A new ancestor algorithm

The old ancestor algorithm could get fooled into returning ancestors
closer to root than it ought to. Hopefully this one, which strictly
orders its search by distance from room, will be foolproof.

mercurial/revlog.py

@@ -8,7 +8,7 @@
 # This software may be used and distributed according to the terms
 # of the GNU General Public License, incorporated herein by reference.
 
-import zlib, struct, sha, os, tempfile, binascii
+import zlib, struct, sha, os, tempfile, binascii, heapq
 from mercurial import mdiff
 
 def hex(node): return binascii.hexlify(node)
@@ -276,38 +276,42 @@ class revlog:
         return node
 
     def ancestor(self, a, b):
-        def expand(list, map):
-            a = []
-            while list:
-                n = list.pop(0)
-                map[n] = 1
-                yield n
-                for p in self.parents(n):
-                    if p != nullid and p not in map:
-                        list.append(p)
-            yield nullid
+        # calculate the distance of every node from root
+        dist = {nullid: 0}
+        for i in xrange(self.count()):
+            n = self.node(i)
+            p1, p2 = self.parents(n)
+            dist[n] = max(dist[p1], dist[p2]) + 1
+        
+        # traverse ancestors in order of decreasing distance from root
+        def ancestors(node):
+            # we store negative distances because heap returns smallest member
+            h = [(-dist[node], node)]
+            seen = {}
+            earliest = self.count()
+            while h:
+                d, n = heapq.heappop(h)
+                r = self.rev(n)
+                if n not in seen:
+                    seen[n] = 1
+                    yield (-d, n)
+                    for p in self.parents(n):
+                        heapq.heappush(h, (-dist[p], p))
 
-        amap = {}
-        bmap = {}
-        ag = expand([a], amap)
-        bg = expand([b], bmap)
-        adone = bdone = 0
+        x = ancestors(a)
+        y = ancestors(b)
+        lx = x.next()
+        ly = y.next()
 
-        while not adone or not bdone:
-            if not adone:
-                an = ag.next()
-                if an == nullid:
-                    adone = 1
-                elif an in bmap:
-                    return an
-            if not bdone:
-                bn = bg.next()
-                if bn == nullid:
-                    bdone = 1
-                elif bn in amap:
-                    return bn
-
-        return nullid
+        # increment each ancestor list until it is closer to root than
+        # the other, or they match
+        while 1:
+            if lx == ly:
+                return lx[1]
+            elif lx < ly:
+                ly = y.next()
+            elif lx > ly:
+                lx = x.next()
 
     def group(self, linkmap):
         # given a list of changeset revs, return a set of deltas and