day 2

day2.hs

@@ -0,0 +1,63 @@
+data Password = Password Int Int Char String
+  deriving Show
+
+main :: IO ()
+main = do
+  raw <- readFile "day2.txt"
+  let ls = map parse $ lines raw
+      fs = case (sequence ls) of
+             Nothing -> error "invalid input"
+             (Just ps) -> ps
+      ansA = length $ filter (== True) $ map validA fs
+      ansB = length $ filter (== True) $ map validB fs
+   in do 
+     putStrLn $ "day2a: " ++ (show ansA)
+     putStrLn $ "day2b: " ++ (show ansB)
+
+validA :: Password -> Bool
+validA (Password l u t p) =
+  let len = length $ filter (== t) p
+   in (len >= l)
+   && (len <= u)
+
+validB :: Password -> Bool
+validB (Password a' b' t p) =
+  case (inBounds a b p) of
+    (False,False) -> False
+    (True,False) -> (p !! a) == t
+    (False,True) -> (p !! b) == t
+    _ -> xor ((p !! a) == t) ((p !! b) == t)
+  where
+    a = (pred a')
+    b = (pred b')
+
+parse :: String -> Maybe Password
+parse [] = error "empty password"
+parse s =
+  do (l,s') <- parseInt s
+     (u,s'') <- parseInt $ dropChar s'
+     (t,s''') <- parseChar $ dropChar s''
+     let p = (dropChar . dropChar) s'''
+      in (Just $ Password l u t p)
+
+parseInt :: String -> Maybe (Int,String)
+parseInt s = case (reads s) of
+               [] -> Nothing
+               [a] -> Just a
+
+parseChar :: String -> Maybe (Char,String)
+parseChar [] = Nothing
+parseChar (t:s) = Just (t,s)
+
+dropChar :: String -> String
+dropChar [] = []
+dropChar (_:ls) = ls
+
+xor :: Bool -> Bool -> Bool
+xor True False = True
+xor False True = True
+xor _ _ = False
+
+inBounds :: Int -> Int -> String -> (Bool, Bool)
+inBounds a b s = ( len >= a, len >= b )
+  where len = length s