rewrite GridSelect.stringToRatio to use randomR (fixes #572)

Due to differences between random-1.1 and random-1.2, on newer systems
stringToRatio returns numbers outside [0, 1] range, which breaks
colorRangeFromClassName colorizers.

This commit fixes the issue by using randomR to directly generate the random number.

Also this fixes the compilation warning (genRange and next are deprecated in random-1.2).

XMonad/Actions/GridSelect.hs

@@ -47,6 +47,7 @@ module XMonad.Actions.GridSelect (
     fromClassName,
     stringColorizer,
     colorRangeFromClassName,
+    stringToRatio,
 
     -- * Navigation Mode assembly
     TwoD,
@@ -93,7 +94,7 @@ import XMonad.Layout.Decoration
 import XMonad.Util.NamedWindows
 import XMonad.Actions.WindowBringer (bringWindow)
 import Text.Printf
-import System.Random (mkStdGen, genRange, next)
+import System.Random (mkStdGen, randomR)
 import Data.Word (Word8)
 
 -- $usage
@@ -628,15 +629,12 @@ mix (r1, g1, b1) (r2, g2, b2) r = (mix' r1 r2, mix' g1 g2, mix' b1 b2)
 
 -- | Generates a Double from a string, trying to
 -- achieve a random distribution.
--- We create a random seed from the sum of all characters
+-- We create a random seed from the hash of all characters
 -- in the string, and use it to generate a ratio between 0 and 1
 stringToRatio :: String -> Double
 stringToRatio "" = 0
-stringToRatio s = let gen = mkStdGen $ sum $ map fromEnum s
-                      range = (\(a, b) -> b - a) $ genRange gen
-                      randomInt = foldr1 combine $ replicate 20 next
-                      combine f1 f2 g = let (_, g') = f1 g in f2 g'
-                  in fi (fst $ randomInt gen) / fi range
+stringToRatio s = let gen = mkStdGen $ foldl' (\t c -> t * 31 + fromEnum c) 0 s
+                  in fst $ randomR (0, 1) gen
 
 -- | Brings up a 2D grid of elements in the center of the screen, and one can
 -- select an element with cursors keys. The selected element is returned.