{-# LANGUAGE CPP #-}
#if __GLASGOW_HASKELL__
{-# LANGUAGE DeriveDataTypeable, StandaloneDeriving #-}
#endif
#if __GLASGOW_HASKELL__ >= 703
{-# LANGUAGE Trustworthy #-}
#endif
#if __GLASGOW_HASKELL__ >= 708
{-# LANGUAGE TypeFamilies #-}
#endif

#include "containers.h"

-----------------------------------------------------------------------------
-- |
-- Module      :  Data.Sequence
-- Copyright   :  (c) Ross Paterson 2005
--                (c) Louis Wasserman 2009
--                (c) David Feuer, Ross Paterson, and Milan Straka 2014
-- License     :  BSD-style
-- Maintainer  :  libraries@haskell.org
-- Stability   :  experimental
-- Portability :  portable
--
-- General purpose finite sequences.
-- Apart from being finite and having strict operations, sequences
-- also differ from lists in supporting a wider variety of operations
-- efficiently.
--
-- An amortized running time is given for each operation, with /n/ referring
-- to the length of the sequence and /i/ being the integral index used by
-- some operations.  These bounds hold even in a persistent (shared) setting.
--
-- The implementation uses 2-3 finger trees annotated with sizes,
-- as described in section 4.2 of
--
--    * Ralf Hinze and Ross Paterson,
--      \"Finger trees: a simple general-purpose data structure\",
--      /Journal of Functional Programming/ 16:2 (2006) pp 197-217.
--      <http://staff.city.ac.uk/~ross/papers/FingerTree.html>
--
-- /Note/: Many of these operations have the same names as similar
-- operations on lists in the "Prelude".  The ambiguity may be resolved
-- using either qualification or the @hiding@ clause.
--
-----------------------------------------------------------------------------

module Data.Sequence (
#if !defined(TESTING)
    Seq,
#else
    Seq(..), Elem(..), FingerTree(..), Node(..), Digit(..),
#endif
    -- * Construction
    empty,          -- :: Seq a
    singleton,      -- :: a -> Seq a
    (<|),           -- :: a -> Seq a -> Seq a
    (|>),           -- :: Seq a -> a -> Seq a
    (><),           -- :: Seq a -> Seq a -> Seq a
    fromList,       -- :: [a] -> Seq a
    fromFunction,   -- :: Int -> (Int -> a) -> Seq a
    fromArray,      -- :: Ix i => Array i a -> Seq a
    -- ** Repetition
    replicate,      -- :: Int -> a -> Seq a
    replicateA,     -- :: Applicative f => Int -> f a -> f (Seq a)
    replicateM,     -- :: Monad m => Int -> m a -> m (Seq a)
    -- ** Iterative construction
    iterateN,       -- :: Int -> (a -> a) -> a -> Seq a
    unfoldr,        -- :: (b -> Maybe (a, b)) -> b -> Seq a
    unfoldl,        -- :: (b -> Maybe (b, a)) -> b -> Seq a
    -- * Deconstruction
    -- | Additional functions for deconstructing sequences are available
    -- via the 'Foldable' instance of 'Seq'.

    -- ** Queries
    null,           -- :: Seq a -> Bool
    length,         -- :: Seq a -> Int
    -- ** Views
    ViewL(..),
    viewl,          -- :: Seq a -> ViewL a
    ViewR(..),
    viewr,          -- :: Seq a -> ViewR a
    -- * Scans
    scanl,          -- :: (a -> b -> a) -> a -> Seq b -> Seq a
    scanl1,         -- :: (a -> a -> a) -> Seq a -> Seq a
    scanr,          -- :: (a -> b -> b) -> b -> Seq a -> Seq b
    scanr1,         -- :: (a -> a -> a) -> Seq a -> Seq a
    -- * Sublists
    tails,          -- :: Seq a -> Seq (Seq a)
    inits,          -- :: Seq a -> Seq (Seq a)
    -- ** Sequential searches
    takeWhileL,     -- :: (a -> Bool) -> Seq a -> Seq a
    takeWhileR,     -- :: (a -> Bool) -> Seq a -> Seq a
    dropWhileL,     -- :: (a -> Bool) -> Seq a -> Seq a
    dropWhileR,     -- :: (a -> Bool) -> Seq a -> Seq a
    spanl,          -- :: (a -> Bool) -> Seq a -> (Seq a, Seq a)
    spanr,          -- :: (a -> Bool) -> Seq a -> (Seq a, Seq a)
    breakl,         -- :: (a -> Bool) -> Seq a -> (Seq a, Seq a)
    breakr,         -- :: (a -> Bool) -> Seq a -> (Seq a, Seq a)
    partition,      -- :: (a -> Bool) -> Seq a -> (Seq a, Seq a)
    filter,         -- :: (a -> Bool) -> Seq a -> Seq a
    -- * Sorting
    sort,           -- :: Ord a => Seq a -> Seq a
    sortBy,         -- :: (a -> a -> Ordering) -> Seq a -> Seq a
    unstableSort,   -- :: Ord a => Seq a -> Seq a
    unstableSortBy, -- :: (a -> a -> Ordering) -> Seq a -> Seq a
    -- * Indexing
    index,          -- :: Seq a -> Int -> a
    adjust,         -- :: (a -> a) -> Int -> Seq a -> Seq a
    update,         -- :: Int -> a -> Seq a -> Seq a
    take,           -- :: Int -> Seq a -> Seq a
    drop,           -- :: Int -> Seq a -> Seq a
    splitAt,        -- :: Int -> Seq a -> (Seq a, Seq a)
    -- ** Indexing with predicates
    -- | These functions perform sequential searches from the left
    -- or right ends of the sequence, returning indices of matching
    -- elements.
    elemIndexL,     -- :: Eq a => a -> Seq a -> Maybe Int
    elemIndicesL,   -- :: Eq a => a -> Seq a -> [Int]
    elemIndexR,     -- :: Eq a => a -> Seq a -> Maybe Int
    elemIndicesR,   -- :: Eq a => a -> Seq a -> [Int]
    findIndexL,     -- :: (a -> Bool) -> Seq a -> Maybe Int
    findIndicesL,   -- :: (a -> Bool) -> Seq a -> [Int]
    findIndexR,     -- :: (a -> Bool) -> Seq a -> Maybe Int
    findIndicesR,   -- :: (a -> Bool) -> Seq a -> [Int]
    -- * Folds
    -- | General folds are available via the 'Foldable' instance of 'Seq'.
    foldlWithIndex, -- :: (b -> Int -> a -> b) -> b -> Seq a -> b
    foldrWithIndex, -- :: (Int -> a -> b -> b) -> b -> Seq a -> b
    -- * Transformations
    mapWithIndex,   -- :: (Int -> a -> b) -> Seq a -> Seq b
    reverse,        -- :: Seq a -> Seq a
    -- ** Zips
    zip,            -- :: Seq a -> Seq b -> Seq (a, b)
    zipWith,        -- :: (a -> b -> c) -> Seq a -> Seq b -> Seq c
    zip3,           -- :: Seq a -> Seq b -> Seq c -> Seq (a, b, c)
    zipWith3,       -- :: (a -> b -> c -> d) -> Seq a -> Seq b -> Seq c -> Seq d
    zip4,           -- :: Seq a -> Seq b -> Seq c -> Seq d -> Seq (a, b, c, d)
    zipWith4,       -- :: (a -> b -> c -> d -> e) -> Seq a -> Seq b -> Seq c -> Seq d -> Seq e
#if TESTING
    Sized(..),
    deep,
    node2,
    node3,
#endif
    ) where

import Prelude hiding (
    Functor(..),
    null, length, take, drop, splitAt, foldl, foldl1, foldr, foldr1,
    scanl, scanl1, scanr, scanr1, replicate, zip, zipWith, zip3, zipWith3,
    takeWhile, dropWhile, iterate, reverse, filter, mapM, sum, all)
import qualified Data.List
import Control.Applicative (Applicative(..), (<$>), Alternative,
                            WrappedMonad(..), liftA, liftA2, liftA3)
import qualified Control.Applicative as Applicative (Alternative(..))
import Control.DeepSeq (NFData(rnf))
import Control.Monad (MonadPlus(..), ap)
import Data.Monoid (Monoid(..))
import Data.Functor (Functor(..))
import Data.Foldable (Foldable(foldl, foldl1, foldr, foldr1, foldMap), foldl', toList)
#if MIN_VERSION_base(4,8,0)
import Data.Foldable (foldr')
#endif
import Data.Traversable
import Data.Typeable

-- GHC specific stuff
#ifdef __GLASGOW_HASKELL__
import GHC.Exts (build)
import Text.Read (Lexeme(Ident), lexP, parens, prec,
    readPrec, readListPrec, readListPrecDefault)
import Data.Data
#endif

-- Array stuff, with GHC.Arr on GHC
import Data.Array (Ix, Array)
#ifdef __GLASGOW_HASKELL__
import qualified GHC.Arr
#endif

-- Coercion on GHC 7.8+
#if __GLASGOW_HASKELL__ >= 708
import Data.Coerce
import qualified GHC.Exts
#else
#endif

-- Identity functor on base 4.8 (GHC 7.10+)
#if MIN_VERSION_base(4,8,0)
import Data.Functor.Identity (Identity(..))
#endif


infixr 5 `consTree`
infixl 5 `snocTree`
infixr 5 `appendTree0`

infixr 5 ><
infixr 5 <|, :<
infixl 5 |>, :>

class Sized a where
    size :: a -> Int

-- | General-purpose finite sequences.
newtype Seq a = Seq (FingerTree (Elem a))

instance Functor Seq where
    fmap = fmapSeq
#ifdef __GLASGOW_HASKELL__
    x <$ s = replicate (length s) x
#endif

fmapSeq :: (a -> b) -> Seq a -> Seq b
fmapSeq f (Seq xs) = Seq (fmap (fmap f) xs)
#ifdef __GLASGOW_HASKELL__
{-# NOINLINE [1] fmapSeq #-}
{-# RULES
"fmapSeq/fmapSeq" forall f g xs . fmapSeq f (fmapSeq g xs) = fmapSeq (f . g) xs
 #-}
#endif
#if __GLASGOW_HASKELL__ >= 709
-- Safe coercions were introduced in 7.8, but did not work well with RULES yet.
{-# RULES
"fmapSeq/coerce" fmapSeq coerce = coerce
 #-}
#endif

instance Foldable Seq where
    foldMap f (Seq xs) = foldMap (foldMap f) xs
    foldr f z (Seq xs) = foldr (flip (foldr f)) z xs
    foldl f z (Seq xs) = foldl (foldl f) z xs

    foldr1 f (Seq xs) = getElem (foldr1 f' xs)
      where f' (Elem x) (Elem y) = Elem (f x y)

    foldl1 f (Seq xs) = getElem (foldl1 f' xs)
      where f' (Elem x) (Elem y) = Elem (f x y)

#if MIN_VERSION_base(4,8,0)
    length = length
    {-# INLINE length #-}
    null   = null
    {-# INLINE null #-}
#endif

instance Traversable Seq where
    traverse f (Seq xs) = Seq <$> traverse (traverse f) xs

instance NFData a => NFData (Seq a) where
    rnf (Seq xs) = rnf xs

instance Monad Seq where
    return = singleton
    xs >>= f = foldl' add empty xs
      where add ys x = ys >< f x
    (>>) = (*>)

instance Applicative Seq where
    pure = singleton

    Seq Empty <*> xs = xs `seq` empty
    fs <*> Seq Empty = fs `seq` empty
    fs <*> Seq (Single (Elem x)) = fmap ($ x) fs
    fs <*> xs
      | length fs < 4 = foldl' add empty fs
      where add ys f = ys >< fmap f xs
    fs <*> xs | length xs < 4 = apShort fs xs
    fs <*> xs = apty fs xs

    xs *> ys = replicateSeq (length xs) ys

-- <*> when the length of the first argument is at least two and
-- the length of the second is two or three.
apShort :: Seq (a -> b) -> Seq a -> Seq b
apShort (Seq fs) xs = Seq $ case toList xs of
            [a,b] -> ap2FT fs (a,b)
            [a,b,c] -> ap3FT fs (a,b,c)
            _ -> error "apShort: not 2-3"

ap2FT :: FingerTree (Elem (a->b)) -> (a,a) -> FingerTree (Elem b)
ap2FT fs (x,y) = Deep (size fs * 2)
                      (Two (Elem $ firstf x) (Elem $ firstf y))
                      (mapMulFT 2 (\(Elem f) -> Node2 2 (Elem (f x)) (Elem (f y))) m)
                      (Two (Elem $ lastf x) (Elem $ lastf y))
  where
    (Elem firstf, m, Elem lastf) = trimTree fs

ap3FT :: FingerTree (Elem (a->b)) -> (a,a,a) -> FingerTree (Elem b)
ap3FT fs (x,y,z) = Deep (size fs * 3)
                        (Three (Elem $ firstf x) (Elem $ firstf y) (Elem $ firstf z))
                        (mapMulFT 3 (\(Elem f) -> Node3 3 (Elem (f x)) (Elem (f y)) (Elem (f z))) m)
                        (Three (Elem $ lastf x) (Elem $ lastf y) (Elem $ lastf z))
  where
    (Elem firstf, m, Elem lastf) = trimTree fs

-- <*> when the length of each argument is at least four.
apty :: Seq (a -> b) -> Seq a -> Seq b
apty (Seq fs) (Seq xs@Deep{}) = Seq $
    Deep (s' * size fs)
         (fmap (fmap firstf) pr')
         (aptyMiddle (fmap firstf) (fmap lastf) fmap fs' xs')
         (fmap (fmap lastf) sf')
  where
    (Elem firstf, fs', Elem lastf) = trimTree fs
    xs'@(Deep s' pr' _m' sf') = rigidify xs
apty _ _ = error "apty: expects a Deep constructor"

-- | 'aptyMiddle' does most of the hard work of computing @fs<*>xs@.
-- It produces the center part of a finger tree, with a prefix corresponding
-- to the prefix of @xs@ and a suffix corresponding to the suffix of @xs@
-- omitted; the missing suffix and prefix are added by the caller.
-- For the recursive call, it squashes the prefix and the suffix into
-- the center tree. Once it gets to the bottom, it turns the tree into
-- a 2-3 tree, applies 'mapMulFT' to produce the main body, and glues all
-- the pieces together.
aptyMiddle
  :: Sized c =>
     (c -> d)
     -> (c -> d)
     -> ((a -> b) -> c -> d)
     -> FingerTree (Elem (a -> b))
     -> FingerTree c
     -> FingerTree (Node d)
-- Not at the bottom yet
aptyMiddle firstf
           lastf
           map23
           fs
           (Deep s pr (Deep sm prm mm sfm) sf)
    = Deep (sm + s * (size fs + 1)) -- note: sm = s - size pr - size sf
           (fmap (fmap firstf) prm)
           (aptyMiddle (fmap firstf)
                       (fmap lastf)
                       (\f -> fmap (map23 f))
                       fs
                       (Deep s (squashL pr prm) mm (squashR sfm sf)))
           (fmap (fmap lastf) sfm)

