131 lines
3.7 KiB
Haskell
131 lines
3.7 KiB
Haskell
{-# LANGUAGE OverloadedStrings #-}
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module Day10 where
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import Data.Void (Void)
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import Data.Text (Text)
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import Data.Char (isSpace)
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import Data.Maybe (mapMaybe)
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import Data.Bifunctor (first, bimap)
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import Control.Monad (join)
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import Text.Megaparsec (Parsec, errorBundlePretty, runParser, many, eof, satisfy)
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import Text.Megaparsec.Char (newline)
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import Misc (withCoords, pairs)
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data Pipe
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= E -- empty
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| S -- start
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| I -- vertical
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| H -- horizontal
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| J -- up-left
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| L -- up-right
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| T -- down-left
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| F -- down-right
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deriving (Show, Eq)
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data Dir = DLeft | DRight | DUp | DDown
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deriving (Show, Eq)
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type Row = Int
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type Col = Int
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type Move = (Dir, (Row, Col))
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type Parser = Parsec Void Text
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parse :: Text -> Either String [[Pipe]]
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parse = first errorBundlePretty . runParser grid ""
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where
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pipe :: Char -> Pipe
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pipe c = case c of
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'.' -> E
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'S' -> S
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'|' -> I
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'-' -> H
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'J' -> J
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'L' -> L
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'7' -> T
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'F' -> F
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bad -> error $ "Invalid character: " <> [bad]
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row :: Parser [Pipe]
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row = map pipe <$> many (satisfy $ not . isSpace) <* newline
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grid :: Parser [[Pipe]]
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grid = many row <* eof
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findStart :: [[Pipe]] -> (Row, Col)
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findStart = fst . head . filter ((S ==) . snd) . withCoords
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get :: (Row, Col) -> [[Pipe]] -> Pipe
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get (r, c) = snd . head . filter ((c ==) . fst) . zip [0..] . snd . head . filter ((r ==) . fst) . zip [0..]
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nextD :: Dir -> Pipe -> Maybe Dir
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nextD dir pipe = case (pipe, dir) of
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(I, DDown) -> Just DDown
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(I, DUp) -> Just DUp
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(H, DRight) -> Just DRight
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(H, DLeft) -> Just DLeft
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(J, DDown) -> Just DLeft
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(J, DRight) -> Just DUp
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(L, DDown) -> Just DRight
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(L, DLeft) -> Just DUp
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(T, DUp) -> Just DLeft
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(T, DRight) -> Just DDown
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(F, DUp) -> Just DRight
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(F, DLeft) -> Just DDown
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_ -> Nothing
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nextC :: (Row, Col) -> Dir -> (Row, Col)
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nextC (r, c) dir = case dir of
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DLeft -> (r, c - 1)
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DRight -> (r, c + 1)
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DUp -> (r - 1, c)
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DDown -> (r + 1, c)
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starts :: [[Pipe]] -> (Move, Move)
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starts grid =
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let s = findStart grid
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ds = mapMaybe (\dir -> check dir $ nextC s dir) [ DUp, DDown, DLeft, DRight ]
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in (head ds, head $ tail ds)
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where
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check :: Dir -> (Row, Col) -> Maybe Move
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check dir rc = fmap (const (dir, rc)) . nextD dir $ get rc grid
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go :: [[Pipe]] -> (Dir, (Row, Col)) -> [(Dir, (Row, Col))]
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go grid = helper
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where
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helper :: (Dir, (Row, Col)) -> [(Dir, (Row, Col))]
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helper (dir, rc) = (dir, rc) : case nextD dir $ get rc grid of
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Just dir' -> helper (dir', nextC rc dir')
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Nothing -> []
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solveA :: [[Pipe]] -> Int
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solveA grid = (1 +) . length . takeWhile (not . isMatch) . uncurry zip . join bimap (go grid) $ starts grid
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where
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isMatch :: (Move, Move) -> Bool
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isMatch (l, r) = snd l == snd r
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verts :: [[Pipe]] -> ((Row, Col), [(Row, Col)])
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verts grid = (findStart grid, findVerts grid)
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where
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isVertex :: (Move, Move) -> Maybe (Row, Col)
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isVertex ((dir, rc), (dir', _)) = if dir /= dir' then Just rc else Nothing
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findVerts :: [[Pipe]] -> [(Row, Col)]
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findVerts = mapMaybe isVertex . pairs . go grid . fst . starts
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-- https://en.wikipedia.org/wiki/Shoelace_formula
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-- A = ((x1 * y2 - y1 * x2) + (x2 * y3 - x3 * y2) + ...) / 2
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trapezoid :: ((Row, Col), [(Row, Col)]) -> Double
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trapezoid (sp, vs) = (/2) . fromIntegral . sum . map inner . pairs $ [sp] <> vs <> [sp]
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where
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inner :: ((Row, Col), (Row, Col)) -> Int
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inner ((r1, c1), (r2, c2)) = r1 * c2 - r2 * c1
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-- https://en.wikipedia.org/wiki/Pick%27s_theorem
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-- A = i + b/2 - 1
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-- i = A - b/2 + 1
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interior :: ((Row, Col), [(Row, Col)]) -> Int -> Double
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interior vs path = abs $ trapezoid vs - fromIntegral path + 1
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solveB :: [[Pipe]] -> Double
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solveB grid = interior (verts grid) (solveA grid)
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