Followup to: <[EMAIL PROTECTED]>
By author: Jesper Juhl <[EMAIL PROTECTED]>
In newsgroup: linux.dev.kernel
>
> On Sun, 23 Jan 2005, Andi Kleen wrote:
>
> > > How about a shell sort? if the data is mostly sorted shell sort beats
> > > qsort lots of times, and since the data sets are often small in-kernel,
> > > shell sorts O(n^2) behaviour won't harm it too much, shell sort is also
> > > faster if the data is already completely sorted. Shell sort is certainly
> > > not the simplest algorithm around, but I think (without having done any
> > > tests) that it would probably do pretty well for in-kernel use... Then
> > > again, I've known to be wrong :)
> >
> > I like shell sort for small data sets too. And I agree it would be
> > appropiate for the kernel.
> >
> 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), and another nice property is that it's iterative so it
> doesn't eat up stack space (as oposed to qsort which is recursive and eats
> stack like ****)...
> Yeah, I think shell sort would be good for the kernel.
>
In klibc, I use combsort:
/*
* qsort.c
*
* This is actually combsort. It's an O(n log n) algorithm with
* simplicity/small code size being its main virtue.
*/
#include <stddef.h>
#include <string.h>
static inline size_t newgap(size_t gap)
{
gap = (gap*10)/13;
if ( gap == 9 || gap == 10 )
gap = 11;
if ( gap < 1 )
gap = 1;
return gap;
}
void qsort(void *base, size_t nmemb, size_t size,
int (*compar)(const void *, const void *))
{
size_t gap = nmemb;
size_t i, j;
char *p1, *p2;
int swapped;
do {
gap = newgap(gap);
swapped = 0;
for ( i = 0, p1 = base ; i < nmemb-gap ; i++, p1 += size ) {
j = i+gap;
if ( compar(p1, p2 = (char *)base+j*size) > 0 ) {
memswap(p1, p2, size);
swapped = 1;
}
}
} while ( gap > 1 || swapped );
}
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