feat: solve day1

app/Day1.hs

@@ -0,0 +1,49 @@
+module Main where
+
+import Debug.Trace (trace)
+import System.Environment (getArgs)
+
+main :: IO ()
+main = do
+  input <- readFile . head =<< getArgs
+  print $ part1 input
+  print $ part2 input
+
+part1 :: String -> Int
+part1 = length . filter (== 0) . scanl rotate 50 . map parse . lines
+
+rotate :: Integer -> Either Integer Integer -> Integer
+rotate pos (Left rot) = (pos - rot) `mod` 100
+rotate pos (Right rot) = (pos + rot) `mod` 100
+
+parse :: String -> Either Integer Integer
+parse ('L' : rest) = Left (read rest :: Integer)
+parse ('R' : rest) = Right (read rest :: Integer)
+parse s = error $ unwords [s, "is not on the form 'L$NUM' or 'R$NUM''"]
+
+test_input = "L68\nL30\nR48\nL5\nR60\nL55\nL1\nL99\nR14\nL82"
+
+part2 :: String -> Integer
+part2 = snd . head . reverse . scanl rotate2 (50, 0) . map parse . lines
+
+rotate2 :: (Integer, Integer) -> Either Integer Integer -> (Integer, Integer)
+rotate2 (pos, zeros) rot =
+  let pos' = rotate pos rot
+      newZeros = go pos rot
+   in trace ("The dial is rotated " ++ show rot ++ " to point at " ++ show pos' ++ if newZeros > 0 then "; during this rotation it points to 0 " ++ (counting newZeros) else ".") (pos', zeros + newZeros)
+  where
+    counting :: Integer -> String
+    counting 1 = "once"
+    counting 2 = "twice"
+    counting n = show n ++ " times"
+
+go :: Integer -> Either Integer Integer -> Integer
+go p (Left r)
+  | p == r = 1
+  | r > 100 = 1 + go p (Left (r - 100))
+  | p /= 0 && p - r < 0 = 1 + go 0 (Left (r - p))
+  | otherwise = 0
+go p (Right r) = (p + r) `div` 100
+
+between :: Integer -> (Integer, Integer) -> Bool
+between x (mn, mx) = x < mx && x > mn