compat: add qsort_s()

The function qsort_s() was introduced with C11 Annex K; it provides the
ability to pass a context pointer to the comparison function, supports
the convention of using a NULL pointer for an empty array and performs a
few safety checks.

Add an implementation based on compat/qsort.c for platforms that lack a
native standards-compliant qsort_s() (i.e. basically everyone).  It
doesn't perform the full range of possible checks: It uses size_t
instead of rsize_t and doesn't check nmemb and size against RSIZE_MAX
because we probably don't have the restricted size type defined.  For
the same reason it returns int instead of errno_t.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 69 insertions, 0 deletions

diff --git a/compat/qsort_s.c b/compat/qsort_s.c
new file mode 100644
index 0000000000..52d1f0a73d
--- /dev/null
+++ b/compat/qsort_s.c
@@ -0,0 +1,69 @@
+#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.
+ * Added context pointer, safety checks and return value.
+ */
+
+static void msort_with_tmp(void *b, size_t n, size_t s,
+			   int (*cmp)(const void *, const void *, void *),
+			   char *t, void *ctx)
+{
+	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, ctx);
+	msort_with_tmp(b2, n2, s, cmp, t, ctx);
+
+	tmp = t;
+
+	while (n1 > 0 && n2 > 0) {
+		if (cmp(b1, b2, ctx) <= 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);
+}
+
+int git_qsort_s(void *b, size_t n, size_t s,
+		int (*cmp)(const void *, const void *, void *), void *ctx)
+{
+	const size_t size = st_mult(n, s);
+	char buf[1024];
+
+	if (!n)
+		return 0;
+	if (!b || !cmp)
+		return -1;
+
+	if (size < sizeof(buf)) {
+		/* The temporary array fits on the small on-stack buffer. */
+		msort_with_tmp(b, n, s, cmp, buf, ctx);
+	} else {
+		/* It's somewhat large, so malloc it.  */
+		char *tmp = xmalloc(size);
+		msort_with_tmp(b, n, s, cmp, tmp, ctx);
+		free(tmp);
+	}
+	return 0;
+}