Shamos-Hoey simple polygon detection bug fix

Author: bosborn

src/main/java/mil/nga/sf/util/sweep/ShamosHoey.java

@@ -1,7 +1,11 @@
 package mil.nga.sf.util.sweep;
 
 import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.HashSet;
 import java.util.List;
+import java.util.Map;
+import java.util.Set;
 
 import mil.nga.sf.LineString;
 import mil.nga.sf.Point;
@@ -115,22 +119,31 @@ public static boolean simplePolygon(List<LineString> rings) {
 				}
 			}
 
-			// Remove duplicate consecutive points
-			for (int j = 0; j < ringCopyPoints.size() - 1; j++) {
-				Point point = ringCopyPoints.get(j);
-				Point next = ringCopyPoints.get(j + 1);
-				if (point.equalsXY(next)) {
-					ringCopyPoints.remove(j + 1);
-					j--;
-				}
-			}
-
 			// Verify enough ring points
 			if (ringCopyPoints.size() < 3) {
 				simple = false;
 				break;
 			}
 
+			// Check for duplicate points (thus connecting more than two edges
+			// and not simple)
+			Map<Double, Set<Double>> pointValues = new HashMap<>();
+			for (Point point : ringCopyPoints) {
+				double x = point.getX();
+				double y = point.getY();
+				Set<Double> xValues = pointValues.get(x);
+				if (xValues == null) {
+					xValues = new HashSet<>();
+					xValues.add(y);
+					pointValues.put(x, xValues);
+				} else if (!xValues.contains(y)) {
+					xValues.add(y);
+				} else {
+					simple = false;
+					break;
+				}
+			}
+
 			// Check holes to make sure the first point is in the polygon
 			if (i > 0) {
 				Point firstPoint = ringCopyPoints.get(0);

src/test/java/mil/nga/sf/util/sweep/ShamosHoeyTest.java

@@ -179,7 +179,22 @@ public void testNonSimple() throws IOException {
 		TestCase.assertFalse(
 				ShamosHoey.simplePolygon(new Polygon(new LineString(points))));
 		TestCase.assertFalse((new LineString(points)).isSimple());
-		TestCase.assertFalse((new Polygon(new LineString(points))).isSimple());
+		complex(new Polygon(new LineString(points)));
+		TestCase.assertEquals(4, points.size());
+
+		points.clear();
+
+		addPoint(points, 1, 0);
+		addPoint(points, 0, 1);
+		addPoint(points, 1, 0);
+		addPoint(points, 2, 2);
+
+		TestCase.assertFalse(ShamosHoey.simplePolygonPoints(points));
+		TestCase.assertFalse(ShamosHoey.simplePolygon(new LineString(points)));
+		TestCase.assertFalse(
+				ShamosHoey.simplePolygon(new Polygon(new LineString(points))));
+		TestCase.assertFalse((new LineString(points)).isSimple());
+		complex(new Polygon(new LineString(points)));
 		TestCase.assertEquals(4, points.size());
 
 	}
@@ -828,6 +843,12 @@ private void validate(Polygon polygon, boolean simple) {
 		Polygon copy = new Polygon(polygon);
 		List<Point> points = copy.getRing(0).getPoints();
 
+		Point first = points.get(0);
+		Point last = points.get(points.size() - 1);
+		if (first.equalsXY(last)) {
+			points.remove(points.size() - 1);
+		}
+
 		for (int i = 1; i < points.size(); i++) {
 
 			points.add(points.remove(0));