from mercurial import revset

def topicset(repo, subset, x):
    """`topic([topic])`
    Specified topic or all changes with any topic specified.

    If `topic` starts with `re:` the remainder of the name is treated
    as a regular expression.

    TODO: make `topic(revset)` work the same as `branch(revset)`.
    """
    args = revset.getargs(x, 0, 1, 'topic takes one or no arguments')
    if args:
        # match a specific topic
        topic = revset.getstring(args[0], 'topic() argument must be a string')
        _kind, _pattern, matcher = revset._stringmatcher(topic)
    else:
        matcher = lambda t: bool(t)
    drafts = subset.filter(lambda r: repo[r].mutable())
    return drafts.filter(lambda r: matcher(repo[r].extra().get('topic', '')))

def modsetup():
    revset.symbols.update({'topic': topicset})