Day 09.

README.md

@@ -15,7 +15,7 @@ To arrive at [⟨ortho|normal⟩](https://hilb.ert.space) as December comes.
 | 05  | ~ 1         |
 | 06  | ~ 3         |
 | 07  | ~ 1         |
-| 08  |             |
+| 08  | ~ 6.5       |
 | 09  |             |
 | 10  |             |
 | 11  |             |

app/Main.hs

@@ -30,4 +30,4 @@ import qualified Day24
 import qualified Day25
 
 main :: IO ()
-main = Day07.main
+main = Day09.main

src/Day09.hs

@@ -1,6 +1,42 @@
+{-# LANGUAGE BangPatterns #-}
+
 module Day09 (main) where
 
+import Data.IntMap (IntMap, empty, insertWith)
+import Data.Sequence (Seq((:<|)), fromList, (><), (<|))
+import qualified Data.Sequence as S (length, drop, take)
+
+players    = 411
+lastMarble = 7105800
+infix 5 %
+(%) = mod
+
+-- (scoreboard, marbles, marble, player)
+type State      = (Scoreboard, Marbles, Int, Int)
+type Scoreboard = IntMap Int
+type Marbles    = Seq Int
+
+-- pop from the front and push in the back n marbles
+spin :: Int -> Marbles -> Marbles
+spin n m = S.drop n m >< S.take n m
+
+-- pop from the back and push in the front n marbles
+unspin :: Int -> Marbles -> Marbles
+unspin n m = S.drop (S.length m - n) m >< S.take (S.length m - n) m
+
+part1 :: State -> Int
+part1 (scores, marbles, m, p)
+    | nextMarble == lastMarble = maximum scores
+    | nextMarble % 23 == 0 =
+        let !(m:<|nextMarbles) = unspin 7 marbles
+            nextScores         = insertWith (+) nextPlayer (nextMarble + m) scores
+        in  part1 (nextScores, nextMarbles, nextMarble, nextPlayer)
+    | otherwise =
+        let !nextMarbles = nextMarble <| spin 2 marbles
+        in  part1 (scores, nextMarbles, nextMarble, nextPlayer)
+    where nextMarble = m + 1
+          nextPlayer = (p % players) + 1
+
 main :: IO ()
 main = do
-    input <- readFile "input/09.txt"
-    print input
+    print $ part1 (empty, fromList [1, 0], 1, 1)