Re: [patch 1/13] Qsort

[James Lamanna <jlamanna@xxxxxxxxx>]

> On Sunday, January 23, 2005, Rafael J. Wysocki wrote:
> > On Sunday, 23 of January 2005 06:05, Jesper Juhl wrote:
> > > On Sun, 23 Jan 2005, Andi Kleen wrote:
> > Even with large data sets that are mostly unsorted shell sorts performance 
> > is close to qsort, and there's an optimization that gives it O(n^(3/2)) 
> > runtime (IIRC),
>
> Yes, there is.

After doing a small amount of research into this, according to the abstract at
http://www.cs.princeton.edu/~rs/shell/paperF.pdf you can get O(n^(4/3)) 
with different increment sequences. (1, 8, 23, 77, 281 ...)

So I guess the sort function could look something like this for XFS use
(for reference only!):

void shellsort(void *array, size_t total_elems, size_t size, 
    int (*cmp)(const void *, const void *))
{
    size_t i, j;
    int k, h;
    register char *a = array;  
    const int incs[3] = {23, 8, 1};
    
    for (k = 0; k < 3; k++) {
        for (h = incs[k], i = h; i < total_elems; i++) {
            j = i;
            while (j >= h && cmp(a + (j-h) * size, a + j * size) > 0) {
                swap(a + j * size, a + (j-h) * size);
                j -= h;
            }
        }
    }
}

-- James Lamanna
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