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
fe21c6b285d (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>

2 files changed, 3 insertions, 70 deletions

diff --git a/compat/mingw.c b/compat/mingw.c
index 738f0a826a..50af33b2b3 100644
--- a/compat/mingw.c
+++ b/compat/mingw.c
@@ -1229,11 +1229,6 @@ static int wenvcmp(const void *a, const void *b)
 	return _wcsnicmp(p, q, p_len);
 }
 
-/* We need a stable sort to convert the environment between UTF-16 <-> UTF-8 */
-#ifndef INTERNAL_QSORT
-#include "qsort.c"
-#endif
-
 /*
  * Build an environment block combining the inherited environment
  * merged with the given list of settings.
@@ -1272,8 +1267,8 @@ static wchar_t *make_environment_block(char **deltaenv)
 
 	/*
 	 * If there is a deltaenv, let's accumulate all keys into `array`,
-	 * sort them using the stable git_qsort() and then copy, skipping
-	 * duplicate keys
+	 * sort them using the stable git_stable_qsort() and then copy,
+	 * skipping duplicate keys
 	 */
 	for (p = wenv; p && *p; ) {
 		ALLOC_GROW(array, nr + 1, alloc);
@@ -1296,7 +1291,7 @@ static wchar_t *make_environment_block(char **deltaenv)
 		p += wlen + 1;
 	}
 
-	git_qsort(array, nr, sizeof(*array), wenvcmp);
+	git_stable_qsort(array, nr, sizeof(*array), wenvcmp);
 	ALLOC_ARRAY(result, size + delta_size);
 
 	for (p = result, i = 0; i < nr; i++) {
diff --git a/compat/qsort.c b/compat/qsort.c
deleted file mode 100644
index 7d071afb70..0000000000
--- a/compat/qsort.c
+++ /dev/null
@@ -1,62 +0,0 @@
-#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_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);
-	}
-}