day 8

Signed-off-by: Amneesh Singh <natto@weirdnatto.in>

day8.hs

@@ -0,0 +1,52 @@
+import Data.Char (digitToInt)
+import Data.List (transpose, zip4, zipWith4)
+
+-- a fun approach, not necessarily the fastest (or cleanest (or smartest))
+
+main :: IO ()
+main = do
+  input <- map (map digitToInt) . lines <$> readFile "day8.in"
+  let (e, w, n, s) = trees input
+  let obscured = zipWith4 zip4 e w n s
+  putStr "Q1: "
+  print $
+    length $
+      filter
+        (not . (\((w, _), (x, _), (y, _), (z, _)) -> w && y && x && z))
+        $ concat obscured
+  putStr "Q2: "
+  print $
+    maximum $
+      map (\((_, w), (_, x), (_, y), (_, z)) -> w * x * y * z) $
+        concat obscured
+
+type Forest = [[(Bool, Int)]]
+
+trees :: [[Int]] -> (Forest, Forest, Forest, Forest)
+trees input =
+  ( trees' input,
+    map reverse $ trees' $ map reverse input,
+    transpose $ trees' $ transpose input,
+    reverse $ transpose $ trees' $ transpose $ reverse input
+  ) -- (east, west, north, south) traversals
+  where
+    trees' :: [[Int]] -> Forest
+    trees' =
+      foldr
+        ( \x xs ->
+            let (_, dist) =
+                  foldr
+                    ( \(cur, y) ((old, m), ys) ->
+                        ( if y > m then (cur, y) else (old, m),
+                          ( y <= m,
+                            let s = length $ takeWhile (< y) $ reverse $ take cur x
+                             in if s == cur then s else s + 1
+                          ) :
+                          ys
+                        )
+                    )
+                    ((-1, -1), [])
+                    (zip [0 ..] x)
+             in dist : xs
+        )
+        []