mescc: Mes C Library: Fix qsort to support duplicate entries.

* lib/stdlib/qsort.c (qpart): Handle dupes.
* scaffold/tests/81-qsort-dupes.c: New file.
* build-aux/check-mescc.sh (tests): Run it.

build-aux/check-mescc.sh

@@ -125,6 +125,7 @@ t
 7s-struct-short
 80-setjmp
 81-qsort
+81-qsort-dupes
 82-define
 83-heterogenoous-init
 84-struct-field-list

lib/stdlib/qsort.c

@@ -37,11 +37,14 @@ qpart (void *base, size_t count, size_t size, int (*compare)(void const *, void
   size_t i = 0;
   for (size_t j = 0; j < count; j++)
     {
-      if (compare (base+j*size, p) < 0)
+      int c = compare (base+j*size, p);
+      if (c < 0)
         {
           qswap (base+i*size, base+j*size, size);
           i++;
         }
+      else if (c == 0)
+        i++;
     }
   if (compare (base+count*size, base+i*size) < 0)
     qswap (base+i*size, base+count*size, size);

scaffold/tests/81-qsort-dupes.c

@@ -0,0 +1,42 @@
+/* -*-comment-start: "//";comment-end:""-*-
+ * GNU Mes --- Maxwell Equations of Software
+ * Copyright © 2018 Jan (janneke) Nieuwenhuizen <janneke@gnu.org>
+ *
+ * This file is part of GNU Mes.
+ *
+ * GNU Mes is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or (at
+ * your option) any later version.
+ *
+ * GNU Mes is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with GNU Mes.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <libmes.h>
+#include <string.h>
+
+int
+qsort_strcmp (void const* a, void const* b)
+{
+  return strcmp (*((char**) a), *((char**) b));
+}
+
+int
+main ()
+{
+  char* list[3] = {"foo", "foo", 0 };
+  oputs ("\nls:\n");
+  qsort (list, 2, sizeof (char*), qsort_strcmp);
+  for (int i = 0; i < 2; i++)
+    {
+      oputs (list[i]); oputs ("\n");
+    }
+
+  return 0;
+}