Problems arise when using LinearRing convenience initializer

[biomiker]

First off, thank you, this library is amazing! Here's the issue I found.

The convenience initializer LinearRing(points: [Coordinate]) gives you back a LinearRing object but not quite a valid one. This is evident in its WKT which describes itself as a LINESTRING. It also causes errors in methods that expect a valid LinearRing, such as the convenience initializer
Polygon(shell: LinearRing, holes: Array<LinearRing>?).

Some sample code and output follows to help illustrate the problem. If I take the LinearRing's WKT, replace LINESTRING with LINEARRING, and pass it to Geometry.create(), I end up with a valid LinearRing.

    func showLinearRingError() {
        
        let origin = MKMapPointForCoordinate(CLLocationCoordinate2D(latitude: 37.9505544833345, longitude: -100.990952407583))

        var mapPoints = [MKMapPoint]()

        mapPoints.append(origin)
        mapPoints.append(MKMapPoint(x:origin.x+10000,y:origin.y+20000))
        mapPoints.append(MKMapPoint(x:origin.x,y:origin.y+20000))
        mapPoints.append(origin)

        let coords = mapPoints.map({ (mp: MKMapPoint) -> Coordinate in
            return CoordinateFromCLLocationCoordinate2D(MKCoordinateForMapPoint(mp))
            })
        
        print(String(describing: coords))
        
        guard let lr1 = LinearRing(points: coords) else {
            print("LR1 failed")
            return
        }
        
        print("-------- LR1 ---------")
        print(String(describing: lr1))

        guard let wkt1 = lr1.WKT else {
            print ("Couldn't get WKT for LR1");
            return
        }
        print(wkt1)
        print("----------------------")
        
        let wkt_new = wkt1.replacingOccurrences(of: "LINESTRING", with: "LINEARRING")
        
        print("NEW WKT = " + wkt_new)
        
        guard let lr2 = Geometry.create(wkt_new) as? LinearRing else {
            print("LR2 failed")
            return
        }
        print("-------- LR2 ---------")
        print(String(describing: lr2))
        
        guard let wkt2 = lr2.WKT else {
            print ("Couldn't get WKT for LR2");
            return
        }
        print(wkt2)
        print("----------------------")
        
        if let polygon1 = Polygon(shell: lr1, holes: nil) {
            print("Created polygon using shell lr1")
            print(String(describing: polygon1))
        }
        else {
            print("Cannot create polygon using shell lr1")
        }

        if let polygon2 = Polygon(shell: lr2, holes: nil) {
            print("Created polygon using shell lr2")
            print(String(describing: polygon2))
        }
        else {
            print("Cannot create polygon using shell lr2")
        }
    }

Output:

[GEOSwift.Coordinate(x: -100.99095240758299, y: 37.950554483334486), GEOSwift.Coordinate(x: -100.97754136250853, y: 37.92940110061317), GEOSwift.Coordinate(x: -100.99095240758299, y: 37.92940110061317), GEOSwift.Coordinate(x: -100.99095240758299, y: 37.950554483334486)]
-------- LR1 ---------
<GEOSwift.LinearRing: 0x608000484420>
LINESTRING (-100.990952407583 37.95055448333449, -100.9775413625085 37.92940110061317, -100.990952407583 37.92940110061317, -100.990952407583 37.95055448333449)
----------------------
NEW WKT = LINEARRING (-100.990952407583 37.95055448333449, -100.9775413625085 37.92940110061317, -100.990952407583 37.92940110061317, -100.990952407583 37.95055448333449)
-------- LR2 ---------
<GEOSwift.LinearRing: 0x600000284f60>
LINEARRING (-100.990952407583 37.95055448333449, -100.9775413625085 37.92940110061317, -100.990952407583 37.92940110061317, -100.990952407583 37.95055448333449)
----------------------
GEOSwift # %s.
Cannot create polygon using shell lr1
Created polygon using shell lr2
<GEOSwift.Polygon: 0x600000285370>

[vfn]

@biomiker have you had a chance to look at this issue? Do you think you could fix it and create a PR?

[johncrenshaw]

I just ran into this and came here looking to see if it was a known issue. The problem is that LinearRing creates the underlying geometry using GEOSGeom_createLineString_r(), but GEOSGeom_createPolygon_r() requires that the shell and holes be created using GEOSGeom_createLinearRing_r().

I can confirm that adding a convenience initializer to LinearRing and and using GEOSGeom_createLinearRing_r() in that initializer resolves the problem.

This is what LinearRing looks like with the fix:

open class LinearRing : LineString {
    
    public convenience init?(points: [Coordinate]) {
        let seq = GEOSCoordSeq_create_r(GEOS_HANDLE, UInt32(points.count), 2)
        for (i,coord) in points.enumerated() {
            GEOSCoordSeq_setX_r(GEOS_HANDLE, seq, UInt32(i), coord.x)
            GEOSCoordSeq_setY_r(GEOS_HANDLE, seq, UInt32(i), coord.y)
        }
        guard let GEOSGeom = GEOSGeom_createLinearRing_r(GEOS_HANDLE, seq) else {
            return nil
        }
        self.init(GEOSGeom: GEOSGeom, destroyOnDeinit: true)
    }
    
}

[vfn]

@johncrenshaw could you create a pull request with this change and tests? Thanks!

[vfn]

@johncrenshaw Thank you for creating the pull request.

I have made a minor change and created a PR on your repo. Please have a look at it, and if you're happy with it, update your PR and we can merge it. Let me know your thoughts

johncrenshaw#1

[vfn]

Fixed by #50