Fastest way to sort byte array using unsafe code?

Question

According to my knowledge quicksort is one of the fastest sorting algorithms, since thats how the Array.Sort() function is implemented in the framework. Is there a way to speed the sorting of a byte array up, probably using unsafe code and pointers?

Using lower level primitives does not improve the complexity of the algorithm. — zerkms
– zerkms, Commented Aug 28, 2016 at 22:46

AlexD · Accepted Answer · 2016-08-28 23:01:58Z

2

For byte array you may consider Counting sort, which sort in linear time.

public static void Sort(byte[] a)
{
    int[] counts = new int[256];
    for (int i = 0; i < a.Length; i++)
        counts[a[i]]++;
    int k = 0;
    for (int i = 0; i < counts.Length; i++)
        for (int j = 0; j < counts[i]; j++)
            a[k++] = (byte)i;
}

edited Aug 28, 2016 at 23:01

answered Aug 28, 2016 at 22:48

AlexD

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4 Comments

yq8 Over a year ago

Your code seems to have a boundary issue. I get this exception when trying your implementation: i.imgur.com/vzofi5W.png

yq8 Over a year ago

This one works, thanks. Can you explain why the creation of a 256-element big int[] is needed? And are there any other tweaks possible, - probably using pointers? :)

zerkms Over a year ago

"probably using pointers" --- using unsafe code does not automagically make anything better. You're trying to optimise something that you cannot measure reliably, it makes very little sense from the performance optimisation point of view.

AlexD Over a year ago

The counting sort utilizes the fact that there are only limited number of possible values in the array (256 in our case). During the first pass, we just count how many zeros, how many ones, how many twos etc. we have, During the second pass we place corresponding numbers or zeroes, ones, twos, etc.

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