diff --git a/mercurial/revlog.py b/mercurial/revlog.py
--- a/mercurial/revlog.py
+++ b/mercurial/revlog.py
@@ -227,3 +227,186 @@ class revlog:
 
         # return the unmerged heads for later resolving
         return (old, self.tip())
+
+    def group(self, linkmap):
+        # given a list of changeset revs, return a set of deltas and
+        # metadata corresponding to nodes the first delta is
+        # parent(nodes[0]) -> nodes[0] the receiver is guaranteed to
+        # have this parent as it has all history before these
+        # changesets. parent is parent[0]
+
+        revs = []
+        needed = {}
+
+        # find file nodes/revs that match changeset revs
+        for i in xrange(0, self.count()):
+            if self.index[i][3] in linkmap:
+                revs.append(i)
+                needed[i] = 1
+
+        # if we don't have any revisions touched by these changesets, bail
+        if not revs: return struct.pack(">l", 0)
+
+        # add the parent of the first rev
+        p = self.parents(self.node(revs[0]))[0]
+        revs.insert(0, self.rev(p))
+
+        # for each delta that isn't contiguous in the log, we need to
+        # reconstruct the base, reconstruct the result, and then
+        # calculate the delta. We also need to do this where we've
+        # stored a full version and not a delta
+        for i in xrange(0, len(revs) - 1):
+            a, b = revs[i], revs[i + 1]
+            if a + 1 != b or self.base(b) == b:
+                for j in xrange(self.base(a), a + 1):
+                    needed[j] = 1
+                for j in xrange(self.base(b), b + 1):
+                    needed[j] = 1
+
+        # calculate spans to retrieve from datafile
+        needed = needed.keys()
+        needed.sort()
+        spans = []
+        for n in needed:
+            if n < 0: continue
+            o = self.start(n)
+            l = self.length(n)
+            spans.append((o, l, [(n, l)]))
+
+        # merge spans
+        merge = [spans.pop(0)]
+        while spans:
+            e = spans.pop(0)
+            f = merge[-1]
+            if e[0] == f[0] + f[1]:
+                merge[-1] = (f[0], f[1] + e[1], f[2] + e[2])
+            else:
+                merge.append(e)
+
+        # read spans in, divide up chunks
+        chunks = {}
+        for span in merge:
+            # we reopen the file for each span to make http happy for now
+            f = self.opener(self.datafile)
+            f.seek(span[0])
+            data = f.read(span[1])
+
+            # divide up the span
+            pos = 0
+            for r, l in span[2]:
+                chunks[r] = data[pos: pos + l]
+                pos += l
+
+        # helper to reconstruct intermediate versions
+        def construct(text, base, rev):
+            for r in range(base + 1, rev + 1):
+                b = decompress(chunks[r])
+                text = self.patch(text, b)
+            return text
+
+        # build deltas
+        deltas = []
+        for d in range(0, len(revs) - 1):
+            a, b = revs[d], revs[d + 1]
+            n = self.node(b)
+            
+            if a + 1 != b or self.base(b) == b:
+                if a >= 0:
+                    base = self.base(a)
+                    ta = decompress(chunks[self.base(a)])
+                    ta = construct(ta, base, a)
+                else:
+                    ta = ""
+                    
+                base = self.base(b)
+                if a > base:
+                    base = a
+                    tb = ta
+                else:
+                    tb = decompress(chunks[self.base(b)])
+                tb = construct(tb, base, b)
+                d = self.diff(ta, tb)
+            else:
+                d = decompress(chunks[b])
+
+            p = self.parents(n)
+            meta = n + p[0] + p[1] + linkmap[self.linkrev(n)]
+            l = struct.pack(">l", len(meta) + len(d) + 4)
+            deltas.append(l + meta + d)
+
+        l = struct.pack(">l", sum(map(len, deltas)) + 4)
+        deltas.insert(0, l)
+        return "".join(deltas)
+        
+    def addgroup(self, data, linkmapper, transaction):
+        # given a set of deltas, add them to the revision log. the
+        # first delta is against its parent, which should be in our
+        # log, the rest are against the previous delta.
+
+        if len(data) <= 4: return
+
+        # retrieve the parent revision of the delta chain
+        chain = data[28:48]
+        text = self.revision(chain)
+
+        # track the base of the current delta log
+        r = self.count()
+        t = r - 1
+        
+        base = prev = -1
+        start = end = 0
+        if r:
+            start = self.start(self.base(t))
+            end = self.end(t)
+            measure = self.length(self.base(t))
+            base = self.base(t)
+            prev = self.tip()
+
+        transaction.add(self.datafile, end)
+        transaction.add(self.indexfile, r * struct.calcsize(indexformat))
+        dfh = self.opener(self.datafile, "a")
+        ifh = self.opener(self.indexfile, "a")
+
+        # loop through our set of deltas
+        pos = 4
+        while pos < len(data):
+            l, node, p1, p2, cs = struct.unpack(">l20s20s20s20s",
+                                                data[pos:pos+84])
+            link = linkmapper(cs)
+            delta = data[pos + 84:pos + l]
+            pos += l
+
+            # full versions are inserted when the needed deltas become
+            # comparable to the uncompressed text or when the previous
+            # version is not the one we have a delta against. We use
+            # the size of the previous full rev as a proxy for the
+            # current size.
+
+            if chain == prev:
+                cdelta = compress(delta)
+
+            if chain != prev or (end - start + len(cdelta)) > measure * 2:
+                # flush our writes here so we can read it in revision
+                dfh.flush()
+                ifh.flush()
+                text = self.revision(self.node(t))
+                text = self.patch(text, delta)
+                chk = self.addrevision(text, transaction, link, p1, p2)
+                if chk != node:
+                    raise "consistency error adding group"
+                measure = len(text)
+            else:
+                e = (end, len(cdelta), self.base(t), link, p1, p2, node)
+                self.index.append(e)
+                self.nodemap[node] = r
+                dfh.write(cdelta)
+                ifh.write(struct.pack(indexformat, *e))
+
+            t, r = r, r + 1
+            chain = prev
+            start = self.start(self.base(t))
+            end = self.end(t)
+
+        dfh.close()
+        ifh.close()
+        return node