diff --git a/contrib/check-code.py b/contrib/check-code.py
--- a/contrib/check-code.py
+++ b/contrib/check-code.py
@@ -169,7 +169,7 @@ pypats = [
      "missing whitespace around operator"),
     (r'\s(\+=|-=|!=|<>|<=|>=|<<=|>>=)\S',
      "missing whitespace around operator"),
-    (r'[^+=*/!<>&| -](\s=|=\s)[^= ]',
+    (r'[^^+=*/!<>&| -](\s=|=\s)[^= ]',
      "wrong whitespace around ="),
     (r'raise Exception', "don't raise generic exceptions"),
     (r' is\s+(not\s+)?["\'0-9-]', "object comparison with literal"),
diff --git a/mercurial/commands.py b/mercurial/commands.py
--- a/mercurial/commands.py
+++ b/mercurial/commands.py
@@ -16,7 +16,7 @@ import sshserver, hgweb, hgweb.server, c
 import merge as mergemod
 import minirst, revset, fileset
 import dagparser, context, simplemerge
-import random, setdiscovery, treediscovery, dagutil
+import random, setdiscovery, treediscovery, dagutil, pvec
 import phases
 
 table = {}
@@ -1963,6 +1963,27 @@ def debugpushkey(ui, repopath, namespace
             ui.write("%s\t%s\n" % (k.encode('string-escape'),
                                    v.encode('string-escape')))
 
+@command('debugpvec', [], _('A B'))
+def debugpvec(ui, repo, a, b=None):
+    ca = scmutil.revsingle(repo, a)
+    cb = scmutil.revsingle(repo, b)
+    pa = pvec.ctxpvec(ca)
+    pb = pvec.ctxpvec(cb)
+    if pa == pb:
+        rel = "="
+    elif pa > pb:
+        rel = ">"
+    elif pa < pb:
+        rel = "<"
+    elif pa | pb:
+        rel = "|"
+    ui.write(_("a: %s\n") % pa)
+    ui.write(_("b: %s\n") % pb)
+    ui.write(_("depth(a): %d depth(b): %d\n") % (pa._depth, pb._depth))
+    ui.write(_("delta: %d hdist: %d distance: %d relation: %s\n") %
+             (abs(pa._depth - pb._depth), pvec._hamming(pa._vec, pb._vec),
+              pa.distance(pb), rel))
+
 @command('debugrebuildstate',
     [('r', 'rev', '', _('revision to rebuild to'), _('REV'))],
     _('[-r REV] [REV]'))
diff --git a/mercurial/pvec.py b/mercurial/pvec.py
new file mode 100644
--- /dev/null
+++ b/mercurial/pvec.py
@@ -0,0 +1,210 @@
+# pvec.py - probabilistic vector clocks for Mercurial
+#
+# Copyright 2012 Matt Mackall <mpm@selenic.com>
+#
+# This software may be used and distributed according to the terms of the
+# GNU General Public License version 2 or any later version.
+
+'''
+A "pvec" is a changeset property based on the theory of vector clocks
+that can be compared to discover relatedness without consulting a
+graph. This can be useful for tasks like determining how a
+disconnected patch relates to a repository.
+
+Currently a pvec consist of 448 bits, of which 24 are 'depth' and the
+remainder are a bit vector. It is represented as a 70-character base85
+string.
+
+Construction:
+
+- a root changeset has a depth of 0 and a bit vector based on its hash
+- a normal commit has a changeset where depth is increased by one and
+  one bit vector bit is flipped based on its hash
+- a merge changeset pvec is constructed by copying changes from one pvec into
+  the other to balance its depth
+
+Properties:
+
+- for linear changes, difference in depth is always <= hamming distance
+- otherwise, changes are probably divergent
+- when hamming distance is < 200, we can reliably detect when pvecs are near
+
+Issues:
+
+- hamming distance ceases to work over distances of ~ 200
+- detecting divergence is less accurate when the common ancestor is very close
+  to either revision or total distance is high
+- this could probably be improved by modeling the relation between
+  delta and hdist
+
+Uses:
+
+- a patch pvec can be used to locate the nearest available common ancestor for
+  resolving conflicts
+- ordering of patches can be established without a DAG
+- two head pvecs can be compared to determine whether push/pull/merge is needed
+  and approximately how many changesets are involved
+- can be used to find a heuristic divergence measure between changesets on
+  different branches
+'''
+
+import base85, util
+from node import nullrev
+
+_size = 448 # 70 chars b85-encoded
+_bytes = _size / 8
+_depthbits = 24
+_depthbytes = _depthbits / 8
+_vecbytes = _bytes - _depthbytes
+_vecbits = _vecbytes * 8
+_radius = (_vecbits - 30) / 2 # high probability vecs are related
+
+def _bin(bs):
+    '''convert a bytestring to a long'''
+    v = 0
+    for b in bs:
+        v = v * 256 + ord(b)
+    return v
+
+def _str(v, l):
