Improve day 5 solution to O(n * log n) from O(n ^ 2)

sort and group are neat :)

Author: Demindiro

2021/5/better.hs

@@ -0,0 +1,31 @@
+import Data.List (group, sort)
+import qualified Data.Text    as Text
+import qualified Data.Text.IO as Text
+import qualified Data.Set as Set
+import Control.Monad
+
+readInput f = Text.readFile f
+          >>= let parsePoint  = map . Text.splitOn
+                  splitLine t = let (l,   r) = Text.break (== ',') $ t
+                                    (x, _:y) = (Text.unpack l, Text.unpack r)
+                                in (read x, read y)
+              in  return . (map $ map splitLine)
+                         . (map $ Text.splitOn $ Text.pack " -> ")
+                         . Text.lines
+
+points d [(x, y), (x', y')] = let b a b' = [a, a + signum (b' - a) .. b']
+                              in  if   x == x' || y == y'
+                                  then if   x == x'
+                                       then zip (repeat x) (b y y')
+                                       else zip (b x x') (repeat y)
+                                  else if   d && abs (x - x') == abs (y - y')
+                                       then zip (b x x') (b y y')
+                                       else []
+
+count' = length . filter (> 1) . map length . group . sort
+
+part12 d f = (readInput f :: IO [[(Int, Int)]])
+         >>= return . count' . concat . map (points d)
+
+main = (part12 False "input.txt" >>= print)
+    >> (part12 True  "input.txt" >>= print)