-- At the bottom
aptyMiddle firstf
           lastf
           map23
           fs
           (Deep s pr m sf)
      = (fmap (fmap firstf) m `snocTree` fmap firstf (digitToNode sf))
        `appendTree0` middle `appendTree0`
        (fmap lastf (digitToNode pr) `consTree`  fmap (fmap lastf) m)
    where middle = case trimTree $ mapMulFT s (\(Elem f) -> fmap (fmap (map23 f)) converted) fs of
                     (firstMapped, restMapped, lastMapped) ->
                        Deep (size firstMapped + size restMapped + size lastMapped)
                             (nodeToDigit firstMapped) restMapped (nodeToDigit lastMapped)
          converted = case m of
                                    Empty -> Node2 s lconv rconv
                                    Single q -> Node3 s lconv q rconv
                                    Deep{} -> error "aptyMiddle: impossible"
          lconv = digitToNode pr
          rconv = digitToNode sf

aptyMiddle _ _ _ _ _ = error "aptyMiddle: expected Deep finger tree"

{-# SPECIALIZE
 aptyMiddle
  :: (Node c -> d)
     -> (Node c -> d)
     -> ((a -> b) -> Node c -> d)
     -> FingerTree (Elem (a -> b))
     -> FingerTree (Node c)
     -> FingerTree (Node d)
 #-}
{-# SPECIALIZE
 aptyMiddle
  :: (Elem c -> d)
     -> (Elem c -> d)
     -> ((a -> b) -> Elem c -> d)
     -> FingerTree (Elem (a -> b))
     -> FingerTree (Elem c)
     -> FingerTree (Node d)
 #-}

digitToNode :: Sized a => Digit a -> Node a
digitToNode (Two a b) = node2 a b
digitToNode (Three a b c) = node3 a b c
digitToNode _ = error "digitToNode: not representable as a node"

type Digit23 = Digit
type Digit12 = Digit

-- Squash the first argument down onto the left side of the second.
squashL :: Sized a => Digit23 a -> Digit12 (Node a) -> Digit23 (Node a)
squashL (Two a b) (One n) = Two (node2 a b) n
squashL (Two a b) (Two n1 n2) = Three (node2 a b) n1 n2
squashL (Three a b c) (One n) = Two (node3 a b c) n
squashL (Three a b c) (Two n1 n2) = Three (node3 a b c) n1 n2
squashL _ _ = error "squashL: wrong digit types"

-- Squash the second argument down onto the right side of the first
squashR :: Sized a => Digit12 (Node a) -> Digit23 a -> Digit23 (Node a)
squashR (One n) (Two a b) = Two n (node2 a b)
squashR (Two n1 n2) (Two a b) = Three n1 n2 (node2 a b)
squashR (One n) (Three a b c) = Two n (node3 a b c)
squashR (Two n1 n2) (Three a b c) = Three n1 n2 (node3 a b c)
squashR _ _ = error "squashR: wrong digit types"

-- | /O(m*n)/ (incremental) Takes an /O(m)/ function and a finger tree of size
-- /n/ and maps the function over the tree leaves. Unlike the usual 'fmap', the
-- function is applied to the "leaves" of the 'FingerTree' (i.e., given a
-- @FingerTree (Elem a)@, it applies the function to elements of type @Elem
-- a@), replacing the leaves with subtrees of at least the same height, e.g.,
-- @Node(Node(Elem y))@. The multiplier argument serves to make the annotations
-- match up properly.
mapMulFT :: Int -> (a -> b) -> FingerTree a -> FingerTree b
mapMulFT _ _ Empty = Empty
mapMulFT _mul f (Single a) = Single (f a)
mapMulFT mul f (Deep s pr m sf) = Deep (mul * s) (fmap f pr) (mapMulFT mul (mapMulNode mul f) m) (fmap f sf)

mapMulNode :: Int -> (a -> b) -> Node a -> Node b
mapMulNode mul f (Node2 s a b)   = Node2 (mul * s) (f a) (f b)
mapMulNode mul f (Node3 s a b c) = Node3 (mul * s) (f a) (f b) (f c)


trimTree :: Sized a => FingerTree a -> (a, FingerTree a, a)
trimTree Empty = error "trim: empty tree"
trimTree Single{} = error "trim: singleton"
trimTree t = case splitTree 0 t of
                 Split _ hd r ->
                   case splitTree (size r - 1) r of
                     Split m tl _ -> (hd, m, tl)

-- | /O(log n)/ (incremental) Takes the extra flexibility out of a 'FingerTree'
-- to make it a genuine 2-3 finger tree. The result of 'rigidify' will have
-- only 'Two' and 'Three' digits at the top level and only 'One' and 'Two'
-- digits elsewhere.  It gives an error if the tree has fewer than four
-- elements.
rigidify :: Sized a => FingerTree a -> FingerTree a
-- Note that 'rigidify' may call itself, but it will do so at most
-- once: each call to 'rigidify' will either fix the whole tree or fix one digit
-- and leave the other alone. The patterns below just fix up the top level of
-- the tree; 'rigidify' delegates the hard work to 'thin'.

-- The top of the tree is fine.
rigidify (Deep s pr@Two{} m sf@Three{}) = Deep s pr (thin m) sf
rigidify (Deep s pr@Three{} m sf@Three{}) = Deep s pr (thin m) sf
rigidify (Deep s pr@Two{} m sf@Two{}) = Deep s pr (thin m) sf
rigidify (Deep s pr@Three{} m sf@Two{}) = Deep s pr (thin m) sf

-- One of the Digits is a Four.
rigidify (Deep s (Four a b c d) m sf) =
   rigidify $ Deep s (Two a b) (node2 c d `consTree` m) sf
rigidify (Deep s pr m (Four a b c d)) =
   rigidify $ Deep s pr (m `snocTree` node2 a b) (Two c d)

-- One of the Digits is a One. If the middle is empty, we can only rigidify the
-- tree if the other Digit is a Three.
rigidify (Deep s (One a) Empty (Three b c d)) = Deep s (Two a b) Empty (Two c d)
rigidify (Deep s (One a) m sf) = rigidify $ case viewLTree m of
   Just2 (Node2 _ b c) m' -> Deep s (Three a b c) m' sf
   Just2 (Node3 _ b c d) m' -> Deep s (Two a b) (node2 c d `consTree` m') sf
   Nothing2 -> error "rigidify: small tree"
rigidify (Deep s (Three a b c) Empty (One d)) = Deep s (Two a b) Empty (Two c d)
rigidify (Deep s pr m (One e)) = rigidify $ case viewRTree m of
   Just2 m' (Node2 _ a b) -> Deep s pr m' (Three a b e)
   Just2 m' (Node3 _ a b c) -> Deep s pr (m' `snocTree` node2 a b) (Two c e)
   Nothing2 -> error "rigidify: small tree"
rigidify Empty = error "rigidify: empty tree"
rigidify Single{} = error "rigidify: singleton"

-- | /O(log n)/ (incremental) Rejigger a finger tree so the digits are all ones
-- and twos.
thin :: Sized a => FingerTree a -> FingerTree a
-- Note that 'thin' may call itself at most once before passing the job on to
-- 'thin12'. 'thin12' will produce a 'Deep' constructor immediately before
-- calling 'thin'.
thin Empty = Empty
thin (Single a) = Single a
thin t@(Deep s pr m sf) =
  case pr of
    One{} -> thin12 t
    Two{} -> thin12 t
    Three a b c  -> thin $ Deep s (One a) (node2 b c `consTree` m) sf
    Four a b c d -> thin $ Deep s (Two a b) (node2 c d `consTree` m) sf

thin12 :: Sized a => FingerTree a -> FingerTree a
thin12 (Deep s pr m sf@One{}) = Deep s pr (thin m) sf
thin12 (Deep s pr m sf@Two{}) = Deep s pr (thin m) sf
thin12 (Deep s pr m (Three a b c)) = Deep s pr (thin $ m `snocTree` node2 a b) (One c)
thin12 (Deep s pr m (Four a b c d)) = Deep s pr (thin $ m `snocTree` node2 a b) (Two c d)
thin12 _ = error "thin12 expects a Deep FingerTree."


instance MonadPlus Seq where
    mzero = empty
    mplus = (><)

instance Alternative Seq where
    empty = empty
    (<|>) = (><)

instance Eq a => Eq (Seq a) where
    xs == ys = length xs == length ys && toList xs == toList ys

instance Ord a => Ord (Seq a) where
    compare xs ys = compare (toList xs) (toList ys)

#if TESTING
instance Show a => Show (Seq a) where
    showsPrec p (Seq x) = showsPrec p x
#else
instance Show a => Show (Seq a) where
    showsPrec p xs = showParen (p > 10) $
        showString "fromList " . shows (toList xs)
#endif

instance Read a => Read (Seq a) where
#ifdef __GLASGOW_HASKELL__
    readPrec = parens $ prec 10 $ do
        Ident "fromList" <- lexP
        xs <- readPrec
        return (fromList xs)

    readListPrec = readListPrecDefault
#else
    readsPrec p = readParen (p > 10) $ \ r -> do
        ("fromList",s) <- lex r
        (xs,t) <- reads s
        return (fromList xs,t)
#endif

instance Monoid (Seq a) where
    mempty = empty
    mappend = (><)

INSTANCE_TYPEABLE1(Seq,seqTc,"Seq")

#if __GLASGOW_HASKELL__
instance Data a => Data (Seq a) where
    gfoldl f z s    = case viewl s of
        EmptyL  -> z empty
        x :< xs -> z (<|) `f` x `f` xs

    gunfold k z c   = case constrIndex c of
        1 -> z empty
        2 -> k (k (z (<|)))
        _ -> error "gunfold"

    toConstr xs
      | null xs     = emptyConstr
      | otherwise   = consConstr

    dataTypeOf _    = seqDataType

    dataCast1 f     = gcast1 f

emptyConstr, consConstr :: Constr
emptyConstr = mkConstr seqDataType "empty" [] Prefix
consConstr  = mkConstr seqDataType "<|" [] Infix

seqDataType :: DataType
seqDataType = mkDataType "Data.Sequence.Seq" [emptyConstr, consConstr]
#endif

-- Finger trees

data FingerTree a
    = Empty
    | Single a
    | Deep {-# UNPACK #-} !Int !(Digit a) (FingerTree (Node a)) !(Digit a)
#if TESTING
    deriving Show
#endif

instance Sized a => Sized (FingerTree a) where
    {-# SPECIALIZE instance Sized (FingerTree (Elem a)) #-}
    {-# SPECIALIZE instance Sized (FingerTree (Node a)) #-}
    size Empty              = 0
    size (Single x)         = size x
    size (Deep v _ _ _)     = v

instance Foldable FingerTree where
    foldMap _ Empty = mempty
    foldMap f (Single x) = f x
    foldMap f (Deep _ pr m sf) =
        foldMap f pr `mappend` (foldMap (foldMap f) m `mappend` foldMap f sf)

    foldr _ z Empty = z
    foldr f z (Single x) = x `f` z
    foldr f z (Deep _ pr m sf) =
        foldr f (foldr (flip (foldr f)) (foldr f z sf) m) pr

    foldl _ z Empty = z
    foldl f z (Single x) = z `f` x
    foldl f z (Deep _ pr m sf) =
        foldl f (foldl (foldl f) (foldl f z pr) m) sf

    foldr1 _ Empty = error "foldr1: empty sequence"
    foldr1 _ (Single x) = x
    foldr1 f (Deep _ pr m sf) =
        foldr f (foldr (flip (foldr f)) (foldr1 f sf) m) pr

    foldl1 _ Empty = error "foldl1: empty sequence"
    foldl1 _ (Single x) = x
    foldl1 f (Deep _ pr m sf) =
        foldl f (foldl (foldl f) (foldl1 f pr) m) sf

instance Functor FingerTree where
    fmap _ Empty = Empty
    fmap f (Single x) = Single (f x)
    fmap f (Deep v pr m sf) =
        Deep v (fmap f pr) (fmap (fmap f) m) (fmap f sf)

instance Traversable FingerTree where
    traverse _ Empty = pure Empty
    traverse f (Single x) = Single <$> f x
    traverse f (Deep v pr m sf) =
        Deep v <$> traverse f pr <*> traverse (traverse f) m <*>
            traverse f sf

instance NFData a => NFData (FingerTree a) where
    rnf (Empty) = ()
    rnf (Single x) = rnf x
    rnf (Deep _ pr m sf) = rnf pr `seq` rnf sf `seq` rnf m

{-# INLINE deep #-}
deep            :: Sized a => Digit a -> FingerTree (Node a) -> Digit a -> FingerTree a
deep pr m sf    =  Deep (size pr + size m + size sf) pr m sf

{-# INLINE pullL #-}
pullL :: Sized a => Int -> FingerTree (Node a) -> Digit a -> FingerTree a
pullL s m sf = case viewLTree m of
    Nothing2        -> digitToTree' s sf
    Just2 pr m'     -> Deep s (nodeToDigit pr) m' sf

{-# INLINE pullR #-}
pullR :: Sized a => Int -> Digit a -> FingerTree (Node a) -> FingerTree a
pullR s pr m = case viewRTree m of
    Nothing2        -> digitToTree' s pr
    Just2 m' sf     -> Deep s pr m' (nodeToDigit sf)

{-# SPECIALIZE deepL :: Maybe (Digit (Elem a)) -> FingerTree (Node (Elem a)) -> Digit (Elem a) -> FingerTree (Elem a) #-}
{-# SPECIALIZE deepL :: Maybe (Digit (Node a)) -> FingerTree (Node (Node a)) -> Digit (Node a) -> FingerTree (Node a) #-}
deepL :: Sized a => Maybe (Digit a) -> FingerTree (Node a) -> Digit a -> FingerTree a
deepL Nothing m sf      = pullL (size m + size sf) m sf
deepL (Just pr) m sf    = deep pr m sf

{-# SPECIALIZE deepR :: Digit (Elem a) -> FingerTree (Node (Elem a)) -> Maybe (Digit (Elem a)) -> FingerTree (Elem a) #-}
{-# SPECIALIZE deepR :: Digit (Node a) -> FingerTree (Node (Node a)) -> Maybe (Digit (Node a)) -> FingerTree (Node a) #-}
deepR :: Sized a => Digit a -> FingerTree (Node a) -> Maybe (Digit a) -> FingerTree a
deepR pr m Nothing      = pullR (size m + size pr) pr m
deepR pr m (Just sf)    = deep pr m sf

