Thread: pgsql: Tweak "line" test to avoid negative zeros on some platforms

Tweak "line" test to avoid negative zeros on some platforms

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/a0a546f0d94ec6cbb3cd6b1c82f58d801046615f

Modified Files
--------------
src/test/regress/expected/line.out |   18 +++++++++---------
src/test/regress/sql/line.sql      |    2 +-
2 files changed, 10 insertions(+), 10 deletions(-)

Peter Eisentraut <peter_e@gmx.net> writes:
> Tweak "line" test to avoid negative zeros on some platforms

I had been playing around with that last night on a PPC-era Mac,
hoping to get rid of the problem by twiddling the expressions in
line_construct_pts().  The only thing I could find to fix it
was the brute-force approach:

diff --git a/src/backend/utils/adt/geo_ops.c b/src/backend/utils/adt/geo_ops.c
index 25f0bfd..41178a6 100644
--- a/src/backend/utils/adt/geo_ops.c
+++ b/src/backend/utils/adt/geo_ops.c
@@ -1116,6 +1116,9 @@ line_construct_pts(LINE *line, Point *pt1, Point *pt2)
         line->A = (pt2->y - pt1->y) / (pt2->x - pt1->x);
         line->B = -1.0;
         line->C = pt1->y - line->A * pt1->x;
+        /* on some platforms, the preceding expression tends to produce -0 */
+        if (line->C == 0.0)
+            line->C = 0.0;
 #ifdef GEODEBUG
         printf("line_construct_pts- line is neither vertical nor horizontal (diffs x=%.*g, y=%.*g\n",
                DBL_DIG, (pt2->x - pt1->x), DBL_DIG, (pt2->y - pt1->y));


While this is surely pretty ugly, I think it is less ugly and more robust
than what you've done here.  Comments?

            regards, tom lane

On 10/25/13, 10:55 AM, Tom Lane wrote:
> diff --git a/src/backend/utils/adt/geo_ops.c b/src/backend/utils/adt/geo_ops.c
> index 25f0bfd..41178a6 100644
> --- a/src/backend/utils/adt/geo_ops.c
> +++ b/src/backend/utils/adt/geo_ops.c
> @@ -1116,6 +1116,9 @@ line_construct_pts(LINE *line, Point *pt1, Point *pt2)
>          line->A = (pt2->y - pt1->y) / (pt2->x - pt1->x);
>          line->B = -1.0;
>          line->C = pt1->y - line->A * pt1->x;
> +        /* on some platforms, the preceding expression tends to produce -0 */
> +        if (line->C == 0.0)
> +            line->C = 0.0;
>  #ifdef GEODEBUG
>          printf("line_construct_pts- line is neither vertical nor horizontal (diffs x=%.*g, y=%.*g\n",
>                 DBL_DIG, (pt2->x - pt1->x), DBL_DIG, (pt2->y - pt1->y));
>
>
> While this is surely pretty ugly, I think it is less ugly and more robust
> than what you've done here.  Comments?

Hehe, if that work's, then yes, it's better.