Allow GeoJSONWriter to force polygons to be CCW

modules/io/common/src/main/java/org/locationtech/jts/io/geojson/GeoJsonWriter.java

@@ -11,14 +11,6 @@
  */
 package org.locationtech.jts.io.geojson;
 
-import java.io.IOException;
-import java.io.StringWriter;
-import java.io.Writer;
-import java.util.ArrayList;
-import java.util.LinkedHashMap;
-import java.util.List;
-import java.util.Map;
-
 import org.json.simple.JSONAware;
 import org.json.simple.JSONObject;
 import org.locationtech.jts.geom.CoordinateSequence;
@@ -32,6 +24,14 @@
 import org.locationtech.jts.geom.Polygon;
 import org.locationtech.jts.util.Assert;
 
+import java.io.IOException;
+import java.io.StringWriter;
+import java.io.Writer;
+import java.util.ArrayList;
+import java.util.LinkedHashMap;
+import java.util.List;
+import java.util.Map;
+
 
 /**
  * Writes {@link Geometry}s as JSON fragments in GeoJSON format.
@@ -59,6 +59,7 @@ public class GeoJsonWriter {
 
   private double scale;
   private boolean isEncodeCRS = true;
+  private boolean isEnforceCCW = false;
 
   /**
    * Constructs a GeoJsonWriter instance.
@@ -88,6 +89,16 @@ public void setEncodeCRS(boolean isEncodeCRS) {
     this.isEncodeCRS  = isEncodeCRS;
   }
 
+  /**
+   * Sets whether the GeoJSON should be output following counter-clockwise orientation aka Right Hand Rule defined in RFC7946
+   * See <a href="https://tools.ietf.org/html/rfc7946#section-3.1.6">RFC 7946 Specification</a> for more context.
+   *
+   * @param enforceCCW true if the GeoJSON should be output following the RFC7946 counter-clockwise orientation aka Right Hand Rule
+   */
+  public void setEnforceCCW(boolean enforceCCW) {
+    this.isEnforceCCW = enforceCCW;
+  }
+
   /**
    * Writes a {@link Geometry} in GeoJson format to a String.
    * 
@@ -158,6 +169,10 @@ public String toJSONString() {
     } else if (geometry instanceof Polygon) {
       Polygon polygon = (Polygon) geometry;
 
+      if (isEnforceCCW) {
+        polygon = (Polygon) OrientationTransformer.transformCCW(polygon);
+      }
+
       result.put(GeoJsonConstants.NAME_COORDINATES, makeJsonAware(polygon));
 
     } else if (geometry instanceof MultiPoint) {
@@ -173,6 +188,10 @@ public String toJSONString() {
     } else if (geometry instanceof MultiPolygon) {
       MultiPolygon multiPolygon = (MultiPolygon) geometry;
 
+      if (isEnforceCCW) {
+        multiPolygon = (MultiPolygon) OrientationTransformer.transformCCW(multiPolygon);
+      }
+
       result.put(GeoJsonConstants.NAME_COORDINATES, makeJsonAware(multiPolygon));
 
     } else if (geometry instanceof GeometryCollection) {

modules/io/common/src/main/java/org/locationtech/jts/io/geojson/OrientationTransformer.java