-- Digits

data Digit a
    = One a
    | Two a a
    | Three a a a
    | Four a a a a
#if TESTING
    deriving Show
#endif

instance Foldable Digit where
    foldMap f (One a) = f a
    foldMap f (Two a b) = f a `mappend` f b
    foldMap f (Three a b c) = f a `mappend` (f b `mappend` f c)
    foldMap f (Four a b c d) = f a `mappend` (f b `mappend` (f c `mappend` f d))

    foldr f z (One a) = a `f` z
    foldr f z (Two a b) = a `f` (b `f` z)
    foldr f z (Three a b c) = a `f` (b `f` (c `f` z))
    foldr f z (Four a b c d) = a `f` (b `f` (c `f` (d `f` z)))

    foldl f z (One a) = z `f` a
    foldl f z (Two a b) = (z `f` a) `f` b
    foldl f z (Three a b c) = ((z `f` a) `f` b) `f` c
    foldl f z (Four a b c d) = (((z `f` a) `f` b) `f` c) `f` d

    foldr1 _ (One a) = a
    foldr1 f (Two a b) = a `f` b
    foldr1 f (Three a b c) = a `f` (b `f` c)
    foldr1 f (Four a b c d) = a `f` (b `f` (c `f` d))

    foldl1 _ (One a) = a
    foldl1 f (Two a b) = a `f` b
    foldl1 f (Three a b c) = (a `f` b) `f` c
    foldl1 f (Four a b c d) = ((a `f` b) `f` c) `f` d

instance Functor Digit where
    {-# INLINE fmap #-}
    fmap f (One a) = One (f a)
    fmap f (Two a b) = Two (f a) (f b)
    fmap f (Three a b c) = Three (f a) (f b) (f c)
    fmap f (Four a b c d) = Four (f a) (f b) (f c) (f d)

instance Traversable Digit where
    {-# INLINE traverse #-}
    traverse f (One a) = One <$> f a
    traverse f (Two a b) = Two <$> f a <*> f b
    traverse f (Three a b c) = Three <$> f a <*> f b <*> f c
    traverse f (Four a b c d) = Four <$> f a <*> f b <*> f c <*> f d

instance NFData a => NFData (Digit a) where
    rnf (One a) = rnf a
    rnf (Two a b) = rnf a `seq` rnf b
    rnf (Three a b c) = rnf a `seq` rnf b `seq` rnf c
    rnf (Four a b c d) = rnf a `seq` rnf b `seq` rnf c `seq` rnf d

instance Sized a => Sized (Digit a) where
    {-# INLINE size #-}
    size = foldl1 (+) . fmap size

{-# SPECIALIZE digitToTree :: Digit (Elem a) -> FingerTree (Elem a) #-}
{-# SPECIALIZE digitToTree :: Digit (Node a) -> FingerTree (Node a) #-}
digitToTree     :: Sized a => Digit a -> FingerTree a
digitToTree (One a) = Single a
digitToTree (Two a b) = deep (One a) Empty (One b)
digitToTree (Three a b c) = deep (Two a b) Empty (One c)
digitToTree (Four a b c d) = deep (Two a b) Empty (Two c d)

-- | Given the size of a digit and the digit itself, efficiently converts
-- it to a FingerTree.
digitToTree' :: Int -> Digit a -> FingerTree a
digitToTree' n (Four a b c d) = Deep n (Two a b) Empty (Two c d)
digitToTree' n (Three a b c) = Deep n (Two a b) Empty (One c)
digitToTree' n (Two a b) = Deep n (One a) Empty (One b)
digitToTree' n (One a) = n `seq` Single a

-- Nodes

data Node a
    = Node2 {-# UNPACK #-} !Int a a
    | Node3 {-# UNPACK #-} !Int a a a
#if TESTING
    deriving Show
#endif

instance Foldable Node where
    foldMap f (Node2 _ a b) = f a `mappend` f b
    foldMap f (Node3 _ a b c) = f a `mappend` (f b `mappend` f c)

    foldr f z (Node2 _ a b) = a `f` (b `f` z)
    foldr f z (Node3 _ a b c) = a `f` (b `f` (c `f` z))

    foldl f z (Node2 _ a b) = (z `f` a) `f` b
    foldl f z (Node3 _ a b c) = ((z `f` a) `f` b) `f` c

instance Functor Node where
    {-# INLINE fmap #-}
    fmap f (Node2 v a b) = Node2 v (f a) (f b)
    fmap f (Node3 v a b c) = Node3 v (f a) (f b) (f c)

instance Traversable Node where
    {-# INLINE traverse #-}
    traverse f (Node2 v a b) = Node2 v <$> f a <*> f b
    traverse f (Node3 v a b c) = Node3 v <$> f a <*> f b <*> f c

instance NFData a => NFData (Node a) where
    rnf (Node2 _ a b) = rnf a `seq` rnf b
    rnf (Node3 _ a b c) = rnf a `seq` rnf b `seq` rnf c

instance Sized (Node a) where
    size (Node2 v _ _)      = v
    size (Node3 v _ _ _)    = v

{-# INLINE node2 #-}
node2           :: Sized a => a -> a -> Node a
node2 a b       =  Node2 (size a + size b) a b

{-# INLINE node3 #-}
node3           :: Sized a => a -> a -> a -> Node a
node3 a b c     =  Node3 (size a + size b + size c) a b c

nodeToDigit :: Node a -> Digit a
nodeToDigit (Node2 _ a b) = Two a b
nodeToDigit (Node3 _ a b c) = Three a b c

-- Elements

newtype Elem a  =  Elem { getElem :: a }
#if TESTING
    deriving Show
#endif

instance Sized (Elem a) where
    size _ = 1

instance Functor Elem where
#if __GLASGOW_HASKELL__ >= 708
-- This cuts the time for <*> by around a fifth.
    fmap = coerce
#else
    fmap f (Elem x) = Elem (f x)
#endif

instance Foldable Elem where
    foldMap f (Elem x) = f x
    foldr f z (Elem x) = f x z
    foldl f z (Elem x) = f z x

instance Traversable Elem where
    traverse f (Elem x) = Elem <$> f x

instance NFData a => NFData (Elem a) where
    rnf (Elem x) = rnf x

-------------------------------------------------------
-- Applicative construction
-------------------------------------------------------
#if !MIN_VERSION_base(4,8,0)
newtype Identity a = Identity {runIdentity :: a}

instance Functor Identity where
    fmap f (Identity x) = Identity (f x)

instance Applicative Identity where
    pure = Identity
    Identity f <*> Identity x = Identity (f x)
#endif

-- | This is essentially a clone of Control.Monad.State.Strict.
newtype State s a = State {runState :: s -> (s, a)}

instance Functor (State s) where
    fmap = liftA

instance Monad (State s) where
    {-# INLINE return #-}
    {-# INLINE (>>=) #-}
    return x = State $ \ s -> (s, x)
    m >>= k = State $ \ s -> case runState m s of
        (s', x) -> runState (k x) s'

instance Applicative (State s) where
    pure = return
    (<*>) = ap

execState :: State s a -> s -> a
execState m x = snd (runState m x)

-- | 'applicativeTree' takes an Applicative-wrapped construction of a
-- piece of a FingerTree, assumed to always have the same size (which
-- is put in the second argument), and replicates it as many times as
-- specified.  This is a generalization of 'replicateA', which itself
-- is a generalization of many Data.Sequence methods.
{-# SPECIALIZE applicativeTree :: Int -> Int -> State s a -> State s (FingerTree a) #-}
{-# SPECIALIZE applicativeTree :: Int -> Int -> Identity a -> Identity (FingerTree a) #-}
-- Special note: the Identity specialization automatically does node sharing,
-- reducing memory usage of the resulting tree to /O(log n)/.
applicativeTree :: Applicative f => Int -> Int -> f a -> f (FingerTree a)
applicativeTree n mSize m = mSize `seq` case n of
    0 -> pure Empty
    1 -> fmap Single m
    2 -> deepA one emptyTree one
    3 -> deepA two emptyTree one
    4 -> deepA two emptyTree two
    5 -> deepA three emptyTree two
    6 -> deepA three emptyTree three
    7 -> deepA four emptyTree three
    8 -> deepA four emptyTree four
    _ -> case n `quotRem` 3 of
           (q,0) -> deepA three (applicativeTree (q - 2) mSize' n3) three
           (q,1) -> deepA four  (applicativeTree (q - 2) mSize' n3) three
           (q,_) -> deepA four  (applicativeTree (q - 2) mSize' n3) four
  where
    one = fmap One m
    two = liftA2 Two m m
    three = liftA3 Three m m m
    four = liftA3 Four m m m <*> m
    deepA = liftA3 (Deep (n * mSize))
    mSize' = 3 * mSize
    n3 = liftA3 (Node3 mSize') m m m
    emptyTree = pure Empty

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

-- | /O(1)/. The empty sequence.
empty           :: Seq a
empty           =  Seq Empty

-- | /O(1)/. A singleton sequence.
singleton       :: a -> Seq a
singleton x     =  Seq (Single (Elem x))

-- | /O(log n)/. @replicate n x@ is a sequence consisting of @n@ copies of @x@.
replicate       :: Int -> a -> Seq a
replicate n x
  | n >= 0      = runIdentity (replicateA n (Identity x))
  | otherwise   = error "replicate takes a nonnegative integer argument"

-- | 'replicateA' is an 'Applicative' version of 'replicate', and makes
-- /O(log n)/ calls to '<*>' and 'pure'.
--
-- > replicateA n x = sequenceA (replicate n x)
replicateA :: Applicative f => Int -> f a -> f (Seq a)
replicateA n x
  | n >= 0      = Seq <$> applicativeTree n 1 (Elem <$> x)
  | otherwise   = error "replicateA takes a nonnegative integer argument"

-- | 'replicateM' is a sequence counterpart of 'Control.Monad.replicateM'.
--
-- > replicateM n x = sequence (replicate n x)
replicateM :: Monad m => Int -> m a -> m (Seq a)
replicateM n x
  | n >= 0      = unwrapMonad (replicateA n (WrapMonad x))
  | otherwise   = error "replicateM takes a nonnegative integer argument"

-- | @'replicateSeq' n xs@ concatenates @n@ copies of @xs@.
replicateSeq :: Int -> Seq a -> Seq a
replicateSeq n s
  | n < 0     = error "replicateSeq takes a nonnegative integer argument"
  | n == 0    = empty
  | otherwise = go n s
  where
    -- Invariant: k >= 1
    go 1 xs = xs
    go k xs | even k    = kxs
            | otherwise = xs >< kxs
            where kxs = go (k `quot` 2) $! (xs >< xs)

-- | /O(1)/. Add an element to the left end of a sequence.
-- Mnemonic: a triangle with the single element at the pointy end.
(<|)            :: a -> Seq a -> Seq a
x <| Seq xs     =  Seq (Elem x `consTree` xs)

{-# SPECIALIZE consTree :: Elem a -> FingerTree (Elem a) -> FingerTree (Elem a) #-}
{-# SPECIALIZE consTree :: Node a -> FingerTree (Node a) -> FingerTree (Node a) #-}
consTree        :: Sized a => a -> FingerTree a -> FingerTree a
consTree a Empty        = Single a
consTree a (Single b)   = deep (One a) Empty (One b)
consTree a (Deep s (Four b c d e) m sf) = m `seq`
    Deep (size a + s) (Two a b) (node3 c d e `consTree` m) sf
consTree a (Deep s (Three b c d) m sf) =
    Deep (size a + s) (Four a b c d) m sf
consTree a (Deep s (Two b c) m sf) =
    Deep (size a + s) (Three a b c) m sf
consTree a (Deep s (One b) m sf) =
    Deep (size a + s) (Two a b) m sf

-- | /O(1)/. Add an element to the right end of a sequence.
-- Mnemonic: a triangle with the single element at the pointy end.
(|>)            :: Seq a -> a -> Seq a
Seq xs |> x     =  Seq (xs `snocTree` Elem x)

{-# SPECIALIZE snocTree :: FingerTree (Elem a) -> Elem a -> FingerTree (Elem a) #-}
{-# SPECIALIZE snocTree :: FingerTree (Node a) -> Node a -> FingerTree (Node a) #-}
snocTree        :: Sized a => FingerTree a -> a -> FingerTree a
snocTree Empty a        =  Single a
snocTree (Single a) b   =  deep (One a) Empty (One b)
snocTree (Deep s pr m (Four a b c d)) e = m `seq`
    Deep (s + size e) pr (m `snocTree` node3 a b c) (Two d e)
snocTree (Deep s pr m (Three a b c)) d =
    Deep (s + size d) pr m (Four a b c d)
snocTree (Deep s pr m (Two a b)) c =
    Deep (s + size c) pr m (Three a b c)
snocTree (Deep s pr m (One a)) b =
    Deep (s + size b) pr m (Two a b)

-- | /O(log(min(n1,n2)))/. Concatenate two sequences.
(><)            :: Seq a -> Seq a -> Seq a
Seq xs >< Seq ys = Seq (appendTree0 xs ys)

-- The appendTree/addDigits gunk below is machine generated

{-# SPECIALIZE appendTree0 :: FingerTree (Elem a) -> FingerTree (Elem a) -> FingerTree (Elem a) #-}
{-# SPECIALIZE appendTree0 :: FingerTree (Node a) -> FingerTree (Node a) -> FingerTree (Node a) #-}
appendTree0 :: Sized a => FingerTree a -> FingerTree a -> FingerTree a
appendTree0 Empty xs =
    xs
appendTree0 xs Empty =
    xs
appendTree0 (Single x) xs =
    x `consTree` xs
appendTree0 xs (Single x) =
    xs `snocTree` x
appendTree0 (Deep s1 pr1 m1 sf1) (Deep s2 pr2 m2 sf2) =
    Deep (s1 + s2) pr1 (addDigits0 m1 sf1 pr2 m2) sf2

