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+-----------------------------------------------------------------------------
+-- |
+-- Module      :  XMonad.Util.Anneal
+-- Copyright   :  (c) David Roundy
+-- License     :  BSD-style (see LICENSE)
+--
+-- Maintainer  :  David Roundy <droundy@darcs.org>
+-- Stability   :  unstable
+-- Portability :  unportable
+--
+-- Requires the 'random' package
+--
+-----------------------------------------------------------------------------
+
+module XMonad.Util.Anneal ( Rated(Rated), the_value, the_rating
+                            , anneal, annealMax ) where
+
+import System.Random ( StdGen, Random, mkStdGen, randomR )
+import Control.Monad.State ( State, runState, put, get, gets, modify )
+
+-- %import XMonad.Util.Anneal
+
+data Rated a b = Rated !a !b
+                 deriving ( Show )
+instance Functor (Rated a) where
+    f `fmap` (Rated v a) = Rated v (f a)
+
+the_value :: Rated a b -> b
+the_value (Rated _ b) = b
+the_rating :: Rated a b -> a
+the_rating (Rated a _) = a
+
+instance Eq a => Eq (Rated a b) where
+    (Rated a _) == (Rated a' _) = a == a'
+instance Ord a => Ord (Rated a b) where
+    compare (Rated a _) (Rated a' _) = compare a a'
+
+anneal :: a -> (a -> Double) -> (a -> [a]) -> Rated Double a
+anneal st r sel = runAnneal st r (do_anneal sel)
+
+annealMax :: a -> (a -> Double) -> (a -> [a]) -> Rated Double a
+annealMax st r sel = runAnneal st (negate . r) (do_anneal sel)
+
+do_anneal :: (a -> [a]) -> State (Anneal a) (Rated Double a)
+do_anneal sel = do sequence_ $ replicate 100 da
+                   gets best
+    where da = do select_metropolis sel
+                  modify $ \s -> s { temperature = temperature s *0.99 }
+
+data Anneal a = A { g :: StdGen
+                  , best :: Rated Double a
+                  , current :: Rated Double a
+                  , rate :: a -> Rated Double a
+                  , temperature :: Double }
+
+runAnneal :: a -> (a -> Double) -> State (Anneal a) b -> b
+runAnneal start r x = fst $ runState x (A { g = mkStdGen 137
+                                          , best = Rated (r start) start
+                                          , current = Rated (r start) start
+                                          , rate = \xx -> Rated (r xx) xx
+                                          , temperature = 1.0 })
+
+select_metropolis :: (a -> [a]) -> State (Anneal a) ()
+select_metropolis x = do c <- gets current
+                         a <- select $ x $ the_value c
+                         metropolis a
+
+metropolis :: a -> State (Anneal a) ()
+metropolis x = do r <- gets rate
+                  c <- gets current
+                  t <- gets temperature
+                  let rx = r x
+                      boltz = exp $ (the_rating c - the_rating rx) / t
+                  if rx < c then do modify $ \s -> s { current = rx, best = rx }
+                            else do p <- getOne (0,1)
+                                    if p < boltz
+                                       then modify $ \s -> s { current = rx }
+                                       else return ()
+
+select :: [a] -> State (Anneal a) a
+select [] = the_value `fmap` gets best
+select [x] = return x
+select xs = do n <- getOne (0,length xs - 1)
+               return (xs !! n)
+
+getOne :: (Random a) => (a,a) -> State (Anneal x) a
+getOne bounds = do s <- get
+                   (x,g') <- return $ randomR bounds (g s)
+                   put $ s { g = g' }
+                   return x