Add polygon area calculation

Turf.xcodeproj/project.pbxproj

@@ -7,6 +7,8 @@
 	objects = {
 
 /* Begin PBXBuildFile section */
+		052521D81FC47A1300DD266A /* polygon.geojson in Resources */ = {isa = PBXBuildFile; fileRef = 052521D71FC47A0300DD266A /* polygon.geojson */; };
+		052521D91FC47A1900DD266A /* polygon.geojson in Resources */ = {isa = PBXBuildFile; fileRef = 052521D71FC47A0300DD266A /* 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 /* polygon.geojson */ = {isa = PBXFileReference; lastKnownFileType = text; path = 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 /* polygon.geojson */,
 			);
 			path = Fixtures;
 			sourceTree = "<group>";
@@ -291,6 +295,7 @@
 			buildActionMask = 2147483647;
 			files = (
 				35CB7F6F1F798A51008A18C8 /* dc-line.geojson in Resources */,
+				052521D91FC47A1900DD266A /* polygon.geojson in Resources */,
 			);
 			runOnlyForDeploymentPostprocessing = 0;
 		};
@@ -306,6 +311,7 @@
 			buildActionMask = 2147483647;
 			files = (
 				356D24591F179B72003BBB9D /* dc-line.geojson in Resources */,
+				052521D81FC47A1300DD266A /* polygon.geojson in Resources */,
 			);
 			runOnlyForDeploymentPostprocessing = 0;
 		};

Turf/Turf.swift

@@ -4,7 +4,10 @@ public typealias LocationRadians = Double
 public typealias RadianDistance = Double
 public typealias RadianDirection = Double
 
-let metersPerRadian = 6_373_000.0
+
+let metersPerRadian: CLLocationDistance = 6_373_000.0
+// WGS84 equatorial radius as specified by the International Union of Geodesy and Geophysics
+let equatorialRadius: CLLocationDistance = 6_378_137
 
 /**
  A `RadianCoordinate2D` is a coordinate represented in radians as opposed to
@@ -285,3 +288,68 @@ public struct Polyline {
         return closestCoordinate
     }
 }
+
+
+/**
+ Creates a `Ring` struct that represents a closed figure that is bounded by three or more straight line segments.
+ */
+public struct Ring {
+    var coordinates: [CLLocationCoordinate2D]
+
+    /**
+     * Calculate the approximate area of the polygon were it projected onto the earth, in square meters.
+     * 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 https://trs.jpl.nasa.gov/handle/2014/41271
+     *
+     */
+    public var area: Double {
+        var area: Double = 0
+        let coordinatesCount: Int = coordinates.count
+
+        if coordinatesCount > 2 {
+            for index in 0..<coordinatesCount {
+
+                let controlPoints: (CLLocationCoordinate2D, CLLocationCoordinate2D, CLLocationCoordinate2D)
+
+                if index == coordinatesCount - 2 {
+                    controlPoints = (coordinates[coordinatesCount - 2],
+                                     coordinates[coordinatesCount - 1],
+                                     coordinates[0])
+                } else if index == coordinatesCount - 1 {
+                    controlPoints = (coordinates[coordinatesCount - 1],
+                                     coordinates[0],
+                                     coordinates[1])
+                } else {
+                    controlPoints = (coordinates[index],
+                                     coordinates[index + 1],
+                                     coordinates[index + 2])
+                }
+
+                area += (controlPoints.2.longitude.toRadians() - controlPoints.0.longitude.toRadians()) * sin(controlPoints.1.latitude.toRadians())
+            }
+
+            area *= equatorialRadius * equatorialRadius / 2
+        }
+        return area
+    }
+}
+
+/**
+ Creates a `Polygon` struct from an outer ring and optional inner rings.
+ Inner rings represent any holes the polygon may have.
+ */
+public struct Polygon {
+    var outerRing: Ring
+    var innerRings: [Ring]
+
+    // Ported from https://github.com/Turfjs/turf/blob/a94151418cb969868fdb42955a19a133512da0fd/packages/turf-area/index.js
+
+    public var area: Double {
+        return abs(outerRing.area) - innerRings
+            .map { abs($0.area) }
+            .reduce(0, +)
+    }
+}

TurfTests/Fixtures/polygon.geojson

@@ -0,0 +1,53 @@
+{
+    "type": "Feature",
+    "properties": {},
+    "geometry": {
+        "type": "Polygon",
+        "coordinates": [
+                        [
+                         [
+                          -109.05029296875,
+                          37.00255267215955
+                          ],
+                         [
+                          -102.0849609375,
+                          37.020098201368114
+                          ],
+                         [
+                          -102.041015625,
+                          41.0130657870063
+                          ],
+                         [
+                          -109.072265625,
+                          40.97989806962013
+                          ],
+                         [
+                          -109.05029296875,
+                          37.00255267215955
+                          ]
+                         ],
+                        [
+                         [
+                          -108.56689453125,
+                          40.6306300839918
+                          ],
+                         [
+                          -108.61083984375,
+                          37.43997405227057
+                          ],
+                         [
+                          -102.50244140624999,
+                          37.405073750176925
+                          ],
+                         [
+                          -102.4365234375,
+                          40.66397287638688
+                          ],
+                         [
+                          -108.56689453125,
+                          40.6306300839918
+                          ]
+                         ]
+                        ]
+    }
+}

TurfTests/TurfTests.swift

@@ -286,4 +286,21 @@ class TurfTests: XCTestCase {
         let b = radian.toDegrees()
         XCTAssertEqual(b, 229, accuracy: 1)
     }
+
+    func testPolygonArea() {
+        let json = Fixture.JSONFromFileNamed(name: "polygon")
+        let geometry = json["geometry"] as! [String: Any]
+        let geoJSONCoordinates = geometry["coordinates"] as! [[[Double]]]
+        let allRings = geoJSONCoordinates.map {
+           $0.map { CLLocationCoordinate2D(latitude: $0[1], longitude: $0[0]) }
+        }
+        let outerRing = Ring(coordinates: allRings.first!)
+        let innerRings = allRings.suffix(from: 1).map {
+            Ring(coordinates: $0)
+        }
+
+        let polygon = Polygon(outerRing: outerRing, innerRings: innerRings)
+
+        XCTAssertEqual(polygon.area, 78588446934.43, accuracy: 0.1)
+    }
 }