{-# LANGUAGE MultiParamTypeClasses, BangPatterns, ScopedTypeVariables #-}
-- |
-- Module      : Data.Vector.Generic.Mutable
-- Copyright   : (c) Roman Leshchinskiy 2008-2010
-- License     : BSD-style
--
-- Maintainer  : Roman Leshchinskiy <rl@cse.unsw.edu.au>
-- Stability   : experimental
-- Portability : non-portable
-- 
-- Generic interface to mutable vectors
--

module Data.Vector.Generic.Mutable (
  -- * Class of mutable vector types
  MVector(..),

  -- * Operations on mutable vectors
  length, overlaps, new, newWith, read, write, swap, clear, set, copy, grow,

  slice, take, drop, init, tail,
  unsafeSlice, unsafeInit, unsafeTail,

  -- * Unsafe operations
  unsafeNew, unsafeNewWith, unsafeRead, unsafeWrite, unsafeSwap,
  unsafeCopy, unsafeGrow,

  -- * Internal operations
  unstream, unstreamR,
  munstream, munstreamR,
  transform, transformR,
  fill, fillR,
  unsafeAccum, accum, unsafeUpdate, update, reverse,
  unstablePartition, unstablePartitionStream, partitionStream
) where

import qualified Data.Vector.Fusion.Stream      as Stream
import           Data.Vector.Fusion.Stream      ( Stream, MStream )
import qualified Data.Vector.Fusion.Stream.Monadic as MStream
import           Data.Vector.Fusion.Stream.Size

import Control.Monad.Primitive ( PrimMonad, PrimState )

import Prelude hiding ( length, reverse, map, read,
                        take, drop, init, tail )

#include "vector.h"

-- | Class of mutable vectors parametrised with a primitive state token.
--
class MVector v a where
  -- | Length of the mutable vector. This method should not be
  -- called directly, use 'length' instead.
  basicLength       :: v s a -> Int

  -- | Yield a part of the mutable vector without copying it. This method
  -- should not be called directly, use 'unsafeSlice' instead.
  basicUnsafeSlice :: Int  -- ^ starting index
                   -> Int  -- ^ length of the slice
                   -> v s a
                   -> v s a

  -- Check whether two vectors overlap. This method should not be
  -- called directly, use 'overlaps' instead.
  basicOverlaps    :: v s a -> v s a -> Bool

  -- | Create a mutable vector of the given length. This method should not be
  -- called directly, use 'unsafeNew' instead.
  basicUnsafeNew   :: PrimMonad m => Int -> m (v (PrimState m) a)

  -- | Create a mutable vector of the given length and fill it with an
  -- initial value. This method should not be called directly, use
  -- 'unsafeNewWith' instead.
  basicUnsafeNewWith :: PrimMonad m => Int -> a -> m (v (PrimState m) a)

  -- | Yield the element at the given position. This method should not be
  -- called directly, use 'unsafeRead' instead.
  basicUnsafeRead  :: PrimMonad m => v (PrimState m) a -> Int -> m a

  -- | Replace the element at the given position. This method should not be
  -- called directly, use 'unsafeWrite' instead.
  basicUnsafeWrite :: PrimMonad m => v (PrimState m) a -> Int -> a -> m ()

  -- | Reset all elements of the vector to some undefined value, clearing all
  -- references to external objects. This is usually a noop for unboxed
  -- vectors. This method should not be called directly, use 'clear' instead.
  basicClear       :: PrimMonad m => v (PrimState m) a -> m ()

  -- | Set all elements of the vector to the given value. This method should
  -- not be called directly, use 'set' instead.
  basicSet         :: PrimMonad m => v (PrimState m) a -> a -> m ()

  -- | Copy a vector. The two vectors may not overlap. This method should not
  -- be called directly, use 'unsafeCopy' instead.
  basicUnsafeCopy  :: PrimMonad m => v (PrimState m) a   -- ^ target
                                  -> v (PrimState m) a   -- ^ source
                                  -> m ()