{-# SPECIALIZE addDigits0 :: FingerTree (Node (Elem a)) -> Digit (Elem a) -> Digit (Elem a) -> FingerTree (Node (Elem a)) -> FingerTree (Node (Elem a)) #-}
{-# SPECIALIZE addDigits0 :: FingerTree (Node (Node a)) -> Digit (Node a) -> Digit (Node a) -> FingerTree (Node (Node a)) -> FingerTree (Node (Node a)) #-}
addDigits0 :: Sized a => FingerTree (Node a) -> Digit a -> Digit a -> FingerTree (Node a) -> FingerTree (Node a)
addDigits0 m1 (One a) (One b) m2 =
    appendTree1 m1 (node2 a b) m2
addDigits0 m1 (One a) (Two b c) m2 =
    appendTree1 m1 (node3 a b c) m2
addDigits0 m1 (One a) (Three b c d) m2 =
    appendTree2 m1 (node2 a b) (node2 c d) m2
addDigits0 m1 (One a) (Four b c d e) m2 =
    appendTree2 m1 (node3 a b c) (node2 d e) m2
addDigits0 m1 (Two a b) (One c) m2 =
    appendTree1 m1 (node3 a b c) m2
addDigits0 m1 (Two a b) (Two c d) m2 =
    appendTree2 m1 (node2 a b) (node2 c d) m2
addDigits0 m1 (Two a b) (Three c d e) m2 =
    appendTree2 m1 (node3 a b c) (node2 d e) m2
addDigits0 m1 (Two a b) (Four c d e f) m2 =
    appendTree2 m1 (node3 a b c) (node3 d e f) m2
addDigits0 m1 (Three a b c) (One d) m2 =
    appendTree2 m1 (node2 a b) (node2 c d) m2
addDigits0 m1 (Three a b c) (Two d e) m2 =
    appendTree2 m1 (node3 a b c) (node2 d e) m2
addDigits0 m1 (Three a b c) (Three d e f) m2 =
    appendTree2 m1 (node3 a b c) (node3 d e f) m2
addDigits0 m1 (Three a b c) (Four d e f g) m2 =
    appendTree3 m1 (node3 a b c) (node2 d e) (node2 f g) m2
addDigits0 m1 (Four a b c d) (One e) m2 =
    appendTree2 m1 (node3 a b c) (node2 d e) m2
addDigits0 m1 (Four a b c d) (Two e f) m2 =
    appendTree2 m1 (node3 a b c) (node3 d e f) m2
addDigits0 m1 (Four a b c d) (Three e f g) m2 =
    appendTree3 m1 (node3 a b c) (node2 d e) (node2 f g) m2
addDigits0 m1 (Four a b c d) (Four e f g h) m2 =
    appendTree3 m1 (node3 a b c) (node3 d e f) (node2 g h) m2

appendTree1 :: FingerTree (Node a) -> Node a -> FingerTree (Node a) -> FingerTree (Node a)
appendTree1 Empty a xs =
    a `consTree` xs
appendTree1 xs a Empty =
    xs `snocTree` a
appendTree1 (Single x) a xs =
    x `consTree` a `consTree` xs
appendTree1 xs a (Single x) =
    xs `snocTree` a `snocTree` x
appendTree1 (Deep s1 pr1 m1 sf1) a (Deep s2 pr2 m2 sf2) =
    Deep (s1 + size a + s2) pr1 (addDigits1 m1 sf1 a pr2 m2) sf2

addDigits1 :: FingerTree (Node (Node a)) -> Digit (Node a) -> Node a -> Digit (Node a) -> FingerTree (Node (Node a)) -> FingerTree (Node (Node a))
addDigits1 m1 (One a) b (One c) m2 =
    appendTree1 m1 (node3 a b c) m2
addDigits1 m1 (One a) b (Two c d) m2 =
    appendTree2 m1 (node2 a b) (node2 c d) m2
addDigits1 m1 (One a) b (Three c d e) m2 =
    appendTree2 m1 (node3 a b c) (node2 d e) m2
addDigits1 m1 (One a) b (Four c d e f) m2 =
    appendTree2 m1 (node3 a b c) (node3 d e f) m2
addDigits1 m1 (Two a b) c (One d) m2 =
    appendTree2 m1 (node2 a b) (node2 c d) m2
addDigits1 m1 (Two a b) c (Two d e) m2 =
    appendTree2 m1 (node3 a b c) (node2 d e) m2
addDigits1 m1 (Two a b) c (Three d e f) m2 =
    appendTree2 m1 (node3 a b c) (node3 d e f) m2
addDigits1 m1 (Two a b) c (Four d e f g) m2 =
    appendTree3 m1 (node3 a b c) (node2 d e) (node2 f g) m2
addDigits1 m1 (Three a b c) d (One e) m2 =
    appendTree2 m1 (node3 a b c) (node2 d e) m2
addDigits1 m1 (Three a b c) d (Two e f) m2 =
    appendTree2 m1 (node3 a b c) (node3 d e f) m2
addDigits1 m1 (Three a b c) d (Three e f g) m2 =
    appendTree3 m1 (node3 a b c) (node2 d e) (node2 f g) m2
addDigits1 m1 (Three a b c) d (Four e f g h) m2 =
    appendTree3 m1 (node3 a b c) (node3 d e f) (node2 g h) m2
addDigits1 m1 (Four a b c d) e (One f) m2 =
    appendTree2 m1 (node3 a b c) (node3 d e f) m2
addDigits1 m1 (Four a b c d) e (Two f g) m2 =
    appendTree3 m1 (node3 a b c) (node2 d e) (node2 f g) m2
addDigits1 m1 (Four a b c d) e (Three f g h) m2 =
    appendTree3 m1 (node3 a b c) (node3 d e f) (node2 g h) m2
addDigits1 m1 (Four a b c d) e (Four f g h i) m2 =
    appendTree3 m1 (node3 a b c) (node3 d e f) (node3 g h i) m2

appendTree2 :: FingerTree (Node a) -> Node a -> Node a -> FingerTree (Node a) -> FingerTree (Node a)
appendTree2 Empty a b xs =
    a `consTree` b `consTree` xs
appendTree2 xs a b Empty =
    xs `snocTree` a `snocTree` b
appendTree2 (Single x) a b xs =
    x `consTree` a `consTree` b `consTree` xs
appendTree2 xs a b (Single x) =
    xs `snocTree` a `snocTree` b `snocTree` x
appendTree2 (Deep s1 pr1 m1 sf1) a b (Deep s2 pr2 m2 sf2) =
    Deep (s1 + size a + size b + s2) pr1 (addDigits2 m1 sf1 a b pr2 m2) sf2

addDigits2 :: FingerTree (Node (Node a)) -> Digit (Node a) -> Node a -> Node a -> Digit (Node a) -> FingerTree (Node (Node a)) -> FingerTree (Node (Node a))
addDigits2 m1 (One a) b c (One d) m2 =
    appendTree2 m1 (node2 a b) (node2 c d) m2
addDigits2 m1 (One a) b c (Two d e) m2 =
    appendTree2 m1 (node3 a b c) (node2 d e) m2
addDigits2 m1 (One a) b c (Three d e f) m2 =
    appendTree2 m1 (node3 a b c) (node3 d e f) m2
addDigits2 m1 (One a) b c (Four d e f g) m2 =
    appendTree3 m1 (node3 a b c) (node2 d e) (node2 f g) m2
addDigits2 m1 (Two a b) c d (One e) m2 =
    appendTree2 m1 (node3 a b c) (node2 d e) m2
addDigits2 m1 (Two a b) c d (Two e f) m2 =
    appendTree2 m1 (node3 a b c) (node3 d e f) m2
addDigits2 m1 (Two a b) c d (Three e f g) m2 =
    appendTree3 m1 (node3 a b c) (node2 d e) (node2 f g) m2
addDigits2 m1 (Two a b) c d (Four e f g h) m2 =
    appendTree3 m1 (node3 a b c) (node3 d e f) (node2 g h) m2
addDigits2 m1 (Three a b c) d e (One f) m2 =
    appendTree2 m1 (node3 a b c) (node3 d e f) m2
addDigits2 m1 (Three a b c) d e (Two f g) m2 =
    appendTree3 m1 (node3 a b c) (node2 d e) (node2 f g) m2
addDigits2 m1 (Three a b c) d e (Three f g h) m2 =
    appendTree3 m1 (node3 a b c) (node3 d e f) (node2 g h) m2
addDigits2 m1 (Three a b c) d e (Four f g h i) m2 =
    appendTree3 m1 (node3 a b c) (node3 d e f) (node3 g h i) m2
addDigits2 m1 (Four a b c d) e f (One g) m2 =
    appendTree3 m1 (node3 a b c) (node2 d e) (node2 f g) m2
addDigits2 m1 (Four a b c d) e f (Two g h) m2 =
    appendTree3 m1 (node3 a b c) (node3 d e f) (node2 g h) m2
addDigits2 m1 (Four a b c d) e f (Three g h i) m2 =
    appendTree3 m1 (node3 a b c) (node3 d e f) (node3 g h i) m2
addDigits2 m1 (Four a b c d) e f (Four g h i j) m2 =
    appendTree4 m1 (node3 a b c) (node3 d e f) (node2 g h) (node2 i j) m2

appendTree3 :: FingerTree (Node a) -> Node a -> Node a -> Node a -> FingerTree (Node a) -> FingerTree (Node a)
appendTree3 Empty a b c xs =
    a `consTree` b `consTree` c `consTree` xs
appendTree3 xs a b c Empty =
    xs `snocTree` a `snocTree` b `snocTree` c
appendTree3 (Single x) a b c xs =
    x `consTree` a `consTree` b `consTree` c `consTree` xs
appendTree3 xs a b c (Single x) =
    xs `snocTree` a `snocTree` b `snocTree` c `snocTree` x
appendTree3 (Deep s1 pr1 m1 sf1) a b c (Deep s2 pr2 m2 sf2) =
    Deep (s1 + size a + size b + size c + s2) pr1 (addDigits3 m1 sf1 a b c pr2 m2) sf2

addDigits3 :: FingerTree (Node (Node a)) -> Digit (Node a) -> Node a -> Node a -> Node a -> Digit (Node a) -> FingerTree (Node (Node a)) -> FingerTree (Node (Node a))
addDigits3 m1 (One a) b c d (One e) m2 =
    appendTree2 m1 (node3 a b c) (node2 d e) m2
addDigits3 m1 (One a) b c d (Two e f) m2 =
    appendTree2 m1 (node3 a b c) (node3 d e f) m2
addDigits3 m1 (One a) b c d (Three e f g) m2 =
    appendTree3 m1 (node3 a b c) (node2 d e) (node2 f g) m2
addDigits3 m1 (One a) b c d (Four e f g h) m2 =
    appendTree3 m1 (node3 a b c) (node3 d e f) (node2 g h) m2
addDigits3 m1 (Two a b) c d e (One f) m2 =
    appendTree2 m1 (node3 a b c) (node3 d e f) m2
addDigits3 m1 (Two a b) c d e (Two f g) m2 =
    appendTree3 m1 (node3 a b c) (node2 d e) (node2 f g) m2
addDigits3 m1 (Two a b) c d e (Three f g h) m2 =
    appendTree3 m1 (node3 a b c) (node3 d e f) (node2 g h) m2
addDigits3 m1 (Two a b) c d e (Four f g h i) m2 =
    appendTree3 m1 (node3 a b c) (node3 d e f) (node3 g h i) m2
addDigits3 m1 (Three a b c) d e f (One g) m2 =
    appendTree3 m1 (node3 a b c) (node2 d e) (node2 f g) m2
addDigits3 m1 (Three a b c) d e f (Two g h) m2 =
    appendTree3 m1 (node3 a b c) (node3 d e f) (node2 g h) m2
addDigits3 m1 (Three a b c) d e f (Three g h i) m2 =
    appendTree3 m1 (node3 a b c) (node3 d e f) (node3 g h i) m2
addDigits3 m1 (Three a b c) d e f (Four g h i j) m2 =
    appendTree4 m1 (node3 a b c) (node3 d e f) (node2 g h) (node2 i j) m2
addDigits3 m1 (Four a b c d) e f g (One h) m2 =
    appendTree3 m1 (node3 a b c) (node3 d e f) (node2 g h) m2
addDigits3 m1 (Four a b c d) e f g (Two h i) m2 =
    appendTree3 m1 (node3 a b c) (node3 d e f) (node3 g h i) m2
addDigits3 m1 (Four a b c d) e f g (Three h i j) m2 =
    appendTree4 m1 (node3 a b c) (node3 d e f) (node2 g h) (node2 i j) m2
addDigits3 m1 (Four a b c d) e f g (Four h i j k) m2 =
    appendTree4 m1 (node3 a b c) (node3 d e f) (node3 g h i) (node2 j k) m2

appendTree4 :: FingerTree (Node a) -> Node a -> Node a -> Node a -> Node a -> FingerTree (Node a) -> FingerTree (Node a)
appendTree4 Empty a b c d xs =
    a `consTree` b `consTree` c `consTree` d `consTree` xs
appendTree4 xs a b c d Empty =
    xs `snocTree` a `snocTree` b `snocTree` c `snocTree` d
appendTree4 (Single x) a b c d xs =
    x `consTree` a `consTree` b `consTree` c `consTree` d `consTree` xs
appendTree4 xs a b c d (Single x) =
    xs `snocTree` a `snocTree` b `snocTree` c `snocTree` d `snocTree` x
appendTree4 (Deep s1 pr1 m1 sf1) a b c d (Deep s2 pr2 m2 sf2) =
    Deep (s1 + size a + size b + size c + size d + s2) pr1 (addDigits4 m1 sf1 a b c d pr2 m2) sf2

addDigits4 :: FingerTree (Node (Node a)) -> Digit (Node a) -> Node a -> Node a -> Node a -> Node a -> Digit (Node a) -> FingerTree (Node (Node a)) -> FingerTree (Node (Node a))
addDigits4 m1 (One a) b c d e (One f) m2 =
    appendTree2 m1 (node3 a b c) (node3 d e f) m2
addDigits4 m1 (One a) b c d e (Two f g) m2 =
    appendTree3 m1 (node3 a b c) (node2 d e) (node2 f g) m2
addDigits4 m1 (One a) b c d e (Three f g h) m2 =
    appendTree3 m1 (node3 a b c) (node3 d e f) (node2 g h) m2
addDigits4 m1 (One a) b c d e (Four f g h i) m2 =
    appendTree3 m1 (node3 a b c) (node3 d e f) (node3 g h i) m2
addDigits4 m1 (Two a b) c d e f (One g) m2 =
    appendTree3 m1 (node3 a b c) (node2 d e) (node2 f g) m2
addDigits4 m1 (Two a b) c d e f (Two g h) m2 =
    appendTree3 m1 (node3 a b c) (node3 d e f) (node2 g h) m2
addDigits4 m1 (Two a b) c d e f (Three g h i) m2 =
    appendTree3 m1 (node3 a b c) (node3 d e f) (node3 g h i) m2
addDigits4 m1 (Two a b) c d e f (Four g h i j) m2 =
    appendTree4 m1 (node3 a b c) (node3 d e f) (node2 g h) (node2 i j) m2
addDigits4 m1 (Three a b c) d e f g (One h) m2 =
    appendTree3 m1 (node3 a b c) (node3 d e f) (node2 g h) m2
addDigits4 m1 (Three a b c) d e f g (Two h i) m2 =
    appendTree3 m1 (node3 a b c) (node3 d e f) (node3 g h i) m2
addDigits4 m1 (Three a b c) d e f g (Three h i j) m2 =
    appendTree4 m1 (node3 a b c) (node3 d e f) (node2 g h) (node2 i j) m2
addDigits4 m1 (Three a b c) d e f g (Four h i j k) m2 =
    appendTree4 m1 (node3 a b c) (node3 d e f) (node3 g h i) (node2 j k) m2
addDigits4 m1 (Four a b c d) e f g h (One i) m2 =
    appendTree3 m1 (node3 a b c) (node3 d e f) (node3 g h i) m2
addDigits4 m1 (Four a b c d) e f g h (Two i j) m2 =
    appendTree4 m1 (node3 a b c) (node3 d e f) (node2 g h) (node2 i j) m2
addDigits4 m1 (Four a b c d) e f g h (Three i j k) m2 =
    appendTree4 m1 (node3 a b c) (node3 d e f) (node3 g h i) (node2 j k) m2
addDigits4 m1 (Four a b c d) e f g h (Four i j k l) m2 =
    appendTree4 m1 (node3 a b c) (node3 d e f) (node3 g h i) (node3 j k l) m2

