Merge pull request #487 from rmanoka/master

Fix Area logic for Polygon with interiors (holes)

Author: urschrei

geo/src/algorithm/area.rs

@@ -6,7 +6,7 @@ use num_traits::Float;
 
 use crate::algorithm::winding_order::twice_signed_ring_area;
 
-/// Signed planar area of a geometry.
+/// Signed and unsigned planar area of a geometry.
 ///
 /// # Examples
 ///
@@ -88,16 +88,31 @@ where
     }
 }
 
+/// **Note.** The implementation handles polygons whose
+/// holes do not all have the same orientation. The sign of
+/// the output is the same as that of the exterior shell.
 impl<T> Area<T> for Polygon<T>
 where
     T: Float,
 {
     fn signed_area(&self) -> T {
-        self.interiors()
+        let area = get_linestring_area(self.exterior());
+
+        // We could use winding order here, but that would
+        // result in computing the shoelace formula twice.
+        let is_negative = area < T::zero();
+
+        let area = self.interiors()
             .iter()
-            .fold(get_linestring_area(self.exterior()), |total, next| {
-                total - get_linestring_area(next)
-            })
+            .fold(area.abs(), |total, next| {
+                total - get_linestring_area(next).abs()
+            });
+
+        if is_negative {
+            -area
+        } else {
+            area
+        }
     }
 
     fn unsigned_area(&self) -> T {
@@ -131,6 +146,12 @@ where
     }
 }
 
+/// **Note.** The implementation is a straight-forward
+/// summation of the signed areas of the individual
+/// polygons. In particular, `unsigned_area` is not
+/// necessarily the sum of the `unsigned_area` of the
+/// constituent polygons unless they are all oriented the
+/// same.
 impl<T> Area<T> for MultiPolygon<T>
 where
     T: Float,
@@ -406,4 +427,102 @@ mod test {
 
         assert_eq!(polygon_area * 2., multi_polygon.unsigned_area());
     }
+
+    #[test]
+    fn area_north_america_cutout() {
+        let poly = polygon![
+            exterior: [
+                (x: -102.902861858977, y: 31.6943450891131),
+                (x: -102.917375513247, y: 31.6990175356827),
+                (x: -102.917887344527, y: 31.7044889522597),
+                (x: -102.938892711173, y: 31.7032871894594),
+                (x: -102.939919687305, y: 31.7142296141915),
+                (x: -102.946922353444, y: 31.713828170995),
+                (x: -102.954642979004, y: 31.7210594956594),
+                (x: -102.960927457803, y: 31.7130240707676),
+                (x: -102.967929895872, y: 31.7126214137469),
+                (x: -102.966383373178, y: 31.6962079209847),
+                (x: -102.973384192133, y: 31.6958049292994),
+                (x: -102.97390013779, y: 31.701276160078),
+                (x: -102.980901394769, y: 31.7008727405409),
+                (x: -102.987902575456, y: 31.7004689164622),
+                (x: -102.986878877087, y: 31.7127206248263),
+                (x: -102.976474089689, y: 31.7054378797983),
+                (x: -102.975448432121, y: 31.7176893134691),
+                (x: -102.96619351228, y: 31.7237224912303),
+                (x: -102.976481009643, y: 31.7286309669534),
+                (x: -102.976997412845, y: 31.7341016591658),
+                (x: -102.978030448215, y: 31.7450427747035),
+                (x: -102.985035821671, y: 31.7446391683265),
+                (x: -102.985552968771, y: 31.7501095683386),
+                (x: -102.992558780682, y: 31.7497055338313),
+                (x: -102.993594334215, y: 31.7606460184322),
+                (x: -102.973746840657, y: 31.7546100958509),
+                (x: -102.966082339116, y: 31.767730116605),
+                (x: -102.959074676589, y: 31.768132602064),
+                (x: -102.95206693787, y: 31.7685346826851),
+                (x: -102.953096767614, y: 31.7794749110023),
+                (x: -102.953611796704, y: 31.7849448911322),
+                (x: -102.952629078076, y: 31.7996518517642),
+                (x: -102.948661251495, y: 31.8072257578725),
+                (x: -102.934638176282, y: 31.8080282207231),
+                (x: -102.927626524626, y: 31.8084288446215),
+                (x: -102.927113253813, y: 31.8029591283411),
+                (x: -102.920102042027, y: 31.8033593239799),
+                (x: -102.919076759513, y: 31.792419577395),
+                (x: -102.912066503301, y: 31.7928193216213),
+                (x: -102.911554491357, y: 31.7873492912889),
+                (x: -102.904544675025, y: 31.7877486073783),
+                (x: -102.904033254331, y: 31.7822784646103),
+                (x: -102.903521909259, y: 31.7768082325431),
+                (x: -102.895800463718, y: 31.7695748336589),
+                (x: -102.889504111843, y: 31.7776055573633),
+                (x: -102.882495099915, y: 31.7780036124077),
+                (x: -102.868476849997, y: 31.7787985077398),
+                (x: -102.866950998738, y: 31.7623869292283),
+                (x: -102.873958615171, y: 31.7619897531194),
+                (x: -102.87888647278, y: 31.7688910039026),
+                (x: -102.879947237315, y: 31.750650764952),
+                (x: -102.886953672823, y: 31.750252825268),
+                (x: -102.89396003296, y: 31.7498544807869),
+                (x: -102.892939355062, y: 31.7389128078806),
+                (x: -102.913954892669, y: 31.7377154844276),
+                (x: -102.913443122277, y: 31.7322445829725),
+                (x: -102.912931427507, y: 31.7267735918962),
+                (x: -102.911908264767, y: 31.7158313407426),
+                (x: -102.904905220014, y: 31.7162307607961),
+                (x: -102.904394266551, y: 31.7107594775392),
+                (x: -102.903372586049, y: 31.6998166417321),
+                (x: -102.902861858977, y: 31.6943450891131),
+            ],
+            interiors: [
+                [
+                    (x: -102.916514879554, y: 31.7650686485918),
+                    (x: -102.921022256876, y: 31.7770831833398),
+                    (x: -102.93367363719, y: 31.771184865332),
+                    (x: -102.916514879554, y: 31.7650686485918),
+                ],
+                [
+                    (x: -102.935483140202, y: 31.7419852607081),
+                    (x: -102.932452314332, y: 31.7328567234689),
+                    (x: -102.918345099146, y: 31.7326099897391),
+                    (x: -102.925566322952, y: 31.7552505533503),
+                    (x: -102.928990700436, y: 31.747856686604),
+                    (x: -102.935996606762, y: 31.7474559134477),
+                    (x: -102.939021176592, y: 31.7539885279379),
+                    (x: -102.944714388971, y: 31.7488395547293),
+                    (x: -102.935996606762, y: 31.7474559134477),
+                    (x: -102.935483140202, y: 31.7419852607081),
+                ],
+                [
+                    (x: -102.956498858767, y: 31.7407805824758),
+                    (x: -102.960959476367, y: 31.7475080456347),
+                    (x: -102.972817445204, y: 31.742072061889),
+                    (x: -102.956498858767, y: 31.7407805824758),
+                ]
+            ],
+        ];
+        // Value from shapely
+        assert_relative_eq!(poly.unsigned_area(), 0.006547948219252177, max_relative = 0.0001);
+    }
 }