Problem:
Score : 400 points

Problem Statement

A maze is composed of a grid of H \times W squares - H vertical, W horizontal.

The square at the i-th row from the top and the j-th column from the left - (i,j) - is a wall if S_{ij} is # and a road if S_{ij} is ..

There is a magician in (C_h,C_w). He can do the following two kinds of moves:

Move A: Walk to a road square that is vertically or horizontally adjacent to the square he is currently in.

Move B: Use magic to warp himself to a road square in the 5\times 5 area centered at the square he is currently in.

In either case, he cannot go out of the maze.

At least how many times does he need to use the magic to reach (D_h, D_w)?

Constraints

1 \leq H,W \leq 10^3

1 \leq C_h,D_h \leq H

1 \leq C_w,D_w \leq W

S_{ij} is # or ..

S_{C_h C_w} and S_{D_h D_w} are ..

(C_h,C_w) \neq (D_h,D_w)

Input

Input is given from Standard Input in the following format:

H W
C_h C_w
D_h D_w
S_{11}\ldots S_{1W}
\vdots
S_{H1}\ldots S_{HW}

Output

Print the minimum number of times the magician needs to use the magic. If he cannot reach (D_h,D_w), print -1 instead.

Sample Input 1

4 4
1 1
4 4
..#.
..#.
.#..
.#..

Sample Output 1

1

For example, by walking to (2,2) and then using the magic to travel to (4,4), just one use of magic is enough.

Note that he cannot walk diagonally.

Sample Input 2

4 4
1 4
4 1
.##.
####
####
.##.

Sample Output 2

-1

He cannot move from there.

Sample Input 3

4 4
2 2
3 3
....
....
....
....

Sample Output 3

0

No use of magic is needed.

Sample Input 4

4 5
1 2
2 5
#.###
####.
#..##
#..##

Sample Output 4

2

Solution in Haskell (GHC 8.8.3) (judge verdict: Accepted)

{-# OPTIONS_GHC -O2 #-}
{-# LANGUAGE BangPatterns, BinaryLiterals, CPP, DerivingStrategies  #-}
{-# LANGUAGE DerivingVia, FlexibleContexts, FlexibleInstances       #-}
{-# LANGUAGE GeneralizedNewtypeDeriving, KindSignatures, LambdaCase #-}
{-# LANGUAGE MagicHash, MultiParamTypeClasses, MultiWayIf           #-}
{-# LANGUAGE NumericUnderscores, OverloadedStrings, PatternSynonyms #-}
{-# LANGUAGE RankNTypes, RecordWildCards, ScopedTypeVariables       #-}
{-# LANGUAGE StandaloneDeriving, TupleSections, TypeApplications    #-}
{-# LANGUAGE TypeFamilies, TypeInType, UnboxedTuples, ViewPatterns  #-}

import           Control.Applicative
import           Control.Exception
import           Control.Monad
import           Control.Monad.Primitive
import           Control.Monad.Reader
import           Control.Monad.ST
import           Control.Monad.State.Strict
import           Data.Bifunctor
import           Data.Bool
import qualified Data.ByteString                   as B
import qualified Data.ByteString.Builder           as B
import qualified Data.ByteString.Char8             as C
import qualified Data.ByteString.Internal          as B
import qualified Data.ByteString.Unsafe            as B
import           Data.Char
import qualified Data.Foldable                     as F
import           Data.Function
import           Data.Functor.Identity
import qualified Data.IntMap.Strict                as IM
import qualified Data.IntSet                       as IS
import qualified Data.List                         as L
import qualified Data.Map.Strict                   as M
import           Data.Monoid
import           Data.Ord
import           Data.Primitive
import           Data.Proxy
import           Data.Ratio
import           Data.Semigroup
import qualified Data.Set                          as S
import           Data.Tuple
import qualified Data.Vector                       as V
import qualified Data.Vector.Algorithms.Intro      as Intro
import qualified Data.Vector.Fusion.Stream.Monadic as MS
import qualified Data.Vector.Fusion.Bundle.Monadic as MB
import           Data.Vector.Fusion.Util
import qualified Data.Vector.Generic               as G
import qualified Data.Vector.Generic.Mutable       as GM
import qualified Data.Vector.Mutable               as VM
import qualified Data.Vector.Primitive             as P
import qualified Data.Vector.Primitive.Mutable     as PM
import qualified Data.Vector.Unboxed               as U
import qualified Data.Vector.Unboxed.Mutable       as UM
import           Debug.Trace
import           Foreign                           hiding (void)
import           GHC.Exts
import           GHC.TypeLits
import           System.IO
import           Unsafe.Coerce

