[geos-devel] QD (Quad-Double) Approach to Robustness

[Martin Davis]

> > In any case, I'm not sure that JTS uses FP equals for 
> anything other 
> > than checking whether two points are *exactly* the same.  So this 
> > should be portable, I assume.

I think you're misreading what I said.  I meant that JTS uses == (via
equals2D) to test whether the *value* of two coordinates is exactly the
same.  Pointers don't enter into it.

You absolutely should NOT try to replace those equality tests with
"approximately equals".  Approx equals may have a place in some of the
algorithms, but this needs to be carefully evaluated on a case-by-case
basis.

Martin Davis, Senior Technical Architect
Vivid Solutions Inc.      www.vividsolutions.com
Suite #1A-2328 Government Street Victoria, B.C. V8T 5G5
Phone: (250) 385 6040 - Local 308 Fax: (250) 385 6046


> -----Original Message-----
> From: geos-devel-bounces at geos.refractions.net 
> [mailto:geos-devel-bounces at geos.refractions.net] On Behalf Of 
> strk at refractions.net
> Sent: June 14, 2006 11:36 AM
> To: GEOS Development List
> Subject: Re: [geos-devel] QD (Quad-Double) Approach to Robustness
> 
> 
> On Wed, Jun 14, 2006 at 11:16:09AM -0700, Martin Davis wrote:
> > > Ok, but equality is actually used in JTS too.
> > > Does Java have an automatic approximate equality ?
> > 
> > I've been working under the assumption that Java uses a single FP 
> > model, so that it's safe to compare FP numbers.
> > 
> > In any case, I'm not sure that JTS uses FP equals for 
> anything other 
> > than checking whether two points are *exactly* the same.  So this 
> > should be portable, I assume.
> 
> As far as you mean memory address comparison that would be 
> portable (pointers are integer values). Unfortunately we 
> don't have garbage collection for free in the C++ world, so 
> in some places your identity checks might have become value 
> comparisons. 
> 
> BTW, a quick grep on JTS sources for equals2D shows this is not true:
> 
>   ./algorithm/CGAlgorithms.java:197:    } while 
> (ring[iPrev].equals2D(hiPt) && iPrev != hiIndex);
>   ./algorithm/CGAlgorithms.java:203:    } while 
> (ring[iNext].equals2D(hiPt) && iNext != hiIndex);
>   ./algorithm/CGAlgorithms.java:214:    if 
> (prev.equals2D(hiPt) || next.equals2D(hiPt) || prev.equals2D(next))
>   ./algorithm/LineIntersector.java:284:      if 
> (intPt[i].equals2D(pt)) {
>   ./algorithm/LineIntersector.java:311:      if (! (   
> intPt[i].equals2D(inputLines[inputLineIndex][0])
>   ./algorithm/LineIntersector.java:312:             || 
> intPt[i].equals2D(inputLines[inputLineIndex][1]) )) {
>   ./geom/Coordinate.java:134:  public boolean 
> equals2D(Coordinate other) {
>   ./geom/Coordinate.java:159:    return equals2D((Coordinate) other);
>   ./geom/CoordinateList.java:141:        if 
> (last.equals2D(coord)) return;
>   ./geom/LineString.java:146:    return 
> getCoordinateN(0).equals2D(getCoordinateN(getNumPoints() - 1));
>   ./geomgraph/Edge.java:193:        if (intPt.equals2D(nextPt)) {
>   ./geomgraph/Edge.java:234:      if (! pts[i].equals2D(e.pts[i])) {
>   ./geomgraph/Edge.java:237:      if (! 
> pts[i].equals2D(e.pts[--iRev])) {
>   ./geomgraph/Edge.java:253:      if (! pts[i].equals2D(e.pts[i])) {
>   ./geomgraph/EdgeIntersectionList.java:145:    boolean 
> useIntPt1 = ei1.dist > 0.0 || ! ei1.coord.equals2D(lastSegStartPt);
>   ./noding/SegmentNode.java:57:    isInterior = ! 
> coord.equals2D(segString.getCoordinate(segmentIndex));
>   ./noding/SegmentNode.java:81:    if 
> (coord.equals2D(other.coord)) return 0;
>   ./noding/SegmentNodeList.java:69:      
> Assert.isTrue(ei.coord.equals2D(intPt), "Found equal nodes 
> with different coordinates");
>   ./noding/SegmentNodeList.java:70://      if (! 
> ei.coord.equals2D(intPt))
>   ./noding/SegmentNodeList.java:126:      if (p0.equals2D(p2)) {
>   ./noding/SegmentNodeList.java:159:    if (! 
> ei0.coord.equals2D(ei1.coord)) return false;
>   ./noding/SegmentNodeList.java:212:    if (! 
> pt0.equals2D(edgePts[0]))
>   ./noding/SegmentNodeList.java:218:    if (! 
> ptn.equals2D(edgePts[edgePts.length - 1]))
>   ./noding/SegmentNodeList.java:238:    boolean useIntPt1 = 
> ei1.isInterior() || ! ei1.coord.equals2D(lastSegStartPt);
>   ./noding/SegmentString.java:157:        if 
> (intPt.equals2D(nextPt)) {
>   ./noding/SegmentPointComparator.java:30:    if 
> (p0.equals2D(p1)) return 0;
> 
> --strk;
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