2 files changed, 79 insertions, 0 deletions
diff --git a/2021/day13/13a.hs b/2021/day13/13a.hs
new file mode 100644
index 0000000..79af7ae
--- /dev/null
+++ b/2021/day13/13a.hs
@@ -0,0 +1,34 @@
+{-# LANGUAGE ViewPatterns #-}
+{-# LANGUAGE OverloadedStrings #-}
+module Main where
+
+import qualified Data.Text as T
+import Data.Set (Set)
+import Data.List
+import qualified Data.Set as S
+
+type Coord = (Int, Int)
+data FoldInstr = Fold Axis Int deriving (Show, Eq, Ord)
+data Axis = XAxis | YAxis deriving (Show, Eq, Ord)
+
+parseCoords :: String -> Coord
+parseCoords s = (read x, read y)
+  where [x, y] = map T.unpack $ T.splitOn "," (T.pack s)
+
+parseFold :: String -> FoldInstr
+parseFold (stripPrefix "fold along y=" -> Just cs) = Fold YAxis (read cs)
+parseFold (stripPrefix "fold along x=" -> Just cs) = Fold XAxis (read cs)
+
+parseFile :: String -> (Set Coord, [FoldInstr])
+parseFile s = (S.fromList $ map parseCoords (lines coordSection), map parseFold (lines foldSection))
+  where [coordSection, foldSection] = map T.unpack $ T.splitOn "\n\n" (T.pack s)
+
+performFold :: Set Coord -> FoldInstr -> Set Coord
+performFold coords (Fold YAxis ye) = S.map (\(x, y) -> (x, -(abs (y - ye)) + ye)) coords
+performFold coords (Fold XAxis xe) = S.map (\(x, y) -> (-(abs (x - xe)) + xe, y)) coords
+
+main :: IO ()
+main = do
+  f <- readFile "./input"
+  let (coords, instrs) = parseFile f;
+  print $ length $ performFold coords (head instrs)
diff --git a/2021/day13/13b.hs b/2021/day13/13b.hs
new file mode 100644
index 0000000..32d29de
--- /dev/null
+++ b/2021/day13/13b.hs
@@ -0,0 +1,45 @@
+{-# LANGUAGE ViewPatterns #-}
+{-# LANGUAGE OverloadedStrings #-}
+module Main where
+
+import qualified Data.Text as T
+import Data.Set (Set)
+import Data.List
+import qualified Data.Set as S
+
+type Coord = (Int, Int)
+data FoldInstr = Fold Axis Int deriving (Show, Eq, Ord)
+data Axis = XAxis | YAxis deriving (Show, Eq, Ord)
+
+parseCoords :: String -> Coord
+parseCoords s = (read x, read y)
+  where [x, y] = map T.unpack $ T.splitOn "," (T.pack s)
+
+parseFold :: String -> FoldInstr
+parseFold (stripPrefix "fold along y=" -> Just cs) = Fold YAxis (read cs)
+parseFold (stripPrefix "fold along x=" -> Just cs) = Fold XAxis (read cs)
+
+parseFile :: String -> (Set Coord, [FoldInstr])
+parseFile s = (S.fromList $ map parseCoords (lines coordSection), map parseFold (lines foldSection))
+  where [coordSection, foldSection] = map T.unpack $ T.splitOn "\n\n" (T.pack s)
+
+performFold :: Set Coord -> FoldInstr -> Set Coord
+performFold coords (Fold YAxis ye) = S.map (\(x, y) -> (x, -(abs (y - ye)) + ye)) coords
+performFold coords (Fold XAxis xe) = S.map (\(x, y) -> (-(abs (x - xe)) + xe, y)) coords
+
+displayCoords :: Set Coord -> String
+displayCoords coords = intercalate "\n" (map displayLine [sy..ey])
+  where sx = minimum (S.map fst coords)
+        ex = maximum (S.map fst coords)
+        sy = minimum (S.map snd coords)
+        ey = maximum (S.map snd coords)
+        displayLine y = map (displayCoord y) [sx..ex]
+        displayCoord y x | S.member (x, y) coords = '#'
+                         | otherwise              = '.'
+
+main :: IO ()
+main = do
+  f <- readFile "./input"
+  let (coords, instrs) = parseFile f;
+  let result = foldl performFold coords instrs;
+  putStrLn $ displayCoords result