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mergesort.hs

 1 {- The authors of this work have released all rights to it and placed it
 2 in the public domain under the Creative Commons CC0 1.0 waiver
 3 (http://creativecommons.org/publicdomain/zero/1.0/).
 4 
 5 THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
 6 EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
 7 MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
 8 IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
 9 CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
10 TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
11 SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
12 
13 Retrieved from: http://en.literateprograms.org/Merge_sort_(Haskell)?oldid=19611
14 -}
15 
16 split :: [a] -> ([a],[a])
17 split xs = go xs xs where
18   go (x:xs) (_:_:zs) = (x:us,vs) where (us,vs)=go xs zs
19   go    xs   _       = ([],xs)
20 merge :: (a -> a -> Bool) -> [a] -> [a] -> [a]
21 merge pred xs []         = xs
22 merge pred [] ys         = ys
23 merge pred (x:xs) (y:ys)
24     | pred x y = x: merge pred xs (y:ys)
25     | otherwise = y: merge pred (x:xs) ys
26 
27 mergesort :: (a -> a -> Bool) -> [a] -> [a]
28 mergesort pred []   = []
29 mergesort pred [x]  = [x]
30 mergesort pred xs = merge pred (mergesort pred xs1) (mergesort pred xs2)
31   where
32     (xs1,xs2) = split xs
33 


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