fix case of single cubic curve closed path including unit test.

BezierKit/BezierKitTests/Path+VectorBooleanTests.swift

@@ -477,6 +477,16 @@ class PathVectorBooleanTests: XCTestCase {
         XCTAssertTrue(componentsEqualAsideFromElementOrdering(result.components[0], square.components[0]))
     }
 
+    func testCrossingsRemovedSingleCurveLoop() {
+        let cgPath = CGMutablePath()
+        cgPath.move(to: CGPoint(x: 0, y: 0))
+        cgPath.addCurve(to: CGPoint(x: 0, y: 0),
+                        control1: CGPoint(x: -1, y: 1),
+                        control2: CGPoint(x: 1, y: 1))
+        let path = Path(cgPath: cgPath)
+        XCTAssertEqual(path.crossingsRemoved(), path)
+    }
+
     func testCrossingsRemovedEdgeCase() {
         // this is an edge cases which caused difficulty in practice
         // the contour, which intersects at (1,1) creates two squares, one with -1 winding count

BezierKit/Library/PathComponent.swift

@@ -371,7 +371,10 @@ open class PathComponent: NSObject, Reversible, Transformable, @unchecked Sendab
             if i1 == i2 {
                 // we are intersecting a path element against itself (only possible with cubic or higher order)
                 if self.order(at: i1) == 3 {
-                    elementIntersections = self.cubic(at: i1).selfIntersections
+                    elementIntersections = self.cubic(at: i1).selfIntersections.filter {
+                        guard self.numberOfElements == 1 else { return true }
+                        return $0.t1 != 0 || $0.t2 != 1 // exclude intersection of single curve path closing itself
+                    }
                 }
             } else if i1 < i2 {
                 // we are intersecting two distinct path elements