Generic C99 partial quicksort.

This is the sort routine largely as described by Bentley & McIlroy's "Engineering a Sort Function", along with some modifications as found in FreeBSD's libc.

"Partial" means pqsort can be used to place only specific items in their final sorted positions, making it useful for pagination, ranking and partitioning.

offset=100, limit=100 will place array[100] - array[199] in the correct position, ensure array[0] - array[99] contains only values smaller than that in array[100], and array[200] - array[nitems - 1] only values larger than array[199].

Since it's a preprocessor macro, comparison functions can be inlined. This can be quite a win if comparisons are cheap compared with function calls, e.g. when sorting integers. It can be easily adapted to handle void *'s and support a runtime-specified comparison function.'

A modified version of this routine was used by Newzbin.com's custom search engine, quickly sorting upwards of 40 million items for result sets between 50 and 20,000.

#include <stdio.h>
#include "pqsort.h"

static inline int IntCmp(const int a, const int b) {
	return (a - b);
}

define_pqsort(my_int, int, IntCmp);

#define DUMP()  printf("{ "); \
	for (int i = 0; i < nvalues; i++)  \
		printf("%d, ", values[i]); \
	printf("}\n");

int main(void) {
	int values[] = { 42, 98, 56, 23, 45, 63, 56, 80, 102, 2 };
	int nvalues = sizeof(values) / sizeof(values[0]);

	// partition the list in two
	my_int_pqsort(values, nvalues, 5, 0);
	DUMP(); // => { 23, 2, 42, 45, 56, 80, 102, 98, 63, 56, }

	// sort the first half
	my_int_pqsort(values, nvalues, 0, 5);
	DUMP(); // => { 2, 23, 42, 45, 56, 102, 98, 63, 80, 56, }

	// sort the rest
	my_int_pqsort(values + 5, nvalues - 5, 0, nvalues);
	DUMP(); // => { 2, 23, 42, 45, 56, 56, 63, 80, 98, 102, }

	return 0;
}