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Day 18 - a really slow solution (~2 mins).

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Jonathan Chan 2018-12-18 12:59:33 -08:00
parent 3251270ee9
commit 16e6c33658
4 changed files with 95 additions and 4 deletions
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2
README.md
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@ -28,7 +28,7 @@ Now located in this repository's wiki.
| 14 | 22.3 | | 14 | 22.3 |
| 15 | < 1 | | 15 | < 1 |
| 16 | ~ 15 | | 16 | ~ 15 |
| 17 | | | 17 | 120 |
| 18 | | | 18 | |
| 19 | | | 19 | |
| 20 | | | 20 | |

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app/Main.hs
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@ -30,4 +30,4 @@ import qualified Day24
import qualified Day25 import qualified Day25
main :: IO () main :: IO ()
main = Day17.main main = Day18.main

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input/18.txt Normal file
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src/Day18.hs
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@ -1,6 +1,47 @@
module Day18 (main) where module Day18 (main) where
import Data.Maybe (catMaybes)
import Data.Matrix (Matrix, fromLists, nrows, ncols, (!), safeGet, setElem)
type Landscape = Matrix Char
type Coordinate = (Int, Int) -- row, col
stepCoord :: Landscape -> Coordinate -> Landscape -> Landscape
stepCoord oldscape (r, c) newscape =
let neighbours = catMaybes $ map (flip (uncurry safeGet) oldscape) [(r', c') | r' <- [r-1 .. r+1], c' <- [c-1 .. c+1], (r', c') /= (r, c)]
newAcre = case oldscape ! (r, c) of
'.' -> if (length $ filter (== '|') neighbours) >= 3 then '|' else '.'
'|' -> if (length $ filter (== '#') neighbours) >= 3 then '#' else '|'
'#' -> if '|' `elem` neighbours && '#' `elem` neighbours then '#' else '.'
in setElem newAcre (r, c) newscape
stepScape :: Landscape -> Landscape
stepScape oldscape = foldr (stepCoord oldscape) oldscape [(r, c) | r <- [1 .. nrows oldscape], c <- [1 .. ncols oldscape]]
part1 :: Landscape -> Int
part1 scape =
let newscape = iterate stepScape scape !! 10
woodedCount = sum $ fmap (fromEnum . (== '|')) newscape
lumberyardCount = sum $ fmap (fromEnum . (== '#')) newscape
in woodedCount * lumberyardCount
part2 :: Landscape -> Int
part2 scape =
let resVals = map getResVal $ iterate stepScape scape
(i, i') = (461, 489) -- cycleIndices (drop 1 resVals) (drop 2 resVals) 1 2
cyclePos = (1000000000 - i) `mod` (i' - i) + i
in getResVal $ iterate stepScape scape !! cyclePos -- resVals !! cyclePos
where
getResVal scape =
let woodedCount = sum $ fmap (fromEnum . (== '|')) scape
lumberyardCount = sum $ fmap (fromEnum . (== '#')) scape
in woodedCount * lumberyardCount
-- theoretically this works but it was just faster to print out the first 500 values
cycleIndices (t:ts) (h:_:hs) ti hi =
if t == h then (ti, hi) else cycleIndices ts hs (ti + 1) (hi + 2)
main :: IO () main :: IO ()
main = do main = do
input <- readFile "input/18.txt" input <- fromLists . lines <$> readFile "input/18.txt"
print input print $ part1 input
print $ part2 input