day3: add haskell solution

README.md

@@ -8,3 +8,4 @@ https://adventofcode.com/2020/
 |---|--------|---------|------|
 | 1 |  `**`  |  `**`   |      |
 | 2 |  `**`  |         | `**` |
+| 3 |  `**`  |         |      |

day3/day3.hs

@@ -0,0 +1,23 @@
+import Control.Applicative (empty)
+import Control.Monad.Zip (mzip)
+import Data.List (intercalate)
+import Data.Maybe (catMaybes)
+
+compose2 :: (a' -> b' -> c) -> (a -> a') -> (b -> b') -> a -> b -> c
+compose2 f g h x y = f (g x) (h y)
+
+(.:) :: (c -> d) -> (a -> b -> c) -> a -> b -> d
+f .: g = (f .) . g
+infixl 8 .:
+
+countHits :: (Int, Int) -> [[Bool]] -> Int
+countHits = length . filter id . map (uncurry $ flip (!!)) . catMaybes .: compose2 (zipWith mzip) (uncurry . flip $ compose2 intercalate (flip replicate empty . pred) (fmap pure . map pure . enumFromThen 0)) (map pure)
+
+treeCharToBool :: Char -> Bool
+treeCharToBool = (== '#')
+
+main = do
+    charLines <- lines <$> readFile "input.txt"
+    let boolLines = cycle <$> map treeCharToBool <$> charLines
+    print $ countHits (3, 1) boolLines
+    print $ product $ flip countHits boolLines <$> [(1,1), (3,1), (5,1), (7,1), (1,2)]

day3/day3.py

@@ -0,0 +1,20 @@
+#!/usr/bin/env python
+
+def count_trees(right, down, lines):
+    n = 0
+    for i, line in enumerate(lines[::down]):
+        c = line[(i*right) % len(line)]
+        if c == '#':
+            n += 1
+    return n
+
+if __name__ == '__main__':
+    with open('input.txt', 'r') as f:
+        lines = [line.strip() for line in f.readlines()]
+
+    print(count_trees(3, 1, lines))
+
+    hits_multiplied = 1
+    for slope in ((1,1), (3,1), (5,1), (7,1), (1,2)):
+        hits_multiplied *= count_trees(*slope, lines)
+    print(hits_multiplied)