@@ -0,0 +1,88 @@
+package org.locationtech.jts.io.geojson;
+
+import org.locationtech.jts.algorithm.Orientation;
+import org.locationtech.jts.geom.*;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * Utilities to modify the ring orientation of polygonal geometries.
+ */
+public class OrientationTransformer {
+
+    /**
+     * Transforms a geometry using the Right Hand Rule specifications defined
+     * in the latest GeoJSON specification.
+     * See <a href="https://tools.ietf.org/html/rfc7946#section-3.1.6">RFC-7946 Specification</a> for more context.
+     *
+     * @param geometry to be transformed
+     * @return Geometry under the Right Hand Rule specifications
+     */
+    public static Geometry transformCCW(final Geometry geometry) {
+
+        if (geometry instanceof MultiPolygon) {
+            MultiPolygon multiPolygon = (MultiPolygon) geometry;
+
+            List<Polygon> polygons = new ArrayList<>();
+            for (int i = 0; i < multiPolygon.getNumGeometries(); i++) {
+                final Geometry polygon = multiPolygon.getGeometryN(i);
+                polygons.add((Polygon) transformCCW(polygon));
+            }
+
+            return new GeometryFactory().createMultiPolygon(polygons.toArray(new Polygon[0]));
+
+        } else if (geometry instanceof Polygon) {
+            return transformCCW((Polygon) geometry);
+
+        } else {
+            return geometry;
+        }
+    }
+
+    /**
+     * Transforms a polygon using the Right Hand Rule specifications defined
+     * in the latest GeoJSON specification.
+     * See <a href="https://tools.ietf.org/html/rfc7946#section-3.1.6">RFC-7946 Specification</a> for more context.
+     *
+     * @param polygon to be transformed
+     * @return Polygon under the Right Hand Rule specifications
+     */
+    public static Polygon transformCCW(Polygon polygon) {
+        LinearRing exteriorRing = polygon.getExteriorRing();
+        LinearRing exteriorRingEnforced = transformCCW(exteriorRing, true);
+
+        List<LinearRing> interiorRings = new ArrayList<>();
+        for (int i = 0; i < polygon.getNumInteriorRing(); i++) {
+            interiorRings.add(transformCCW(polygon.getInteriorRingN(i), false));
+        }
+
+        return new GeometryFactory(polygon.getPrecisionModel(), polygon.getSRID())
+                .createPolygon(exteriorRingEnforced, interiorRings.toArray(new LinearRing[0]));
+    }
+
+    /**
+     * Transforms a polygon using the Right Hand Rule specifications defined
+     * in the latest GeoJSON specification.
+     * A linear ring MUST follow the right-hand rule with respect to the
+     * area it bounds, i.e., exterior rings are counterclockwise, and
+     * holes are clockwise.
+     *
+     * See <a href="https://tools.ietf.org/html/rfc7946#section-3.1.6">RFC 7946 Specification</a> for more context.
+     *
+     * @param ring the LinearRing, a constraint specific to Polygons
+     * @param isExteriorRing true if the LinearRing is the exterior polygon ring, the one that defines the boundary
+     * @return LinearRing under the Right Hand Rule specifications
+     */
+    public static LinearRing transformCCW(LinearRing ring, boolean isExteriorRing) {
+        final boolean isRingClockWise = !Orientation.isCCW(ring.getCoordinateSequence());
+
+        final LinearRing rightHandRuleRing;
+        if (isExteriorRing) {
+            rightHandRuleRing = isRingClockWise? ring.reverse() : (LinearRing) ring.copy();
+        } else {
+            rightHandRuleRing = isRingClockWise? (LinearRing) ring.copy() : ring.reverse();
+        }
+        return rightHandRuleRing;
+    }
+}

modules/io/common/src/test/java/org/locationtech/jts/io/geojson/GeoJsonWriterTest.java

@@ -76,6 +76,16 @@ public void testPolygonWithHole() throws ParseException {
         "{'type':'Polygon','coordinates':[[[0.0,0.0],[100,0.0],[100,100],[0.0,100],[0.0,0.0]],[[1,1],[1,10],[10,10],[10,1],[1,1]]]}");
   }
 
+  public void testPolygonRightHandRule() throws ParseException {
+    runTest("POLYGON ((0 0, 0 100, 100 100, 100 0, 0 0))", true,
+            "{'type':'Polygon','coordinates':[[[0.0,0.0],[100,0.0],[100,100],[0.0,100],[0.0,0.0]]]}");
+  }
+
+  public void testPolygonWithHoleRightHandRule() throws ParseException {
+    runTest("POLYGON ((0 0, 0 100, 100 100, 100 0, 0 0), (1 1, 10 1, 10 10, 1 10, 1 1) )", true,
+            "{'type':'Polygon','coordinates':[[[0.0,0.0],[100,0.0],[100,100],[0.0,100],[0.0,0.0]],[[1,1],[1,10],[10,10],[10,1],[1,1]]]}");
+  }
+
   public void testMultiPoint() throws ParseException {
     runTest("MULTIPOINT ((0 0), (1 4), (100 200))",
         "{'type':'MultiPoint','coordinates':[[0.0,0.0],[1,4],[100,200]]}");
@@ -125,20 +135,25 @@ private void runTest(String wkt) throws ParseException {
   }
 
   private void runTest(String wkt, String expectedGeojson) throws ParseException {
-    runTest(wkt, 0, false, expectedGeojson);
+    runTest(wkt, 0, false, false, expectedGeojson);
   }
 
   private void runTest(String wkt, int srid, String expectedGeojson) throws ParseException {
-    runTest(wkt, srid, true, expectedGeojson);
+    runTest(wkt, srid, true, false, expectedGeojson);
+  }
+
+  private void runTest(String wkt, boolean enforceRHR, String expectedGeojson) throws ParseException {
+    runTest(wkt, 0, false, enforceRHR, expectedGeojson);
   }
 
-  private void runTest(String wkt, int srid, boolean encodeCRS, String expectedGeojson) throws ParseException {
+  private void runTest(String wkt, int srid, boolean encodeCRS, boolean enforceRHR, String expectedGeojson) throws ParseException {
     Geometry geom = read(wkt);
     geom.setSRID(srid);
     geoJsonWriter.setEncodeCRS(encodeCRS);
+    geoJsonWriter.setEnforceCCW(enforceRHR);
     String json = this.geoJsonWriter.write(geom);
     json = json.replace('"', '\'');
-    assertEquals(json, expectedGeojson);
+    assertEquals(expectedGeojson, json);
   }
 
 }