  -- | Grow a vector by the given number of elements. This method should not be
  -- called directly, use 'unsafeGrow' instead.
  basicUnsafeGrow  :: PrimMonad m => v (PrimState m) a -> Int
                                                       -> m (v (PrimState m) a)

  {-# INLINE basicUnsafeNewWith #-}
  basicUnsafeNewWith n x
    = do
        v <- basicUnsafeNew n
        basicSet v x
        return v

  {-# INLINE basicClear #-}
  basicClear _ = return ()

  {-# INLINE basicSet #-}
  basicSet v x = do_set 0
    where
      n = basicLength v

      do_set i | i < n = do
                           basicUnsafeWrite v i x
                           do_set (i+1)
                | otherwise = return ()

  {-# INLINE basicUnsafeCopy #-}
  basicUnsafeCopy dst src = do_copy 0
    where
      n = basicLength src

      do_copy i | i < n = do
                            x <- basicUnsafeRead src i
                            basicUnsafeWrite dst i x
                            do_copy (i+1)
                | otherwise = return ()

  {-# INLINE basicUnsafeGrow #-}
  basicUnsafeGrow v by
    = do
        v' <- basicUnsafeNew (n+by)
        basicUnsafeCopy (basicUnsafeSlice 0 n v') v
        return v'
    where
      n = basicLength v

-- ------------------
-- Internal functions
-- ------------------

-- Check whether two vectors overlap.
overlaps :: MVector v a => v s a -> v s a -> Bool
{-# INLINE overlaps #-}
overlaps = basicOverlaps

unsafeAppend1 :: (PrimMonad m, MVector v a)
        => v (PrimState m) a -> Int -> a -> m (v (PrimState m) a)
{-# INLINE_INNER unsafeAppend1 #-}
    -- NOTE: The case distinction has to be on the outside because
    -- GHC creates a join point for the unsafeWrite even when everything
    -- is inlined. This is bad because with the join point, v isn't getting
    -- unboxed.
unsafeAppend1 v i x
  | i < length v = do
                     unsafeWrite v i x
                     return v
  | otherwise    = do
                     v' <- enlarge v
                     INTERNAL_CHECK(checkIndex) "unsafeAppend1" i (length v')
                       $ unsafeWrite v' i x
                     return v'

unsafePrepend1 :: (PrimMonad m, MVector v a)
        => v (PrimState m) a -> Int -> a -> m (v (PrimState m) a, Int)
{-# INLINE_INNER unsafePrepend1 #-}
unsafePrepend1 v i x
  | i /= 0    = do
                  let i' = i-1
                  unsafeWrite v i' x
                  return (v, i')
  | otherwise = do
                  (v', i) <- enlargeFront v
                  let i' = i-1
                  INTERNAL_CHECK(checkIndex) "unsafePrepend1" i' (length v')
                    $ unsafeWrite v' i' x
                  return (v', i')

mstream :: (PrimMonad m, MVector v a) => v (PrimState m) a -> MStream m a
{-# INLINE mstream #-}
mstream v = v `seq` (MStream.unfoldrM get 0 `MStream.sized` Exact n)
  where
    n = length v

    {-# INLINE_INNER get #-}
    get i | i < n     = do x <- unsafeRead v i
                           return $ Just (x, i+1)
          | otherwise = return $ Nothing

fill :: (PrimMonad m, MVector v a)
           => v (PrimState m) a -> MStream m a -> m (v (PrimState m) a)
{-# INLINE fill #-}
fill v s = v `seq` do
                     n' <- MStream.foldM put 0 s
                     return $ unsafeSlice 0 n' v
  where
    {-# INLINE_INNER put #-}
    put i x = do
                INTERNAL_CHECK(checkIndex) "fill" i (length v)
                  $ unsafeWrite v i x
                return (i+1)

transform :: (PrimMonad m, MVector v a)
  => (MStream m a -> MStream m a) -> v (PrimState m) a -> m (v (PrimState m) a)
{-# INLINE_STREAM transform #-}
transform f v = fill v (f (mstream v))

mstreamR :: (PrimMonad m, MVector v a) => v (PrimState m) a -> MStream m a
{-# INLINE mstreamR #-}
mstreamR v = v `seq` (MStream.unfoldrM get n `MStream.sized` Exact n)
  where
    n = length v

