day 4

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

lib/AoC.hs

@@ -1,10 +1,29 @@
 module AoC where
 
-import Text.Parsec
+import Data.List (transpose)
+import Text.Parsec (ParseError)
 
+-- extract Right value after parsing
 extract :: Either ParseError a -> a
 extract (Left err) = error ("Parsing failed: " ++ show err)
 extract (Right val) = val
 
+-- count elements in a list
 count :: (Eq a) => a -> [a] -> Int
 count x = length . filter (x ==)
+
+-- extract diagonals from a matrix
+diagonals :: [[a]] -> [[a]]
+diagonals =
+  (++)
+    <$> reverse . transpose . zipWith drop [0 ..]
+    <*> transpose . zipWith drop [1 ..] . transpose
+
+-- get indices of substring sub in str
+findSubstrings :: [Char] -> [Char] -> [Int]
+findSubstrings sub str = findSubstrings' sub str 0
+  where
+    findSubstrings' _ [] _ = []
+    findSubstrings' sub str@(x : xs) idx
+      | take (length sub) str == sub = idx : findSubstrings' sub xs (idx + 1)
+      | otherwise = findSubstrings' sub xs (idx + 1)