2024-11

common/src/Advent/Input.hs

@@ -15,6 +15,7 @@ module Advent.Input where
 
 import Advent.Coord (Coord(..), coordLines)
 import Data.Array.Unboxed qualified as A
+import Data.List (findIndex)
 import Data.Map (Map)
 import Data.Map.Strict qualified as SMap
 import System.Environment (getArgs)
@@ -38,22 +39,23 @@ getRawInput y d =
        "+":input:_ -> pure input
        fn:_  -> readFile fn
 
--- | Default input filename given a day number
-inputFileName :: Int {- ^ day -} -> FilePath
-inputFileName = printf "inputs/%02d.txt"
-
 -- | Load input file as a list of lines.
 getInputLines :: Int {- ^ year -} -> Int {- ^ day -} -> IO [String]
 getInputLines y d = lines <$> getRawInput y d
 
 -- | Load input file as a rectangular array of characters.
 getInputArray :: Int {- ^ year -} -> Int {- ^ day -} -> IO (A.UArray Coord Char)
 getInputArray y d =
-  do xs <- getInputLines y d
-     pure $! A.listArray (C 0 0, C (length xs - 1) (length (head xs) - 1)) (concat xs)
+ do xs <- getInputLines y d
+    w <- case xs of
+      [] -> fail "getInputArray: empty grid"
+      x : xs ->
+        case findIndex (\y -> length y /= w) xs of
+          Just i  -> fail ("getInputArray: bad length on line " ++ show (i+2))
+          Nothing -> pure w
+        where w = length x
+    pure $! A.listArray (C 0 0, C (length xs - 1) (w - 1)) (concat xs)
 
 -- | Load input file as a 2-dimensional map of characters.
 getInputMap :: Int {- ^ year -} -> Int {- ^ day -} -> IO (Map Coord Char)
-getInputMap y d =
-  do xs <- getInputLines y d
-     pure $! SMap.fromList (coordLines xs)
+getInputMap y d = SMap.fromList . coordLines <$> getInputLines y d

inputs/2024/11.txt

@@ -0,0 +1 @@
+2 72 8949 0 981038 86311 246 7636740

solutions/solutions.cabal

@@ -1162,3 +1162,8 @@ executable sln_2024_10
   import:              day
   main-is:             2024/10.hs
   build-depends:       array, containers
+
+executable sln_2024_11
+  import:              day
+  main-is:             2024/11.hs
+  build-depends:       containers

solutions/src/2024/11.hs

@@ -0,0 +1,55 @@
+{-# Language QuasiQuotes, ImportQualifiedPost #-}
+{-|
+Module      : Main
+Description : Day 11 solution
+Copyright   : (c) Eric Mertens, 2024
+License     : ISC
+Maintainer  : [email protected]
+
+<https://adventofcode.com/2024/day/11>
+
+This solution runs efficiently by remembering how many of each stone
+there are and blinking all of that same kind of stone all at once.
+While the problem does state that order is preserved, the question
+it asks about the stones does not depend on order, so we forget that
+order!
+
+>>> :main + "125 17\n"
+55312
+65601038650482
+
+-}
+module Main (main) where
+
+import Advent (format, times)
+import Data.Map (Map)
+import Data.Map.Strict qualified as Map
+
+-- | >>> :main
+-- 202019
+-- 239321955280205
+main :: IO ()
+main =
+ do input <- [format|2024 11 %u& %n|]
+    print (solve 25 input)
+    print (solve 75 input)
+
+-- | Compute the number of stones resulting from a starting set of stones
+-- and a number of blink iterations.
+solve :: Int -> [Int] -> Int
+solve n input = sum (times n blinks (Map.fromListWith (+) [(i, 1) | i <- input]))
+
+-- | Blink all the stones at once. Stone numbers are mapped to multiplicity.
+blinks :: Map Int Int -> Map Int Int
+blinks stones = Map.fromListWith (+) [(stone', n) | (stone, n) <- Map.assocs stones, stone' <- blink stone]
+
+-- | Blink a single stone and figure out what stones it turns into.
+blink :: Int -> [Int]
+blink 0 = [1]         -- 0 -> 1
+blink n               -- split in half if even length
+  | let str = show n
+  , let len = length str
+  , even len
+  , (l, r) <- splitAt (len `quot` 2) str
+  = [read l, read r]
+blink n = [n * 2024]  -- otherwise multiply by 2024