#define MOD 1000000007

main :: IO ()
main = do
    [h, w] <- map read.words <$> getLine
    [sx,sy] <- map (subtract 1.read).words <$> getLine
    [gx,gy] <- map (subtract 1.read).words <$> getLine
    mat <- U.unfoldrN (h*w) (runParser char) . C.filter (not.isSpace) <$> C.getContents
    print . maybe (-1) id$ solve h w (sx,sy) (gx,gy) mat

solve :: Int -> Int -> (Int, Int) -> (Int, Int) -> U.Vector Char -> Maybe Int
solve h w (sx,sy) (gx,gy) mat = runST $ do
    dist <- UM.replicate (h*w) maxBound
    heap <- newMinBinaryHeap 4400400
    UM.write dist start 0
    insertBH (0, start) heap
    fix $ \loop -> deleteFindTopBH heap >>= \case
        Just (dv,xy) -> do
          dv' <- UM.unsafeRead dist xy
          when (dv == dv') $ do
            let (x, y) = quotRem xy w
            neighbor4 h w x y $ \nx ny -> when (U.unsafeIndex mat (ix nx ny) == '.') $ do
                dnxny <- UM.unsafeRead dist (ix nx ny)
                when (dv < dnxny) $ do
                    UM.unsafeWrite dist (ix nx ny) dv
                    insertBH (dv, ix nx ny) heap
            flip MS.mapM_ (stream (max 0 $ x-2) (min h $ x+3)) $ \nx -> do
                flip MS.mapM_ (stream (max 0 $ y-2) (min w $ y+3)) $ \ny -> do
                    let nxny = ix nx ny
                    when (U.unsafeIndex mat nxny == '.' ) $ do
                        dnxny <- UM.unsafeRead dist nxny
                        when (dv + 1 < dnxny) $ do
                            UM.unsafeWrite dist nxny (dv + 1)
                            insertBH (dv+1, nxny) heap
          loop
        Nothing -> return ()
    dg <- UM.read dist goal
    if dg == maxBound
    then return Nothing
    else return $ Just dg
  where
    ix i j = i * w + j
    {-# INLINE ix #-}
    start = ix sx sy
    goal = ix gx gy


inGrid :: Int -> Int -> Int -> Int -> Bool
inGrid h w x y = 0 <= x && x < h && 0 <= y && y < w
{-# INLINE inGrid #-}

neighbor4 :: (Applicative f) => Int -> Int -> Int -> Int -> (Int -> Int -> f ()) -> f ()
neighbor4 h w x y f
    =  when (x > 0) (f (x - 1) y)
    *> when (y > 0) (f x (y - 1))
    *> when (y < w - 1) (f x (y + 1))
    *> when (x < h - 1) (f (x + 1) y)
{-# INLINE neighbor4 #-}

