commit fce814616623d597974ab1958a6ee0f6f875536c · drubery.com/aoc2025

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src/Day4.hs

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       module Day4 where

     

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       part1 :: String -> String

     

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       part1 _ = "Day 4 part 1"

     

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       part2 :: String -> String

     

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       part2 _ = "Day 4 part 2"

     

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       module Day4 where

     

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       import qualified Data.Map as Map

     

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       import Data.Map (Map)

     

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       type Grid = Map (Int,Int) Char

     

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       enumerate :: [a] -> [(Int,a)]

     

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       enumerate l = zip [0..] l

     

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       addRow :: Int -> String -> Grid -> Grid

     

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       addRow y row grid =

     

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         foldl

     

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           (\g (x,c) -> Map.insert (x,y) c g)

     

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           grid

     

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           (enumerate row)

     

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       parse :: String -> Grid

     

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       parse s =

     

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         foldl

     

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           (\g (x,row) -> addRow x row g)

     

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           Map.empty

     

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           (enumerate $ lines s)

     

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       neighbors :: (Int,Int) -> [(Int,Int)]

     

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       neighbors (x,y) =

     

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         [(x-1,y-1),(x,y-1),(x+1,y-1),(x-1,y),(x+1,y),(x-1,y+1),(x,y+1),(x+1,y+1)]

     

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       isOccupied :: Grid -> (Int,Int) -> Bool

     

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       isOccupied grid p = Map.lookup p grid == Just '@'

     

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       isAccessible :: Grid -> (Int,Int) -> Char -> Bool

     

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       isAccessible grid p '@' =

     

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         length (filter (isOccupied grid) (neighbors p)) < 4

     

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       isAccessible _ _ _ = False

     

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       part1 :: String -> String

     

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       part1 input =

     

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         let grid = parse input

     

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          in show $

     

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             length $

     

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             Map.filterWithKey (isAccessible grid) grid

     

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       removeAccessible :: Grid -> Grid

     

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       removeAccessible grid =

     

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         let accessible = Map.filterWithKey (isAccessible grid) grid

     

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          in Map.difference grid accessible

     

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       removeAll :: Grid -> Grid

     

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       removeAll grid =

     

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         let grids = iterate removeAccessible grid

     

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             stabilized (x : y : gs) =

     

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               if Map.size x == Map.size y then

     

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                 x

     

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               else

     

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                 stabilized (y : gs)

     

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          in stabilized grids

     

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       part2 :: String -> String

     

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       part2 input =

     

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         let grid = parse input

     

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          in show $ Map.size $ Map.difference grid (removeAll grid)