diff --git a/src/Diff.hs b/src/Diff.hs
index 01f7df6..78db394 100644
--- a/src/Diff.hs
+++ b/src/Diff.hs
@@ -32,31 +32,240 @@ data Op
   | Add
   deriving (Show, Eq)
 
+data DiffEnv =
+  DiffEnv
+    { deD1 :: BS
+    , deD2 :: BS
+    , deT1 :: V.Vector Tok
+    , deT2 :: V.Vector Tok
+    , deS :: Int
+    , deE :: Int
+    , deL :: Int
+    , deW :: Int
+    , deA :: Int
+    , deB :: Int
+    , deVS :: V.Vector Int
+    , deVE :: V.Vector Int
+    , deTrans :: Bool
+    }
+  deriving (Show)
+
 substr b e = B.take (e - b) . B.drop b
 
-toksMatch d1 d2 t1 t2 x y =
-  let (tok1, (b1, e1)) = t1 V.! x
-      (tok2, (b2, e2)) = t2 V.! y
-   in tok1 == tok2 && substr b1 e1 d1 == substr b2 e2 d2
+toksMatch x y DiffEnv {deT1 = t1, deT2 = t2, deD1 = d1, deD2 = d2} =
+  let (isTok1, (b1, e1)) = t1 V.! x
+      (isTok2, (b2, e2)) = t2 V.! y
+   in isTok1 == isTok2 && substr b1 e1 d1 == substr b2 e2 d2
 
-stripEqToks :: BS -> BS -> V.Vector Tok -> V.Vector Tok -> (Int, Int, Int)
-stripEqToks d1 d2 t1 t2 = (b, l1 - e, l2 - e)
+stripEqToks :: DiffEnv -> (Int, Int, Int)
+stripEqToks de@DiffEnv {deT1 = t1, deT2 = t2} = (b, l1 - e, l2 - e)
   where
     l1 = V.length t1
     l2 = V.length t2
     firstDiff i
-      | i < l1 && i < l2 && toksMatch d1 d2 t1 t2 i i = firstDiff (i + 1)
+      | i < l1 && i < l2 && toksMatch i i de = firstDiff (i + 1)
       | otherwise = i
     b = firstDiff 0
     lastDiff i
       | l1 - i - 1 >= b &&
-          l2 - i - 1 >= b && toksMatch d1 d2 t1 t2 (l1 - i - 1) (l2 - i - 1) =
+          l2 - i - 1 >= b && toksMatch (l1 - i - 1) (l2 - i - 1) de =
         lastDiff (i + 1)
       | otherwise = i
     e = lastDiff 0
 
