algorithm from http://blog.liw.fi/posts/rsync-in-python/

rsync.py

@@ -0,0 +1,48 @@
+block_size = 4096
+
+def signature(f):
+    while True:
+        block_data = f.read(block_size)
+        if not block_data:
+            break
+        yield (zlib.adler32(block_data), hashlib.md5(block_data).digest())
+
+class RsyncLookupTable(object):
+
+    def __init__(self, checksums):
+        self.dict = {}
+        for block_number, c in enumerate(checksums):
+            weak, strong = c
+            if weak not in self.dict:
+                self.dict[weak] = dict()
+            self.dict[weak][strong] = block_number
+
+    def __getitem__(self, block_data):
+        weak = zlib.adler32(block_data)
+        subdict = self.dict.get(weak)
+        if subdict:
+            strong = hashlib.md5(block_data).digest()
+            return subdict.get(strong)
+        return None
+
+def delta(sigs, f):
+    table = RsyncLookupTable(sigs)
+    block_data = f.read(block_size)
+    while block_data:
+        block_number = table[block_data]
+        if block_number:
+            yield (block_number * block_size, len(block_data))
+            block_data = f.read(block_size)
+        else:
+            yield block_data[0]
+            block_data = block_data[1:] + f.read(1)
+
+def patch(outputf, deltas, old_file):
+    for x in deltas:
+        if type(x) == str:
+            outputf.write(x)
+        else:
+            offset, length = x
+            old_file.seek(offset)
+            outputf.write(old_file.read(length))
+