diff --git a/mercurial/revlog.py b/mercurial/revlog.py
new file mode 100644
--- /dev/null
+++ b/mercurial/revlog.py
@@ -0,0 +1,199 @@
+# revlog.py - storage back-end for mercurial
+#
+# This provides efficient delta storage with O(1) retrieve and append
+# and O(changes) merge between branches
+#
+# Copyright 2005 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 zlib, struct, sha, binascii, os, tempfile
+from mercurial import mdiff
+
+def compress(text):
+    return zlib.compress(text)
+
+def decompress(bin):
+    return zlib.decompress(bin)
+
+def hash(text, p1, p2):
+    l = [p1, p2]
+    l.sort()
+    return sha.sha(l[0] + l[1] + text).digest()
+
+nullid = "\0" * 20
+indexformat = ">4l20s20s20s"
+
+class revlog:
+    def __init__(self, opener, indexfile, datafile):
+        self.indexfile = indexfile
+        self.datafile = datafile
+        self.index = []
+        self.opener = opener
+        self.cache = None
+        self.nodemap = { -1: nullid, nullid: -1 }
+        # read the whole index for now, handle on-demand later
+        try:
+            n = 0
+            i = self.opener(self.indexfile).read()
+            s = struct.calcsize(indexformat)
+            for f in range(0, len(i), s):
+                # offset, size, base, linkrev, p1, p2, nodeid, changeset
+                e = struct.unpack(indexformat, i[f:f + s])
+                self.nodemap[e[6]] = n
+                self.index.append(e)
+                n += 1
+        except IOError: pass
+
+    def tip(self): return self.node(len(self.index) - 1)
+    def count(self): return len(self.index)
+    def node(self, rev): return rev < 0 and nullid or self.index[rev][6]
+    def rev(self, node): return self.nodemap[node]
+    def linkrev(self, node): return self.index[self.nodemap[node]][3]
+    def parents(self, node): return self.index[self.nodemap[node]][4:6]
+
+    def start(self, rev): return self.index[rev][0]
+    def length(self, rev): return self.index[rev][1]
+    def end(self, rev): return self.start(rev) + self.length(rev)
+    def base(self, rev): return self.index[rev][2]
+
+    def revisions(self, list):
+        # this can be optimized to do spans, etc
+        # be stupid for now
+        for r in list:
+            yield self.revision(r)
+
+    def diff(self, a, b):
+        return mdiff.textdiff(a, b)
+
+    def patch(self, text, patch):
+        return mdiff.patch(text, patch)
+
+    def revision(self, node):
+        if node is nullid: return ""
+        if self.cache and self.cache[0] == node: return self.cache[2]
+
+        text = None
+        rev = self.rev(node)
+        base = self.base(rev)
+        start = self.start(base)
+        end = self.end(rev)
+
+        if self.cache and self.cache[1] >= base and self.cache[1] < rev:
+            base = self.cache[1]
+            start = self.start(base + 1)
+            text = self.cache[2]
+            last = 0
+
+        f = self.opener(self.datafile)
+        f.seek(start)
+        data = f.read(end - start)
+
+        if not text:
+            last = self.length(base)
+            text = decompress(data[:last])
+
+        for r in range(base + 1, rev + 1):
+            s = self.length(r)
+            b = decompress(data[last:last + s])
+            text = self.patch(text, b)
+            last = last + s
+
+        (p1, p2) = self.parents(node)
+        if self.node(rev) != hash(text, p1, p2):
+            raise "Consistency check failed on %s:%d" % (self.datafile, rev)
+
+        self.cache = (node, rev, text)
+        return text  
+
+    def addrevision(self, text, transaction, link, p1=None, p2=None):
+        if text is None: text = ""
+        if p1 is None: p1 = self.tip()
+        if p2 is None: p2 = nullid
+
+        node = hash(text, p1, p2)
+
+        n = self.count()
+        t = n - 1
+
+        if n:
+            start = self.start(self.base(t))
+            end = self.end(t)
+            prev = self.revision(self.tip())
+            if 0:
+                dd = self.diff(prev, text)
+                tt = self.patch(prev, dd)
+                if tt != text:
+                    print prev
+                    print text
+                    print tt
+                    raise "diff+patch failed"
+            data = compress(self.diff(prev, text))
+
+        # full versions are inserted when the needed deltas
+        # become comparable to the uncompressed text
+        if not n or (end + len(data) - start) > len(text) * 2:
+            data = compress(text)
+            base = n
+        else:
+            base = self.base(t)
+
+        offset = 0
+        if t >= 0:
+            offset = self.end(t)
+
+        e = (offset, len(data), base, link, p1, p2, node)
+        
+        self.index.append(e)
+        self.nodemap[node] = n
+        entry = struct.pack(indexformat, *e)
+
+        transaction.add(self.datafile, e[0])
+        self.opener(self.datafile, "a").write(data)
+        transaction.add(self.indexfile, n * len(entry))
+        self.opener(self.indexfile, "a").write(entry)
+
+        self.cache = (node, n, text)
+        return node
+
+    def ancestor(self, a, b):
+        def expand(e1, e2, a1, a2):
+            ne = []
+            for n in e1:
+                (p1, p2) = self.parents(n)
+                if p1 in a2: return p1
+                if p2 in a2: return p2
+                if p1 != nullid and p1 not in a1:
+                    a1[p1] = 1
+                    ne.append(p1)
+                if p2 != nullid and p2 not in a1:
+                    a1[p2] = 1
+                    ne.append(p2)
+            return expand(e2, ne, a2, a1)
+        return expand([a], [b], {a:1}, {b:1})
+
+    def mergedag(self, other, transaction, linkseq, accumulate = None):
+        """combine the nodes from other's DAG into ours"""
+        old = self.tip()
+        i = self.count()
+        l = []
+
+        # merge the other revision log into our DAG
+        for r in range(other.count()):
+            id = other.node(r)
+            if id not in self.nodemap:
+                (xn, yn) = other.parents(id)
+                l.append((id, xn, yn))
+                self.nodemap[id] = i
+                i += 1
+
+        # merge node date for new nodes
+        r = other.revisions([e[0] for e in l])
+        for e in l:
+            t = r.next()
+            if accumulate: accumulate(t)
+            self.addrevision(t, transaction, linkseq.next(), e[1], e[2])
+
+        # return the unmerged heads for later resolving
+        return (old, self.tip())