Add ol.geom.flatLinearRingIsClockwise

src/ol/geom/geometry.js

@@ -360,6 +360,31 @@ ol.geom.flatLinearRingContainsXY =
 };
 
 
+/**
+ * @param {Array.<number>} flatCoordinates Flat coordinates.
+ * @param {number} offset Offset.
+ * @param {number} end End.
+ * @param {number} stride Stride.
+ * @return {boolean} Is clockwise.
+ */
+ol.geom.flatLinearRingIsClockwise =
+    function(flatCoordinates, offset, end, stride) {
+  // http://tinyurl.com/clockwise-method
+  // https://github.com/OSGeo/gdal/blob/trunk/gdal/ogr/ogrlinearring.cpp
+  var edge = 0;
+  var x1 = flatCoordinates[end - stride];
+  var y1 = flatCoordinates[end - stride + 1];
+  for (; offset < end; offset += stride) {
+    var x2 = flatCoordinates[offset];
+    var y2 = flatCoordinates[offset + 1];
+    edge += (x2 - x1) * (y2 + y1);
+    x1 = x2;
+    y1 = y2;
+  }
+  return edge > 0;
+};
+
+
 /**
  * @param {Array.<number>} flatCoordinates Flat coordinates.
  * @param {number} offset Offset.

test/spec/ol/geom/geom.test.js

@@ -54,6 +54,24 @@ describe('ol.geom', function() {
 
   });
 
+  describe('ol.geom.flatLinearRingIsClockwise', function() {
+
+    it('identifies clockwise rings', function() {
+      var flatCoordinates = [0, 1, 1, 4, 4, 3, 3, 0];
+      var isClockwise = ol.geom.flatLinearRingIsClockwise(
+          flatCoordinates, 0, flatCoordinates.length, 2);
+      expect(isClockwise).to.be(true);
+    });
+
+    it('identifies anti-clockwise rings', function() {
+      var flatCoordinates = [2, 2, 3, 2, 3, 3, 2, 3];
+      var isClockwise = ol.geom.flatLinearRingIsClockwise(
+          flatCoordinates, 0, flatCoordinates.length, 2);
+      expect(isClockwise).to.be(false);
+    });
+
+  });
+
   describe('ol.geom.reverseFlatCoordinates', function() {
 
     describe('with a stride of 2', function() {