From: Ben Pfaff <blp@cs.stanford.edu>
Date: Wed, 20 Sep 2017 15:36:18 +0000 (-0700)
Subject: SORT VARIABLES: Improve stability of sort.
X-Git-Url: https://pintos-os.org/cgi-bin/gitweb.cgi?p=pspp;a=commitdiff_plain;h=85e8724e69cf54946c7b40628e88f2ec12368d7c

SORT VARIABLES: Improve stability of sort.

The sort was being made "stable" by comparing variables' addresses in
memory.  There's nothing particularly predictable about that, especially
from one malloc implementation to another.  This commit fixes the stability
by comparing the variables' dictionary indexes instead.

Bug #52072.
---

diff --git a/src/language/dictionary/sort-variables.c b/src/language/dictionary/sort-variables.c
index 8c653af724..aee341bda6 100644
--- a/src/language/dictionary/sort-variables.c
+++ b/src/language/dictionary/sort-variables.c
@@ -190,7 +190,11 @@ compare_vars (const void *a_, const void *b_, const void *c_)
 
   /* Make this a stable sort. */
   if (!retval)
-    retval = a < b ? -1 : a > b;
+    {
+      size_t a_index = var_get_dict_index (a);
+      size_t b_index = var_get_dict_index (b);
+      retval = a_index < b_index ? -1 : a_index > b_index;
+    }
 
   if (c->descending)
     retval = -retval;