-------------------------------------------------------------------------------
-- Utils
-------------------------------------------------------------------------------
rep :: (Monad m) => Int -> (Int -> m ()) -> m ()
rep n = flip MS.mapM_ (stream 0 n)
{-# INLINE rep #-}
rev :: (Monad m) => Int -> (Int -> m ()) -> m ()
rev !n = flip MS.mapM_ (streamR 0 n)
{-# INLINE rev #-}
stream :: (Monad m) => Int -> Int -> MS.Stream m Int
stream !l !r = MS.Stream step l where { step x | x < r = return $ MS.Yield x (x + 1) | otherwise = return MS.Done; {-# INLINE [0] step #-}}
{-# INLINE [1] stream #-}
streamR :: (Monad m) => Int -> Int -> MS.Stream m Int
streamR !l !r = MS.Stream step (r - 1) where { step x | x >= l = return $ MS.Yield x (x - 1) | otherwise = return MS.Done; {-# INLINE [0] step #-}}
{-# INLINE [1] streamR #-}
stream' :: (Monad m) => Int -> Int -> Int -> MS.Stream m Int
stream' !l !r !d = MS.Stream step l where { step x | x < r = return $ MS.Yield x (x + d) | otherwise = return MS.Done; {-# INLINE [0] step #-}}
{-# INLINE [1] stream' #-}
infixl 8 `shiftRL`, `unsafeShiftRL`
shiftRL :: Int -> Int -> Int
shiftRL = unsafeShiftRL
{-# INLINE shiftRL #-}
unsafeShiftRL :: Int -> Int -> Int
unsafeShiftRL (I# x#) (I# i#) = I# (uncheckedIShiftRL# x# i#)
{-# INLINE unsafeShiftRL #-}
type Parser a = StateT C.ByteString Maybe a
runParser :: Parser a -> C.ByteString -> Maybe (a, C.ByteString)
runParser = runStateT
{-# INLINE runParser #-}
int :: Parser Int
int = coerce $ C.readInt . C.dropWhile isSpace
{-# INLINE int #-}
int1 :: Parser Int
int1 = fmap (subtract 1) int
{-# INLINE int1 #-}
char :: Parser Char
char = coerce C.uncons
{-# INLINE char #-}
byte :: Parser Word8
byte = coerce B.uncons
{-# INLINE byte #-}
skipSpaces :: Parser ()
skipSpaces = modify' (C.dropWhile isSpace)
{-# INLINE skipSpaces #-}
lowerBoundM :: (Monad m) => Int -> Int -> (Int -> m Bool) -> m Int
lowerBoundM low high p = go low high where { go !low !high | high <= low = return high | otherwise = p mid >>= bool (go (mid + 1) high) (go low mid) where { mid = low + unsafeShiftRL (high - low) 1}}
{-# INLINE lowerBoundM #-}
upperBoundM :: (Monad m) => Int -> Int -> (Int -> m Bool) -> m Int
upperBoundM low high p = do { flg <- p high; if flg then return high else subtract 1 <$!> lowerBoundM low high (fmap not . p)}
{-# INLINE upperBoundM #-}
lowerBound :: Int -> Int -> (Int -> Bool) -> Int
lowerBound low high p = runIdentity (lowerBoundM low high (return . p))
{-# INLINE lowerBound #-}
upperBound :: Int -> Int -> (Int -> Bool) -> Int
upperBound low high p = runIdentity (upperBoundM low high (return . p))
{-# INLINE upperBound #-}
-------------------------------------------------------------------------------
-- Data.VecQueue
-------------------------------------------------------------------------------
data VecQueue s a = VecQueue{intVarsVQ :: !(UM.MVector s Int), internalVecQueue :: !(UM.MVector s a)}
_dequeueCount :: Int
_dequeueCount = 0
{-# INLINE _dequeueCount #-}
_enqueueCount :: Int
_enqueueCount = 1
{-# INLINE _enqueueCount #-}
newVecQueue :: (PrimMonad m, UM.Unbox a) => Int -> m (VecQueue (PrimState m) a)
newVecQueue n = VecQueue <$> UM.replicate 2 0 <*> UM.unsafeNew n
defaultVecQueueSize :: Int
defaultVecQueueSize = 1024 * 1024
lengthVQ :: (PrimMonad m, UM.Unbox a) => VecQueue (PrimState m) a -> m Int
lengthVQ (VecQueue info _) = (-) <$> UM.unsafeRead info _enqueueCount <*> UM.unsafeRead info _dequeueCount
