[postgis-devel] closest_segment_2d corner case (32bit failure)

[Sandro Santilli]

Reguarding https://trac.osgeo.org/postgis/ticket/5004

We have an liblwgeom function returning the "closest segment"
to a given point. One of our test uses a query point which
is on the continuation of the first segment of an L-shaped
geometry:

            ^
            |
            |
            |
            |
   ---------'  . Query point

The test is expecting the first segment to be returned (seg 0)
as the distance from both segments should be equal, closest point
being the lower-right vertex of that LineString.

But on a 32bit system, and under certain conditions, the second
segment is returned, although the distance from it is the same
up to the 20th significant digits, so there must be a very very
subtle difference in distance calculation based on the
segment-to-point distance function (distance2d_sqr_pt_seg)

Do you think we should make an effort to avoid divisions
when points have the same Y or X (as in this case) to gain
more numerical stability or should we just accept that
the code gives unstable results there and thus change
the unit test and ensure no promise is given on the matter ?

BTW, I see there are no unit tests for distance2d_sqr_pt_seg()
function, which was last touched by Raúl in commit
86057e2e46a272838a54eff9e6ebb5e56f33fab7

--strk;

  Libre GIS consultant/developer
  https://strk.kbt.io/services.html