    {-# INLINE_INNER get #-}
    get i | j >= 0    = do x <- unsafeRead v j
                           return $ Just (x,j)
          | otherwise = return Nothing
      where
        j = i-1

fillR :: (PrimMonad m, MVector v a)
           => v (PrimState m) a -> MStream m a -> m (v (PrimState m) a)
{-# INLINE fillR #-}
fillR v s = v `seq` do
                      i <- MStream.foldM put n s
                      return $ unsafeSlice i (n-i) v
  where
    n = length v

    {-# INLINE_INNER put #-}
    put i x = do
                unsafeWrite v j x
                return j
      where
        j = i-1

transformR :: (PrimMonad m, MVector v a)
  => (MStream m a -> MStream m a) -> v (PrimState m) a -> m (v (PrimState m) a)
{-# INLINE_STREAM transformR #-}
transformR f v = fillR v (f (mstreamR v))

-- | Create a new mutable vector and fill it with elements from the 'Stream'.
-- The vector will grow exponentially if the maximum size of the 'Stream' is
-- unknown.
unstream :: (PrimMonad m, MVector v a) => Stream a -> m (v (PrimState m) a)
-- NOTE: replace INLINE_STREAM by INLINE? (also in unstreamR)
{-# INLINE_STREAM unstream #-}
unstream s = munstream (Stream.liftStream s)

-- | Create a new mutable vector and fill it with elements from the monadic
-- stream. The vector will grow exponentially if the maximum size of the stream
-- is unknown.
munstream :: (PrimMonad m, MVector v a) => MStream m a -> m (v (PrimState m) a)
{-# INLINE_STREAM munstream #-}
munstream s = case upperBound (MStream.size s) of
               Just n  -> munstreamMax     s n
               Nothing -> munstreamUnknown s

-- FIXME: I can't think of how to prevent GHC from floating out
-- unstreamUnknown. That is bad because SpecConstr then generates two
-- specialisations: one for when it is called from unstream (it doesn't know
-- the shape of the vector) and one for when the vector has grown. To see the
-- problem simply compile this:
--
-- fromList = Data.Vector.Unboxed.unstream . Stream.fromList
--
-- I'm not sure this still applies (19/04/2010)

munstreamMax
  :: (PrimMonad m, MVector v a) => MStream m a -> Int -> m (v (PrimState m) a)
{-# INLINE munstreamMax #-}
munstreamMax s n
  = do
      v <- INTERNAL_CHECK(checkLength) "munstreamMax" n
           $ unsafeNew n
      let put i x = do
                       INTERNAL_CHECK(checkIndex) "munstreamMax" i n
                         $ unsafeWrite v i x
                       return (i+1)
      n' <- MStream.foldM' put 0 s
      return $ INTERNAL_CHECK(checkSlice) "munstreamMax" 0 n' n
             $ unsafeSlice 0 n' v

munstreamUnknown
  :: (PrimMonad m, MVector v a) => MStream m a -> m (v (PrimState m) a)
{-# INLINE munstreamUnknown #-}
munstreamUnknown s
  = do
      v <- unsafeNew 0
      (v', n) <- MStream.foldM put (v, 0) s
      return $ INTERNAL_CHECK(checkSlice) "munstreamUnknown" 0 n (length v')
             $ unsafeSlice 0 n v'
  where
    {-# INLINE_INNER put #-}
    put (v,i) x = do
                    v' <- unsafeAppend1 v i x
                    return (v',i+1)