-- | Builds a sequence from a seed value.  Takes time linear in the
-- number of generated elements.  /WARNING:/ If the number of generated
-- elements is infinite, this method will not terminate.
unfoldr :: (b -> Maybe (a, b)) -> b -> Seq a
unfoldr f = unfoldr' empty
  -- uses tail recursion rather than, for instance, the List implementation.
  where unfoldr' as b = maybe as (\ (a, b') -> unfoldr' (as |> a) b') (f b)

-- | @'unfoldl' f x@ is equivalent to @'reverse' ('unfoldr' ('fmap' swap . f) x)@.
unfoldl :: (b -> Maybe (b, a)) -> b -> Seq a
unfoldl f = unfoldl' empty
  where unfoldl' as b = maybe as (\ (b', a) -> unfoldl' (a <| as) b') (f b)

-- | /O(n)/.  Constructs a sequence by repeated application of a function
-- to a seed value.
--
-- > iterateN n f x = fromList (Prelude.take n (Prelude.iterate f x))
iterateN :: Int -> (a -> a) -> a -> Seq a
iterateN n f x
  | n >= 0      = replicateA n (State (\ y -> (f y, y))) `execState` x
  | otherwise   = error "iterateN takes a nonnegative integer argument"

------------------------------------------------------------------------
-- Deconstruction
------------------------------------------------------------------------

-- | /O(1)/. Is this the empty sequence?
null            :: Seq a -> Bool
null (Seq Empty) = True
null _          =  False

-- | /O(1)/. The number of elements in the sequence.
length          :: Seq a -> Int
length (Seq xs) =  size xs

-- Views

data Maybe2 a b = Nothing2 | Just2 a b

-- | View of the left end of a sequence.
data ViewL a
    = EmptyL        -- ^ empty sequence
    | a :< Seq a    -- ^ leftmost element and the rest of the sequence
#if __GLASGOW_HASKELL__
    deriving (Eq, Ord, Show, Read, Data)
#else
    deriving (Eq, Ord, Show, Read)
#endif

INSTANCE_TYPEABLE1(ViewL,viewLTc,"ViewL")

instance Functor ViewL where
    {-# INLINE fmap #-}
    fmap _ EmptyL       = EmptyL
    fmap f (x :< xs)    = f x :< fmap f xs

instance Foldable ViewL where
    foldr _ z EmptyL = z
    foldr f z (x :< xs) = f x (foldr f z xs)

    foldl _ z EmptyL = z
    foldl f z (x :< xs) = foldl f (f z x) xs

    foldl1 _ EmptyL = error "foldl1: empty view"
    foldl1 f (x :< xs) = foldl f x xs

instance Traversable ViewL where
    traverse _ EmptyL       = pure EmptyL
    traverse f (x :< xs)    = (:<) <$> f x <*> traverse f xs

-- | /O(1)/. Analyse the left end of a sequence.
viewl           ::  Seq a -> ViewL a
viewl (Seq xs)  =  case viewLTree xs of
    Nothing2 -> EmptyL
    Just2 (Elem x) xs' -> x :< Seq xs'

{-# SPECIALIZE viewLTree :: FingerTree (Elem a) -> Maybe2 (Elem a) (FingerTree (Elem a)) #-}
{-# SPECIALIZE viewLTree :: FingerTree (Node a) -> Maybe2 (Node a) (FingerTree (Node a)) #-}
viewLTree       :: Sized a => FingerTree a -> Maybe2 a (FingerTree a)
viewLTree Empty                 = Nothing2
viewLTree (Single a)            = Just2 a Empty
viewLTree (Deep s (One a) m sf) = Just2 a (pullL (s - size a) m sf)
viewLTree (Deep s (Two a b) m sf) =
    Just2 a (Deep (s - size a) (One b) m sf)
viewLTree (Deep s (Three a b c) m sf) =
    Just2 a (Deep (s - size a) (Two b c) m sf)
viewLTree (Deep s (Four a b c d) m sf) =
    Just2 a (Deep (s - size a) (Three b c d) m sf)

-- | View of the right end of a sequence.
data ViewR a
    = EmptyR        -- ^ empty sequence
    | Seq a :> a    -- ^ the sequence minus the rightmost element,
            -- and the rightmost element
#if __GLASGOW_HASKELL__
    deriving (Eq, Ord, Show, Read, Data)
#else
    deriving (Eq, Ord, Show, Read)
#endif

INSTANCE_TYPEABLE1(ViewR,viewRTc,"ViewR")

instance Functor ViewR where
    {-# INLINE fmap #-}
    fmap _ EmptyR       = EmptyR
    fmap f (xs :> x)    = fmap f xs :> f x

instance Foldable ViewR where
    foldMap _ EmptyR = mempty
    foldMap f (xs :> x) = foldMap f xs `mappend` f x

    foldr _ z EmptyR = z
    foldr f z (xs :> x) = foldr f (f x z) xs

    foldl _ z EmptyR = z
    foldl f z (xs :> x) = foldl f z xs `f` x

    foldr1 _ EmptyR = error "foldr1: empty view"
    foldr1 f (xs :> x) = foldr f x xs
#if MIN_VERSION_base(4,8,0)
    -- The default definitions are sensible for ViewL, but not so much for
    -- ViewR.
    null EmptyR = True
    null (_ :> _) = False

    length = foldr' (\_ k -> k+1) 0
#endif

instance Traversable ViewR where
    traverse _ EmptyR       = pure EmptyR
    traverse f (xs :> x)    = (:>) <$> traverse f xs <*> f x

-- | /O(1)/. Analyse the right end of a sequence.
viewr           ::  Seq a -> ViewR a
viewr (Seq xs)  =  case viewRTree xs of
    Nothing2 -> EmptyR
    Just2 xs' (Elem x) -> Seq xs' :> x

{-# SPECIALIZE viewRTree :: FingerTree (Elem a) -> Maybe2 (FingerTree (Elem a)) (Elem a) #-}
{-# SPECIALIZE viewRTree :: FingerTree (Node a) -> Maybe2 (FingerTree (Node a)) (Node a) #-}
viewRTree       :: Sized a => FingerTree a -> Maybe2 (FingerTree a) a
viewRTree Empty                 = Nothing2
viewRTree (Single z)            = Just2 Empty z
viewRTree (Deep s pr m (One z)) = Just2 (pullR (s - size z) pr m) z
viewRTree (Deep s pr m (Two y z)) =
    Just2 (Deep (s - size z) pr m (One y)) z
viewRTree (Deep s pr m (Three x y z)) =
    Just2 (Deep (s - size z) pr m (Two x y)) z
viewRTree (Deep s pr m (Four w x y z)) =
    Just2 (Deep (s - size z) pr m (Three w x y)) z

------------------------------------------------------------------------
-- Scans
--
-- These are not particularly complex applications of the Traversable
-- functor, though making the correspondence with Data.List exact
-- requires the use of (<|) and (|>).
--
-- Note that save for the single (<|) or (|>), we maintain the original
-- structure of the Seq, not having to do any restructuring of our own.
--
-- wasserman.louis@gmail.com, 5/23/09
------------------------------------------------------------------------

-- | 'scanl' is similar to 'foldl', but returns a sequence of reduced
-- values from the left:
--
-- > scanl f z (fromList [x1, x2, ...]) = fromList [z, z `f` x1, (z `f` x1) `f` x2, ...]
scanl :: (a -> b -> a) -> a -> Seq b -> Seq a
scanl f z0 xs = z0 <| snd (mapAccumL (\ x z -> let x' = f x z in (x', x')) z0 xs)

-- | 'scanl1' is a variant of 'scanl' that has no starting value argument:
--
-- > scanl1 f (fromList [x1, x2, ...]) = fromList [x1, x1 `f` x2, ...]
scanl1 :: (a -> a -> a) -> Seq a -> Seq a
scanl1 f xs = case viewl xs of
    EmptyL          -> error "scanl1 takes a nonempty sequence as an argument"
    x :< xs'        -> scanl f x xs'

-- | 'scanr' is the right-to-left dual of 'scanl'.
scanr :: (a -> b -> b) -> b -> Seq a -> Seq b
scanr f z0 xs = snd (mapAccumR (\ z x -> let z' = f x z in (z', z')) z0 xs) |> z0

-- | 'scanr1' is a variant of 'scanr' that has no starting value argument.
scanr1 :: (a -> a -> a) -> Seq a -> Seq a
scanr1 f xs = case viewr xs of
    EmptyR          -> error "scanr1 takes a nonempty sequence as an argument"
    xs' :> x        -> scanr f x xs'

-- Indexing

-- | /O(log(min(i,n-i)))/. The element at the specified position,
-- counting from 0.  The argument should thus be a non-negative
-- integer less than the size of the sequence.
-- If the position is out of range, 'index' fails with an error.
index           :: Seq a -> Int -> a
index (Seq xs) i
  | 0 <= i && i < size xs = case lookupTree i xs of
                Place _ (Elem x) -> x
  | otherwise   = error "index out of bounds"

data Place a = Place {-# UNPACK #-} !Int a
#if TESTING
    deriving Show
#endif

{-# SPECIALIZE lookupTree :: Int -> FingerTree (Elem a) -> Place (Elem a) #-}
{-# SPECIALIZE lookupTree :: Int -> FingerTree (Node a) -> Place (Node a) #-}
lookupTree :: Sized a => Int -> FingerTree a -> Place a
lookupTree _ Empty = error "lookupTree of empty tree"
lookupTree i (Single x) = Place i x
lookupTree i (Deep totalSize pr m sf)
  | i < spr     =  lookupDigit i pr
  | i < spm     =  case lookupTree (i - spr) m of
                   Place i' xs -> lookupNode i' xs
  | otherwise   =  lookupDigit (i - spm) sf
  where
    spr     = size pr
    spm     = totalSize - size sf

{-# SPECIALIZE lookupNode :: Int -> Node (Elem a) -> Place (Elem a) #-}
{-# SPECIALIZE lookupNode :: Int -> Node (Node a) -> Place (Node a) #-}
lookupNode :: Sized a => Int -> Node a -> Place a
lookupNode i (Node2 _ a b)
  | i < sa      = Place i a
  | otherwise   = Place (i - sa) b
  where
    sa      = size a
lookupNode i (Node3 _ a b c)
  | i < sa      = Place i a
  | i < sab     = Place (i - sa) b
  | otherwise   = Place (i - sab) c
  where
    sa      = size a
    sab     = sa + size b

{-# SPECIALIZE lookupDigit :: Int -> Digit (Elem a) -> Place (Elem a) #-}
{-# SPECIALIZE lookupDigit :: Int -> Digit (Node a) -> Place (Node a) #-}
lookupDigit :: Sized a => Int -> Digit a -> Place a
lookupDigit i (One a) = Place i a
lookupDigit i (Two a b)
  | i < sa      = Place i a
  | otherwise   = Place (i - sa) b
  where
    sa      = size a
lookupDigit i (Three a b c)
  | i < sa      = Place i a
  | i < sab     = Place (i - sa) b
  | otherwise   = Place (i - sab) c
  where
    sa      = size a
    sab     = sa + size b
lookupDigit i (Four a b c d)
  | i < sa      = Place i a
  | i < sab     = Place (i - sa) b
  | i < sabc    = Place (i - sab) c
  | otherwise   = Place (i - sabc) d
  where
    sa      = size a
    sab     = sa + size b
    sabc    = sab + size c

-- | /O(log(min(i,n-i)))/. Replace the element at the specified position.
-- If the position is out of range, the original sequence is returned.
update          :: Int -> a -> Seq a -> Seq a
update i x      = adjust (const x) i

-- | /O(log(min(i,n-i)))/. Update the element at the specified position.
-- If the position is out of range, the original sequence is returned.
adjust          :: (a -> a) -> Int -> Seq a -> Seq a
adjust f i (Seq xs)
  | 0 <= i && i < size xs = Seq (adjustTree (const (fmap f)) i xs)
  | otherwise   = Seq xs

{-# SPECIALIZE adjustTree :: (Int -> Elem a -> Elem a) -> Int -> FingerTree (Elem a) -> FingerTree (Elem a) #-}
{-# SPECIALIZE adjustTree :: (Int -> Node a -> Node a) -> Int -> FingerTree (Node a) -> FingerTree (Node a) #-}
adjustTree      :: Sized a => (Int -> a -> a) ->
            Int -> FingerTree a -> FingerTree a
adjustTree _ _ Empty = error "adjustTree of empty tree"
adjustTree f i (Single x) = Single (f i x)
adjustTree f i (Deep s pr m sf)
  | i < spr     = Deep s (adjustDigit f i pr) m sf
  | i < spm     = Deep s pr (adjustTree (adjustNode f) (i - spr) m) sf
  | otherwise   = Deep s pr m (adjustDigit f (i - spm) sf)
  where
    spr     = size pr
    spm     = spr + size m

{-# SPECIALIZE adjustNode :: (Int -> Elem a -> Elem a) -> Int -> Node (Elem a) -> Node (Elem a) #-}
{-# SPECIALIZE adjustNode :: (Int -> Node a -> Node a) -> Int -> Node (Node a) -> Node (Node a) #-}
adjustNode      :: Sized a => (Int -> a -> a) -> Int -> Node a -> Node a
adjustNode f i (Node2 s a b)
  | i < sa      = Node2 s (f i a) b
  | otherwise   = Node2 s a (f (i - sa) b)
  where
    sa      = size a
adjustNode f i (Node3 s a b c)
  | i < sa      = Node3 s (f i a) b c
  | i < sab     = Node3 s a (f (i - sa) b) c
  | otherwise   = Node3 s a b (f (i - sab) c)
  where
    sa      = size a
    sab     = sa + size b

{-# SPECIALIZE adjustDigit :: (Int -> Elem a -> Elem a) -> Int -> Digit (Elem a) -> Digit (Elem a) #-}
{-# SPECIALIZE adjustDigit :: (Int -> Node a -> Node a) -> Int -> Digit (Node a) -> Digit (Node a) #-}
adjustDigit     :: Sized a => (Int -> a -> a) -> Int -> Digit a -> Digit a
adjustDigit f i (One a) = One (f i a)
adjustDigit f i (Two a b)
  | i < sa      = Two (f i a) b
  | otherwise   = Two a (f (i - sa) b)
  where
    sa      = size a
adjustDigit f i (Three a b c)
  | i < sa      = Three (f i a) b c
  | i < sab     = Three a (f (i - sa) b) c
  | otherwise   = Three a b (f (i - sab) c)
  where
    sa      = size a
    sab     = sa + size b
adjustDigit f i (Four a b c d)
  | i < sa      = Four (f i a) b c d
  | i < sab     = Four a (f (i - sa) b) c d
  | i < sabc    = Four a b (f (i - sab) c) d
  | otherwise   = Four a b c (f (i- sabc) d)
  where
    sa      = size a
    sab     = sa + size b
    sabc    = sab + size c

