    {-
    (c) The University of Glasgow 2006
    (c) The AQUA Project, Glasgow University, 1993-1998

    This is useful, general stuff for the Native Code Generator.

    Provide trees (of instructions), so that lists of instructions
    can be appended in linear time.
    -}

    {-# LANGUAGE CPP #-}
    module OrdList (
            OrdList,
            nilOL, isNilOL, unitOL, appOL, consOL, snocOL, concatOL, lastOL,
            mapOL, fromOL, toOL, foldrOL, foldlOL
    ) where

    import Outputable

    #if __GLASGOW_HASKELL__ < 709
    import Data.Monoid ( Monoid(..) )
    #endif

    infixl 5  `appOL`
    infixl 5  `snocOL`
    infixr 5  `consOL`

    data OrdList a
      = None
      | One a
      | Many [a]          -- Invariant: non-empty
      | Cons a (OrdList a)
      | Snoc (OrdList a) a
      | Two (OrdList a) -- Invariant: non-empty
            (OrdList a) -- Invariant: non-empty

    instance Outputable a => Outputable (OrdList a) where
      ppr ol = ppr (fromOL ol)  -- Convert to list and print that

    instance Monoid (OrdList a) where
      mempty = nilOL
      mappend = appOL
      mconcat = concatOL

    nilOL    :: OrdList a
    isNilOL  :: OrdList a -> Bool

    unitOL   :: a           -> OrdList a
    snocOL   :: OrdList a   -> a         -> OrdList a
    consOL   :: a           -> OrdList a -> OrdList a
    appOL    :: OrdList a   -> OrdList a -> OrdList a
    concatOL :: [OrdList a] -> OrdList a
    lastOL   :: OrdList a   -> a

    nilOL        = None
    unitOL as    = One as
    snocOL as   b    = Snoc as b
    consOL a    bs   = Cons a bs
    concatOL aas = foldr appOL None aas

    lastOL None        = panic "lastOL"
    lastOL (One a)     = a
    lastOL (Many as)   = last as
    lastOL (Cons _ as) = lastOL as
    lastOL (Snoc _ a)  = a
    lastOL (Two _ as)  = lastOL as

    isNilOL None = True
    isNilOL _    = False

    None  `appOL` b     = b
    a     `appOL` None  = a
    One a `appOL` b     = Cons a b
    a     `appOL` One b = Snoc a b
    a     `appOL` b     = Two a b

    fromOL :: OrdList a -> [a]
    fromOL a = go a []
      where go None       acc = acc
            go (One a)    acc = a : acc
            go (Cons a b) acc = a : go b acc
            go (Snoc a b) acc = go a (b:acc)
            go (Two a b)  acc = go a (go b acc)
            go (Many xs)  acc = xs ++ acc

    mapOL :: (a -> b) -> OrdList a -> OrdList b
    mapOL _ None = None
    mapOL f (One x) = One (f x)
    mapOL f (Cons x xs) = Cons (f x) (mapOL f xs)
    mapOL f (Snoc xs x) = Snoc (mapOL f xs) (f x)
    mapOL f (Two x y) = Two (mapOL f x) (mapOL f y)
    mapOL f (Many xs) = Many (map f xs)

    instance Functor OrdList where
      fmap = mapOL

    foldrOL :: (a->b->b) -> b -> OrdList a -> b
    foldrOL _ z None        = z
    foldrOL k z (One x)     = k x z
    foldrOL k z (Cons x xs) = k x (foldrOL k z xs)
    foldrOL k z (Snoc xs x) = foldrOL k (k x z) xs
    foldrOL k z (Two b1 b2) = foldrOL k (foldrOL k z b2) b1
    foldrOL k z (Many xs)   = foldr k z xs

    foldlOL :: (b->a->b) -> b -> OrdList a -> b
    foldlOL _ z None        = z
    foldlOL k z (One x)     = k z x
    foldlOL k z (Cons x xs) = foldlOL k (k z x) xs
    foldlOL k z (Snoc xs x) = k (foldlOL k z xs) x
    foldlOL k z (Two b1 b2) = foldlOL k (foldlOL k z b1) b2
    foldlOL k z (Many xs)   = foldl k z xs

    toOL :: [a] -> OrdList a
    toOL [] = None
    toOL xs = Many xs