From f80cad531bb24c135b1b17fdca88e0f5b4cc9f27 Mon Sep 17 00:00:00 2001
From: Tom Payne <twpayne@gmail.com>
Date: Mon, 2 Dec 2013 12:05:04 +0100
Subject: [PATCH] Add ol.geom.flat.linearRingArea

---
 src/ol/geom/flatgeom.js            | 22 ++++++++++++++++++++++
 test/spec/ol/geom/flatgeom.test.js | 14 ++++++++++++++
 2 files changed, 36 insertions(+)

diff --git a/src/ol/geom/flatgeom.js b/src/ol/geom/flatgeom.js
index 4add38c2d8..93913e83e0 100644
--- a/src/ol/geom/flatgeom.js
+++ b/src/ol/geom/flatgeom.js
@@ -257,6 +257,28 @@ ol.geom.flat.lineStringLength = function(flatCoordinates, offset, end, stride) {
 };
 
 
+/**
+ * @param {Array.<number>} flatCoordinates Flat coordinates.
+ * @param {number} offset Offset.
+ * @param {number} end End.
+ * @param {number} stride Stride.
+ * @return {number} Area.
+ */
+ol.geom.flat.linearRingArea = function(flatCoordinates, offset, end, stride) {
+  var twiceArea = 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];
+    twiceArea += x1 * y2 - y1 * x2;
+    x1 = x2;
+    y1 = y2;
+  }
+  return Math.abs(twiceArea / 2);
+};
+
+
 /**
  * @param {Array.<number>} flatCoordinates Flat coordinates.
  * @param {number} offset Offset.
diff --git a/test/spec/ol/geom/flatgeom.test.js b/test/spec/ol/geom/flatgeom.test.js
index 0f1ca8ea77..9d709d12dd 100644
--- a/test/spec/ol/geom/flatgeom.test.js
+++ b/test/spec/ol/geom/flatgeom.test.js
@@ -127,6 +127,20 @@ describe('ol.geom.flat', function() {
 
   });
 
+  describe('ol.geom.flat.linearRingArea', function() {
+
+    it('calcaultes the area of a triangle', function() {
+      var area = ol.geom.flat.linearRingArea([0, 0, 0.5, 1, 1, 0], 0, 6, 2);
+      expect(area).to.be(0.5);
+    });
+
+    it('calculates the area of a unit square', function() {
+      var area = ol.geom.flat.linearRingArea([0, 0, 0, 1, 1, 1, 1, 0], 0, 8, 2);
+      expect(area).to.be(1);
+    });
+
+  });
+
   describe('ol.geom.flat.linearRingIsClockwise', function() {
 
     it('identifies clockwise rings', function() {