-- | /O(n)/. A generalization of 'fmap', 'mapWithIndex' takes a mapping
-- function that also depends on the element's index, and applies it to every
-- element in the sequence.
mapWithIndex :: (Int -> a -> b) -> Seq a -> Seq b
mapWithIndex f' (Seq xs') = Seq $ mapWithIndexTree (\s (Elem a) -> Elem (f' s a)) 0 xs'
 where
  {-# SPECIALIZE mapWithIndexTree :: (Int -> Elem y -> b) -> Int -> FingerTree (Elem y) -> FingerTree b #-}
  {-# SPECIALIZE mapWithIndexTree :: (Int -> Node y -> b) -> Int -> FingerTree (Node y) -> FingerTree b #-}
  mapWithIndexTree :: Sized a => (Int -> a -> b) -> Int -> FingerTree a -> FingerTree b
  mapWithIndexTree _ s Empty = s `seq` Empty
  mapWithIndexTree f s (Single xs) = Single $ f s xs
  mapWithIndexTree f s (Deep n pr m sf) = sPspr `seq` sPsprm `seq`
          Deep n
               (mapWithIndexDigit f s pr)
               (mapWithIndexTree (mapWithIndexNode f) sPspr m)
               (mapWithIndexDigit f sPsprm sf)
    where
      sPspr = s + size pr
      sPsprm = s + n - size sf

  {-# SPECIALIZE mapWithIndexDigit :: (Int -> Elem y -> b) -> Int -> Digit (Elem y) -> Digit b #-}
  {-# SPECIALIZE mapWithIndexDigit :: (Int -> Node y -> b) -> Int -> Digit (Node y) -> Digit b #-}
  mapWithIndexDigit :: Sized a => (Int -> a -> b) -> Int -> Digit a -> Digit b
  mapWithIndexDigit f s (One a) = One (f s a)
  mapWithIndexDigit f s (Two a b) = sPsa `seq` Two (f s a) (f sPsa b)
    where
      sPsa = s + size a
  mapWithIndexDigit f s (Three a b c) = sPsa `seq` sPsab `seq`
                                      Three (f s a) (f sPsa b) (f sPsab c)
    where
      sPsa = s + size a
      sPsab = sPsa + size b
  mapWithIndexDigit f s (Four a b c d) = sPsa `seq` sPsab `seq` sPsabc `seq`
                          Four (f s a) (f sPsa b) (f sPsab c) (f sPsabc d)
    where
      sPsa = s + size a
      sPsab = sPsa + size b
      sPsabc = sPsab + size c

  {-# SPECIALIZE mapWithIndexNode :: (Int -> Elem y -> b) -> Int -> Node (Elem y) -> Node b #-}
  {-# SPECIALIZE mapWithIndexNode :: (Int -> Node y -> b) -> Int -> Node (Node y) -> Node b #-}
  mapWithIndexNode :: Sized a => (Int -> a -> b) -> Int -> Node a -> Node b
  mapWithIndexNode f s (Node2 ns a b) = sPsa `seq` Node2 ns (f s a) (f sPsa b)
    where
      sPsa = s + size a
  mapWithIndexNode f s (Node3 ns a b c) = sPsa `seq` sPsab `seq`
                                     Node3 ns (f s a) (f sPsa b) (f sPsab c)
    where
      sPsa = s + size a
      sPsab = sPsa + size b

#ifdef __GLASGOW_HASKELL__
{-# NOINLINE [1] mapWithIndex #-}
{-# RULES
"mapWithIndex/mapWithIndex" forall f g xs . mapWithIndex f (mapWithIndex g xs) =
  mapWithIndex (\k a -> f k (g k a)) xs
"mapWithIndex/fmapSeq" forall f g xs . mapWithIndex f (fmapSeq g xs) =
  mapWithIndex (\k a -> f k (g a)) xs
"fmapSeq/mapWithIndex" forall f g xs . fmapSeq f (mapWithIndex g xs) =
  mapWithIndex (\k a -> f (g k a)) xs
 #-}
#endif

-- | /O(n)/. Convert a given sequence length and a function representing that
-- sequence into a sequence.
fromFunction :: Int -> (Int -> a) -> Seq a
fromFunction len f | len < 0 = error "Data.Sequence.fromFunction called with negative len"
                   | len == 0 = empty
                   | otherwise = Seq $ create (lift_elem f) 1 0 len
  where
    create :: (Int -> a) -> Int -> Int -> Int -> FingerTree a
    create b{-tree_builder-} s{-tree_size-} i{-start_index-} trees = i `seq` s `seq` case trees of
       1 -> Single $ b i
       2 -> Deep (2*s) (One (b i)) Empty (One (b (i+s)))
       3 -> Deep (3*s) (createTwo i) Empty (One (b (i+2*s)))
       4 -> Deep (4*s) (createTwo i) Empty (createTwo (i+2*s))
       5 -> Deep (5*s) (createThree i) Empty (createTwo (i+3*s))
       6 -> Deep (6*s) (createThree i) Empty (createThree (i+3*s))
       _ -> case trees `quotRem` 3 of
           (trees', 1) -> Deep (trees*s) (createTwo i)
                              (create mb (3*s) (i+2*s) (trees'-1))
                              (createTwo (i+(2+3*(trees'-1))*s))
           (trees', 2) -> Deep (trees*s) (createThree i)
                              (create mb (3*s) (i+3*s) (trees'-1))
                              (createTwo (i+(3+3*(trees'-1))*s))
           (trees', _) -> Deep (trees*s) (createThree i)
                              (create mb (3*s) (i+3*s) (trees'-2))
                              (createThree (i+(3+3*(trees'-2))*s))
      where
        createTwo j = Two (b j) (b (j + s))
        {-# INLINE createTwo #-}
        createThree j = Three (b j) (b (j + s)) (b (j + 2*s))
        {-# INLINE createThree #-}
        mb j = Node3 (3*s) (b j) (b (j + s)) (b (j + 2*s))
        {-# INLINE mb #-}

    lift_elem :: (Int -> a) -> (Int -> Elem a)
#if __GLASGOW_HASKELL__ >= 708
    lift_elem g = coerce g
#else
    lift_elem g = Elem . g
#endif
    {-# INLINE lift_elem #-}

-- | /O(n)/. Create a sequence consisting of the elements of an 'Array'.
-- Note that the resulting sequence elements may be evaluated lazily (as on GHC),
-- so you must force the entire structure to be sure that the original array
-- can be garbage-collected.
fromArray :: Ix i => Array i a -> Seq a
#ifdef __GLASGOW_HASKELL__
fromArray a = fromFunction (GHC.Arr.numElements a) (GHC.Arr.unsafeAt a)
#else
fromArray a = fromList2 (Data.Array.rangeSize (Data.Array.bounds a)) (Data.Array.elems a)
#endif

-- Splitting

-- | /O(log(min(i,n-i)))/. The first @i@ elements of a sequence.
-- If @i@ is negative, @'take' i s@ yields the empty sequence.
-- If the sequence contains fewer than @i@ elements, the whole sequence
-- is returned.
take            :: Int -> Seq a -> Seq a
take i          =  fst . splitAt' i

-- | /O(log(min(i,n-i)))/. Elements of a sequence after the first @i@.
-- If @i@ is negative, @'drop' i s@ yields the whole sequence.
-- If the sequence contains fewer than @i@ elements, the empty sequence
-- is returned.
drop            :: Int -> Seq a -> Seq a
drop i          =  snd . splitAt' i

-- | /O(log(min(i,n-i)))/. Split a sequence at a given position.
-- @'splitAt' i s = ('take' i s, 'drop' i s)@.
splitAt                 :: Int -> Seq a -> (Seq a, Seq a)
splitAt i (Seq xs)      =  (Seq l, Seq r)
  where (l, r)          =  split i xs

-- | /O(log(min(i,n-i))) A strict version of 'splitAt'.
splitAt'                 :: Int -> Seq a -> (Seq a, Seq a)
splitAt' i (Seq xs)      = case split i xs of
                             (l, r) -> (Seq l, Seq r)

split :: Int -> FingerTree (Elem a) ->
    (FingerTree (Elem a), FingerTree (Elem a))
split i Empty   = i `seq` (Empty, Empty)
split i xs
  | size xs > i = case splitTree i xs of
                    Split l x r -> (l, consTree x r)
  | otherwise   = (xs, Empty)

data Split t a = Split t a t
#if TESTING
    deriving Show
#endif

{-# SPECIALIZE splitTree :: Int -> FingerTree (Elem a) -> Split (FingerTree (Elem a)) (Elem a) #-}
{-# SPECIALIZE splitTree :: Int -> FingerTree (Node a) -> Split (FingerTree (Node a)) (Node a) #-}
splitTree :: Sized a => Int -> FingerTree a -> Split (FingerTree a) a
splitTree _ Empty = error "splitTree of empty tree"
splitTree i (Single x) = i `seq` Split Empty x Empty
splitTree i (Deep _ pr m sf)
  | i < spr     = case splitDigit i pr of
            Split l x r -> Split (maybe Empty digitToTree l) x (deepL r m sf)
  | i < spm     = case splitTree im m of
            Split ml xs mr -> case splitNode (im - size ml) xs of
                Split l x r -> Split (deepR pr ml l) x (deepL r mr sf)
  | otherwise   = case splitDigit (i - spm) sf of
            Split l x r -> Split (deepR pr m l) x (maybe Empty digitToTree r)
  where
    spr     = size pr
    spm     = spr + size m
    im      = i - spr

{-# SPECIALIZE splitNode :: Int -> Node (Elem a) -> Split (Maybe (Digit (Elem a))) (Elem a) #-}
{-# SPECIALIZE splitNode :: Int -> Node (Node a) -> Split (Maybe (Digit (Node a))) (Node a) #-}
splitNode :: Sized a => Int -> Node a -> Split (Maybe (Digit a)) a
splitNode i (Node2 _ a b)
  | i < sa      = Split Nothing a (Just (One b))
  | otherwise   = Split (Just (One a)) b Nothing
  where
    sa      = size a
splitNode i (Node3 _ a b c)
  | i < sa      = Split Nothing a (Just (Two b c))
  | i < sab     = Split (Just (One a)) b (Just (One c))
  | otherwise   = Split (Just (Two a b)) c Nothing
  where
    sa      = size a
    sab     = sa + size b

{-# SPECIALIZE splitDigit :: Int -> Digit (Elem a) -> Split (Maybe (Digit (Elem a))) (Elem a) #-}
{-# SPECIALIZE splitDigit :: Int -> Digit (Node a) -> Split (Maybe (Digit (Node a))) (Node a) #-}
splitDigit :: Sized a => Int -> Digit a -> Split (Maybe (Digit a)) a
splitDigit i (One a) = i `seq` Split Nothing a Nothing
splitDigit i (Two a b)
  | i < sa      = Split Nothing a (Just (One b))
  | otherwise   = Split (Just (One a)) b Nothing
  where
    sa      = size a
splitDigit i (Three a b c)
  | i < sa      = Split Nothing a (Just (Two b c))
  | i < sab     = Split (Just (One a)) b (Just (One c))
  | otherwise   = Split (Just (Two a b)) c Nothing
  where
    sa      = size a
    sab     = sa + size b
splitDigit i (Four a b c d)
  | i < sa      = Split Nothing a (Just (Three b c d))
  | i < sab     = Split (Just (One a)) b (Just (Two c d))
  | i < sabc    = Split (Just (Two a b)) c (Just (One d))
  | otherwise   = Split (Just (Three a b c)) d Nothing
  where
    sa      = size a
    sab     = sa + size b
    sabc    = sab + size c

-- | /O(n)/.  Returns a sequence of all suffixes of this sequence,
-- longest first.  For example,
--
-- > tails (fromList "abc") = fromList [fromList "abc", fromList "bc", fromList "c", fromList ""]
--
-- Evaluating the /i/th suffix takes /O(log(min(i, n-i)))/, but evaluating
-- every suffix in the sequence takes /O(n)/ due to sharing.
tails                   :: Seq a -> Seq (Seq a)
tails (Seq xs)          = Seq (tailsTree (Elem . Seq) xs) |> empty

-- | /O(n)/.  Returns a sequence of all prefixes of this sequence,
-- shortest first.  For example,
--
-- > inits (fromList "abc") = fromList [fromList "", fromList "a", fromList "ab", fromList "abc"]
--
-- Evaluating the /i/th prefix takes /O(log(min(i, n-i)))/, but evaluating
-- every prefix in the sequence takes /O(n)/ due to sharing.
inits                   :: Seq a -> Seq (Seq a)
inits (Seq xs)          = empty <| Seq (initsTree (Elem . Seq) xs)

-- This implementation of tails (and, analogously, inits) has the
-- following algorithmic advantages:
--      Evaluating each tail in the sequence takes linear total time,
--      which is better than we could say for
--              @fromList [drop n xs | n <- [0..length xs]]@.
--      Evaluating any individual tail takes logarithmic time, which is
--      better than we can say for either
--              @scanr (<|) empty xs@ or @iterateN (length xs + 1) (\ xs -> let _ :< xs' = viewl xs in xs') xs@.
--
-- Moreover, if we actually look at every tail in the sequence, the
-- following benchmarks demonstrate that this implementation is modestly
-- faster than any of the above:
--
-- Times (ms)
--               min      mean    +/-sd    median    max
-- Seq.tails:   21.986   24.961   10.169   22.417   86.485
-- scanr:       85.392   87.942    2.488   87.425  100.217
-- iterateN:       29.952   31.245    1.574   30.412   37.268
--
-- The algorithm for tails (and, analogously, inits) is as follows:
--
-- A Node in the FingerTree of tails is constructed by evaluating the
-- corresponding tail of the FingerTree of Nodes, considering the first
-- Node in this tail, and constructing a Node in which each tail of this
-- Node is made to be the prefix of the remaining tree.  This ends up
-- working quite elegantly, as the remainder of the tail of the FingerTree
-- of Nodes becomes the middle of a new tail, the suffix of the Node is
-- the prefix, and the suffix of the original tree is retained.
--
-- In particular, evaluating the /i/th tail involves making as
-- many partial evaluations as the Node depth of the /i/th element.
-- In addition, when we evaluate the /i/th tail, and we also evaluate
-- the /j/th tail, and /m/ Nodes are on the path to both /i/ and /j/,
-- each of those /m/ evaluations are shared between the computation of
-- the /i/th and /j/th tails.
--
-- wasserman.louis@gmail.com, 7/16/09

tailsDigit :: Digit a -> Digit (Digit a)
tailsDigit (One a) = One (One a)
tailsDigit (Two a b) = Two (Two a b) (One b)
tailsDigit (Three a b c) = Three (Three a b c) (Two b c) (One c)
tailsDigit (Four a b c d) = Four (Four a b c d) (Three b c d) (Two c d) (One d)

initsDigit :: Digit a -> Digit (Digit a)
initsDigit (One a) = One (One a)
initsDigit (Two a b) = Two (One a) (Two a b)
initsDigit (Three a b c) = Three (One a) (Two a b) (Three a b c)
initsDigit (Four a b c d) = Four (One a) (Two a b) (Three a b c) (Four a b c d)

tailsNode :: Node a -> Node (Digit a)
tailsNode (Node2 s a b) = Node2 s (Two a b) (One b)
tailsNode (Node3 s a b c) = Node3 s (Three a b c) (Two b c) (One c)

initsNode :: Node a -> Node (Digit a)
initsNode (Node2 s a b) = Node2 s (One a) (Two a b)
initsNode (Node3 s a b c) = Node3 s (One a) (Two a b) (Three a b c)

