Error in difference between two polygons

[DGuidi]

copied from GoogleCode Issue 193
What steps will reproduce the problem?

var precisionModel = new GeometryFactory(new PrecisionModel(100));
var reader = new WKTReader(precisionModel);
var p1 = reader.Read(@"POLYGON ((504927.9 6228865.64, 504969.88 6228833.89, 504980.82 6228861.76, 504927.9 6228865.64))");
var p2 = reader.Read(@"POLYGON ((504927.9 6228865.64, 504951.14 6228848.06, 504957.42 6228863.47, 504927.9 6228865.64))");
var test = p1.Difference(p2);

What is the expected output? What do you see instead?
The two polygons share a vertex (504927.9 6228865.64) at the left tip of the triangle, and when creating the difference this point does not get removed, leaving a strange "tail" on the resulting polygon. Expected to get only the blue(ish) colored polygon as a result
See the image atached.

What version of the product are you using? On what operating system?
It seems to be the same problem with verison 13.1 and 13.2
Its tested on Windows 7 x64 and .NET framework 4.5.1

[FObermaier]

What is JTS' result? I suspect it is a precision problem.

[DGuidi]

@FObermaier same suspect. I wanna check jts results first.

[DGuidi]

checked: jts'trunk returns same results

[svalentinh]

We also checked JTS with the same result. After some investigation it turns out its due to our implementation. We did calculate an intersection and then in a second step tried to remove the intersection from the geometry.
The problem is that one point is exactly on the line between two vertices and then the pricision model when rounding the coordinates moves the line between the two vertices slightly, and then if the intersection point is inside the geometry it creates a tail.

So strictly speeking the algorithm is correct and you just have to be carefull when using calculated geometries and instead of calculating an overlap and then removing that overlap, then just remove it by using the difference between the two original geometries