2022-23 part 2 in haskell
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2022/23-Unstable-Diffusion/second.hs
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2022/23-Unstable-Diffusion/second.hs
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-- requires cabal install --lib megaparsec parser-combinators vector
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module Main (main) where
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import Control.Monad (void, when)
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import Data.Functor
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import Data.List qualified as L
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import Data.Map qualified as M
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import Data.Void (Void)
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import Text.Megaparsec
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import Text.Megaparsec.Char
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import System.Exit (die)
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exampleExpectedOutput = 20
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type Input = M.Map (Int, Int) Bool -- True if there is an elf
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type Parser = Parsec Void String
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inputToList :: String -> [((Int, Int), Bool)]
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inputToList = snd . L.foldl' nextElf ((0, 0), [])
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where
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nextElf :: ((Int, Int), [((Int, Int), Bool)]) -> Char -> ((Int, Int), [((Int, Int), Bool)])
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nextElf ((x, y), acc) '\n' = ((0, y+1), acc)
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nextElf ((x, y), acc) '.' = ((x+1, y), acc)
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nextElf ((x, y), acc) '#' = ((x+1, y), acc ++ [((x, y), True)])
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parseInput :: String -> IO Input
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parseInput filename = do
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input <- readFile filename
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return $ M.fromList (inputToList input)
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type Proposals = M.Map (Int, Int) (Int, Int, Bool)
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firstHalfStep :: Input -> Int -> Proposals
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firstHalfStep input iter = M.filter (\(_, _, e) -> e) $ M.foldrWithKey eval M.empty input
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where
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eval :: (Int, Int) -> Bool -> Proposals -> Proposals
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eval (x, y) _ = eval' (x, y, 0)
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eval' :: (Int, Int, Int) -> Proposals -> Proposals
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eval' (x, y, 4) acc = acc
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eval' (x, y, r) acc | alone = acc
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| otherwise = case (iter + r) `mod` 4 of
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0 -> if M.member (x-1, y-1) input || M.member (x, y-1) input || M.member (x+1, y-1) input
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then next
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else if M.member (x, y-1) acc
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then M.insert (x, y-1) (x, y, False) acc -- two or more elves attempted to move to the same spot
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else M.insert (x, y-1) (x, y, True) acc
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1 -> if M.member (x-1, y+1) input || M.member (x, y+1) input || M.member (x+1, y+1) input
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then next
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else if M.member (x, y+1) acc
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then M.insert (x, y+1) (x, y, False) acc -- two or more elves attempted to move to the same spot
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else M.insert (x, y+1) (x, y, True) acc
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2 -> if M.member (x-1, y-1) input || M.member (x-1, y) input || M.member (x-1, y+1) input
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then next
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else if M.member (x-1, y) acc
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then M.insert (x-1, y) (x, y, False) acc -- two or more elves attempted to move to the same spot
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else M.insert (x-1, y) (x, y, True) acc
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3 -> if M.member (x+1, y-1) input || M.member (x+1, y) input || M.member (x+1, y+1) input
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then next
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else if M.member (x+1, y) acc
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then M.insert (x+1, y) (x, y, False) acc -- two or more elves attempted to move to the same spot
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else M.insert (x+1, y) (x, y, True) acc
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where
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alone = M.notMember (x-1, y-1) input
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&& M.notMember (x, y-1) input
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&& M.notMember (x+1, y-1) input
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&& M.notMember (x-1, y) input
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&& M.notMember (x+1, y) input
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&& M.notMember (x-1, y+1) input
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&& M.notMember (x, y+1) input
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&& M.notMember (x+1, y+1) input
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next = eval' (x, y, r+1) acc
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secondHalfStep :: Input -> Proposals -> Input
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secondHalfStep = M.foldrWithKey eval
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where
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eval :: (Int, Int) -> (Int, Int, Bool) -> Input -> Input
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eval (x', y') (x, y, _) acc = M.insert (x', y') True $ M.delete (x, y) acc
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compute :: Input -> Int
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compute = compute' 0
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where
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compute' :: Int -> Input -> Int
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compute' i input | next == input = i + 1
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| otherwise = compute' (i+1) next
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where
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next = secondHalfStep input (firstHalfStep input i)
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keys = M.keys input
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xs = L.map fst keys
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ys = L.map snd keys
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xmax = maximum xs
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xmin = minimum xs
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ymax = maximum ys
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ymin = minimum ys
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main :: IO ()
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main = do
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example <- parseInput "example"
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putStr $ showMap example
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let exampleOutput = compute example
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when (exampleOutput /= exampleExpectedOutput) (die $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
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input <- parseInput "input"
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print $ compute input
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showMap :: Input -> String
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showMap input = printMap' xmin ymin ""
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where
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printMap' :: Int -> Int -> String -> String
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printMap' x y acc | M.member (x, y) input = printMap' (x+1) y (acc ++ "#")
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| y > ymax = acc
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| x <= xmax = printMap' (x+1) y (acc ++ ".")
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| otherwise = printMap' xmin (y+1) (acc ++ "\n")
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keys = M.keys input
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xs = L.map fst keys
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ys = L.map snd keys
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xmax = maximum xs
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xmin = minimum xs
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ymax = maximum ys
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ymin = minimum ys
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