{-# SPECIALIZE tailsTree :: (FingerTree (Elem a) -> Elem b) -> FingerTree (Elem a) -> FingerTree (Elem b) #-}
{-# SPECIALIZE tailsTree :: (FingerTree (Node a) -> Node b) -> FingerTree (Node a) -> FingerTree (Node b) #-}
-- | Given a function to apply to tails of a tree, applies that function
-- to every tail of the specified tree.
tailsTree :: (Sized a, Sized b) => (FingerTree a -> b) -> FingerTree a -> FingerTree b
tailsTree _ Empty = Empty
tailsTree f (Single x) = Single (f (Single x))
tailsTree f (Deep n pr m sf) =
    Deep n (fmap (\ pr' -> f (deep pr' m sf)) (tailsDigit pr))
        (tailsTree f' m)
        (fmap (f . digitToTree) (tailsDigit sf))
  where
    f' ms = let Just2 node m' = viewLTree ms in
        fmap (\ pr' -> f (deep pr' m' sf)) (tailsNode node)

{-# SPECIALIZE initsTree :: (FingerTree (Elem a) -> Elem b) -> FingerTree (Elem a) -> FingerTree (Elem b) #-}
{-# SPECIALIZE initsTree :: (FingerTree (Node a) -> Node b) -> FingerTree (Node a) -> FingerTree (Node b) #-}
-- | Given a function to apply to inits of a tree, applies that function
-- to every init of the specified tree.
initsTree :: (Sized a, Sized b) => (FingerTree a -> b) -> FingerTree a -> FingerTree b
initsTree _ Empty = Empty
initsTree f (Single x) = Single (f (Single x))
initsTree f (Deep n pr m sf) =
    Deep n (fmap (f . digitToTree) (initsDigit pr))
        (initsTree f' m)
        (fmap (f . deep pr m) (initsDigit sf))
  where
    f' ms =  let Just2 m' node = viewRTree ms in
             fmap (\ sf' -> f (deep pr m' sf')) (initsNode node)

{-# INLINE foldlWithIndex #-}
-- | 'foldlWithIndex' is a version of 'foldl' that also provides access
-- to the index of each element.
foldlWithIndex :: (b -> Int -> a -> b) -> b -> Seq a -> b
foldlWithIndex f z xs = foldl (\ g x i -> i `seq` f (g (i - 1)) i x) (const z) xs (length xs - 1)

{-# INLINE foldrWithIndex #-}
-- | 'foldrWithIndex' is a version of 'foldr' that also provides access
-- to the index of each element.
foldrWithIndex :: (Int -> a -> b -> b) -> b -> Seq a -> b
foldrWithIndex f z xs = foldr (\ x g i -> i `seq` f i x (g (i+1))) (const z) xs 0

{-# INLINE listToMaybe' #-}
-- 'listToMaybe\'' is a good consumer version of 'listToMaybe'.
listToMaybe' :: [a] -> Maybe a
listToMaybe' = foldr (\ x _ -> Just x) Nothing

-- | /O(i)/ where /i/ is the prefix length.  'takeWhileL', applied
-- to a predicate @p@ and a sequence @xs@, returns the longest prefix
-- (possibly empty) of @xs@ of elements that satisfy @p@.
takeWhileL :: (a -> Bool) -> Seq a -> Seq a
takeWhileL p = fst . spanl p

-- | /O(i)/ where /i/ is the suffix length.  'takeWhileR', applied
-- to a predicate @p@ and a sequence @xs@, returns the longest suffix
-- (possibly empty) of @xs@ of elements that satisfy @p@.
--
-- @'takeWhileR' p xs@ is equivalent to @'reverse' ('takeWhileL' p ('reverse' xs))@.
takeWhileR :: (a -> Bool) -> Seq a -> Seq a
takeWhileR p = fst . spanr p

-- | /O(i)/ where /i/ is the prefix length.  @'dropWhileL' p xs@ returns
-- the suffix remaining after @'takeWhileL' p xs@.
dropWhileL :: (a -> Bool) -> Seq a -> Seq a
dropWhileL p = snd . spanl p

-- | /O(i)/ where /i/ is the suffix length.  @'dropWhileR' p xs@ returns
-- the prefix remaining after @'takeWhileR' p xs@.
--
-- @'dropWhileR' p xs@ is equivalent to @'reverse' ('dropWhileL' p ('reverse' xs))@.
dropWhileR :: (a -> Bool) -> Seq a -> Seq a
dropWhileR p = snd . spanr p

-- | /O(i)/ where /i/ is the prefix length.  'spanl', applied to
-- a predicate @p@ and a sequence @xs@, returns a pair whose first
-- element is the longest prefix (possibly empty) of @xs@ of elements that
-- satisfy @p@ and the second element is the remainder of the sequence.
spanl :: (a -> Bool) -> Seq a -> (Seq a, Seq a)
spanl p = breakl (not . p)

-- | /O(i)/ where /i/ is the suffix length.  'spanr', applied to a
-- predicate @p@ and a sequence @xs@, returns a pair whose /first/ element
-- is the longest /suffix/ (possibly empty) of @xs@ of elements that
-- satisfy @p@ and the second element is the remainder of the sequence.
spanr :: (a -> Bool) -> Seq a -> (Seq a, Seq a)
spanr p = breakr (not . p)

{-# INLINE breakl #-}
-- | /O(i)/ where /i/ is the breakpoint index.  'breakl', applied to a
-- predicate @p@ and a sequence @xs@, returns a pair whose first element
-- is the longest prefix (possibly empty) of @xs@ of elements that
-- /do not satisfy/ @p@ and the second element is the remainder of
-- the sequence.
--
-- @'breakl' p@ is equivalent to @'spanl' (not . p)@.
breakl :: (a -> Bool) -> Seq a -> (Seq a, Seq a)
breakl p xs = foldr (\ i _ -> splitAt i xs) (xs, empty) (findIndicesL p xs)

{-# INLINE breakr #-}
-- | @'breakr' p@ is equivalent to @'spanr' (not . p)@.
breakr :: (a -> Bool) -> Seq a -> (Seq a, Seq a)
breakr p xs = foldr (\ i _ -> flipPair (splitAt (i + 1) xs)) (xs, empty) (findIndicesR p xs)
  where flipPair (x, y) = (y, x)

-- | /O(n)/.  The 'partition' function takes a predicate @p@ and a
-- sequence @xs@ and returns sequences of those elements which do and
-- do not satisfy the predicate.
partition :: (a -> Bool) -> Seq a -> (Seq a, Seq a)
partition p = foldl part (empty, empty)
  where
    part (xs, ys) x
      | p x         = (xs |> x, ys)
      | otherwise   = (xs, ys |> x)

-- | /O(n)/.  The 'filter' function takes a predicate @p@ and a sequence
-- @xs@ and returns a sequence of those elements which satisfy the
-- predicate.
filter :: (a -> Bool) -> Seq a -> Seq a
filter p = foldl (\ xs x -> if p x then xs |> x else xs) empty

-- Indexing sequences

-- | 'elemIndexL' finds the leftmost index of the specified element,
-- if it is present, and otherwise 'Nothing'.
elemIndexL :: Eq a => a -> Seq a -> Maybe Int
elemIndexL x = findIndexL (x ==)

-- | 'elemIndexR' finds the rightmost index of the specified element,
-- if it is present, and otherwise 'Nothing'.
elemIndexR :: Eq a => a -> Seq a -> Maybe Int
elemIndexR x = findIndexR (x ==)

-- | 'elemIndicesL' finds the indices of the specified element, from
-- left to right (i.e. in ascending order).
elemIndicesL :: Eq a => a -> Seq a -> [Int]
elemIndicesL x = findIndicesL (x ==)

-- | 'elemIndicesR' finds the indices of the specified element, from
-- right to left (i.e. in descending order).
elemIndicesR :: Eq a => a -> Seq a -> [Int]
elemIndicesR x = findIndicesR (x ==)

-- | @'findIndexL' p xs@ finds the index of the leftmost element that
-- satisfies @p@, if any exist.
findIndexL :: (a -> Bool) -> Seq a -> Maybe Int
findIndexL p = listToMaybe' . findIndicesL p

-- | @'findIndexR' p xs@ finds the index of the rightmost element that
-- satisfies @p@, if any exist.
findIndexR :: (a -> Bool) -> Seq a -> Maybe Int
findIndexR p = listToMaybe' . findIndicesR p

{-# INLINE findIndicesL #-}
-- | @'findIndicesL' p@ finds all indices of elements that satisfy @p@,
-- in ascending order.
findIndicesL :: (a -> Bool) -> Seq a -> [Int]
#if __GLASGOW_HASKELL__
findIndicesL p xs = build (\ c n -> let g i x z = if p x then c i z else z in
                foldrWithIndex g n xs)
#else
findIndicesL p xs = foldrWithIndex g [] xs
  where g i x is = if p x then i:is else is
#endif

{-# INLINE findIndicesR #-}
-- | @'findIndicesR' p@ finds all indices of elements that satisfy @p@,
-- in descending order.
findIndicesR :: (a -> Bool) -> Seq a -> [Int]
#if __GLASGOW_HASKELL__
findIndicesR p xs = build (\ c n ->
    let g z i x = if p x then c i z else z in foldlWithIndex g n xs)
#else
findIndicesR p xs = foldlWithIndex g [] xs
  where g is i x = if p x then i:is else is
#endif

------------------------------------------------------------------------
-- Lists
------------------------------------------------------------------------

-- The implementation below, by Ross Paterson, avoids the rebuilding
-- the previous (|>)-based implementation suffered from.

-- | /O(n)/. Create a sequence from a finite list of elements.
-- There is a function 'toList' in the opposite direction for all
-- instances of the 'Foldable' class, including 'Seq'.
fromList        :: [a] -> Seq a
fromList = Seq . mkTree 1 . map_elem
  where
    {-# SPECIALIZE mkTree :: Int -> [Elem a] -> FingerTree (Elem a) #-}
    {-# SPECIALIZE mkTree :: Int -> [Node a] -> FingerTree (Node a) #-}
    mkTree :: (Sized a) => Int -> [a] -> FingerTree a
    STRICT_1_OF_2(mkTree)
    mkTree _ [] = Empty
    mkTree _ [x1] = Single x1
    mkTree s [x1, x2] = Deep (2*s) (One x1) Empty (One x2)
    mkTree s [x1, x2, x3] = Deep (3*s) (One x1) Empty (Two x2 x3)
    mkTree s (x1:x2:x3:x4:xs) = case getNodes (3*s) x4 xs of
      (ns, sf) -> case mkTree (3*s) ns of
        m -> m `seq` Deep (3*size x1 + size m + size sf) (Three x1 x2 x3) m sf

    getNodes :: Int -> a -> [a] -> ([Node a], Digit a)
    STRICT_1_OF_3(getNodes)
    getNodes _ x1 [] = ([], One x1)
    getNodes _ x1 [x2] = ([], Two x1 x2)
    getNodes _ x1 [x2, x3] = ([], Three x1 x2 x3)
    getNodes s x1 (x2:x3:x4:xs) = (Node3 s x1 x2 x3:ns, d)
       where (ns, d) = getNodes s x4 xs

    map_elem :: [a] -> [Elem a]
#if __GLASGOW_HASKELL__ >= 708
    map_elem xs = coerce xs
#else
    map_elem xs = Data.List.map Elem xs
#endif
    {-# INLINE map_elem #-}

#if __GLASGOW_HASKELL__ >= 708
instance GHC.Exts.IsList (Seq a) where
    type Item (Seq a) = a
    fromList = fromList
    fromListN = fromList2
    toList = toList
#endif

------------------------------------------------------------------------
-- Reverse
------------------------------------------------------------------------

-- | /O(n)/. The reverse of a sequence.
reverse :: Seq a -> Seq a
reverse (Seq xs) = Seq (reverseTree id xs)

reverseTree :: (a -> a) -> FingerTree a -> FingerTree a
reverseTree _ Empty = Empty
reverseTree f (Single x) = Single (f x)
reverseTree f (Deep s pr m sf) =
    Deep s (reverseDigit f sf)
        (reverseTree (reverseNode f) m)
        (reverseDigit f pr)

{-# INLINE reverseDigit #-}
reverseDigit :: (a -> a) -> Digit a -> Digit a
reverseDigit f (One a) = One (f a)
reverseDigit f (Two a b) = Two (f b) (f a)
reverseDigit f (Three a b c) = Three (f c) (f b) (f a)
reverseDigit f (Four a b c d) = Four (f d) (f c) (f b) (f a)

reverseNode :: (a -> a) -> Node a -> Node a
reverseNode f (Node2 s a b) = Node2 s (f b) (f a)
reverseNode f (Node3 s a b c) = Node3 s (f c) (f b) (f a)

------------------------------------------------------------------------
-- Mapping with a splittable value
------------------------------------------------------------------------

-- For zipping, it is useful to build a result by
-- traversing a sequence while splitting up something else.  For zipping, we
-- traverse the first sequence while splitting up the second.
--
-- What makes all this crazy code a good idea:
--
-- Suppose we zip together two sequences of the same length:
--
-- zs = zip xs ys
--
-- We want to get reasonably fast indexing into zs immediately, rather than
-- needing to construct the entire thing first, as the previous implementation
-- required. The first aspect is that we build the result "outside-in" or
-- "top-down", rather than left to right. That gives us access to both ends
-- quickly. But that's not enough, by itself, to give immediate access to the
-- center of zs. For that, we need to be able to skip over larger segments of
-- zs, delaying their construction until we actually need them. The way we do
-- this is to traverse xs, while splitting up ys according to the structure of
-- xs. If we have a Deep _ pr m sf, we split ys into three pieces, and hand off
-- one piece to the prefix, one to the middle, and one to the suffix of the
-- result. The key point is that we don't need to actually do anything further
-- with those pieces until we actually need them; the computations to split
-- them up further and zip them with their matching pieces can be delayed until
-- they're actually needed. We do the same thing for Digits (splitting into
-- between one and four pieces) and Nodes (splitting into two or three). The
-- ultimate result is that we can index into, or split at, any location in zs
-- in polylogarithmic time *immediately*, while still being able to force all
-- the thunks in O(n) time.
--
-- Benchmark info, and alternatives:
--
-- The old zipping code used mapAccumL to traverse the first sequence while
-- cutting down the second sequence one piece at a time.
--
-- An alternative way to express that basic idea is to convert both sequences
-- to lists, zip the lists, and then convert the result back to a sequence.
-- I'll call this the "listy" implementation.
--
-- I benchmarked two operations: Each started by zipping two sequences
-- constructed with replicate and/or fromList. The first would then immediately
-- index into the result. The second would apply deepseq to force the entire
-- result.  The new implementation worked much better than either of the others
-- on the immediate indexing test, as expected. It also worked better than the
-- old implementation for all the deepseq tests. For short sequences, the listy
-- implementation outperformed all the others on the deepseq test. However, the
-- splitting implementation caught up and surpassed it once the sequences grew
-- long enough. It seems likely that by avoiding rebuilding, it interacts
-- better with the cache hierarchy.
--
-- David Feuer, with excellent guidance from Carter Schonwald, December 2014

