solve: day 2, part 2

2020/02/part2.hs

@@ -0,0 +1,45 @@
+#!/usr/bin/env runghc
+
+import Data.Char
+
+data PasswordEntry = PasswordEntry { pos1 :: Int, pos2 :: Int, char :: Char, password :: String }
+  deriving (Show)
+
+-- This could be a lot better...
+instance Read PasswordEntry where
+  readsPrec _ input =
+    let (pos1', rest) = span isDigit input
+        pos1 = read pos1' :: Int
+        (_, rest') = splitAt 1 rest
+        (pos2', rest'') = span isDigit rest'
+        pos2 = read pos2' :: Int
+        (_:char:_:_:password) = rest''
+      in
+      [(PasswordEntry pos1 pos2 char password, "")]
+
+
+main = interact solve
+
+solve = show . length . filter (isValidPasswordEntry . readPasswordEntry) . lines
+
+readPasswordEntry :: String -> PasswordEntry
+readPasswordEntry = read
+
+(!!?) :: [a] -> Int -> Maybe a
+[] !!? _ = Nothing
+(x:xs) !!? 0 = Just x
+(_:xs) !!? idx = xs !!? (idx - 1)
+
+maybeToBool (Just a) = a
+maybeToBool Nothing = False
+
+xor :: Bool -> Bool -> Bool
+xor l r = l /= r
+
+isValidPasswordEntry (PasswordEntry pos1 pos2 char pass) =
+    (maybeToBool $ fmap (==char) char1) `xor`
+    (maybeToBool $ fmap (==char) char2)
+  where
+    char1 = pass !!? (pos1 - 1)
+    char2 = pass !!? (pos2 - 1)
+