-- | Create a new mutable vector and fill it with elements from the 'Stream'
-- from right to left. The vector will grow exponentially if the maximum size
-- of the 'Stream' is unknown.
unstreamR :: (PrimMonad m, MVector v a) => Stream a -> m (v (PrimState m) a)
-- NOTE: replace INLINE_STREAM by INLINE? (also in unstream)
{-# INLINE_STREAM unstreamR #-}
unstreamR s = munstreamR (Stream.liftStream s)

-- | Create a new mutable vector and fill it with elements from the monadic
-- stream from right to left. The vector will grow exponentially if the maximum
-- size of the stream is unknown.
munstreamR :: (PrimMonad m, MVector v a) => MStream m a -> m (v (PrimState m) a)
{-# INLINE_STREAM munstreamR #-}
munstreamR s = case upperBound (MStream.size s) of
               Just n  -> munstreamRMax     s n
               Nothing -> munstreamRUnknown s

munstreamRMax
  :: (PrimMonad m, MVector v a) => MStream m a -> Int -> m (v (PrimState m) a)
{-# INLINE munstreamRMax #-}
munstreamRMax s n
  = do
      v <- INTERNAL_CHECK(checkLength) "munstreamRMax" n
           $ unsafeNew n
      let put i x = do
                      let i' = i-1
                      INTERNAL_CHECK(checkIndex) "munstreamRMax" i' n
                        $ unsafeWrite v i' x
                      return i'
      i <- MStream.foldM' put n s
      return $ INTERNAL_CHECK(checkSlice) "munstreamRMax" i (n-i) n
             $ unsafeSlice i (n-i) v

munstreamRUnknown
  :: (PrimMonad m, MVector v a) => MStream m a -> m (v (PrimState m) a)
{-# INLINE munstreamRUnknown #-}
munstreamRUnknown s
  = do
      v <- unsafeNew 0
      (v', i) <- MStream.foldM put (v, 0) s
      let n = length v'
      return $ INTERNAL_CHECK(checkSlice) "unstreamRUnknown" i (n-i) n
             $ unsafeSlice i (n-i) v'
  where
    {-# INLINE_INNER put #-}
    put (v,i) x = unsafePrepend1 v i x

-- Length
-- ------

-- | Length of the mutable vector.
length :: MVector v a => v s a -> Int
{-# INLINE length #-}
length = basicLength

-- | Check whether the vector is empty
null :: MVector v a => v s a -> Bool
{-# INLINE null #-}
null v = length v == 0


-- Construction
-- ------------

-- | Create a mutable vector of the given length.
new :: (PrimMonad m, MVector v a) => Int -> m (v (PrimState m) a)
{-# INLINE new #-}
new n = BOUNDS_CHECK(checkLength) "new" n
      $ unsafeNew n

-- | Create a mutable vector of the given length and fill it with an
-- initial value.
newWith :: (PrimMonad m, MVector v a) => Int -> a -> m (v (PrimState m) a)
{-# INLINE newWith #-}
newWith n x = BOUNDS_CHECK(checkLength) "newWith" n
            $ unsafeNewWith n x

-- | Create a mutable vector of the given length. The length is not checked.
unsafeNew :: (PrimMonad m, MVector v a) => Int -> m (v (PrimState m) a)
{-# INLINE unsafeNew #-}
unsafeNew n = UNSAFE_CHECK(checkLength) "unsafeNew" n
            $ basicUnsafeNew n

-- | Create a mutable vector of the given length and fill it with an
-- initial value. The length is not checked.
unsafeNewWith :: (PrimMonad m, MVector v a) => Int -> a -> m (v (PrimState m) a)
{-# INLINE unsafeNewWith #-}
unsafeNewWith n x = UNSAFE_CHECK(checkLength) "unsafeNewWith" n
                  $ basicUnsafeNewWith n x


