Look into clipper2-java for polygon union/buffer/unbuffer ops

[msbarry]

@bdon pointed out this project https://github.com/micycle1/Clipper2-java which seems like it might have faster implementations of expensive JTS polygon union and buffer/unbuffer operations (including ones that work in integer coordinates). Together these JTS operations are responsible for about 45% of overall CPU time (20% for snap/fix, and 25% for buffer/unbuffer)

[msbarry]

I did a quick test trying to use it for snapAndFixPolygon and bufferUnionUnbuffer in FeatureMerge and it looks like it does what we want most of the time, but it doesn't appear to fix single-point self-intersections, has inconsistent order for output winding, and visible artifacts when rendered with maplibre gl:

I might not have been using the library correctly. Here's what I tried for snapAndFixPolygon https://github.com/onthegomap/planetiler/commit/20818934ce5877e50ac4d310083df45b1baaa82e and for bufferUnionUnbuffer:

   private static Geometry bufferUnionUnbuffer(double buffer, List<Geometry> polygonGroup) {
+    double amount = buffer * 4096d / 256d;
+    var merged = new Paths64();
     for (int i = 0; i < polygonGroup.size(); i++) {
-      polygonGroup.set(i, buffer(buffer, polygonGroup.get(i)));
+      var clipper = GeoUtils.toClipper2(polygonGroup.get(i));
+      merged.addAll(clipper);
     }
-    Geometry merged = GeoUtils.createGeometryCollection(polygonGroup);
-    merged = union(merged);
-    merged = unbuffer(buffer, merged);
-    return merged;
+    merged = Clipper.InflatePaths(merged, amount, JoinType.Miter, EndType.Polygon);
+    merged = Clipper.InflatePaths(merged, -amount, JoinType.Miter, EndType.Polygon);
+    return GeoUtils.fromClipper2(merged);
    }

Let me know if anyone has any suggestions!

[bdon]

It's worth checking if https://github.com/lightbringer/clipper-java fails in the same way; supposedly Clipper2 is strictly better than Clipper1, but possible failures are:

• differences (bugs?) in Clipper2 java port vs reference implementations in C++, Delphi and C#
• bug or unhandled case in Clipper2 algorithm itself
• Fundamental difference between Clipper2 validity model and OGC Simple + Valid