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POsorting integers fast in haskell
 

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    <p>Is there any function in haskell libraries that sorts integers in O(n) time?? [By, O(n) I mean faster than comparison sort and specific for integers]</p> <p>Basically I find that the following code takes a lot of time with the sort (as compared to summing the list without sorting) :</p> <pre><code>import System.Random import Control.DeepSeq import Data.List (sort) genlist gen = id $!! sort $!! take (2^22) ((randoms gen)::[Int]) main = do gen &lt;- newStdGen putStrLn $ show $ sum $ genlist gen </code></pre> <p>Summing a list doesn't require deepseq but what I am trying for does, but the above code is good enough for the pointers I am seeking. </p> <p>Time : 6 seconds (without sort); about 35 seconds (with sort)</p> <p>Memory : about 80 MB (without sort); about 310 MB (with sort)</p> <p><strong>Note 1 :</strong> memory is a bigger issue than time for me here as for the task at hand I am getting out of memory errors (memory usage becomes 3GB! after 30 minutes of run-time)</p> <p>I am assuming faster algorithms will provide bettor memory print too, hence looking for O(n) time.</p> <p><strong>Note 2 :</strong> I am looking for fast algorithms for Int64, though fast algorithms for other specific types will also be helpful.</p> <hr> <p><strong>Solution Used :</strong> IntroSort with unboxed vectors was good enough for my task:</p> <pre><code>import qualified Data.Vector.Unboxed as V import qualified Data.Vector.Algorithms.Intro as I sort :: [Int] -&gt; [Int] sort = V.toList . V.modify I.sort . V.fromList </code></pre>
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    1. CO`O(n)` sorting? I guess you could try implementing [spaghetti sort](https://en.wikipedia.org/wiki/Spaghetti_sort).
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      1. UShuon
    2. COA comparison sort cannot have lesser complexity than `O(n * log n)`. Since the range is finite, you could use a bucket sort (but that wouldn't reduce the memory usage here ;). Have you tried building a `Data.IntSet` and `toList` on that?
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      1. USDaniel Fischer
    3. COusing Data.IntSet it takes about 24 seconds, so it does seem faster but the memory footprint is 320 MB !! [ `genlist gen = id $!! toList $!! ((fromList $!! take (2^22) ((randoms gen)::[Int])) :: IntSet)`]
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