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|
--------------------------------------------------------------------------------
module Transforms where
--------------------------------------------------------------------------------
import Control.Applicative ((<|>))
import Text.ParserCombinators.ReadP
--------------------------------------------------------------------------------
data Transform = VerticalFlip
| HorizontalFlip
| Shift Int
deriving (Eq, Show)
digit :: ReadP Char
digit =
satisfy (\c -> c >= '0' && c <= '9')
command :: ReadP Transform
command = vertical
<|> horizontal
<|> shift
where
vertical =
char 'V' >> pure VerticalFlip
horizontal =
char 'H' >> pure HorizontalFlip
shift = do
_ <- char 'S'
negative <- option Nothing $ fmap Just (satisfy (== '-'))
n <- read <$> many1 digit
case negative of
Nothing -> pure $ Shift n
Just _ -> pure $ Shift (-1 * n)
-- | Attempt to remove redundant transformations.
-- | Here are some rules that I'd like to support but may not have time for:
-- | - All even-numbered flips (w/o intermittent shifts) can become zero
-- | - All odd-numbered flips (w/o intermittent shifts) can become 1
-- | - All shifts can be be reduce to the absolute value of shifts
optimize :: [Transform] -> [Transform]
optimize [] = []
optimize [x] = [x]
optimize (VerticalFlip:VerticalFlip:xs) = optimize xs
optimize (HorizontalFlip:HorizontalFlip:xs) = optimize xs
optimize xs = xs
fromString :: String -> Maybe [Transform]
fromString x =
case readP_to_S (manyTill command eof) x of
[(res, "")] -> Just res
_ -> Nothing
|