-diffToks :: BS -> BS -> V.Vector Tok -> V.Vector Tok -> IO [(Op, Tok)]
-diffToks d1 d2 t1 t2 = do
+diffToks :: BS -> BS -> V.Vector Tok -> V.Vector Tok -> [(Op, Tok)]
+diffToks d1 d2 t1 t2 =
+  diffToks' $
+  if V.length t1 >= V.length t2
+    then DiffEnv
+           { deD1 = d1
+           , deD2 = d2
+           , deT1 = t1
+           , deT2 = t2
+           , deS = 0
+           , deE = V.length t1
+           , deL = V.length t1
+           , deW = V.length t2
+           , deA = 0
+           , deB = V.length t2
+           , deVS = V.fromList [0 .. V.length t2]
+           , deVE = V.fromList $ reverse [0 .. V.length t2]
+           , deTrans = False
+           }
+    else DiffEnv
+           { deD1 = d2
+           , deD2 = d1
+           , deT1 = t2
+           , deT2 = t1
+           , deS = 0
+           , deE = V.length t2
+           , deL = V.length t2
+           , deW = V.length t1
+           , deA = 0
+           , deB = V.length t1
+           , deVS = V.fromList [0 .. V.length t1]
+           , deVE = V.fromList $ reverse [0 .. V.length t1]
+           , deTrans = True
+           }
+
+minIndexFwd =
+  V.minIndexBy
+    (\x y ->
+       if x <= y
+         then LT
+         else GT --basically normal V.minIndex
+     )
+
+minIndexRev =
+  V.minIndexBy
+    (\x y ->
+       if x < y
+         then LT
+         else GT --picks the last minimum
+     )
+
+diffToks' :: DiffEnv -> [(Op, Tok)]
+diffToks' de@DiffEnv {deS = s, deE = e, deA = a, deB = b, deTrans = trans} =
+  diff
+  where
+    mid = (s + e) `div` 2
+    vecSmid = vecS mid
+    vecEmid = vecE mid
+    vecS = vec -- "forward" operation
+      where
+        vec i
+          | i == s = deVS de
+          | i > s = V.fromList . upd i . vec $ pred i
+          | otherwise = error "Internal bounds check failure"
+        upd i v = i : go 1 i
+          where
+            go j up
+              | j > deW de = []
+              | otherwise =
+                let left = v V.! j
+                    upleft = v V.! pred j
+                    keep
+                      | toksMatch (pred i) (pred j) de = min upleft
+                      | otherwise = id
+                    res = keep $ min (succ up) (succ left)
+                 in res : go (succ j) res
+    vecE = vec -- "backward" operation
+      where
+        vec i
+          | i == e = deVE de
+          | i < e = V.fromList . reverse . upd i . vec $ succ i
+          | otherwise = error "Internal bounds check failure"
+        upd i v = (deL de - i) : go (pred $ deW de) (deL de - i)
+          where
+            go j down
+              | j < 0 = []
+              | otherwise =
+                let right = v V.! j
+                    downright = v V.! succ j
+                    keep
+                      | toksMatch i j de = min downright
+                      | otherwise = id
+                    res = keep $ min (succ down) (succ right)
+                 in res : go (pred j) res
+    {- Now, find the optimal point for splitting.
+     -
+     - Heuristics A: if 2 paths are completely same, prefer deletion first;
+     - which is done by choosing the 'upper' of two possibilities
+     - preferentially (or 'lower' ie 'more to the right' in case of transposed
+     - matrix) -}
+    opt =
+      (a +) .
+      (if trans
+         then minIndexRev
+         else minIndexFwd) $
+      V.zipWith (+) (slice vecSmid) (slice vecEmid)
+      where
+        slice = V.slice a (succ $ b - a)
+    diff
+      | s > e =
+        error $
+        "Internal failure -- recursion off limits: " <>
+        show s <> " vs " <> show e
+      | s == e = map (\i -> (Add, deT2 de V.! i)) [a .. pred b]
+      | succ s == e =
+        let vecLS = deVS de
+            vecRE = deVE de
+            vecLE = vecE s
+            vecRS = vecS e
+            sumL = V.zipWith (+) vecLS vecLE
+            sumR = V.zipWith (+) vecRS vecRE
+            {- This is getting a bit complicted. In the non-transposed case, we
+             - want to select one Remove/Keep surrounded by 0-n Add ops, possibly
+             - from both sides. The chosen path must belong to the best paths
+             - (bidirectional matrix sums must match the minimum at (s,a) and
+             - (b,e) on all steps), AND at the same time the path must be
+             - admissible for the edit operations (ie, it has to `backtrack
+             - well`). Also, it should follow Heuristic A that says that Remove
+             - and Keep ops should go earlier than Add ops (or vice versa if
+             - transposed). -}
+            totalCost = sumL V.! a
+            sCost = vecLS V.! a
+            eCost = vecRS V.! b
+            doKeep
+              | eCost - sCost == succ (b - a) = False
+              | eCost - sCost == pred (b - a) = True
+              | otherwise =
+                error $
+                "Internal check failure -- costs seem broken: " <>
+                show [sCost, eCost, a, b]
+            jumpPos =
+              (if trans {- Heuristic A applies here -}
+                 then last
+                 else head)
+                [ i
+                | i <-
+                    [a .. if doKeep
+                            then pred b
+                            else b]
+                , vecLS V.! i == sCost - a + i
+                , sumL V.! i == totalCost
+                , if doKeep
+                    then vecLS V.! i == vecRS V.! succ i
+                    else succ (vecLS V.! i) == vecRS V.! i
+                , if doKeep
+                    then sumR V.! succ i == totalCost
+                    else sumR V.! i == totalCost
+                , not doKeep || toksMatch s i de
+                ]
+            jumpEnd =
+              if doKeep
+                then jumpPos + 1
+                else jumpPos
+         in map
+              (\i ->
+                 ( if trans
+                     then Remove
+                     else Add
+                 , deT2 de V.! i))
+              [a .. pred jumpPos] ++
+            [ if trans
+                then if doKeep
+                       then (Keep, deT2 de V.! jumpPos)
+                       else (Add, deT1 de V.! s)
+                else ( if doKeep
+                         then Keep
+                         else Remove
+                     , deT1 de V.! s)
+            ] ++
+            map
+              (\i ->
+                 ( if trans
+                     then Remove
+                     else Add
+                 , deT2 de V.! i))
+              [jumpEnd .. pred b]
+      | otherwise =
+        diffToks' de {deE = mid, deVE = vecEmid, deB = opt} ++
+        diffToks' de {deS = mid, deVS = vecSmid, deA = opt}
+
+{-
+diffToksXX :: BS -> BS -> V.Vector Tok -> V.Vector Tok -> IO [(Op, Tok)]
+diffToksXX d1 d2 t1 t2 = do
   let (b, e1, e2) = stripEqToks d1 d2 t1 t2
       ms1 = e1 - b
       ms2 = e2 - b
@@ -96,7 +305,7 @@ diffToks d1 d2 t1 t2 = do
   return $
     map ((,) Keep) (take b $ V.toList t1) ++
     diff ++ map ((,) Keep) (drop e1 $ V.toList t1)
-
+-}
 pprDiff :: BS -> BS -> [(Op, Tok)] -> [BS]
 pprDiff d1 d2 = map (pprDiff1 d1 d2)
 
