util: introduce merge sort routine

In some cases it's important to preserve order of elements with equal
keys (stable sort). qsort(3) doesn't define order of elements with
equal keys.

git__msort() implements merge sort which is stable sort.

Implementation taken from git. Function renamed git_qsort() -> git__msort().

Signed-off-by: Kirill A. Shutemov <kirill@shutemov.name>

src/util.c

@@ -332,3 +332,67 @@ uint32_t git__hash(const void *key, int len, uint32_t seed)
 	return h1;
 }
 #endif
+
+/*
+ * 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);
+}
+
+int git__msort(void *b, size_t n, size_t s,
+		int (*cmp)(const void *, const void *))
+{
+	const size_t size = 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 = git__malloc(size);
+		if (tmp == NULL)
+			return GIT_ENOMEM;
+		msort_with_tmp(b, n, s, cmp, tmp);
+		free(tmp);
+	}
+
+	return GIT_SUCCESS;
+}

src/util.h

@@ -118,4 +118,7 @@ extern int git__fnmatch(const char *pattern, const char *name, int flags);
 		} \
 	} while (0)
 
+extern int git__msort(void *b, size_t n, size_t s,
+		int (*cmp)(const void *, const void *));
+
 #endif /* INCLUDE_util_h__ */