new file mode 100644
--- /dev/null
+++ b/mercurial/ancestor.py
@@ -0,0 +1,83 @@
+# ancestor.py - generic DAG ancestor algorithm for mercurial
+#
+# Copyright 2006 Matt Mackall <mpm@selenic.com>
+#
+# This software may be used and distributed according to the terms
+# of the GNU General Public License, incorporated herein by reference.
+
+import heapq
+
+def ancestor(a, b, pfunc):
+    """
+    return the least common ancestor of nodes a and b or None if there
+    is no such ancestor.
+
+    pfunc must return a list of parent vertices
+    """
+
+    if a == b:
+        return a
+
+    # find depth from root of all ancestors
+    visit = [a, b]
+    depth = {}
+    while visit:
+        vertex = visit[-1]
+        pl = pfunc(vertex)
+        if not pl:
+            depth[vertex] = 0
+            visit.pop()
+        else:
+            for p in pl:
+                if p == a or p == b: # did we find a or b as a parent?
+                    return p # we're done
+                if p not in depth:
+                    visit.append(p)
+            if visit[-1] == vertex:
+                depth[vertex] = min([depth[p] for p in pl]) - 1
+                visit.pop()
+
+    # traverse ancestors in order of decreasing distance from root
+    def ancestors(vertex):
+        h = [(depth[vertex], vertex)]
+        seen = {}
+        while h:
+            d, n = heapq.heappop(h)
+            if n not in seen:
+                seen[n] = 1
+                yield (d, n)
+                for p in pfunc(n):
+                    heapq.heappush(h, (depth[p], p))
+
+    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]:
+                for v in gx[1]:
+                    if v in gy[1]:
+                        return v
+                gy = y.next()
+                gx = x.next()
+            elif gx[0] > gy[0]:
+                gy = y.next()
+            else:
+                gx = x.next()
+    except StopIteration:
+        return None