Add polygon area calculation

Turf.xcodeproj/project.pbxproj

@@ -7,6 +7,8 @@
 	objects = {
 
 /* Begin PBXBuildFile section */
+		052521D81FC47A1300DD266A /* dc-polygon.geojson in Resources */ = {isa = PBXBuildFile; fileRef = 052521D71FC47A0300DD266A /* dc-polygon.geojson */; };
+		052521D91FC47A1900DD266A /* dc-polygon.geojson in Resources */ = {isa = PBXBuildFile; fileRef = 052521D71FC47A0300DD266A /* dc-polygon.geojson */; };
 		353E9B101F3E093A007CFA23 /* Turf.framework in Frameworks */ = {isa = PBXBuildFile; fileRef = 353E9B071F3E093A007CFA23 /* Turf.framework */; };
 		353E9B1E1F3E09D3007CFA23 /* CoreLocation.swift in Sources */ = {isa = PBXBuildFile; fileRef = 356D24531F17948C003BBB9D /* CoreLocation.swift */; };
 		353E9B1F1F3E09D8007CFA23 /* Turf.swift in Sources */ = {isa = PBXBuildFile; fileRef = 35650B0A1F150DDB00B5C158 /* Turf.swift */; };
@@ -41,6 +43,7 @@
 /* End PBXContainerItemProxy section */
 
 /* Begin PBXFileReference section */
+		052521D71FC47A0300DD266A /* dc-polygon.geojson */ = {isa = PBXFileReference; lastKnownFileType = text; path = "dc-polygon.geojson"; sourceTree = "<group>"; };
 		353E9B071F3E093A007CFA23 /* Turf.framework */ = {isa = PBXFileReference; explicitFileType = wrapper.framework; includeInIndex = 0; path = Turf.framework; sourceTree = BUILT_PRODUCTS_DIR; };
 		353E9B0F1F3E093A007CFA23 /* TurfMacTests.xctest */ = {isa = PBXFileReference; explicitFileType = wrapper.cfbundle; includeInIndex = 0; path = TurfMacTests.xctest; sourceTree = BUILT_PRODUCTS_DIR; };
 		35650AF01F150DC500B5C158 /* Turf.framework */ = {isa = PBXFileReference; explicitFileType = wrapper.framework; includeInIndex = 0; path = Turf.framework; sourceTree = BUILT_PRODUCTS_DIR; };
@@ -135,6 +138,7 @@
 			isa = PBXGroup;
 			children = (
 				356D24581F179B72003BBB9D /* dc-line.geojson */,
+				052521D71FC47A0300DD266A /* dc-polygon.geojson */,
 			);
 			path = Fixtures;
 			sourceTree = "<group>";
@@ -291,6 +295,7 @@
 			buildActionMask = 2147483647;
 			files = (
 				35CB7F6F1F798A51008A18C8 /* dc-line.geojson in Resources */,
+				052521D91FC47A1900DD266A /* dc-polygon.geojson in Resources */,
 			);
 			runOnlyForDeploymentPostprocessing = 0;
 		};
@@ -306,6 +311,7 @@
 			buildActionMask = 2147483647;
 			files = (
 				356D24591F179B72003BBB9D /* dc-line.geojson in Resources */,
+				052521D81FC47A1300DD266A /* dc-polygon.geojson in Resources */,
 			);
 			runOnlyForDeploymentPostprocessing = 0;
 		};

Turf/Turf.swift

@@ -5,6 +5,7 @@ public typealias RadianDistance = Double
 public typealias RadianDirection = Double
 
 let metersPerRadian = 6_373_000.0
+let equitorialRadius:Double = 6378137
 
 /**
  A `RadianCoordinate2D` is a coordinate represented in radians as opposed to
@@ -285,3 +286,61 @@ public struct Polyline {
         return closestCoordinate
     }
 }
+
+public struct Polygon {
+    var coordinates: [CLLocationCoordinate2D]
+
+    var polygonArea: Double {
+        return 0;
+    }
+
+    /**
+     * Calculate the approximate area of the polygon were it projected onto the earth.
+     * Note that this area will be positive if ring is oriented clockwise, otherwise it will be negative.
+     *
+     * Reference:
+     * Robert. G. Chamberlain and William H. Duquette, "Some Algorithms for Polygons on a Sphere", JPL Publication 07-03, Jet Propulsion
+     * Laboratory, Pasadena, CA, June 2007 http://trs-new.jpl.nasa.gov/dspace/handle/2014/40409
+     *
+     */
+
+    var ringArea: Double {
+        var p1:CLLocationCoordinate2D
+        var p2:CLLocationCoordinate2D
+        var p3:CLLocationCoordinate2D
+        var lowerIndex:Int
+        var middleIndex:Int
+        var upperIndex:Int
+        var i:Int
+        var area:Double = 0
+        var coordsLength:Int = coordinates.count
+
+        if (coordsLength > 2) {
+            for(index, coordinate) in coordinates.enumerated() {
+                if (index == coordsLength - 2) {
+                    lowerIndex = coordsLength - 2
+                    middleIndex = coordsLength - 1
+                    upperIndex = 0
+                } else if(index == coordsLength - 1) {
+                    lowerIndex = coordsLength - 1;
+                    middleIndex = 0;
+                    upperIndex = 1;
+                } else {
+                    lowerIndex = index
+                    middleIndex = index + 1
+                    upperIndex = index + 2
+                }
+
+                p1 = coordinates[lowerIndex]
+                p2 = coordinates[middleIndex]
+                p3 = coordinates[upperIndex]
+                area += (p3.longitude.toRadians() - p1.longitude.toRadians()) * sin(p2.latitude.toRadians())
+            }
+
+            area = area * equitorialRadius * equitorialRadius / 2;
+
+        }
+
+        return area
+    }
+}

TurfTests/Fixtures/dc-polygon.geojson

@@ -0,0 +1,25 @@
+{
+    "type": "Feature",
+    "properties": {},
+    "geometry": {
+        "type": "Polygon",
+        "coordinates": [
+            [
+             -77.0416259765625,
+             39.06611426153784
+             ],
+            [
+             -77.32177734375,
+             38.843986129756615
+             ],
+            [
+             -76.7669677734375,
+             38.831149809348744
+             ],
+            [
+             -77.0416259765625,
+             39.06611426153784
+             ]
+            ]
+    }
+}

TurfTests/TurfTests.swift

@@ -286,4 +286,12 @@ class TurfTests: XCTestCase {
         let b = radian.toDegrees()
         XCTAssertEqual(b, 229, accuracy: 1)
     }
+
+    func testPolygonArea() {
+        let json = Fixture.JSONFromFileNamed(name: "dc-polygon")
+        let coordinates = ((json["geometry"] as! [String: Any])["coordinates"] as! [[Double]]).map { CLLocationCoordinate2D(latitude: $0[0], longitude: $0[1]) }
+        let polygon = Polygon(coordinates: coordinates)
+
+        XCTAssertEqual(polygon.ringArea, 611152631.86)
+    }
 }