Add 2015 day 18

haskellAoC/src/Y2015/Day18.hs

@@ -0,0 +1,48 @@
+module Y2015.Day18 (y15day18) where
+
+import Data.Maybe
+import qualified Data.Set as S
+
+type Coords = (Int, Int)
+type Lights = S.Set Coords
+
+rows :: Int
+rows = 99
+
+cols :: Int
+cols = 99
+
+corners :: [Coords]
+corners = [(0, 0), (0, 99), (99, 0), (99, 99)]
+
+parseLine :: (Int, String) -> [Coords]
+parseLine (x, str) = concatMap (maybeToList . parseChar) $ zip (iterate (+1) 0) str
+  where parseChar (y, '#') = Just (x, y)
+        parseChar (_, '.') = Nothing
+        parseChar entry    = error ("Error in input: " ++ show entry)
+
+nbNeighbors :: Lights -> Coords -> Int
+nbNeighbors ls (x, y)   = length $ filter (== True) $ map (`S.member` ls) $ neighborsCoords
+  where neighborsCoords = [ (x - 1, y - 1), (x - 1, y), (x - 1, y + 1),
+                            (    x, y - 1),             (    x, y + 1),
+                            (x + 1, y - 1), (x + 1, y), (x + 1, y + 1) ]
+
+nextStep :: Bool -> Lights -> Lights
+nextStep isP2 lights = S.fromList $ concatMap (maybeToList . getNewState) $ allCoords
+  where allCoords     = [(x, y) | x <- [0..cols], y <- [0..rows]]
+        getNewState c
+          | isP2 == True && c `elem` corners = Just c
+          | otherwise = case (isLight, lightNeighbors) of
+                          (True,  2) -> Just c
+                          (_,  3)    -> Just c
+                          _          -> Nothing
+          where isLight        = c `S.member` lights
+                lightNeighbors = nbNeighbors lights c
+
+y15day18 :: [String] -> (String, String)
+y15day18 input = (part1, part2)
+  where part1  = show $ S.size $ head $ drop 100 $ iterate (nextStep False) state
+        part2  = show $ S.size $ head $ drop 100 $ iterate (nextStep True) stateP2
+
+        state   = S.fromList $ concatMap parseLine $ zip (iterate (+1) 0) input
+        stateP2 = S.union state $ S.fromList corners

haskellAoC/src/Y2015/Days.hs

@@ -17,6 +17,8 @@ import Y2015.Day14
 import Y2015.Day15
 import Y2015.Day16
 import Y2015.Day17
+import Y2015.Day18
+
 
 year2015 :: String -> [String] -> (String, String)
 year2015 "01" = y15day01
@@ -36,3 +38,4 @@ year2015 "14" = y15day14
 year2015 "15" = y15day15
 year2015 "16" = y15day16
 year2015 "17" = y15day17
+year2015 "18" = y15day18