Day 12.

README.md

@@ -19,7 +19,7 @@ To arrive at [⟨ortho|normal⟩](https://hilb.ert.space) as December comes.
 | 09  | ~ 7         |
 | 10  | ~ 6.5       |
 | 11  | ~ 7         |
-| 12  |             |
+| 12  | ~ 1.5       |
 | 13  |             |
 | 14  |             |
 | 15  |             |

app/Main.hs

@@ -30,4 +30,4 @@ import qualified Day24
 import qualified Day25
 
 main :: IO ()
-main = Day11.main
+main = Day12.main

input/12.txt

@@ -0,0 +1,34 @@
+initial state: #.##.#.##..#.#...##...#......##..#..###..##..#.#.....##..###...#.#..#...######...#####..##....#..###
+
+##.## => .
+##... => #
+..#.# => #
+#.... => .
+#..#. => #
+.#### => .
+.#..# => .
+.##.# => .
+#.##. => #
+####. => .
+..##. => .
+##..# => .
+.#.## => #
+.#... => .
+.##.. => #
+..#.. => #
+#..## => #
+#.#.. => #
+..### => #
+...#. => #
+###.. => .
+##.#. => #
+#.#.# => #
+##### => #
+....# => .
+#.### => .
+.#.#. => #
+.###. => #
+...## => .
+..... => .
+###.# => #
+#...# => .

src/Day12.hs

@@ -1,6 +1,61 @@
 module Day12 (main) where
 
+import Prelude hiding (replicate, fromList)
+import Data.Bits ((.|.), shift)
+import Data.List (sort)
+import Data.Foldable (toList)
+import Data.Vector (Vector, (!))
+import qualified Data.Vector as V (fromList)
+import Data.Sequence (Seq(..), (><), fromList, takeWhileL, takeWhileR, replicate, empty)
+import Debug.Trace (traceShow)
+
+type PotState = (Int, Pots)
+type Pots     = Seq Char
+type Notes    = Vector Char
+
+parse :: String -> (Notes, Pots)
+parse str =
+    let init:_:ns = lines str
+        seq   = fromList . drop 15 $ init
+        notes = V.fromList . map last . reverse . sort $ ns
+    in  (notes, seq)
+
+potsToInt :: Seq Char -> Int
+potsToInt = potsToIntRec 0 5
+    where
+        potsToIntRec val 0 _       = val
+        potsToIntRec val n (c:<|cs) = potsToIntRec (val `shift` 1 .|. fromEnum (c == '#')) (n - 1) cs
+
+appendEmptyPots :: PotState -> PotState
+appendEmptyPots potState@(lp, ps) =
+    let lPots = max 0 . (5 -) . length . takeWhileL (== '.') $ ps
+        rPots = max 0 . (5 -) . length . takeWhileR (== '.') $ ps
+    in  (lp - lPots, replicate lPots '.' >< ps >< replicate rPots '.')
+
+grow :: Notes -> PotState -> PotState
+grow notes potState =
+    let (leftPot, pots) = appendEmptyPots potState
+        (_, newPots) = foldr (\_ (op@(p:<|ps), np) -> (ps, np :|> notes ! potsToInt op)) (pots, empty) [0 .. length pots - 5]
+    in  (leftPot, fromList ".." >< newPots >< fromList "..")
+
+sumPotNums :: PotState -> Int
+sumPotNums (leftPot, pots) = sum . map fst . filter ((== '#') . snd) . zip [leftPot..] . toList $ pots
+
+part1 :: Notes -> Pots -> Int
+part1 notes pots = sumPotNums $ iterate (grow notes) (0, pots) !! 20
+
+part2 :: Notes -> Pots -> Int
+part2 notes pots =
+    let sums  = map sumPotNums $ iterate (grow notes) (0, pots)
+        diffs = zipWith (-) (tail sums) sums
+        index = findRepeatIndex 1 diffs
+        diff  = sums !! (index + 1) - sums !! index
+    in  sums !! index + diff * (50000000000 - index)
+    where findRepeatIndex n (d1:d2:ds) =
+            if d1 == d2 then n else findRepeatIndex (n + 1) (d2:ds)
+
 main :: IO ()
 main = do
-    input <- readFile "input/12.txt"
-    print input
+    (notes, pots) <- parse <$> readFile "input/12.txt"
+    print $ part1 notes pots
+    print $ part2 notes pots