From: Ramakrishnan Muthukrishnan <ram@rkrishnan.org>
Date: Thu, 31 Dec 2015 13:28:53 +0000 (+0530)
Subject: add rolling checksum implementation
X-Git-Url: https://git.rkrishnan.org/vdrive/%22news.html/class-simplejson.JSONDecoder-index.html?a=commitdiff_plain;h=180950d844550cbe5cdc47a6ce4d3c5fa50d7709;p=hs-rsync.git

add rolling checksum implementation
---

diff --git a/src/Lib.hs b/src/Lib.hs
index 260de94..2500097 100644
--- a/src/Lib.hs
+++ b/src/Lib.hs
@@ -83,3 +83,10 @@ recreate f0 blockSize ins =
             RBlk i  -> (f0blocks !! i) `mappend` go f0blocks insts
             RChar w -> BL.singleton w `mappend` go f0blocks insts
 
+rollingChecksum :: Int -> Int -> BL.ByteString -> Word32
+rollingChecksum strtIdx lenBS bs = a `mod` m + ((fromIntegral b) `mod` m) * m
+  where a    = BL.foldl (\acc x -> acc + (fromIntegral x)) 0 bs'
+        b    = BL.foldl (\acc x -> acc + x) 0 (BL.pack wbs')
+        bs'  = BL.take (fromIntegral lenBS) $ BL.drop (fromIntegral strtIdx) bs
+        m    = 2^16
+        wbs' = BL.zipWith (*) (BL.pack (reverse (map fromIntegral [1..(lenBS - strtIdx + 1)]))) bs'