diff --git a/SudokuSolver.hs b/SudokuSolver.hs
new file mode 100644
index 0000000..0cf5000
--- /dev/null
+++ b/SudokuSolver.hs
@@ -0,0 +1,144 @@
+{-# LANGUAGE BangPatterns #-}
+
+module Main where
+
+import qualified Data.Array as A
+import qualified Data.Map as M
+import qualified Data.Set as S
+import Control.Monad (foldM, forM_)
+import Data.Char (digitToInt, intToDigit)
+import Data.List (foldl', intersperse, (\\), sortBy, groupBy)
+import Data.List.Split (splitEvery)
+import Data.Maybe (fromJust)
+import Data.Ord (comparing)
+import System.CPUTime (getCPUTime)
+import Text.Printf (printf)
+
+countingSortBy f lo hi =
+  concatMap snd
+  . A.assocs . A.accumArray (\ e a -> a : e) [] (lo, hi) . map (\i -> (f i, i))
+
+data Cell = Cell !Int ![Int] deriving (Eq, Ord)
+type Board = M.Map Int Cell
+
+instance Show Cell where
+  show cell@(Cell ix val) = "<" ++ show ix ++ " " ++ show val ++ ">"
+
+emptyBoard =
+  foldl' (\m i -> M.insert i (Cell i [1..9]) m) M.empty [0..80]
+
+constrainCell cell@(Cell ix val) board c@(Cell i pos) =
+  case () of _
+              | c == cell                          -> return board
+              | null pos' && length val == 1       -> Nothing
+              | null pos'                          -> return board
+              | length pos' == 1 && length pos > 1 -> constrainBoard board (Cell i pos')
+              | otherwise                          -> return $ M.insert i (Cell i pos') board
+  where pos' = pos \\ val
+
+constrainCells :: Cell -> S.Set Cell -> Board -> [Cell] -> Maybe Board
+constrainCells cell@(Cell ix val) seen board unit = do
+  board' <- foldM (constrainCell cell) board unit
+  let unit' = map (\(Cell ix _) -> fromJust $ M.lookup ix board') $ unit
+
+  foldM (\board'' cell' ->
+            constrainCells cell' (S.insert cell' seen) board'' unit')
+        board' (dups unit')
+  where
+    dups = filter (not . flip S.member seen) . map head
+           . filter (\xs@(Cell _ v : _) -> length xs == length v)
+           . groupBy (\(Cell _ v1) (Cell _ v2) -> v1 == v2)
+           . sortBy (comparing (\(Cell _ val) -> val))
+           . filter (\(Cell _ v) -> length v > 1 && length v < 4)
+
+constrainBoard :: Board -> Cell -> Maybe Board
+constrainBoard board cell@(Cell ix _) =
+  foldM (\board'' unitf ->
+            constrainCells cell (S.singleton cell) board'' (unitf cell board''))
+        (M.insert ix cell board) [row, column, box]
+  where
+    (rowIx, colIx) = ix `divMod` 9
+    (rowIx', colIx') = ((rowIx `div` 3) * 3, (colIx `div` 3) * 3)
+
+    cells board = map (fromJust . flip M.lookup board)
+    row (Cell ix _) board = cells board $ take 9 [rowIx * 9 ..]
+    column (Cell ix _) board = cells board $ take 9 [colIx, colIx + 9 ..]
+    box (Cell ix _) board =
+      cells board [r * 9 + c | r <- [rowIx' .. rowIx' + 2], c <- [colIx' .. colIx' + 2]]
+
+readBoard :: String -> Maybe Board
+readBoard str =
+  foldM constrainBoard emptyBoard
+  . map (\(ix, n) -> Cell ix [digitToInt n])
+  . filter ((/= '.') . snd)
+  . zip [0..] $ str
+
+showBoard :: Board -> String
+showBoard board =
+  zipWith (\(Cell _ val) dot ->
+              if length val == 1 then intToDigit (head val) else dot)
+          (map snd . M.toList $ board)
+          (repeat '.')
+
+printBoard :: Board -> IO ()
+printBoard board =
+  putStrLn
+  . (\t -> line ++ "\n" ++ t ++ line ++ "\n")
+  . unlines . concat . intersperse [line] . splitEvery 3
+  . map ((\r -> "| " ++ r ++ " |") . concat
+         . intersperse " | " . map (intersperse ' ') . splitEvery 3)
+  . splitEvery 9
+  . showBoard $ board
+  where line = "+-------+-------+-------+"
+
+solveSudoku :: Board -> Maybe Board
+solveSudoku = fst . flip solve S.empty
+  where
+    solve board invalid = go (cells board) board invalid
+
+    cells board =
+    -- countingSortBy (\(Cell _ val) -> length val) 2 9
+      sortBy (comparing (\(Cell _ val) -> length val))
+      . filter (\(Cell _ val) -> length val /= 1)
+      . map snd . M.toList $ board
+
+    isSolved = all (\(Cell _ val) -> length val == 1) . M.elems
+
+    go [] board invalid = (Nothing, S.insert board invalid)
+    go (Cell ix val : cs) board invalid
+      | S.member board invalid = (Nothing, invalid)
+      | null val               = go cs board (S.insert board invalid)
+      | otherwise              = let
+          nextPos = Cell ix [head val]
+          restPos = Cell ix (tail val)
+          board' = M.insert ix nextPos board
+          in case constrainBoard board nextPos of
+               Nothing -> go (restPos : cs) board (S.insert board' invalid)
+               Just board'' | isSolved board'' -> (Just board'', invalid)
+                            | otherwise        -> let
+                                (mBoard', invalid') = solve board'' invalid
+                                in case mBoard' of
+                                     Just board''' -> (Just board''', invalid')
+                                     Nothing       ->
+                                       go (restPos : cs) board (S.insert board'' invalid')
+
+main = do
+  lns <- fmap lines getContents
+  forM_ lns $ \line -> do
+    start <- getCPUTime
+    let sudoku = readBoard line
+    case sudoku of
+      Nothing -> putStrLn ("Invalid input sudoku: " ++ line)
+      Just board -> do
+        let !res = solveSudoku board
+        end <- getCPUTime
+        let diff = (fromIntegral (end - start)) / (10^12)
+
+        putStrLn (printf "%s -> %s [%0.3f sec]" line (maybe "Unsolvable" showBoard res) (diff :: Double))
+
+        --putStrLn (printf "Time taken: %0.3f sec" (diff :: Double))
+        --printBoard board
+
+        --case res of
+        --  Nothing -> putStrLn "Unsolvable"
+        --  Just board' -> printBoard board'
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diff --git a/rubyquiz.cabal b/rubyquiz.cabal
index f55a4cb..a5151ee 100644
--- a/rubyquiz.cabal
+++ b/rubyquiz.cabal
@@ -78,4 +78,13 @@ executable AmazingMazes
                       random == 1.0.*,
                       pqueue == 1.2.*
   main-is           : AmazingMazes.hs
+  default-language  : Haskell2010
+
+executable SudokuSolver
+  build-depends     : base == 4.*,
+                      containers == 0.4.*,
+                      mtl == 2.1.*,
+                      split == 0.1.4.*,
+                      array == 0.4.*
+  main-is           : SudokuSolver.hs
   default-language  : Haskell2010
\ No newline at end of file