-- | /O(n)/. Constructs a new sequence with the same structure as an existing
-- sequence using a user-supplied mapping function along with a splittable
-- value and a way to split it. The value is split up lazily according to the
-- structure of the sequence, so one piece of the value is distributed to each
-- element of the sequence. The caller should provide a splitter function that
-- takes a number, @n@, and a splittable value, breaks off a chunk of size @n@
-- from the value, and returns that chunk and the remainder as a pair. The
-- following examples will hopefully make the usage clear:
--
-- > zipWith :: (a -> b -> c) -> Seq a -> Seq b -> Seq c
-- > zipWith f s1 s2 = splitMap splitAt (\b a -> f a (b `index` 0)) s2' s1'
-- >   where
-- >     minLen = min (length s1) (length s2)
-- >     s1' = take minLen s1
-- >     s2' = take minLen s2
--
-- > mapWithIndex :: (Int -> a -> b) -> Seq a -> Seq b
-- > mapWithIndex f = splitMap (\n i -> (i, n+i)) f 0
splitMap :: (Int -> s -> (s,s)) -> (s -> a -> b) -> s -> Seq a -> Seq b
splitMap splt' = go
 where
  go f s (Seq xs) = Seq $ splitMapTree splt' (\s' (Elem a) -> Elem (f s' a)) s xs

  {-# SPECIALIZE splitMapTree :: (Int -> s -> (s,s)) -> (s -> Elem y -> b) -> s -> FingerTree (Elem y) -> FingerTree b #-}
  {-# SPECIALIZE splitMapTree :: (Int -> s -> (s,s)) -> (s -> Node y -> b) -> s -> FingerTree (Node y) -> FingerTree b #-}
  splitMapTree :: Sized a => (Int -> s -> (s,s)) -> (s -> a -> b) -> s -> FingerTree a -> FingerTree b
  splitMapTree _    _ _ Empty = Empty
  splitMapTree _    f s (Single xs) = Single $ f s xs
  splitMapTree splt f s (Deep n pr m sf) = Deep n (splitMapDigit splt f prs pr) (splitMapTree splt (splitMapNode splt f) ms m) (splitMapDigit splt f sfs sf)
    where
      (prs, r) = splt (size pr) s
      (ms, sfs) = splt (n - size pr - size sf) r

  {-# SPECIALIZE splitMapDigit :: (Int -> s -> (s,s)) -> (s -> Elem y -> b) -> s -> Digit (Elem y) -> Digit b #-}
  {-# SPECIALIZE splitMapDigit :: (Int -> s -> (s,s)) -> (s -> Node y -> b) -> s -> Digit (Node y) -> Digit b #-}
  splitMapDigit :: Sized a => (Int -> s -> (s,s)) -> (s -> a -> b) -> s -> Digit a -> Digit b
  splitMapDigit _    f s (One a) = One (f s a)
  splitMapDigit splt f s (Two a b) = Two (f first a) (f second b)
    where
      (first, second) = splt (size a) s
  splitMapDigit splt f s (Three a b c) = Three (f first a) (f second b) (f third c)
    where
      (first, r) = splt (size a) s
      (second, third) = splt (size b) r
  splitMapDigit splt f s (Four a b c d) = Four (f first a) (f second b) (f third c) (f fourth d)
    where
      (first, s') = splt (size a) s
      (middle, fourth) = splt (size b + size c) s'
      (second, third) = splt (size b) middle

  {-# SPECIALIZE splitMapNode :: (Int -> s -> (s,s)) -> (s -> Elem y -> b) -> s -> Node (Elem y) -> Node b #-}
  {-# SPECIALIZE splitMapNode :: (Int -> s -> (s,s)) -> (s -> Node y -> b) -> s -> Node (Node y) -> Node b #-}
  splitMapNode :: Sized a => (Int -> s -> (s,s)) -> (s -> a -> b) -> s -> Node a -> Node b
  splitMapNode splt f s (Node2 ns a b) = Node2 ns (f first a) (f second b)
    where
      (first, second) = splt (size a) s
  splitMapNode splt f s (Node3 ns a b c) = Node3 ns (f first a) (f second b) (f third c)
    where
      (first, r) = splt (size a) s
      (second, third) = splt (size b) r

{-# INLINE splitMap #-}

getSingleton :: Seq a -> a
getSingleton (Seq (Single (Elem a))) = a
getSingleton (Seq Empty) = error "getSingleton: Empty"
getSingleton _ = error "getSingleton: Not a singleton."

------------------------------------------------------------------------
-- Zipping
------------------------------------------------------------------------

-- | /O(min(n1,n2))/.  'zip' takes two sequences and returns a sequence
-- of corresponding pairs.  If one input is short, excess elements are
-- discarded from the right end of the longer sequence.
zip :: Seq a -> Seq b -> Seq (a, b)
zip = zipWith (,)

-- | /O(min(n1,n2))/.  'zipWith' generalizes 'zip' by zipping with the
-- function given as the first argument, instead of a tupling function.
-- For example, @zipWith (+)@ is applied to two sequences to take the
-- sequence of corresponding sums.
zipWith :: (a -> b -> c) -> Seq a -> Seq b -> Seq c
zipWith f s1 s2 = zipWith' f s1' s2'
  where
    minLen = min (length s1) (length s2)
    s1' = take minLen s1
    s2' = take minLen s2

-- | A version of zipWith that assumes the sequences have the same length.
zipWith' :: (a -> b -> c) -> Seq a -> Seq b -> Seq c
zipWith' f s1 s2 = splitMap splitAt' (\s a -> f a (getSingleton s)) s2 s1

-- | /O(min(n1,n2,n3))/.  'zip3' takes three sequences and returns a
-- sequence of triples, analogous to 'zip'.
zip3 :: Seq a -> Seq b -> Seq c -> Seq (a,b,c)
zip3 = zipWith3 (,,)

-- | /O(min(n1,n2,n3))/.  'zipWith3' takes a function which combines
-- three elements, as well as three sequences and returns a sequence of
-- their point-wise combinations, analogous to 'zipWith'.
zipWith3 :: (a -> b -> c -> d) -> Seq a -> Seq b -> Seq c -> Seq d
zipWith3 f s1 s2 s3 = zipWith' ($) (zipWith' f s1' s2') s3'
  where
    minLen = minimum [length s1, length s2, length s3]
    s1' = take minLen s1
    s2' = take minLen s2
    s3' = take minLen s3

zipWith3' :: (a -> b -> c -> d) -> Seq a -> Seq b -> Seq c -> Seq d
zipWith3' f s1 s2 s3 = zipWith' ($) (zipWith' f s1 s2) s3

-- | /O(min(n1,n2,n3,n4))/.  'zip4' takes four sequences and returns a
-- sequence of quadruples, analogous to 'zip'.
zip4 :: Seq a -> Seq b -> Seq c -> Seq d -> Seq (a,b,c,d)
zip4 = zipWith4 (,,,)

-- | /O(min(n1,n2,n3,n4))/.  'zipWith4' takes a function which combines
-- four elements, as well as four sequences and returns a sequence of
-- their point-wise combinations, analogous to 'zipWith'.
zipWith4 :: (a -> b -> c -> d -> e) -> Seq a -> Seq b -> Seq c -> Seq d -> Seq e
zipWith4 f s1 s2 s3 s4 = zipWith' ($) (zipWith3' f s1' s2' s3') s4'
  where
    minLen = minimum [length s1, length s2, length s3, length s4]
    s1' = take minLen s1
    s2' = take minLen s2
    s3' = take minLen s3
    s4' = take minLen s4

------------------------------------------------------------------------
-- Sorting
--
-- sort and sortBy are implemented by simple deforestations of
--      \ xs -> fromList2 (length xs) . Data.List.sortBy cmp . toList
-- which does not get deforested automatically, it would appear.
--
-- Unstable sorting is performed by a heap sort implementation based on
-- pairing heaps.  Because the internal structure of sequences is quite
-- varied, it is difficult to get blocks of elements of roughly the same
-- length, which would improve merge sort performance.  Pairing heaps,
-- on the other hand, are relatively resistant to the effects of merging
-- heaps of wildly different sizes, as guaranteed by its amortized
-- constant-time merge operation.  Moreover, extensive use of SpecConstr
-- transformations can be done on pairing heaps, especially when we're
-- only constructing them to immediately be unrolled.
--
-- On purely random sequences of length 50000, with no RTS options,
-- I get the following statistics, in which heapsort is about 42.5%
-- faster:  (all comparisons done with -O2)
--
-- Times (ms)            min      mean    +/-sd    median    max
-- to/from list:       103.802  108.572    7.487  106.436  143.339
-- unstable heapsort:   60.686   62.968    4.275   61.187   79.151
--
-- Heapsort, it would seem, is less of a memory hog than Data.List.sortBy.
-- The gap is narrowed when more memory is available, but heapsort still
-- wins, 15% faster, with +RTS -H128m:
--
-- Times (ms)            min    mean    +/-sd  median    max
-- to/from list:       42.692  45.074   2.596  44.600  56.601
-- unstable heapsort:  37.100  38.344   3.043  37.715  55.526
--
-- In addition, on strictly increasing sequences the gap is even wider
-- than normal; heapsort is 68.5% faster with no RTS options:
-- Times (ms)            min    mean    +/-sd  median    max
-- to/from list:       52.236  53.574   1.987  53.034  62.098
-- unstable heapsort:  16.433  16.919   0.931  16.681  21.622
--
-- This may be attributed to the elegant nature of the pairing heap.
--
-- wasserman.louis@gmail.com, 7/20/09
------------------------------------------------------------------------

-- | /O(n log n)/.  'sort' sorts the specified 'Seq' by the natural
-- ordering of its elements.  The sort is stable.
-- If stability is not required, 'unstableSort' can be considerably
-- faster, and in particular uses less memory.
sort :: Ord a => Seq a -> Seq a
sort = sortBy compare

-- | /O(n log n)/.  'sortBy' sorts the specified 'Seq' according to the
-- specified comparator.  The sort is stable.
-- If stability is not required, 'unstableSortBy' can be considerably
-- faster, and in particular uses less memory.
sortBy :: (a -> a -> Ordering) -> Seq a -> Seq a
sortBy cmp xs = fromList2 (length xs) (Data.List.sortBy cmp (toList xs))

-- | /O(n log n)/.  'unstableSort' sorts the specified 'Seq' by
-- the natural ordering of its elements, but the sort is not stable.
-- This algorithm is frequently faster and uses less memory than 'sort',
-- and performs extremely well -- frequently twice as fast as 'sort' --
-- when the sequence is already nearly sorted.
unstableSort :: Ord a => Seq a -> Seq a
unstableSort = unstableSortBy compare

-- | /O(n log n)/.  A generalization of 'unstableSort', 'unstableSortBy'
-- takes an arbitrary comparator and sorts the specified sequence.
-- The sort is not stable.  This algorithm is frequently faster and
-- uses less memory than 'sortBy', and performs extremely well --
-- frequently twice as fast as 'sortBy' -- when the sequence is already
-- nearly sorted.
unstableSortBy :: (a -> a -> Ordering) -> Seq a -> Seq a
unstableSortBy cmp (Seq xs) =
    fromList2 (size xs) $ maybe [] (unrollPQ cmp) $
        toPQ cmp (\ (Elem x) -> PQueue x Nil) xs

-- | fromList2, given a list and its length, constructs a completely
-- balanced Seq whose elements are that list using the applicativeTree
-- generalization.
fromList2 :: Int -> [a] -> Seq a
fromList2 n = execState (replicateA n (State ht))
  where
    ht (x:xs) = (xs, x)
    ht []     = error "fromList2: short list"

-- | A 'PQueue' is a simple pairing heap.
data PQueue e = PQueue e (PQL e)
data PQL e = Nil | {-# UNPACK #-} !(PQueue e) :& PQL e

infixr 8 :&

#if TESTING

instance Functor PQueue where
    fmap f (PQueue x ts) = PQueue (f x) (fmap f ts)

instance Functor PQL where
    fmap f (q :& qs) = fmap f q :& fmap f qs
    fmap _ Nil = Nil

instance Show e => Show (PQueue e) where
    show = unlines . draw . fmap show

-- borrowed wholesale from Data.Tree, as Data.Tree actually depends
-- on Data.Sequence
draw :: PQueue String -> [String]
draw (PQueue x ts0) = x : drawSubTrees ts0
  where
    drawSubTrees Nil = []
    drawSubTrees (t :& Nil) =
        "|" : shift "`- " "   " (draw t)
    drawSubTrees (t :& ts) =
        "|" : shift "+- " "|  " (draw t) ++ drawSubTrees ts

    shift first other = Data.List.zipWith (++) (first : repeat other)
#endif

-- | 'unrollPQ', given a comparator function, unrolls a 'PQueue' into
-- a sorted list.
unrollPQ :: (e -> e -> Ordering) -> PQueue e -> [e]
unrollPQ cmp = unrollPQ'
  where
    {-# INLINE unrollPQ' #-}
    unrollPQ' (PQueue x ts) = x:mergePQs0 ts
    (<>) = mergePQ cmp
    mergePQs0 Nil = []
    mergePQs0 (t :& Nil) = unrollPQ' t
    mergePQs0 (t1 :& t2 :& ts) = mergePQs (t1 <> t2) ts
    mergePQs t ts = t `seq` case ts of
        Nil             -> unrollPQ' t
        t1 :& Nil       -> unrollPQ' (t <> t1)
        t1 :& t2 :& ts' -> mergePQs (t <> (t1 <> t2)) ts'

-- | 'toPQ', given an ordering function and a mechanism for queueifying
-- elements, converts a 'FingerTree' to a 'PQueue'.
toPQ :: (e -> e -> Ordering) -> (a -> PQueue e) -> FingerTree a -> Maybe (PQueue e)
toPQ _ _ Empty = Nothing
toPQ _ f (Single x) = Just (f x)
toPQ cmp f (Deep _ pr m sf) = Just (maybe (pr' <> sf') ((pr' <> sf') <>) (toPQ cmp fNode m))
  where
    fDigit digit = case fmap f digit of
        One a           -> a
        Two a b         -> a <> b
        Three a b c     -> a <> b <> c
        Four a b c d    -> (a <> b) <> (c <> d)
    (<>) = mergePQ cmp
    fNode = fDigit . nodeToDigit
    pr' = fDigit pr
    sf' = fDigit sf

-- | 'mergePQ' merges two 'PQueue's.
mergePQ :: (a -> a -> Ordering) -> PQueue a -> PQueue a -> PQueue a
mergePQ cmp q1@(PQueue x1 ts1) q2@(PQueue x2 ts2)
  | cmp x1 x2 == GT     = PQueue x2 (q1 :& ts2)
  | otherwise           = PQueue x1 (q2 :& ts1)
