Day 8

src/Day8.hs

@@ -0,0 +1,84 @@
+module Day8 (
+  Forest,
+  Visibillity,
+  parseForest,
+  checkVisibility,
+  countVisible,
+  findMaxScenicScore,
+  viewingDistance,
+  day8
+)where
+
+import qualified Data.Matrix as M
+import Data.Matrix ((!))
+import Data.Char (isDigit)
+import qualified Data.Vector as V
+import Data.Monoid (Sum (..), getSum, Product (..), getProduct)
+import Control.Arrow ((>>>))
+
+type Forest = M.Matrix Int
+type Visibillity = M.Matrix Bool
+
+parseForest :: String -> Forest
+parseForest input = M.fromList rows cols asNumbers
+                     where
+                       ls = lines input
+                       cols = length (head ls)
+                       rows = length ls
+                       asNumbers :: [Int]
+                       asNumbers = map (read . (: [])) . filter isDigit $ input
+
+checkAgainstNeighbours :: (Int -> [V.Vector Int] -> a) -> Forest -> M.Matrix a
+checkAgainstNeighbours f forest  = M.matrix (M.nrows forest) (M.ncols forest) check
+  where
+    check(x, y) =
+      let row = M.getRow x forest
+          col = M.getCol y forest
+          -- left and top are reversed to order the as seen from (x,y)
+          left   = V.reverse $ V.take (y - 1) row 
+          right  = V.drop y row
+          top    = V.reverse $ V.take (x - 1) col 
+          bottom = V.drop x col
+          v = forest ! (x,y)
+      in f v [left, right, top, bottom]
+
+checkVisibility :: Forest -> Visibillity
+checkVisibility = checkAgainstNeighbours isVisible
+  where
+    isVisible v = any (all (< v))
+
+countVisible :: Visibillity -> Int
+countVisible vis = getSum $ foldMap (Sum . isTrue) vis
+  where
+    isTrue True = 1
+    isTrue False = 0
+
+checkScenicScores :: Forest -> M.Matrix Int
+checkScenicScores = checkAgainstNeighbours allDistances
+  where
+    allDistances tree neighbors = getProduct $ foldMap (Product . viewingDistance tree) neighbors
+
+viewingDistance :: Int -> V.Vector Int -> Int
+viewingDistance treeHeight neighbors
+  | V.length neighbors == 0 = 0
+  | V.head neighbors >= treeHeight = 1
+  | V.head neighbors < treeHeight = 1 + viewingDistance treeHeight (V.tail neighbors)
+  | otherwise = 0
+
+findMaxScenicScore :: Forest -> Int
+findMaxScenicScore forest = maximum $ M.toList scores
+  where
+    scores = checkScenicScores forest
+
+day8 :: IO ()
+day8 = do
+   input <- readFile "ressources/day08-input"
+   putStrLn "Day8"
+   let forest = parseForest input
+   let visibleTrees = checkVisibility >>>
+                      countVisible $ forest
+   putStrLn ("Number of visible trees is " ++ show visibleTrees)
+   let highestScenicScore = findMaxScenicScore forest
+   putStrLn ("Highest scenic score is " ++ show highestScenicScore)
+   
+

src/Lib.hs

@@ -9,6 +9,7 @@ import Day4
 import Day5
 import Day6
 import Day7
+import Day8
 
 someFunc :: IO ()
 someFunc = do
@@ -25,5 +26,7 @@ someFunc = do
   day6
   putStrLn "-----------"
   day7
+  putStrLn "-----------"
+  day8