diff --git a/mercurial/context.py b/mercurial/context.py
--- a/mercurial/context.py
+++ b/mercurial/context.py
@@ -7,7 +7,7 @@
 
 from node import *
 from demandload import demandload
-demandload(globals(), "heapq")
+demandload(globals(), "ancestor")
 
 class changectx(object):
     """A changecontext object makes access to data related to a particular
@@ -161,103 +161,26 @@ class filectx(object):
         find the common ancestor file context, if any, of self, and fc2
         """
 
-        a, b = (self._path, self._filenode), (fc2._path, fc2._filenode)
-
-        if a == b:
-            return self
-
-        if a[0] == b[0]:
-            n = self._filelog.ancestor(a[1], b[1])
-            if n != nullid:
-                return filectx(self._repo, self._path,
-                               fileid=n, filelog=self._filelog)
-
-        # build a graph of all ancestors, crossing renames
-        ag = {}
-        fv = [a, b]
+        acache = {}
         flcache = {self._path:self._filelog, fc2._path:fc2._filelog}
-
-        while fv:
-            f,n = fv.pop()
-            try:
-                fl = flcache[f]
-            except KeyError:
+        def parents(vertex):
+            if vertex in acache:
+                return acache[vertex]
+            f, n = vertex
+            if f not in flcache:
                 flcache[f] = self._repo.file(f)
-                fl = flcache[f]
-            v = [n]
-            while v:
-                n = v.pop()
-                c = (f, n)
-                if c in ag:
-                    continue
-
-                pl = [ n for n in fl.parents(n) if n != nullid ]
-                v += pl
-                pl = [ (f, n) for n in pl ]
-                re = fl.renamed(n)
-                if re:
-                    pl.append(re)
-                    if re not in ag:
-                        fv.append(re)
-                ag[c] = pl
+            fl = flcache[f]
+            pl = [ (f,p) for p in fl.parents(n) if p != nullid ]
+            re = fl.renamed(n)
+            if re:
+                pl.append(re)
+            acache[vertex]=pl
+            return pl
 
-        dist = {}
-        def depth(node):
-            try:
-                return dist[node]
-            except KeyError:
-                pl = ag[node]
-                if not pl:
-                    dist[node] = 0
-                else:
-                    dist[node] = max([depth(p) for p in pl]) + 1
-                return dist[node]
-
-        # traverse ancestors in order of decreasing distance from root
-        def ancestors(vertex):
-            h = [(-depth(vertex), vertex)]
-            seen = {}
-            while h:
-                d, v = heapq.heappop(h)
-                if v not in seen:
-                    seen[v] = 1
-                    yield (-d, v)
-                    for p in ag[v]:
-                        heapq.heappush(h, (-depth(p), p))
+        a, b = (self._path, self._filenode), (fc2._path, fc2._filenode)
+        v = ancestor.ancestor(a, b, parents)
+        if v:
+            f,n = v
+            return filectx(self._repo, f, fileid=n, filelog=flcache[f])
 
-        def generations(vertex):
-            sg, s = None, {}
-            for g,v in ancestors(vertex):
-                if g != sg:
-                    if sg:
-                        yield sg, s
-                    sg, s = g, {v:1}
-                else:
-                    s[v] = 1
-            yield sg, s
-
-        x = generations(a)
-        y = generations(b)
-        gx = x.next()
-        gy = y.next()
-
-        # increment each ancestor list until it is closer to root than
-        # the other, or they match
-        try:
-            while 1:
-                if gx[0] == gy[0]:
-                    # find the intersection
-                    i = [ n for n in gx[1] if n in gy[1] ]
-                    if i:
-                        fp,fn = i[0]
-                        fl = flcache[fp]
-                        return filectx(self._repo, fp, fileid=fn, filelog=fl)
-                    else:
-                        gy = y.next()
-                        gx = x.next()
-                elif gx[0] < gy[0]:
-                    gy = y.next()
-                else:
-                    gx = x.next()
-        except StopIteration:
-            return None
+        return None