praxis

commit d6a1d6c2114f5a076ddcc828de869806bb765d65
parent 93c1c0aa179fb8e13fcbe100a75168acc1c48fbf
Author: Jared Tobin <jared@jtobin.ca>
Date:   Wed, 27 Jun 2018 11:41:38 +1200

Add tally exercise.

Diffstat:
A
20180514_tally/Tally.hs
 | 
29
+++++++++++++++++++++++++++++

1 file changed, 29 insertions(+), 0 deletions(-)
diff --git a/20180514_tally/Tally.hs b/20180514_tally/Tally.hs
@@ -0,0 +1,29 @@
+{-# OPTIONS_GHC -Wall #-}
+{-# LANGUAGE BangPatterns #-}
+
+import Data.Char (isLower, toLower)
+import Data.List as L (foldl', sortBy)
+import Data.Function (on)
+import Data.Monoid (mempty)
+import qualified Data.Map.Strict as MS
+
+tally :: String -> [(Char, Int)]
+tally = sortBy (flip compare `on` snd) . MS.toList . accum
+
+accum :: String -> MS.Map Char Int
+accum = L.foldl' alg mempty where
+  alg !acc score =
+    let adjuster val = case (val, isLower score) of
+          (Nothing, True)  -> Just 1
+          (Nothing, False) -> Just (negate 1)
+          (Just s, True)   -> Just (succ s)
+          (Just s, False)  -> Just (pred s)
+
+    in  MS.alter adjuster (toLower score) acc
+
+test :: String
+test = "EbAAdbBEaBaaBBdAccbeebaec"
+
+main :: IO ()
+main = print (tally test)
+