Uploaded new day04 with friendly improvements

day04/main_improved.hs

@@ -0,0 +1,54 @@
+  -- main.hs as written by me, thuroughly fixed by <https://github.com/h7x4>
+main :: IO ()
+main = do  
+  file <- readFile "input.txt"
+  let
+    pairs :: [((Int, Int), (Int, Int))]
+    pairs = inputLinesToPairs file
+
+  putStrLn "Part 1: "
+  print $ part1 pairs
+
+  putStrLn "Part 2: "
+  print $ part2 pairs
+
+-- Map each line of input from "1-2,3-4" to ((1,2),(3,4))
+inputLinesToPairs :: String -> [((Int, Int), (Int, Int))]
+inputLinesToPairs input = result
+  where
+  stringpairs :: [[String]]
+  stringpairs = map (splitWhen (==',')) $ lines input 
+
+  parseRange :: String -> (Int, Int)
+  parseRange pair = (read first, read second)
+    where
+      (first:second:_) = splitWhen (=='-') pair 
+
+  result :: [((Int, Int), (Int, Int))]
+  result = map (\x -> (head x, last x)) $ map (map parseRange) stringpairs
+
+
+part1 :: [((Int, Int), (Int, Int))] -> Int
+part1 = length . filter (uncurry rangeIsContained)
+
+part2 :: [((Int, Int), (Int, Int))] -> Int
+part2 = length . filter (uncurry rangeOverlaps)
+
+-- True if any part of the two ranges overlap
+rangeOverlaps :: (Int, Int) -> (Int, Int) -> Bool
+rangeOverlaps (lo1, hi1) (lo2, hi2) = rangeIsContained (lo1, lo1) (lo2, hi2)
+                                   || (lo2 >= lo1 && lo2 <= hi1)
+                                   || (hi2 >= lo1 && hi2 <= hi1)
+
+-- True if the first range is entirely contained in the second range, or vice versa
+rangeIsContained :: (Int, Int) -> (Int, Int) -> Bool
+rangeIsContained (lo1, hi1) (lo2, hi2) = (lo1 <= lo2 && hi1 >= hi2)
+                                      || (lo1 >= lo2 && hi1 <= hi2)
+
+-- Modified from Prelude.words
+splitWhen :: (Char -> Bool) -> String -> [String]
+splitWhen p s = case dropWhile p s of
+  "" -> []
+  s' -> w : splitWhen p s''
+    where (w, s'') = break p s'
+