diff -r 8a53f99d9061 -r d49f75eab6a3 src/topic/topicmap.py
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/topic/topicmap.py	Sat Mar 12 15:36:17 2016 +0000
@@ -0,0 +1,61 @@
+from mercurial import branchmap
+
+def _filename(repo):
+    """name of a branchcache file for a given repo or repoview"""
+    filename = "cache/topicmap"
+    if repo.filtername:
+        filename = '%s-%s' % (filename, repo.filtername)
+    return filename
+
+oldbranchcache = branchmap.branchcache
+
+class topiccache(oldbranchcache):
+
+    def __init__(self, *args, **kwargs):
+        otherbranchcache = branchmap.branchcache
+        try:
+            # super() call may fail otherwise
+            branchmap.branchcache = oldbranchcache
+            return super(topiccache, self).__init__(*args, **kwargs)
+        finally:
+            branchmap.branchcache = otherbranchcache
+
+    def branchtip(self, branch, topic=''):
+        '''Return the tipmost open head on branch head, otherwise return the
+        tipmost closed head on branch.
+        Raise KeyError for unknown branch.'''
+        if topic:
+            branch = '%s:%s' % (branch, topic)
+        return super(topiccache, self).branchtip(branch)
+
+    def branchheads(self, branch, closed=False, topic=''):
+        if topic:
+            branch = '%s:%s' % (branch, topic)
+        return super(topiccache, self).branchheads(branch, closed=closed)
+
+    def write(self, repo):
+        # The cache key needs to take phase root in account because change to
+        # what is public affect topics. We can't write on disk until we have this.
+        return
+
+    def update(self, repo, revgen):
+        """Given a branchhead cache, self, that may have extra nodes or be
+        missing heads, and a generator of nodes that are strictly a superset of
+        heads missing, this function updates self to be correct.
+        """
+        oldgetbranchinfo = repo.revbranchcache().branchinfo
+        try:
+            def branchinfo(r):
+                info = oldgetbranchinfo(r)
+                topic = ''
+                ctx = repo[r]
+                if ctx.mutable():
+                    topic = ctx.topic()
+                branch = info[0]
+                if topic:
+                    branch = '%s:%s' % (branch, topic)
+                return (branch, info[1])
+            repo.revbranchcache().branchinfo = branchinfo
+            return super(topiccache, self).update(repo, revgen)
+        finally:
+            repo.revbranchcache().branchinfo = oldgetbranchinfo