advent03/Main.hs

   1 -- Writeup at https://work.njae.me.uk/2022/12/14/advent-of-code-2022-day-12/
   2
   3 import AoC
   4
   5
   6 import Data.Char
   7 import Data.List
   8 import Linear (V2(..), (^+^))
   9 import Data.Array.IArray
  10
  11 type Position = V2 Int -- r, c
  12 type Engine = Array Position Char
  13 type NumPos = [Position]
  14
  15 data NumberSeek =
  16   NumberSeek { inNumber :: Bool
  17              , positions :: NumPos
  18              , foundNumbers :: [NumPos]
  19              } deriving (Show)
  20
  21
  22 main :: IO ()
  23 main =
  24   do  dataFileName <- getDataFileName
  25       text <- readFile dataFileName
  26       let engine = mkEngine text
  27       let allNums = findNumbers engine
  28       let symbols = findSymbols engine
  29       let partNums = filter (touchRegion symbols) allNums
  30       -- print partNums
  31       -- print engine
  32       -- print $ findNumbers engine
  33       print $ part1 engine partNums
  34       print $ part2 engine partNums
  35
  36
  37 part1, part2 :: Engine -> [NumPos] -> Int
  38 part1 engine partNums = sum partNumValues
  39   where partNumValues = map (readNumber engine) partNums
  40
  41 part2 engine partNums = sum $ fmap product gearRatios
  42   where stars = findStars engine
  43         touchingStars = fmap (possibleGears partNums) stars
  44         gears = filter ((==) 2 . length) touchingStars
  45         gearRatios = [[readNumber engine n | n <- pNs] | pNs <- gears]
  46
  47 mkEngine :: String -> Engine
  48 mkEngine text = grid
  49   where rows = lines text
  50         r = length rows - 1
  51         c = (length $ head rows) - 1
  52         grid = listArray ((V2 0 0), (V2 r c)) $ concat rows
  53
  54 isEngineSymbol :: Char -> Bool
  55 isEngineSymbol c = (not $ isDigit c) && (c /= '.')
  56
  57 findNumbers :: Engine -> [NumPos]
  58 findNumbers engine = numbers
  59   where ((V2 r1 _), (V2 r2 _)) = bounds engine
  60         rows = [r1..r2]
  61         numbers = concatMap (foundNumbers . (findNumbersInRow engine)) rows
  62
  63 findNumbersInRow :: Engine -> Int -> NumberSeek
  64 findNumbersInRow engine r
  65   | inNumber finalSeek = NumberSeek False [] ((reverse $ positions finalSeek):foundNumbers finalSeek)
  66   | otherwise = finalSeek
  67   where finalSeek = foldl' (buildNumber engine)
  68                            (NumberSeek False [] [])
  69                            $ range $ rowBounds engine r
  70
  71 buildNumber :: Engine -> NumberSeek -> Position -> NumberSeek
  72 buildNumber engine NumberSeek{..} p
  73   | inNumber && isDigit c = NumberSeek True (p:positions) foundNumbers
  74   | inNumber && not (isDigit c) = NumberSeek False [] ((reverse positions):foundNumbers)
  75   | not inNumber && isDigit c = NumberSeek True [p] foundNumbers
  76   | otherwise = NumberSeek False [] foundNumbers
  77   where c = engine ! p
  78
  79 rowBounds :: Engine -> Int -> (V2 Int, V2 Int)
  80 rowBounds engine r = (V2 r c1, V2 r c2)
  81   where ((V2 _ c1), (V2 _ c2)) = bounds engine
  82
  83 neighbours :: Position -> [Position]
  84 neighbours p = [p ^+^ V2 dr dc | dr <- [-1..1], dc <- [-1..1]
  85                                , (dr, dc) /= (0, 0) ]
  86
  87
  88 touchPoint :: [Position] -> Position -> Bool
  89 touchPoint region point = not $ null $ intersect region $ neighbours point
  90
  91 touchRegion :: [Position] -> [Position] -> Bool
  92 touchRegion region1 region2 = any (touchPoint region2) region1
  93
  94
  95 readNumber :: Engine -> NumPos -> Int
  96 readNumber engine ps = read $ map (engine !) ps
  97
  98 findSymbols :: Engine -> [Position]
  99 findSymbols engine = filter (isEngineSymbol . (engine !)) $ indices engine
 100
 101 findStars :: Engine -> [Position]
 102 findStars engine = filter ((==) '*' . (engine !)) $ indices engine
 103
 104 possibleGears :: [NumPos] -> Position -> [NumPos]
 105 possibleGears nums star = filter (flip touchPoint star) nums