+{-
+GHC's mergesort replaced by a better implementation, 24/12/2009.
+This code originally contributed to the nhc12 compiler by Thomas Nordin
+in 2002. Rumoured to have been based on code by Lennart Augustsson, e.g.
+ http://www.mail-archive.com/haskell@haskell.org/msg01822.html
+and possibly to bear similarities to a 1982 paper by Richard O'Keefe:
+"A smooth applicative merge sort".
+
+Benchmarks show it to be often 2x the speed of the previous implementation.
+Fixes ticket http://hackage.haskell.org/trac/ghc/ticket/2143
+-}
+
+sort = sortBy compare
+sortBy cmp = mergeAll . sequences
+ where
+ sequences (a:b:xs)
+ | a `cmp` b == GT = descending b [a] xs
+ | otherwise = ascending b (a:) xs
+ sequences xs = [xs]
+
+ descending a as (b:bs)
+ | a `cmp` b == GT = descending b (a:as) bs
+ descending a as bs = (a:as): sequences bs
+
+ ascending a as (b:bs)
+ | a `cmp` b /= GT = ascending b (\ys -> as (a:ys)) bs
+ ascending a as bs = as [a]: sequences bs
+
+ mergeAll [x] = x
+ mergeAll xs = mergeAll (mergePairs xs)
+
+ mergePairs (a:b:xs) = merge a b: mergePairs xs
+ mergePairs xs = xs
+
+ merge as@(a:as') bs@(b:bs')
+ | a `cmp` b == GT = b:merge as bs'
+ | otherwise = a:merge as' bs
+ merge [] bs = bs
+ merge as [] = as
+
+{-