--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/hgext3rd/topic/destination.py	Thu Mar 17 09:12:18 2016 -0700
@@ -0,0 +1,87 @@
+from mercurial import error
+from mercurial import util
+from mercurial import destutil
+from mercurial import extensions
+from mercurial import bookmarks
+from mercurial.i18n import _
+
+def _destmergebranch(orig, repo, action='merge', sourceset=None, onheadcheck=True):
+    p1 = repo['.']
+    top = p1.topic()
+    if top:
+        heads = repo.revs('heads(topic(.)::topic(.))')
+        if p1.rev() not in heads:
+            raise error.Abort(_("not at topic head, update or explicit"))
+        elif 1 == len(heads):
+            # should look at all branch involved but... later
+            bhead = ngtip(repo, p1.branch(), all=True)
+            if not bhead:
+                raise error.Abort(_("nothing to merge"))
+            elif 1 == len(bhead):
+                return bhead.first()
+            else:
+                raise error.Abort(_("branch '%s' has %d heads - "
+                                   "please merge with an explicit rev")
+                                 % (p1.branch(), len(bhead)),
+                                 hint=_("run 'hg heads .' to see heads"))
+        elif 2 == len(heads):
+            heads = [r for r in heads if r != p1.rev()]
+            # XXX: bla bla bla bla bla
+            if 1 < len(heads):
+                raise error.Abort(_('working directory not at a head revision'),
+                                 hint=_("use 'hg update' or merge with an "
+                                        "explicit revision"))
+            return heads[0]
+        elif 2 < len(heads):
+            raise error.Abort(_("topic '%s' has %d heads - "
+                                "please merge with an explicit rev")
+                              % (top, len(heads)))
+        else:
+            assert False # that's impossible
+    if orig.func_default: # version above hg-3.7
+        return orig(repo, action, sourceset, onheadcheck)
+    else:
+        return orig(repo)
+
+def _destupdatetopic(repo, clean, check):
+    """decide on an update destination from current topic"""
+    movemark = node = None
+    topic = repo.currenttopic
+    revs = repo.revs('.::topic("%s")' % topic)
+    if not revs:
+        return None, None, None
+    node = revs.last()
+    if bookmarks.isactivewdirparent(repo):
+        movemark = repo['.'].node()
+    return node, movemark, None
+
+def setupdest():
+    if util.safehasattr(destutil, '_destmergebranch'):
+        extensions.wrapfunction(destutil, '_destmergebranch', _destmergebranch)
+    rebase = extensions.find('rebase')
+    if (util.safehasattr(rebase, '_destrebase')
+            # logic not shared with merge yet < hg-3.8
+            and not util.safehasattr(rebase, '_definesets')):
+        extensions.wrapfunction(rebase, '_destrebase', _destmergebranch)
+    if util.safehasattr(destutil, 'destupdatesteps'):
+        bridx = destutil.destupdatesteps.index('branch')
+        destutil.destupdatesteps.insert(bridx, 'topic')
+        destutil.destupdatestepmap['topic'] = _destupdatetopic
+
+def ngtip(repo, branch, all=False):
+    """tip new generation"""
+    ## search for untopiced heads of branch
+    # could be heads((::branch(x) - topic()))
+    # but that is expensive
+    #
+    # we should write plain code instead
+    subquery = '''heads(
+                    parents(
+                       ancestor(
+                         (head() and branch(%s)
+                         or (topic() and branch(%s)))))
+                   ::(head() and branch(%s))
+                   - topic())'''
+    if not all:
+        subquery = 'max(%s)' % subquery
+    return repo.revs(subquery, branch, branch, branch)