Add comment that ll_sort is stable,

which is verified by the existing set of tests.

diff --git a/src/libpspp/ll.c b/src/libpspp/ll.c
index 25b4462b9bdd34218ad4bf5583b38ea4b1042eac..525dcc5b2434e623358b5f41f4318fc4a8190255 100644 (file)
--- a/src/libpspp/ll.c
+++ b/src/libpspp/ll.c
@@ -429,6 +429,8 @@ ll_prev_permutation (struct ll *r0, struct ll *r1,
    In use, keep in mind that R0 may move during the sort, so that
    afterward R0...R1 may denote a different range.
    (On the other hand, R1 is fixed in place.)
+   The sort is stable; that is, it will not change the relative
+   order of nodes that compare equal.
    Runs in O(n lg n) time in the number of nodes in the range. */
 void
 ll_sort (struct ll *r0, struct ll *r1, ll_compare_func *compare, void *aux)