Add OffsetCurve

geos/geom.go

@@ -166,6 +166,13 @@ func (g *Geometry) BufferWithOpts(width float64, opts BufferOpts) (*Geometry, er
 	return geomFromC("BufferWithOpts", cGEOSBufferWithParams_r(handle, g.g, parms, C.double(width)))
 }
 
+// OffsetCurve computes a new linestring that is offset from the input
+// linestring by the given distance and buffer options.  A negative distance is
+// offset on the right side; positive distance offset on the left side.
+func (g *Geometry) OffsetCurve(distance float64, opts BufferOpts) (*Geometry, error) {
+	return geomFromC("OffsetCurve", cGEOSOffsetCurve_r(handle, g.g, C.double(distance), C.int(opts.QuadSegs), C.int(opts.JoinStyle), C.double(opts.MitreLimit)))
+}
+
 // Geometry Constructors
 
 // NewPoint returns a new geometry of type Point, initialized with the given

geos/geom_test.go

@@ -104,6 +104,18 @@ func TestGeometryBufferWithOpts(t *testing.T) {
 	}
 }
 
+func TestOffsetCurve(t *testing.T) {
+	g := Must(FromWKT("LINESTRING (0 10, 5 0, 10 10)"))
+	opts := BufferOpts{QuadSegs: 8, JoinStyle: JoinRound, MitreLimit: 5.0}
+	curve := Must(g.OffsetCurve(1.0, opts))
+	expected := Must(FromWKT(offsetCurve))
+	if !mustEqual(curve.EqualsExact(expected, 0.000001)) {
+		t.Errorf("want %v, got %v", expected, curve)
+	}
+}
+
+const offsetCurve = `LINESTRING (0.8944271909999159 10.4472135954999583, 5.0000000000000000 2.2360679774997907, 9.1055728090000834 10.4472135954999583)`
+
 func reconstructGeom(g *Geometry) *Geometry {
 	typeId, err := g.Type()
 	if err != nil {