final day 1

src/Day2501.hs

@@ -1,17 +1,27 @@
 module Main where
 
+import Debug.Trace (trace)
+
+convertToInts = map (\it -> (if head it == 'L' then -1 else 1) * read (drop 1 it))
+
 part1 :: String -> Int
 part1 = length . dropNonZero . scanl (+) 50 . convertToInts . lines
   where
-  convertToInts = map (\it -> (if head it == 'L' then -1 else 1) * read (drop 1 it))
   dropNonZero = filter ((0==) . (`mod` 100))
 
 part2 :: String -> Int
-part2 = length . dropNonZero . scanl (+) 50 . makeSingleSteps . convertToInts . lines
+part2 = go 50 . convertToInts . lines
   where
-  convertToInts = map (\it -> (if head it == 'L' then -1 else 1) * read (drop 1 it))
+  go :: Int -> [Int] -> Int
-  dropNonZero = filter ((0==) . (`mod` 100))
+  go p []     = 0
-  makeSingleSteps = concatMap (\it -> replicate (abs it) (signum it))
+  go p (x:xs) = n + edgeCase + go p' xs
+    where
+    n = abs $ p' `div` 100 - p `div` 100
+
+    edgeCase = if x < 0 && p' `mod` 100 == 0 then 1 else 0
+             + if x < 0 && p  `mod` 100 == 0 then -1 else 0
+
+    p' = (p + x)
 
 main :: IO ()
 main = do