-- Growing
-- -------

-- | Grow a vector by the given number of elements. The number must be
-- positive.
grow :: (PrimMonad m, MVector v a)
                => v (PrimState m) a -> Int -> m (v (PrimState m) a)
{-# INLINE grow #-}
grow v by = BOUNDS_CHECK(checkLength) "grow" by
          $ unsafeGrow v by

growFront :: (PrimMonad m, MVector v a)
                => v (PrimState m) a -> Int -> m (v (PrimState m) a)
{-# INLINE growFront #-}
growFront v by = BOUNDS_CHECK(checkLength) "growFront" by
               $ unsafeGrowFront v by

enlarge_delta v = max (length v) 1

-- | Grow a vector logarithmically
enlarge :: (PrimMonad m, MVector v a)
                => v (PrimState m) a -> m (v (PrimState m) a)
{-# INLINE enlarge #-}
enlarge v = unsafeGrow v (enlarge_delta v)

enlargeFront :: (PrimMonad m, MVector v a)
                => v (PrimState m) a -> m (v (PrimState m) a, Int)
{-# INLINE enlargeFront #-}
enlargeFront v = do
                   v' <- unsafeGrowFront v by
                   return (v', by)
  where
    by = enlarge_delta v

-- | Grow a vector by the given number of elements. The number must be
-- positive but this is not checked.
unsafeGrow :: (PrimMonad m, MVector v a)
                        => v (PrimState m) a -> Int -> m (v (PrimState m) a)
{-# INLINE unsafeGrow #-}
unsafeGrow v n = UNSAFE_CHECK(checkLength) "unsafeGrow" n
               $ basicUnsafeGrow v n

unsafeGrowFront :: (PrimMonad m, MVector v a)
                        => v (PrimState m) a -> Int -> m (v (PrimState m) a)
{-# INLINE unsafeGrowFront #-}
unsafeGrowFront v by = UNSAFE_CHECK(checkLength) "unsafeGrowFront" by
                     $ do
                         let n = length v
                         v' <- basicUnsafeNew (by+n)
                         basicUnsafeCopy (basicUnsafeSlice by n v') v
                         return v'

-- Accessing individual elements
-- -----------------------------

-- | Yield the element at the given position.
read :: (PrimMonad m, MVector v a) => v (PrimState m) a -> Int -> m a
{-# INLINE read #-}
read v i = BOUNDS_CHECK(checkIndex) "read" i (length v)
         $ unsafeRead v i

-- | Replace the element at the given position.
write :: (PrimMonad m, MVector v a) => v (PrimState m) a -> Int -> a -> m ()
{-# INLINE write #-}
write v i x = BOUNDS_CHECK(checkIndex) "write" i (length v)
            $ unsafeWrite v i x

-- | Swap the elements at the given positions.
swap :: (PrimMonad m, MVector v a) => v (PrimState m) a -> Int -> Int -> m ()
{-# INLINE swap #-}
swap v i j = BOUNDS_CHECK(checkIndex) "swap" i (length v)
           $ BOUNDS_CHECK(checkIndex) "swap" j (length v)
           $ unsafeSwap v i j

-- | Replace the element at the give position and return the old element.
exchange :: (PrimMonad m, MVector v a) => v (PrimState m) a -> Int -> a -> m a
{-# INLINE exchange #-}
exchange v i x = BOUNDS_CHECK(checkIndex) "exchange" i (length v)
               $ unsafeExchange v i x

-- | Yield the element at the given position. No bounds checks are performed.
unsafeRead :: (PrimMonad m, MVector v a) => v (PrimState m) a -> Int -> m a
{-# INLINE unsafeRead #-}
unsafeRead v i = UNSAFE_CHECK(checkIndex) "unsafeRead" i (length v)
               $ basicUnsafeRead v i

-- | Replace the element at the given position. No bounds checks are performed.
unsafeWrite :: (PrimMonad m, MVector v a)
                                => v (PrimState m) a -> Int -> a -> m ()
{-# INLINE unsafeWrite #-}
unsafeWrite v i x = UNSAFE_CHECK(checkIndex) "unsafeWrite" i (length v)
                  $ basicUnsafeWrite v i x

