praxis

Various programming exercises.
git clone git://git.jtobin.io/praxis.git
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Tally.hs (770B)

      1 {-# OPTIONS_GHC -Wall #-}
      2 {-# LANGUAGE BangPatterns #-}
      3 
      4 import Data.Char (isLower, toLower)
      5 import Data.List as L (foldl', sortBy)
      6 import Data.Function (on)
      7 import Data.Monoid (mempty)
      8 import qualified Data.Map.Strict as MS
      9 
     10 tally :: String -> [(Char, Int)]
     11 tally = sortBy (flip compare `on` snd) . MS.toList . accum
     12 
     13 accum :: String -> MS.Map Char Int
     14 accum = L.foldl' alg mempty where
     15   alg !acc score =
     16     let adjuster val = case (val, isLower score) of
     17           (Nothing, True)  -> Just 1
     18           (Nothing, False) -> Just (negate 1)
     19           (Just s, True)   -> Just (succ s)
     20           (Just s, False)  -> Just (pred s)
     21 
     22     in  MS.alter adjuster (toLower score) acc
     23 
     24 test :: String
     25 test = "EbAAdbBEaBaaBBdAccbeebaec"
     26 
     27 main :: IO ()
     28 main = print (tally test)
     29