+    bs = ""
+    for p in xrange(l):
+        bs = chr(v & 255) + bs
+        v >>= 8
+    return bs
+
+def _split(b):
+    '''depth and bitvec'''
+    return _bin(b[:_depthbytes]), _bin(b[_depthbytes:])
+
+def _join(depth, bitvec):
+    return _str(depth, _depthbytes) + _str(bitvec, _vecbytes)
+
+def _hweight(x):
+    c = 0
+    while x:
+        if x & 1:
+            c += 1
+        x >>= 1
+    return c
+_htab = [_hweight(x) for x in xrange(256)]
+
+def _hamming(a, b):
+    '''find the hamming distance between two longs'''
+    d = a ^ b
+    c = 0
+    while d:
+        c += _htab[d & 0xff]
+        d >>= 8
+    return c
+
+def _mergevec(x, y, c):
+    # Ideally, this function would be x ^ y ^ ancestor, but finding
+    # ancestors is a nuisance. So instead we find the minimal number
+    # of changes to balance the depth and hamming distance
+
+    d1, v1 = x
+    d2, v2 = y
+    if d1 < d2:
+        d1, d2, v1, v2 = d2, d1, v2, v1
+
+    hdist = _hamming(v1, v2)
+    ddist = d1 - d2
+    v = v1
+    m = v1 ^ v2 # mask of different bits
+    i = 1
+
+    if hdist > ddist:
+        # if delta = 10 and hdist = 100, then we need to go up 55 steps
+        # to the ancestor and down 45
+        changes = (hdist - ddist + 1) / 2
+    else:
+        # must make at least one change
+        changes = 1
+    depth = d1 + changes
+
+    # copy changes from v2
+    if m:
+        while changes:
+            if m & i:
+                v ^= i
+                changes -= 1
+            i <<= 1
+    else:
+        v = _flipbit(v, c)
+
+    return depth, v
+
+def _flipbit(v, node):
+    # converting bit strings to longs is slow
+    bit = (hash(node) & 0xffffffff) % _vecbits
+    return v ^ (1<<bit)
+
+def ctxpvec(ctx):
+    '''construct a pvec for ctx while filling in the cache'''
+    r = ctx._repo
+    if not util.safehasattr(r, "_pveccache"):
+        r._pveccache = {}
+    pvc = r._pveccache
+    if ctx.rev() not in pvc:
+        cl = r.changelog
+        for n in xrange(ctx.rev() + 1):
+            if n not in pvc:
+                node = cl.node(n)
+                p1, p2 = cl.parentrevs(n)
+                if p1 == nullrev:
+                    # start with a 'random' vector at root
+                    pvc[n] = (0, _bin((node * 3)[:_vecbytes]))
+                elif p2 == nullrev:
+                    d, v = pvc[p1]
+                    pvc[n] = (d + 1, _flipbit(v, node))
+                else:
+                    pvc[n] = _mergevec(pvc[p1], pvc[p2], node)
+    bs = _join(*pvc[ctx.rev()])
+    return pvec(base85.b85encode(bs))
+
+class pvec(object):
+    def __init__(self, hashorctx):
+        if isinstance(hashorctx, str):
+            self._bs = hashorctx
+            self._depth, self._vec = _split(base85.b85decode(hashorctx))
+        else:
+            self._vec = ctxpvec(ctx)
+
+    def __str__(self):
+        return self._bs
+
+    def __eq__(self, b):
+        return self._vec == b._vec and self._depth == b._depth
+
+    def __lt__(self, b):
+        delta = b._depth - self._depth
+        if delta < 0:
+            return False # always correct
+        if _hamming(self._vec, b._vec) > delta:
+            return False
+        return True
+
+    def __gt__(self, b):
+        return b < self
+
+    def __or__(self, b):
+        delta = abs(b._depth - self._depth)
+        if _hamming(self._vec, b._vec) <= delta:
+            return False
+        return True
+
+    def __sub__(self, b):
+        if self | b:
+            raise ValueError("concurrent pvecs")
+        return self._depth - b._depth
+
+    def distance(self, b):
+        d = abs(b._depth - self._depth)
+        h = _hamming(self._vec, b._vec)
+        return max(d, h)
+
+    def near(self, b):
+        dist = abs(b.depth - self._depth)
+        if dist > _radius or _hamming(self._vec, b._vec) > _radius:
+            return False
diff --git a/tests/test-debugcomplete.t b/tests/test-debugcomplete.t
--- a/tests/test-debugcomplete.t
+++ b/tests/test-debugcomplete.t
@@ -87,6 +87,7 @@ Show debug commands if there are no othe
   debuginstall
   debugknown
   debugpushkey
+  debugpvec
   debugrebuildstate
   debugrename
   debugrevlog
@@ -236,6 +237,7 @@ Show all commands + options
   debuginstall: 
   debugknown: 
   debugpushkey: 
+  debugpvec: 
   debugrebuildstate: rev
   debugrename: rev
   debugrevlog: changelog, manifest, dump