-- | Swap the elements at the given positions. No bounds checks are performed.
unsafeSwap :: (PrimMonad m, MVector v a)
                => v (PrimState m) a -> Int -> Int -> m ()
{-# INLINE unsafeSwap #-}
unsafeSwap v i j = UNSAFE_CHECK(checkIndex) "unsafeSwap" i (length v)
                 $ UNSAFE_CHECK(checkIndex) "unsafeSwap" j (length v)
                 $ do
                     x <- unsafeRead v i
                     y <- unsafeRead v j
                     unsafeWrite v i y
                     unsafeWrite v j x

-- | Replace the element at the give position and return the old element. No
-- bounds checks are performed.
unsafeExchange :: (PrimMonad m, MVector v a)
                                => v (PrimState m) a -> Int -> a -> m a
{-# INLINE unsafeExchange #-}
unsafeExchange v i x = UNSAFE_CHECK(checkIndex) "unsafeExchange" i (length v)
                     $ do
                         y <- unsafeRead v i
                         unsafeWrite v i x
                         return y

-- Block operations
-- ----------------

-- | Reset all elements of the vector to some undefined value, clearing all
-- references to external objects. This is usually a noop for unboxed vectors. 
clear :: (PrimMonad m, MVector v a) => v (PrimState m) a -> m ()
{-# INLINE clear #-}
clear = basicClear

-- | Set all elements of the vector to the given value.
set :: (PrimMonad m, MVector v a) => v (PrimState m) a -> a -> m ()
{-# INLINE set #-}
set = basicSet

-- | Copy a vector. The two vectors must have the same length and may not
-- overlap.
copy :: (PrimMonad m, MVector v a)
                => v (PrimState m) a -> v (PrimState m) a -> m ()
{-# INLINE copy #-}
copy dst src = BOUNDS_CHECK(check) "copy" "overlapping vectors"
                                          (not (dst `overlaps` src))
             $ BOUNDS_CHECK(check) "copy" "length mismatch"
                                          (length dst == length src)
             $ unsafeCopy dst src

-- | Copy a vector. The two vectors must have the same length and may not
-- overlap. This is not checked.
unsafeCopy :: (PrimMonad m, MVector v a) => v (PrimState m) a   -- ^ target
                                         -> v (PrimState m) a   -- ^ source
                                         -> m ()
{-# INLINE unsafeCopy #-}
unsafeCopy dst src = UNSAFE_CHECK(check) "unsafeCopy" "length mismatch"
                                         (length dst == length src)
                   $ UNSAFE_CHECK(check) "unsafeCopy" "overlapping vectors"
                                         (not (dst `overlaps` src))
                   $ (dst `seq` src `seq` basicUnsafeCopy dst src)

-- Subvectors
-- ----------

-- | Yield a part of the mutable vector without copying it.
slice :: MVector v a => Int -> Int -> v s a -> v s a
{-# INLINE slice #-}
slice i n v = BOUNDS_CHECK(checkSlice) "slice" i n (length v)
            $ unsafeSlice i n v

take :: MVector v a => Int -> v s a -> v s a
{-# INLINE take #-}
take n v = unsafeSlice 0 (min (max n 0) (length v)) v

drop :: MVector v a => Int -> v s a -> v s a
{-# INLINE drop #-}
drop n v = unsafeSlice (min m n') (max 0 (m - n')) v
  where
    n' = max n 0
    m  = length v

init :: MVector v a => v s a -> v s a
{-# INLINE init #-}
init v = slice 0 (length v - 1) v

tail :: MVector v a => v s a -> v s a
{-# INLINE tail #-}
tail v = slice 1 (length v - 1) v

