fix duplicate points returned in polygon intersection (libgdx#7519)

* fix duplicate points returned in polygon intersection

* apply spotless

* improve comments

gdx/src/com/badlogic/gdx/math/Intersector.java

@@ -197,6 +197,9 @@ public static boolean intersectPolygons (Polygon p1, Polygon p2, Polygon overlap
 			floatArray2.addAll(floatArray);
 			floatArray.clear();
 		}
+		// Ensure first and last point are different
+		if (floatArray2.size >= 6 && floatArray2.get(0) == floatArray2.get(floatArray2.size - 2)
+			&& floatArray2.get(1) == floatArray2.get(floatArray2.size - 1)) floatArray2.setSize(floatArray2.size - 2);
 		// Check for 3 or more vertices needed due to floating point precision errors
 		if (floatArray2.size >= 6) {
 			if (overlap != null) {
@@ -210,14 +213,14 @@ public static boolean intersectPolygons (Polygon p1, Polygon p2, Polygon overlap
 		return false;
 	}
 
-	/** Returns true if the specified poygons intersect. */
+	/** Returns true if the specified polygons intersect. */
 	static public boolean intersectPolygons (FloatArray polygon1, FloatArray polygon2) {
 		if (Intersector.isPointInPolygon(polygon1.items, 0, polygon1.size, polygon2.items[0], polygon2.items[1])) return true;
 		if (Intersector.isPointInPolygon(polygon2.items, 0, polygon2.size, polygon1.items[0], polygon1.items[1])) return true;
 		return intersectPolygonEdges(polygon1, polygon2);
 	}
 
-	/** Returns true if the lines of the specified poygons intersect. */
+	/** Returns true if the lines of the specified polygons intersect. */
 	static public boolean intersectPolygonEdges (FloatArray polygon1, FloatArray polygon2) {
 		int last1 = polygon1.size - 2, last2 = polygon2.size - 2;
 		float[] p1 = polygon1.items, p2 = polygon2.items;

gdx/test/com/badlogic/gdx/math/IntersectorTest.java

@@ -231,4 +231,20 @@ public void testIntersectPolygons () {
 				new Polygon(new float[] {3213.0f, 131.0f, 3214.0f, 131.0f, 3214.0f, 130.0f, 3213.0f, 130.0f}), intersectionPolygon));
 		assertEquals(0, intersectionPolygon.getVertexCount());
 	}
+
+	@Test
+	public void testIntersectPolygonsWithVertexLyingOnEdge () {
+		Polygon p1 = new Polygon(new float[] {1, -1, 2, -1, 2, -2, 1, -2});
+		Polygon p2 = new Polygon(new float[] {0.5f, -1.5f, 1.5f, -1.5f, 1.5f, -2.5f});
+
+		Polygon intersectionPolygon = new Polygon();
+		boolean checkResult = Intersector.intersectPolygons(p1, p2, intersectionPolygon);
+
+		assertTrue(checkResult);
+		assertEquals(4, intersectionPolygon.getVertexCount());
+		assertEquals(new Vector2(1.0f, -2.0f), intersectionPolygon.getVertex(0, new Vector2()));
+		assertEquals(new Vector2(1.0f, -1.5f), intersectionPolygon.getVertex(1, new Vector2()));
+		assertEquals(new Vector2(1.5f, -1.5f), intersectionPolygon.getVertex(2, new Vector2()));
+		assertEquals(new Vector2(1.5f, -2.0f), intersectionPolygon.getVertex(3, new Vector2()));
+	}
 }