def tolist(val):

    """

    a convenience method to return an empty list instead of None

    """

    if val is None:

        return []

    else:

        return [val]

class namespaces(object):

    """provides an interface to register and operate on multiple namespaces. See

    the namespace class below for details on the namespace object.

    """

    _names_version = 0

    def __init__(self):

        self._names = util.sortdict()

        # we need current mercurial named objects (bookmarks, tags, and

        # branches) to be initialized somewhere, so that place is here

        bmknames = lambda repo: repo._bookmarks.keys()

        bmknamemap = lambda repo, name: tolist(repo._bookmarks.get(name))

        bmknodemap = lambda repo, name: repo.nodebookmarks(name)

        n = namespace("bookmarks", templatename="bookmark",

                      # i18n: column positioning for "hg log"

                      logfmt=_("bookmark:    %s\n"),

                      listnames=bmknames,

                      namemap=bmknamemap, nodemap=bmknodemap)

        self.addnamespace(n)

        tagnames = lambda repo: [t for t, n in repo.tagslist()]

        tagnamemap = lambda repo, name: tolist(repo._tagscache.tags.get(name))

        tagnodemap = lambda repo, name: repo.nodetags(name)

        n = namespace("tags", templatename="tag",

                      # i18n: column positioning for "hg log"

                      logfmt=_("tag:         %s\n"),

                      listnames=tagnames,

                      namemap=tagnamemap, nodemap=tagnodemap,

                      deprecated=set(['tip']))

        self.addnamespace(n)

        bnames = lambda repo: repo.branchmap().keys()

        bnamemap = lambda repo, name: tolist(repo.branchtip(name, True))

        bnodemap = lambda repo, node: [repo[node].branch()]

        n = namespace("branches", templatename="branch",

                      # i18n: column positioning for "hg log"

                      logfmt=_("branch:      %s\n"),

                      listnames=bnames,

                      namemap=bnamemap, nodemap=bnodemap)

        self.addnamespace(n)

    def __getitem__(self, namespace):

        """returns the namespace object"""

        return self._names[namespace]

    def __iter__(self):

        return self._names.__iter__()

    def iteritems(self):

        return self._names.iteritems()

    def addnamespace(self, namespace, order=None):

        """register a namespace

        namespace: the name to be registered (in plural form)

        order: optional argument to specify the order of namespaces

               (e.g. 'branches' should be listed before 'bookmarks')

        """

        if order is not None:

            self._names.insert(order, namespace.name, namespace)

        else:

            self._names[namespace.name] = namespace

        # we only generate a template keyword if one does not already exist

        if namespace.name not in templatekw.keywords:

            def generatekw(**args):

                return templatekw.shownames(namespace.name, **args)

            templatekw.keywords[namespace.name] = generatekw

    def singlenode(self, repo, name):

        """

        Return the 'best' node for the given name. Best means the first node

        in the first nonempty list returned by a name-to-nodes mapping function

        in the defined precedence order.

        Raises a KeyError if there is no such node.

        """

        for ns, v in self._names.iteritems():

            n = v.namemap(repo, name)

            if n:

                # return max revision number

                if len(n) > 1:

                    cl = repo.changelog

                    maxrev = max(cl.rev(node) for node in n)

                    return cl.node(maxrev)

                return n[0]

        raise KeyError(_('no such name: %s') % name)

class namespace(object):

    """provides an interface to a namespace

    Namespaces are basically generic many-to-many mapping between some

    (namespaced) names and nodes. The goal here is to control the pollution of

    jamming things into tags or bookmarks (in extension-land) and to simplify

    internal bits of mercurial: log output, tab completion, etc.

    More precisely, we define a mapping of names to nodes, and a mapping from

    nodes to names. Each mapping returns a list.

    Furthermore, each name mapping will be passed a name to lookup which might

    not be in its domain. In this case, each method should return an empty list

    and not raise an error.

    This namespace object will define the properties we need:

      'name': the namespace (plural form)

      'templatename': name to use for templating (usually the singular form

                      of the plural namespace name)

      'listnames': list of all names in the namespace (usually the keys of a

                   dictionary)

      'namemap': function that takes a name and returns a list of nodes

      'nodemap': function that takes a node and returns a list of names

      'deprecated': set of names to be masked for ordinary use

    """

    def __init__(self, name, templatename=None, logname=None, colorname=None,

                 logfmt=None, listnames=None, namemap=None, nodemap=None,

                 deprecated=None):

        """create a namespace

        name: the namespace to be registered (in plural form)

        templatename: the name to use for templating

        logname: the name to use for log output; if not specified templatename

                 is used

        colorname: the name to use for colored log output; if not specified

                   logname is used

        logfmt: the format to use for (i18n-ed) log output; if not specified

                it is composed from logname

        listnames: function to list all names

        namemap: function that inputs a node, output name(s)

        nodemap: function that inputs a name, output node(s)

        deprecated: set of names to be masked for ordinary use

        """

        self.name = name

        self.templatename = templatename

        self.logname = logname

        self.colorname = colorname

        self.logfmt = logfmt

        self.listnames = listnames

        self.namemap = namemap

        self.nodemap = nodemap

        # if logname is not specified, use the template name as backup

        if self.logname is None:

            self.logname = self.templatename

        # if colorname is not specified, just use the logname as a backup

        if self.colorname is None:

            self.colorname = self.logname

        # if logfmt is not specified, compose it from logname as backup

        if self.logfmt is None:

            # i18n: column positioning for "hg log"

            self.logfmt = ("%s:" % self.logname).ljust(13) + "%s\n"

        if deprecated is None:

            self.deprecated = set()

        else:

            self.deprecated = deprecated

    def names(self, repo, node):

        """method that returns a (sorted) list of names in a namespace that

        match a given node"""

        return sorted(self.nodemap(repo, node))

    def nodes(self, repo, name):

        """method that returns a list of nodes in a namespace that

        match a given name.

        """

        return sorted(self.namemap(repo, name))