src/advent06/advent06.hs

   1 {-# LANGUAGE OverloadedStrings #-}
   2
   3 import Data.List
   4
   5 import Data.Text (Text)
   6 import qualified Data.Text.IO as TIO
   7
   8 import Data.Void (Void)
   9
  10 import Text.Megaparsec
  11 import Text.Megaparsec.Char
  12 import qualified Text.Megaparsec.Char.Lexer as L
  13 import qualified Control.Applicative as CA
  14
  15 import qualified Data.Map.Strict as M
  16
  17 type Coord = (Integer, Integer) -- x, y
  18 type Bounds = (Integer, Integer, Integer, Integer) -- minX, maxX, minY, maxY
  19 type Region = M.Map Coord Int
  20
  21 main :: IO ()
  22 main = do
  23         text <- TIO.readFile "data/advent06.txt"
  24         let coords = successfulParse text
  25         let boundingBox = findBounds coords
  26         print $ part1 coords boundingBox
  27         print $ part2 coords boundingBox
  28
  29
  30 part1 coords bounds = largestRegion $ regionSizes $ finite edgeLabels regions
  31     where regions = findRegions coords bounds
  32           edgeLabels = infinite regions bounds
  33
  34 part2 coords bounds = M.size $ M.filter (< 10000) $ safeCells coords bounds
  35
  36 findRegions :: [Coord] -> Bounds -> Region
  37 findRegions coords (minX, maxX, minY, maxY) = M.fromList labelledCells
  38     where cells = [(x, y) | x <- [minX .. maxX], y <- [minY .. maxY] ]
  39           starts = zip [1..] coords
  40           labelledCells = map (\c -> (c, nearestStart 0 c starts)) cells
  41
  42 nearestStart :: Int -> Coord -> [(Int, Coord)] -> Int
  43 nearestStart tieLabel cell starts = nearestLabel
  44     where distances = sort $ map (\(l, s) -> (distance s cell , l)) starts
  45           nearestLabel = if fst (distances!!0) == fst (distances!!1)
  46                          then tieLabel
  47                          else snd (distances!!0)
  48
  49 safeCells :: [Coord] -> Bounds -> Region
  50 safeCells coords (minX, maxX, minY, maxY) = M.fromList distanceCells
  51     where cells = [(x, y) | x <- [minX .. maxX], y <- [minY .. maxY] ]
  52           distanceCells = map (\c -> (c, fromIntegral $ sumDistance c coords) ) cells
  53
  54 sumDistance :: Coord -> [Coord] -> Integer
  55 sumDistance here others = sum $ map (\c -> distance here c) others
  56
  57 infinite :: Region -> Bounds -> [Int]
  58 infinite regions (minX, maxX, minY, maxY) = nub $ sort $ M.elems $ M.filterWithKey onEdge regions
  59     where onEdge (x, y) _ = (x == minX) || (x == maxX) || (y == minY) || (y == maxY)
  60
  61 finite :: [Int] -> Region -> Region
  62 finite excluded regions = M.filter (\r -> r `notElem` excludedTied) regions
  63     where excludedTied = (0:excluded)
  64
  65 regionSizes :: Region -> [(Int, Int)]
  66 regionSizes regions = map (\g -> (g!!0, length g)) $ group $ sort $ M.elems regions
  67
  68 largestRegion :: [(Int, Int)] -> Int
  69 largestRegion = maximum . map snd
  70
  71
  72 findBounds :: [Coord] -> (Integer, Integer, Integer, Integer)
  73 findBounds coords = ( minX - (maxY - minY) `div` 2 -- small x edge
  74                     , maxX + (maxY - minY) `div` 2 -- large x edge
  75                     , minY - (maxX - minX) `div` 2-- small x edge
  76                     , maxY + (maxX - minX) `div` 2 -- large y edge
  77                     )
  78     where maxX = maximum $ map fst coords
  79           minX = minimum $ map fst coords
  80           maxY = maximum $ map snd coords
  81           minY = minimum $ map snd coords
  82
  83 -- Manhattan distance
  84 distance :: Coord -> Coord -> Integer
  85 distance (x1, y1) (x2, y2) = (abs (x1 - x2)) + (abs (y1 - y2))
  86
  87
  88 -- Parse the input file
  89
  90 type Parser = Parsec Void Text
  91
  92 sc :: Parser ()
  93 sc = L.space (skipSome spaceChar) CA.empty CA.empty
  94
  95 lexeme  = L.lexeme sc
  96 integer = lexeme L.decimal
  97 symb = L.symbol sc
  98
  99 commaP = symb ","
 100
 101 coordFileP = many coordP
 102 coordP = (,) <$> integer <* commaP <*> integer
 103
 104 successfulParse :: Text -> [Coord]
 105 successfulParse input =
 106         case parse coordFileP "input" input of
 107                 Left  _error -> [] -- TIO.putStr $ T.pack $ parseErrorPretty err
 108                 Right coords -> coords