git/stable-qsort.c
Johannes Schindelin 97fff61012 Move git_sort(), a stable sort, into into libgit.a
The `qsort()` function is not guaranteed to be stable, i.e. it does not
promise to maintain the order of items it is told to consider equal. In
contrast, the `git_sort()` function we carry in `compat/qsort.c` _is_
stable, by virtue of implementing a merge sort algorithm.

In preparation for using a stable sort in Git's rename detection, move
the stable sort into `libgit.a` so that it is compiled in
unconditionally, and rename it to `git_stable_qsort()`.

Note: this also makes the hack obsolete that was introduced in
fe21c6b285 (mingw: reencode environment variables on the fly (UTF-16
<-> UTF-8), 2018-10-30), where we included `compat/qsort.c` directly in
`compat/mingw.c` to use the stable sort.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2019-10-02 14:44:51 +09:00

63 lines
1.2 KiB
C

#include "git-compat-util.h"
/*
* A merge sort implementation, simplified from the qsort implementation
* by Mike Haertel, which is a part of the GNU C Library.
*/
static void msort_with_tmp(void *b, size_t n, size_t s,
int (*cmp)(const void *, const void *),
char *t)
{
char *tmp;
char *b1, *b2;
size_t n1, n2;
if (n <= 1)
return;
n1 = n / 2;
n2 = n - n1;
b1 = b;
b2 = (char *)b + (n1 * s);
msort_with_tmp(b1, n1, s, cmp, t);
msort_with_tmp(b2, n2, s, cmp, t);
tmp = t;
while (n1 > 0 && n2 > 0) {
if (cmp(b1, b2) <= 0) {
memcpy(tmp, b1, s);
tmp += s;
b1 += s;
--n1;
} else {
memcpy(tmp, b2, s);
tmp += s;
b2 += s;
--n2;
}
}
if (n1 > 0)
memcpy(tmp, b1, n1 * s);
memcpy(b, t, (n - n2) * s);
}
void git_stable_qsort(void *b, size_t n, size_t s,
int (*cmp)(const void *, const void *))
{
const size_t size = st_mult(n, s);
char buf[1024];
if (size < sizeof(buf)) {
/* The temporary array fits on the small on-stack buffer. */
msort_with_tmp(b, n, s, cmp, buf);
} else {
/* It's somewhat large, so malloc it. */
char *tmp = xmalloc(size);
msort_with_tmp(b, n, s, cmp, tmp);
free(tmp);
}
}