diff --git a/src/MainDiff.hs b/src/MainDiff.hs
index 33f9e7a..ea60c54 100644
--- a/src/MainDiff.hs
+++ b/src/MainDiff.hs
@@ -71,8 +71,8 @@ patchCmdOptions =
     auto
     (short 'p' <>
      long "strip" <>
-     metavar "NUM" <> help "Strip NUM leading components from the paths" <>
-     value 0) <*>
+     metavar "NUM" <>
+     help "Strip NUM leading components from the paths" <> value 0) <*>
   mergeOption True
 
 diff3CmdOptions =
@@ -103,11 +103,15 @@ main =
            progDesc
              "Compare, patch and merge files on arbitrarily-tokenized sequences." <>
            header "adiff: arbitrary-token diff utilities")
-   in do o <- execParser opts
-         redfa <- redfaPrepareRules (diffRedfaOpt o)
-         data1 <- mmapFileByteString (diffFile1 o) Nothing
-         data2 <- mmapFileByteString (diffFile2 o) Nothing
-         toks1 <- V.fromList <$> redfaTokenize redfa data1
-         toks2 <- V.fromList <$> redfaTokenize redfa data2
-         hs <- hunks (max 0 $ context o) <$> diffToks data1 data2 toks1 toks2
-         B8.putStrLn $ pprHunks data1 data2 hs
+   in do ADiffOptions {adiffRedfaOpt = ropt, adiffCmdOpts = copt} <-
+           execParser opts
+         redfa <- redfaPrepareRules ropt
+         case copt of
+           CmdDiff {diffFile1 = f1, diffFile2 = f2, context = ctxt} -> do
+             data1 <- mmapFileByteString f1 Nothing
+             data2 <- mmapFileByteString f2 Nothing
+             toks1 <- V.fromList <$> redfaTokenize redfa data1
+             toks2 <- V.fromList <$> redfaTokenize redfa data2
+             let hs = hunks (max 0 ctxt) $ diffToks data1 data2 toks1 toks2
+             B8.putStrLn $ pprHunks data1 data2 hs
+           CmdPatch {} -> putStrLn "not supported yet"
diff --git a/src/Redfa.hs b/src/Redfa.hs
index 4f8d3e5..42b1db2 100644
--- a/src/Redfa.hs
+++ b/src/Redfa.hs
@@ -59,7 +59,7 @@ redfaOptionToRuleStrings (RedfaOptionFile fn) =
   B8.lines <$> B.readFile fn -- TODO improve
 
 splitFirst :: Char -> BS -> (BS, BS)
-splitFirst c s = B.splitAt (maybe (B.length s) id $ B8.elemIndex c s) s
+splitFirst c s = B.splitAt (fromMaybe (B.length s) $ B8.elemIndex c s) s
 
 redfaRuleStringToRuleStr :: BS -> Maybe (BS, BS, BS, Bool)
 redfaRuleStringToRuleStr s =
@@ -68,7 +68,7 @@ redfaRuleStringToRuleStr s =
       sf = B8.strip from
       (cleanFrom, isToken)
         | B.null sf = (sf, True)
-        | B.head sf == (fromIntegral $ fromEnum '_') = (B.tail sf, False)
+        | B.head sf == fromIntegral (fromEnum '_') = (B.tail sf, False)
         | otherwise = (sf, True)
       go
         | B.null s = Nothing
@@ -89,7 +89,7 @@ unescapeRegex s = BL.toStrict . BB.toLazyByteString <$> unescape' s
         fail "incomplete escape sequence"
       | B.head s == BI.c2w '\\' =
         let rest = B.tail s
-            cc = B.head (rest)
+            cc = B.head rest
             thechar =
               BB.stringUtf8 $
               case BI.w2c cc of
@@ -108,12 +108,12 @@ unescapeRegex s = BL.toStrict . BB.toLazyByteString <$> unescape' s
                 '\\' -> "\\\\"
                 a -> [a] --TODO add support for \x and \u
          in (thechar <>) <$> unescape' (B.tail rest)
-      | otherwise = (mappend $ BB.word8 $ B.head s) <$> unescape' (B.tail s)
+      | otherwise = mappend (BB.word8 $ B.head s) <$> unescape' (B.tail s)
 
 redfaPrepareRules :: RedfaOption -> IO RedfaSpec
 redfaPrepareRules opt = do
   (states, jumps, regexes, isToken) <-
-    unzip4 . catMaybes . fmap redfaRuleStringToRuleStr <$>
+    unzip4 . mapMaybe redfaRuleStringToRuleStr <$>
     redfaOptionToRuleStrings opt
   uRegexes <- traverse unescapeRegex regexes
   startREs <- traverse (makeRegexM . (fromString "\\`" <>)) uRegexes
@@ -148,7 +148,7 @@ redfaTokenize' spec s state off visited
   | off >= B.length s = pure []
   | otherwise =
     let (ooff, reg) =
-          if (off == 0)
+          if off == 0
             then (0, rrRegexStart)
             else (1, rrRegexMid)
         matchString = B.drop (off - ooff) s
@@ -159,7 +159,7 @@ redfaTokenize' spec s state off visited
         contOK (RedfaRule {rrJump = j}, (off', len))
           | off' /= 0 = False
           | len > ooff = True
-          | otherwise = not $ j `elem` visited
+          | otherwise = j `notElem` visited
      in case matches of
           [] ->
             fail $