Sebastian's day 14.

day 14/day_14_sebastian.hs

+-- 13 + c * 101 = 65 + d * 103 
+-- i mod 101 = 13
+-- i mod 103 = 65
+module Main where
+
+import qualified Data.Set as S
+
+wh :: (Int, Int)
+-- wh = (11, 7)
+wh = (101, 103)
+
+nimg :: Int
+nimg = head $ filter ((== 13) . (`mod` 101)) [65, 168 ..]
+
+main :: IO ()
+main = do
+  s <- getContents
+  putStrLn ("Part 1: " ++ part1 s)
+  -- mapM_ (\i -> writeFile (show i ++ ".pbm") (toPbm $ map (simulate i) $ getRobots s)) [0 .. 999]
+  writeFile (show nimg ++ ".pbm") (toPbm $ map (simulate nimg) $ getRobots s)
+
+toPbm :: [(Int, Int)] -> String
+toPbm l = unlines ["P1", unwords [show w, show h]] ++ img where
+  (w, h) = wh
+  pixels = S.fromList l
+  indices = [[(i, j) | i <- [0 .. w-1]] | j <- [0 .. h-1]]
+  img = unlines $ map (unwords . map toPixel) indices
+  toPixel (i, j) = if (i, j) `S.member` pixels then "1" else "0"
+
+-- solve :: String -> String
+-- solve s = unlines ["Part 1: " ++ part1 s, "Part 2: " ++ part2 s]
+
+part1 :: String -> String
+part1 = show . safetyFactor . map (quadrant . simulate 100) . getRobots
+
+-- part2 :: String -> String
+-- part2 = const ""
+
+parseRobot :: String -> ((Int, Int), (Int, Int))
+parseRobot s = ((read x', read y'), (read dx', read dy')) where
+  r1 = drop 2 s
+  (x', r2) = span (/= ',') r1
+  (y', r3) = span (/= ' ') (tail r2)
+  (dx', r4) = span (/= ',') (drop 3 r3)
+  dy' = tail r4
+
+getRobots :: String -> [((Int, Int), (Int, Int))]
+getRobots = map parseRobot . lines
+
+simulate :: Int -> ((Int, Int), (Int, Int)) -> (Int, Int)
+simulate n ((x, y), (dx, dy)) = ((x + n* dx) `mod` w, (y + n * dy) `mod` h) where (w, h) = wh
+
+quadrant :: (Int, Int) -> Maybe Int
+quadrant (x, y) | x < w `div` 2 && y < h `div` 2 = Just 1
+  | x > w `div` 2 && y < h `div` 2 = Just 2
+  | x < w `div` 2 && y > h `div` 2 = Just 3
+  | x > w `div` 2 && y > h `div` 2 = Just 4
+  | otherwise = Nothing where (w, h) = wh
+
+safetyFactor :: [Maybe Int] -> Int
+safetyFactor l = product $ map (\i -> length (filter (== Just i) l)) [1, 2, 3, 4]
+