Day 5, parts 1 & 2: Optimize `reacts` to only use one pass.

Day5/Part1.hs

@@ -1,21 +1,18 @@
 #! /usr/bin/env stack
 -- stack --resolver lts-12.20 --install-ghc script
-{-# LANGUAGE FlexibleContexts, LambdaCase #-}
+{-# LANGUAGE ViewPatterns #-}
 module Main where
 
-import Data.Function (fix)
 import qualified Data.Char as C
 import qualified Data.List as L
 
 main :: IO ()
-main = interact $ show . react . takeWhile C.isAlpha
+main = interact $ show . length . react . takeWhile C.isAlpha
 
-react input =
-   let scan (x:y:zs) = if x `reactsWith` y then scan zs else (x : scan (y:zs))
-       scan zs = zs
-       input' = scan input
-       result = (input', length input')
-   in if input' == input then result else react input'
+react [] = []
+react (x:(react -> ys))
+   | (y:ys') <- ys, x `reactsWith` y = ys'
+   | otherwise                       = x:ys
 
 x `reactsWith` y = (C.isUpper x && C.toLower x == y) ||
                    (C.isLower x && C.toUpper x == y)

Day5/Part2.hs

@@ -1,6 +1,6 @@
 #! /usr/bin/env stack
 -- stack --resolver lts-12.20 --install-ghc script
-{-# LANGUAGE FlexibleContexts, LambdaCase #-}
+{-# LANGUAGE ViewPatterns #-}
 module Main where
 
 import Data.Function (fix, on)
@@ -9,16 +9,15 @@ import qualified Data.List as L
 
 main :: IO ()
 main = interact $ \input -> show $ L.minimumBy (compare `on` snd) $
-   [ react $ filter (\c -> C.isAlpha c && c /= r && c /= C.toUpper r) input
+   [ let rs = react input' in ((rs, r), length rs)
    | r <- ['a'..'z']
+   , let input' = [ c | c <- input, C.isAlpha c, c /= r, c /= C.toUpper r ]
    ]
 
-react input =
-   let scan (x:y:zs) = if x `reactsWith` y then scan zs else (x : scan (y:zs))
-       scan zs = zs
-       input' = scan input
-       result = (input', length input')
-   in if input' == input then result else react input'
+react [] = []
+react (x:(react -> ys))
+   | (y:ys') <- ys, x `reactsWith` y = ys'
+   | otherwise                       = x:ys
 
 x `reactsWith` y = (C.isUpper x && C.toLower x == y) ||
                    (C.isLower x && C.toUpper x == y)