src/advent11/advent11.hs

   1 {-# LANGUAGE OverloadedStrings #-}
   2
   3 -- Using a summed-area table, e.g. see https://en.wikipedia.org/wiki/Summed-area_table
   4
   5 import Data.List
   6 import qualified Data.Map.Strict as M
   7 import Data.Map.Strict ((!))
   8 import Data.Ord (comparing)
   9
  10 type Coord = (Integer, Integer) -- x, y
  11 type Grid = M.Map Coord Integer
  12
  13 serialNumber = 5719
  14
  15 main :: IO ()
  16 main = do
  17         let g = makeGrid serialNumber
  18         print $ part1 g
  19         print $ part2 g
  20
  21
  22 part1 grid = keyOfMaxValue sg
  23     where sg = allSubCellPower 3 grid
  24
  25 part2 grid = maximumBy (comparing snd) [bestInGrid size grid | size <- [3..300]]
  26
  27 makeGrid :: Integer -> Grid
  28 -- makeGrid key = M.fromList [((x, y), powerLevel x y key) | x <- [1..300], y <- [1..300] ]
  29 makeGrid key = foldl' addSummedArea M.empty [((x, y), powerLevel x y key) | x <- [0..300], y <- [0..300] ]
  30
  31 addSummedArea :: Grid -> ((Integer, Integer), Integer) -> Grid
  32 addSummedArea grid ((x, y), power) = M.insert (x, y) (power + upper + left - upperLeft) grid
  33     where upper = M.findWithDefault 0 (x-1, y) grid
  34           left  = M.findWithDefault 0 (x, y-1) grid
  35           upperLeft = M.findWithDefault 0 (x-1, y-1) grid
  36
  37
  38 powerLevel :: Integer -> Integer -> Integer -> Integer
  39 powerLevel 0 _ _ = 0
  40 powerLevel _ 0 _ = 0
  41 powerLevel x y key = ((interim `div` 100) `mod` 10) - 5
  42     where rackID = x + 10
  43           interim = ((rackID) * y + key) * rackID
  44
  45 subCellPower :: Integer -> Integer -> Integer -> Grid -> Integer
  46 -- subCellPower size x y grid = sum [grid!(sx, sy) | sx <- [x..(x+size-1)], sy <- [y..(y+size-1)]]
  47 subCellPower size x0 y0 grid = grid!(x,y) + grid!(x',y') - grid!(x,y') - grid!(x',y)
  48     where x  = x0 - 1
  49           x' = x + size
  50           y  = y0 - 1
  51           y' = y + size
  52
  53 allSubCellPower :: Integer -> Grid -> Grid
  54 allSubCellPower size grid = M.fromList [((x, y), subCellPower size x y grid)| x <- [1..(301-size)], y <- [1..(301-size)]]
  55
  56
  57 keyAndMaxValue :: Ord b => M.Map a b -> (a, b)
  58 keyAndMaxValue m = M.foldrWithKey mergeKV (M.findMin m) m
  59     where mergeKV k v (bestK, bestV) =
  60             if v > bestV then (k, v) else (bestK, bestV)
  61
  62 keyOfMaxValue :: Ord b => M.Map a b -> a
  63 keyOfMaxValue m = fst $ keyAndMaxValue m
  64
  65 bestInGrid :: Integer -> Grid -> ((Integer, Integer, Integer), Integer)
  66 bestInGrid size grid = ((x, y, size), p)
  67     where ((x, y), p) = keyAndMaxValue $ allSubCellPower size grid