{-# INLINE lengthVQ #-}
dequeueVQ :: (PrimMonad m, UM.Unbox a) => VecQueue (PrimState m) a -> m (Maybe a)
dequeueVQ (VecQueue info q) = do { f <- UM.unsafeRead info _dequeueCount; r <- UM.unsafeRead info _enqueueCount; if f < r then do { UM.unsafeWrite info _dequeueCount (f + 1); pure <$> UM.unsafeRead q f} else return Nothing}
{-# INLINE dequeueVQ #-}
enqueueVQ :: (PrimMonad m, UM.Unbox a) => a -> VecQueue (PrimState m) a -> m ()
enqueueVQ x (VecQueue info q) = do { r <- UM.unsafeRead info _enqueueCount; UM.unsafeWrite q r x; UM.unsafeWrite info _enqueueCount (r + 1)}
{-# INLINE enqueueVQ #-}
enqueuesVQ :: (PrimMonad m, UM.Unbox a) => U.Vector a -> VecQueue (PrimState m) a -> m ()
enqueuesVQ vec (VecQueue info q) = do { r <- UM.unsafeRead info _enqueueCount; UM.unsafeWrite info _enqueueCount (r + U.length vec); U.unsafeCopy (UM.unsafeSlice r (U.length vec) q) vec}
{-# INLINE enqueuesVQ #-}
clearVQ :: (UM.Unbox a, PrimMonad m) => VecQueue (PrimState m) a -> m ()
clearVQ (VecQueue info _) = do { UM.unsafeWrite info _dequeueCount 0; UM.unsafeWrite info _enqueueCount 0}
freezeVecQueue :: (PrimMonad m, UM.Unbox a) => VecQueue (PrimState m) a -> m (U.Vector a)
freezeVecQueue (VecQueue info q) = do { f <- UM.unsafeRead info _dequeueCount; r <- UM.unsafeRead info _enqueueCount; U.unsafeFreeze $ UM.unsafeSlice f (r - f) q}
-------------------------------------------------------------------------------
-- Data.Heap.Binary
-------------------------------------------------------------------------------
data BinaryHeap (f :: * -> *) s a = BinaryHeap{priorityBH :: a -> f a, intVarsBH :: !(UM.MVector s Int), internalVecBH :: !(UM.MVector s a)}
_sizeBH :: Int
_sizeBH = 0
{-# INLINE _sizeBH #-}
type MinBinaryHeap s a = BinaryHeap Identity s a
type MaxBinaryHeap s a = BinaryHeap Down s a
newBinaryHeap :: (U.Unbox a, PrimMonad m) => (a -> f a) -> Int -> m (BinaryHeap f (PrimState m) a)
newBinaryHeap prio n = BinaryHeap prio <$> UM.replicate 1 0 <*> UM.unsafeNew n
newMinBinaryHeap :: (U.Unbox a, PrimMonad m) => Int -> m (MinBinaryHeap (PrimState m) a)
newMinBinaryHeap = newBinaryHeap Identity
newMaxBinaryHeap :: (U.Unbox a, PrimMonad m) => Int -> m (MaxBinaryHeap (PrimState m) a)
newMaxBinaryHeap = newBinaryHeap Down
getBinaryHeapSize :: (PrimMonad m) => BinaryHeap f (PrimState m) a -> m Int
getBinaryHeapSize BinaryHeap{..} = UM.unsafeRead intVarsBH _sizeBH
{-# INLINE getBinaryHeapSize #-}
siftUpBy :: (U.Unbox a, PrimMonad m) => (a -> a -> Ordering) -> Int -> UM.MVector (PrimState m) a -> m ()
siftUpBy cmp k vec = do { x <- UM.unsafeRead vec k; flip fix k $ \ loop !i -> if i > 0 then do { let { parent = (i - 1) `unsafeShiftR` 1}; p <- UM.unsafeRead vec parent; case cmp p x of { GT -> UM.unsafeWrite vec i p >> loop parent; _ -> UM.unsafeWrite vec i x}} else UM.unsafeWrite vec 0 x}
{-# INLINE siftUpBy #-}
siftDownBy :: (U.Unbox a, PrimMonad m) => (a -> a -> Ordering) -> Int -> UM.MVector (PrimState m) a -> m ()
siftDownBy cmp k vec = do { x <- UM.unsafeRead vec k; let { !n = UM.length vec}; flip fix k $ \ loop !i -> do { let { l = unsafeShiftL i 1 .|. 1}; let { r = l + 1}; if n <= l then UM.unsafeWrite vec i x else do { vl <- UM.unsafeRead vec l; if r < n then do { vr <- UM.unsafeRead vec r; case cmp vr vl of { LT -> case cmp x vr of { GT -> UM.unsafeWrite vec i vr >> loop r; _ -> UM.unsafeWrite vec i x}; _ -> case cmp x vl of { GT -> UM.unsafeWrite vec i vl >> loop l; _ -> UM.unsafeWrite vec i x}}} else case cmp x vl of { GT -> UM.unsafeWrite vec i vl >> loop l; _ -> UM.unsafeWrite vec i x}}}}