-- | Yield a part of the mutable vector without copying it. No bounds checks
-- are performed.
unsafeSlice :: MVector v a => Int  -- ^ starting index
                           -> Int  -- ^ length of the slice
                           -> v s a
                           -> v s a
{-# INLINE unsafeSlice #-}
unsafeSlice i n v = UNSAFE_CHECK(checkSlice) "unsafeSlice" i n (length v)
                  $ basicUnsafeSlice i n v

unsafeInit :: MVector v a => v s a -> v s a
{-# INLINE unsafeInit #-}
unsafeInit v = unsafeSlice 0 (length v - 1) v

unsafeTail :: MVector v a => v s a -> v s a
{-# INLINE unsafeTail #-}
unsafeTail v = unsafeSlice 1 (length v - 1) v

unsafeTake :: MVector v a => Int -> v s a -> v s a
{-# INLINE unsafeTake #-}
unsafeTake n v = unsafeSlice 0 n v

unsafeDrop :: MVector v a => Int -> v s a -> v s a
{-# INLINE unsafeDrop #-}
unsafeDrop n v = unsafeSlice n (length v - n) v

-- Permutations
-- ------------

accum :: (PrimMonad m, MVector v a)
        => (a -> b -> a) -> v (PrimState m) a -> Stream (Int, b) -> m ()
{-# INLINE accum #-}
accum f !v s = Stream.mapM_ upd s
  where
    {-# INLINE_INNER upd #-}
    upd (i,b) = do
                  a <- BOUNDS_CHECK(checkIndex) "accum" i (length v)
                     $ unsafeRead v i
                  unsafeWrite v i (f a b)

update :: (PrimMonad m, MVector v a)
                        => v (PrimState m) a -> Stream (Int, a) -> m ()
{-# INLINE update #-}
update !v s = Stream.mapM_ upd s
  where
    {-# INLINE_INNER upd #-}
    upd (i,b) = BOUNDS_CHECK(checkIndex) "update" i (length v)
              $ unsafeWrite v i b

unsafeAccum :: (PrimMonad m, MVector v a)
            => (a -> b -> a) -> v (PrimState m) a -> Stream (Int, b) -> m ()
{-# INLINE unsafeAccum #-}
unsafeAccum f !v s = Stream.mapM_ upd s
  where
    {-# INLINE_INNER upd #-}
    upd (i,b) = do
                  a <- UNSAFE_CHECK(checkIndex) "accum" i (length v)
                     $ unsafeRead v i
                  unsafeWrite v i (f a b)

unsafeUpdate :: (PrimMonad m, MVector v a)
                        => v (PrimState m) a -> Stream (Int, a) -> m ()
{-# INLINE unsafeUpdate #-}
unsafeUpdate !v s = Stream.mapM_ upd s
  where
    {-# INLINE_INNER upd #-}
    upd (i,b) = UNSAFE_CHECK(checkIndex) "accum" i (length v)
                  $ unsafeWrite v i b

reverse :: (PrimMonad m, MVector v a) => v (PrimState m) a -> m ()
{-# INLINE reverse #-}
reverse !v = reverse_loop 0 (length v - 1)
  where
    reverse_loop i j | i < j = do
                                 unsafeSwap v i j
                                 reverse_loop (i + 1) (j - 1)
    reverse_loop _ _ = return ()

unstablePartition :: forall m v a. (PrimMonad m, MVector v a)
                  => (a -> Bool) -> v (PrimState m) a -> m Int
{-# INLINE unstablePartition #-}
unstablePartition f !v = from_left 0 (length v)
  where
    -- NOTE: GHC 6.10.4 panics without the signatures on from_left and
    -- from_right
    from_left :: Int -> Int -> m Int
    from_left i j
      | i == j    = return i
      | otherwise = do
                      x <- unsafeRead v i
                      if f x
                        then from_left (i+1) j
                        else from_right i (j-1)

