day 9 slightly faster by not searching all neighbors every time

Author: mstksg

src/AOC/Challenge/Day09.hs

@@ -14,16 +14,13 @@ module AOC.Challenge.Day09 (
  , day09b
  ) where
 
-import AOC.Common (freqs, digitToIntSafe)
+import AOC.Common (freqs, digitToIntSafe, firstJust)
 import AOC.Common.Point (Point, cardinalNeighbs, parseAsciiMap)
 import AOC.Solver ((:~>)(..))
 import Control.Monad (mfilter)
 import Data.Foldable (toList)
 import Data.List (sortBy)
 import Data.Map (Map)
-import Data.Maybe (mapMaybe)
-import Data.Ord (comparing)
-import Safe.Foldable (minimumByMay)
 import qualified Data.Map as M
 
 -- | Find low points and their heights
@@ -60,7 +57,6 @@ day09b = MkSol
 flowMap :: Map Point Int -> Map Point (Maybe Point)
 flowMap mp = M.mapWithKey go mp
  where
- go p i = fmap fst . minimumByMay (comparing snd) $
- mapMaybe getGrad (cardinalNeighbs p)
+ go p i = firstJust getGrad (cardinalNeighbs p)
  where
- getGrad q = (q,) <$> mfilter (< i) (M.lookup q mp)
+ getGrad q = q <$ mfilter (< i) (M.lookup q mp)