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.