    from_right :: Int -> Int -> m Int
    from_right i j
      | i == j    = return i
      | otherwise = do
                      x <- unsafeRead v j
                      if f x
                        then do
                               y <- unsafeRead v i
                               unsafeWrite v i x
                               unsafeWrite v j y
                               from_left (i+1) j
                        else from_right i (j-1)

unstablePartitionStream :: (PrimMonad m, MVector v a)
        => (a -> Bool) -> Stream a -> m (v (PrimState m) a, v (PrimState m) a)
{-# INLINE unstablePartitionStream #-}
unstablePartitionStream f s
  = case upperBound (Stream.size s) of
      Just n  -> unstablePartitionMax f s n
      Nothing -> partitionUnknown f s

unstablePartitionMax :: (PrimMonad m, MVector v a)
        => (a -> Bool) -> Stream a -> Int
        -> m (v (PrimState m) a, v (PrimState m) a)
{-# INLINE unstablePartitionMax #-}
unstablePartitionMax f s n
  = do
      v <- INTERNAL_CHECK(checkLength) "unstablePartitionMax" n
           $ unsafeNew n
      let {-# INLINE_INNER put #-}
          put (i, j) x
            | f x       = do
                            unsafeWrite v i x
                            return (i+1, j)
            | otherwise = do
                            unsafeWrite v (j-1) x
                            return (i, j-1)
                                
      (i,j) <- Stream.foldM' put (0, n) s
      return (unsafeSlice 0 i v, unsafeSlice j (n-j) v)

partitionStream :: (PrimMonad m, MVector v a)
        => (a -> Bool) -> Stream a -> m (v (PrimState m) a, v (PrimState m) a)
{-# INLINE partitionStream #-}
partitionStream f s
  = case upperBound (Stream.size s) of
      Just n  -> partitionMax f s n
      Nothing -> partitionUnknown f s

partitionMax :: (PrimMonad m, MVector v a)
  => (a -> Bool) -> Stream a -> Int -> m (v (PrimState m) a, v (PrimState m) a)
{-# INLINE partitionMax #-}
partitionMax f s n
  = do
      v <- INTERNAL_CHECK(checkLength) "unstablePartitionMax" n
         $ unsafeNew n

      let {-# INLINE_INNER put #-}
          put (i,j) x
            | f x       = do
                            unsafeWrite v i x
                            return (i+1,j)

            | otherwise = let j' = j-1 in
                          do
                            unsafeWrite v j' x
                            return (i,j') 
                            
      (i,j) <- Stream.foldM' put (0,n) s
      INTERNAL_CHECK(check) "partitionMax" "invalid indices" (i <= j)
        $ return ()
      let l = unsafeSlice 0 i v
          r = unsafeSlice j (n-j) v
      reverse r
      return (l,r)

partitionUnknown :: (PrimMonad m, MVector v a)
        => (a -> Bool) -> Stream a -> m (v (PrimState m) a, v (PrimState m) a)
{-# INLINE partitionUnknown #-}
partitionUnknown f s
  = do
      v1 <- unsafeNew 0
      v2 <- unsafeNew 0
      (v1', n1, v2', n2) <- Stream.foldM' put (v1, 0, v2, 0) s
      INTERNAL_CHECK(checkSlice) "partitionUnknown" 0 n1 (length v1')
        $ INTERNAL_CHECK(checkSlice) "partitionUnknown" 0 n2 (length v2')
        $ return (unsafeSlice 0 n1 v1', unsafeSlice 0 n2 v2')
  where
    -- NOTE: The case distinction has to be on the outside because
    -- GHC creates a join point for the unsafeWrite even when everything
    -- is inlined. This is bad because with the join point, v isn't getting
    -- unboxed.
    {-# INLINE_INNER put #-}
    put (v1, i1, v2, i2) x
      | f x       = do
                      v1' <- unsafeAppend1 v1 i1 x
                      return (v1', i1+1, v2, i2)
      | otherwise = do
                      v2' <- unsafeAppend1 v2 i2 x
                      return (v1, i1, v2', i2+1)