path.endswith(b".yml"):
                flow_ids.append(entry.path[:-4])

    return sorted(flow_ids)

# vim: foldmethod=marker