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IndecisionTreeIndecisionTree
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feat(day8): solve day 8
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‎.inputs/8.txt

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‎advent-of-haskell.cabal

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build-depends:
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, aoc
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, array ^>=0.5.4.0
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, base ^>=4.16.3.0
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, containers ^>=0.6.5
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, megaparsec ^>=9.3.0

‎data/examples/8/part1.txt

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- Example 1
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==
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‎data/examples/8/part2.txt

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- Example 1
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==
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‎solutions/Days/Day08.hs

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module Days.Day08 (day08) where
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import AOC (Solution (..))
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import AOC (Solution(..))
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import Control.Monad (foldM)
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import Control.Monad.Trans.State.Strict
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import Data.Array (Array)
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import Data.Char (digitToInt)
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import Data.List (foldl')
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import Data.Text (Text)
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import qualified Data.Array as A
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import qualified Data.Map.Strict as M
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import qualified Data.Text as T
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-- TODO whole day needs a refactor and a performance check. Part 1
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-- seems fine, but part 2 is entirely bruteforce. Still runs fast on
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-- my machine, but there has to be a better way.
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day08 :: Solution
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day08 = Solution parseInput part1 part2
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parseInput :: T.Text -> a
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parseInput = error "parseInput not defined for day 08"
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type Index = (Int, Int)
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type Grid = Array Index Int
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type VisibleMap = M.Map Index Bool
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parseInput :: Text -> Grid
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parseInput input = A.listArray is $ digitToInt <$> T.unpack (T.concat lns)
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where
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lns = T.lines input
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rows = length lns
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cols = T.length (head lns)
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is = ((0, 0), (cols-1, rows-1))
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part1 :: Grid -> Int
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part1 g = M.size . M.filter id . bottom . top . right . left $ M.empty
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where
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(_, (rows, cols)) = A.bounds g
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lefts = [[(i, j) | j <- [0..cols]] | i <- [0..rows]]
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rights = [[(i, j) | j <- [cols, cols-1..0]] | i <- [0..rows]]
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tops = [[(i, j) | i <- [0..rows]] | j <- [0..cols]]
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bottoms = [[(i, j) | i <- [rows, rows-1..0]] | j <- [0..cols]]
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left m = foldl' computeVisible m lefts
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right m = foldl' computeVisible m rights
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top m = foldl' computeVisible m tops
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bottom m = foldl' computeVisible m bottoms
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computeVisible :: VisibleMap -> [Index] -> VisibleMap
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computeVisible vm = flip evalState (-1) . foldM go vm
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where
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go :: VisibleMap -> Index -> State Int VisibleMap
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go m i = do
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mx <- get
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let v = g A.! i
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visible = M.findWithDefault False i m
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if v > mx
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then put v >> return (M.insert i True m)
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else return (M.insert i visible m)
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-- TODO find a way to not bruteforce this
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part2 :: Grid -> Int
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part2 g = maximum $ scenic <$> A.indices g
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where
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(_, (rows, cols)) = A.bounds g
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scenic :: Index -> Int
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scenic x@(i, j) = lefts * rights * tops * bottoms
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where
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lefts = cnt + if cnt < j && j > 0 then 1 else 0
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where cnt = countScenic x [(i, j') | j' <- [j-1, j-2..0]]
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rights = cnt + if cnt < (cols-j) && j < cols then 1 else 0
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where cnt = countScenic x [(i, j') | j' <- [j+1..cols]]
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tops = cnt + if cnt < i && i > 0 then 1 else 0
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where cnt = countScenic x [(i', j) | i' <- [i-1, i-2..0]]
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bottoms = cnt + if cnt < (rows-i) && i < rows then 1 else 0
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where cnt = countScenic x [(i', j) | i' <- [i+1..rows]]
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part1 :: a -> Int
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part1 = error "part1 not defined for day 08"
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countScenic :: Index -> [Index] -> Int
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countScenic ind = length . takeWhile ((< val) . (g A.!))
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where
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val = g A.! ind
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part2 :: a -> Int
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part2 = error "part2 not defined for day 08"

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