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):
+        # calculate the distance of every node from root
-            a = []
+        dist = {nullid: 0}
-            while list:
+        for i in xrange(self.count()):
-                n = list.pop(0)
+            n = self.node(i)
-                map[n] = 1
+            p1, p2 = self.parents(n)
-                yield n
+            dist[n] = max(dist[p1], dist[p2]) + 1
-                for p in self.parents(n):
+        
-                    if p != nullid and p not in map:
+        # traverse ancestors in order of decreasing distance from root
-                        list.append(p)
+        def ancestors(node):
-            yield nullid
+            # 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 = {}
+        x = ancestors(a)
-        bmap = {}
+        y = ancestors(b)
-        ag = expand([a], amap)
+        lx = x.next()
-        bg = expand([b], bmap)
+        ly = y.next()
-        adone = bdone = 0
 
-        while not adone or not bdone:
+        # increment each ancestor list until it is closer to root than
-            if not adone:
+        # the other, or they match
-                an = ag.next()
+        while 1:
-                if an == nullid:
+            if lx == ly:
-                    adone = 1
+                return lx[1]
-                elif an in bmap:
+            elif lx < ly:
-                    return an
+                ly = y.next()
-            if not bdone:
+            elif lx > ly:
-                bn = bg.next()
+                lx = x.next()
-                if bn == nullid:
-                    bdone = 1
-                elif bn in amap:
-                    return bn
-
-        return nullid
 
     def group(self, linkmap):
         # given a list of changeset revs, return a set of deltas and