{-# INLINE siftDownBy #-}
heapifyBy :: (U.Unbox a, PrimMonad m) => (a -> a -> Ordering) -> UM.MVector (PrimState m) a -> m ()
heapifyBy cmp vec = do { rev (UM.length vec `quot` 2) $ \ i -> do { siftDownBy cmp i vec}}
{-# INLINE heapifyBy #-}
class OrdVia f a where { compareVia :: (a -> f a) -> a -> a -> Ordering}
instance (Ord a) => OrdVia Identity a where { compareVia _ = coerce (compare :: Identity a -> Identity a -> Ordering); {-# INLINE compareVia #-}}
instance (Ord a) => OrdVia Down a where { compareVia _ = coerce (compare :: Down a -> Down a -> Ordering); {-# INLINE compareVia #-}}
buildBinaryHeapVia :: (OrdVia f a, U.Unbox a, PrimMonad m) => (a -> f a) -> U.Vector a -> m (BinaryHeap f (PrimState m) a)
buildBinaryHeapVia ~priorityBH vec = do { intVarsBH <- UM.replicate 1 $ U.length vec; internalVecBH <- U.thaw vec; heapifyBy (compareVia priorityBH) internalVecBH; return $! BinaryHeap{..}}
{-# INLINE buildBinaryHeapVia #-}
buildMinBinaryHeap :: (Ord a, U.Unbox a, PrimMonad m) => U.Vector a -> m (BinaryHeap Identity (PrimState m) a)
buildMinBinaryHeap = buildBinaryHeapVia Identity
{-# INLINE buildMinBinaryHeap #-}
buildMaxBinaryHeap :: (Ord a, U.Unbox a, PrimMonad m) => U.Vector a -> m (BinaryHeap Down (PrimState m) a)
buildMaxBinaryHeap = buildBinaryHeapVia Down
{-# INLINE buildMaxBinaryHeap #-}
unsafeViewBH :: (U.Unbox a, PrimMonad m) => BinaryHeap f (PrimState m) a -> m a
unsafeViewBH BinaryHeap{..} = UM.unsafeRead internalVecBH 0
{-# INLINE unsafeViewBH #-}
viewBH :: (U.Unbox a, PrimMonad m) => BinaryHeap f (PrimState m) a -> m (Maybe a)
viewBH bh = do { size <- getBinaryHeapSize bh; if size > 0 then Just <$!> unsafeViewBH bh else return $! Nothing}
{-# INLINE viewBH #-}
insertBH :: (OrdVia f a, U.Unbox a, PrimMonad m) => a -> BinaryHeap f (PrimState m) a -> m ()
insertBH x BinaryHeap{..} = do { size <- UM.unsafeRead intVarsBH _sizeBH; UM.unsafeWrite intVarsBH _sizeBH (size + 1); UM.unsafeWrite internalVecBH size x; siftUpBy (compareVia priorityBH) size internalVecBH}
{-# INLINE insertBH #-}
unsafeDeleteBH :: (OrdVia f a, U.Unbox a, PrimMonad m) => BinaryHeap f (PrimState m) a -> m ()
unsafeDeleteBH BinaryHeap{..} = do { size' <- subtract 1 <$!> UM.unsafeRead intVarsBH _sizeBH; UM.unsafeWrite intVarsBH _sizeBH size'; UM.unsafeSwap internalVecBH 0 size'; siftDownBy (compareVia priorityBH) 0 (UM.unsafeTake size' internalVecBH)}
{-# INLINE unsafeDeleteBH #-}
modifyTopBH :: (OrdVia f a, U.Unbox a, PrimMonad m) => (a -> a) -> BinaryHeap f (PrimState m) a -> m ()
modifyTopBH f BinaryHeap{..} = do { UM.unsafeModify internalVecBH f 0; size <- UM.unsafeRead intVarsBH _sizeBH; siftDownBy (compareVia priorityBH) 0 (UM.unsafeTake size internalVecBH)}
{-# INLINE modifyTopBH #-}
deleteFindTopBH :: (Ord a, U.Unbox a, PrimMonad m) => MinBinaryHeap (PrimState m) a -> m (Maybe a)
deleteFindTopBH bh = do { size <- getBinaryHeapSize bh; if size > 0 then do { !top <- unsafeViewBH bh <* unsafeDeleteBH bh; return $ Just top} else return Nothing}
{-# INLINE deleteFindTopBH #-}
clearBH :: (PrimMonad m) => BinaryHeap f (PrimState m) a -> m ()
clearBH BinaryHeap{..} = UM.unsafeWrite intVarsBH 0 0
freezeInternalVecBH :: (U.Unbox a, PrimMonad m) => BinaryHeap f (PrimState m) a -> m (U.Vector a)
freezeInternalVecBH BinaryHeap{..} = do { size <- UM.unsafeRead intVarsBH _sizeBH; U.unsafeFreeze (UM.unsafeTake size internalVecBH)}