[hs] Solve 2020_15 part 1

hs/src/Aoc/Y2020.hs

@@ -17,6 +17,7 @@ import qualified Aoc.Y2020.D11 as D11
 import qualified Aoc.Y2020.D12 as D12
 import qualified Aoc.Y2020.D13 as D13
 import qualified Aoc.Y2020.D14 as D14
+import qualified Aoc.Y2020.D15 as D15
 
 year :: Year
 year = Year 2020
@@ -34,4 +35,5 @@ year = Year 2020
   , (12, D12.day)
   , (13, D13.day)
   , (14, D14.day)
+  , (15, D15.day)
   ]

hs/src/Aoc/Y2020/D15.hs

@@ -0,0 +1,28 @@
+module Aoc.Y2020.D15
+  ( day
+  ) where
+
+import           Data.List
+
+import           Aoc.Day
+import           Aoc.Parse
+
+parser :: Parser [Int]
+parser = (decimal `sepBy` char ',') <* newline
+
+step :: [Int] -> [Int]
+step []     = error "list must not be empty"
+step (x:xs) = case elemIndex x xs of
+  Nothing -> 0 : x : xs
+  Just i  -> (i + 1) : x : xs
+
+stepUntil :: Int -> [Int] -> [Int]
+stepUntil amount nums = iterate step nums !! (amount - length nums)
+
+solver :: [Int] -> IO ()
+solver nums = do
+  putStrLn ">> Part 1"
+  print $ head $ stepUntil 2020 $ reverse nums
+
+day :: Day
+day = dayParse parser solver