--- a/mercurial/branchmap.py	Wed Dec 19 14:47:38 2012 +0100
+++ b/mercurial/branchmap.py	Wed Dec 19 14:49:06 2012 +0100
@@ -50,3 +50,64 @@
         f.close()
     except (IOError, OSError):
         pass
+
+def update(repo, partial, ctxgen):
+    """Given a branchhead cache, partial, that may have extra nodes or be
+    missing heads, and a generator of nodes that are at least a superset of
+    heads missing, this function updates partial to be correct.
+    """
+    # collect new branch entries
+    newbranches = {}
+    for c in ctxgen:
+        newbranches.setdefault(c.branch(), []).append(c.node())
+    # if older branchheads are reachable from new ones, they aren't
+    # really branchheads. Note checking parents is insufficient:
+    # 1 (branch a) -> 2 (branch b) -> 3 (branch a)
+    for branch, newnodes in newbranches.iteritems():
+        bheads = partial.setdefault(branch, [])
+        # Remove candidate heads that no longer are in the repo (e.g., as
+        # the result of a strip that just happened).  Avoid using 'node in
+        # self' here because that dives down into branchcache code somewhat
+        # recursively.
+        bheadrevs = [repo.changelog.rev(node) for node in bheads
+                     if repo.changelog.hasnode(node)]
+        newheadrevs = [repo.changelog.rev(node) for node in newnodes
+                       if repo.changelog.hasnode(node)]
+        ctxisnew = bheadrevs and min(newheadrevs) > max(bheadrevs)
+        # Remove duplicates - nodes that are in newheadrevs and are already
+        # in bheadrevs.  This can happen if you strip a node whose parent
+        # was already a head (because they're on different branches).
+        bheadrevs = sorted(set(bheadrevs).union(newheadrevs))
+
+        # Starting from tip means fewer passes over reachable.  If we know
+        # the new candidates are not ancestors of existing heads, we don't
+        # have to examine ancestors of existing heads
+        if ctxisnew:
+            iterrevs = sorted(newheadrevs)
+        else:
+            iterrevs = list(bheadrevs)
+
+        # This loop prunes out two kinds of heads - heads that are
+        # superseded by a head in newheadrevs, and newheadrevs that are not
+        # heads because an existing head is their descendant.
+        while iterrevs:
+            latest = iterrevs.pop()
+            if latest not in bheadrevs:
+                continue
+            ancestors = set(repo.changelog.ancestors([latest],
+                                                     bheadrevs[0]))
+            if ancestors:
+                bheadrevs = [b for b in bheadrevs if b not in ancestors]
+        partial[branch] = [repo.changelog.node(rev) for rev in bheadrevs]
+
+    # There may be branches that cease to exist when the last commit in the
+    # branch was stripped.  This code filters them out.  Note that the
+    # branch that ceased to exist may not be in newbranches because
+    # newbranches is the set of candidate heads, which when you strip the
+    # last commit in a branch will be the parent branch.
+    for branch in partial.keys():
+        nodes = [head for head in partial[branch]
+                 if repo.changelog.hasnode(head)]
+        if not nodes:
+            del partial[branch]
+