Create texteditor.hs

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Abhinav Sarkar 2016-04-06 10:04:57 +05:30
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module TextEditor where
import Data.Monoid ((<>), Sum(..))
data JoinList m a = Empty | Value m a | JoinList m (JoinList m a) (JoinList m a)
deriving (Show, Eq)
meta :: (Monoid m) => JoinList m a -> m
meta Empty = mempty
meta (Value m _) = m
meta (JoinList m _ _ ) = m
instance Monoid m => Monoid (JoinList m a) where
mempty = Empty
mappend Empty x = x
mappend x Empty = x
mappend left right = JoinList (meta left <> meta right) left right
instance Functor (JoinList m) where
fmap _ Empty = Empty
fmap f (Value m x) = Value m (f x)
fmap f (JoinList m left right) = JoinList m (fmap f left) (fmap f right)
instance Foldable (JoinList m) where
foldMap _ Empty = mempty
foldMap f (Value _ x) = f x
foldMap f (JoinList _ left right) = foldMap f left <> foldMap f right
data Meta = Meta { lineCount :: Sum Int
, letterCount :: Sum Int
, wordCount :: Sum Int
} deriving (Show)
instance Monoid Meta where
mempty = Meta mempty mempty mempty
mappend (Meta a b c) (Meta x y z) = Meta (a <> x) (b <> y) (c <> z)
mkMeta :: String -> Meta
mkMeta x = Meta (Sum 1) (Sum . length $ x) (Sum . length . words $ x)
type Text = JoinList Meta String
fromList :: [String] -> Text
fromList [] = Empty
fromList [x] = Value (mkMeta x) x
fromList xs =
let (left, right) = splitAt (length xs `div` 2) xs
in fromList left <> fromList right
getAt :: Int -> Text -> Maybe String
getAt 1 (Value _ x) = Just x
getAt n (JoinList _ left right) =
let l = getSum . lineCount . meta $ left
in if n <= l
then getAt n left
else getAt (n - l) right
getAt _ _ = Nothing
transform :: Int -> String -> Text -> (Int -> String -> Text -> Maybe Text) -> Maybe Text
transform n s (JoinList _ left right) f =
let l = getSum . lineCount . meta $ left
in if n <= l
then fmap (\e -> JoinList (meta e <> meta right) e right) $ f n s left
else fmap (\e -> JoinList (meta left <> meta e) left e) $ f (n - l) s right
transform _ _ _ _ = Nothing
editAt :: Int -> String -> Text -> Maybe Text
editAt 1 s (Value _ _) = Just (Value (mkMeta s) s)
editAt n s l@JoinList {} = transform n s l editAt
editAt _ _ _ = Nothing
insertAt :: Int -> String -> Text -> Maybe Text
insertAt 1 s Empty = Just $ Value (mkMeta s) s
insertAt 1 s v@(Value _ _) =
let left = Value (mkMeta s) s
in Just (JoinList (meta left <> meta v) left v)
insertAt n s l@JoinList {} = transform n s l insertAt
insertAt _ _ _ = Nothing