Support parsing the list of transforms

Using Haskell's Text.ParserCombinators.ReadP library for the first time, and I
enjoyed it thoroughly! It's nice avoiding a third-party library like MegaParsec.

1 files changed, 40 insertions, 0 deletions

diff --git a/scratch/brilliant/Transforms.hs b/scratch/brilliant/Transforms.hs
new file mode 100644
index 000000000000..e707defda796
--- /dev/null
+++ b/scratch/brilliant/Transforms.hs
@@ -0,0 +1,40 @@
+--------------------------------------------------------------------------------
+module Transforms where
+--------------------------------------------------------------------------------
+import Control.Applicative ((<|>))
+import Text.ParserCombinators.ReadP
+--------------------------------------------------------------------------------
+
+data Transform = VerticalFlip
+               | HorizontalFlip
+               | Shift Integer
+               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)
+
+fromString :: String -> Maybe [Transform]
+fromString x =
+  case readP_to_S (manyTill command eof) x of
+   [(res, "")] -> Just res
+   _           -> Nothing