Fix incorrect Intersection algorithm

geofence.go

@@ -4,7 +4,7 @@ import (
 	"github.com/kellydunn/golang-geo"
 )
 
-// Geofence represents a point on the Earth with an accuracy radius.
+// Geofence represents a point on the Earth with an accuracy radius in meters.
 type Geofence struct {
 	Latitude, Longitude, Radius float64
 }
@@ -29,17 +29,19 @@ func (mi *Geofence) Intersection(tu *Geofence) (i SetIntersection) {
 	tuPoint := geo.NewPoint(tu.Latitude, tu.Longitude)
 	distance := miPoint.GreatCircleDistance(tuPoint) * 1000
 
-	ourRadius := mi.Radius + tu.Radius
-	if ourRadius > distance {
+	radiusSum := mi.Radius + tu.Radius
+	radiusDiff := mi.Radius - tu.Radius
+
+	if distance-radiusSum > 0 {
 		i = IsDisjoint
 		return
 	}
 
-	if mi.Radius-tu.Radius > distance {
+	if -distance+radiusDiff >= 0 {
 		i |= IsSuperset
 	}
 
-	if tu.Radius-mi.Radius > distance {
+	if -distance-radiusDiff >= 0 {
 		i |= IsSubset
 	}
 

geofence_test.go

@@ -0,0 +1,26 @@
+package main
+
+import "testing"
+
+func TestIntersection(t *testing.T) {
+	tests := []struct {
+		Mi     Geofence
+		Tu     Geofence
+		Result SetIntersection
+	}{
+		{Geofence{36.1699, -115.1398, 1000.0}, Geofence{36.1699, -115.1398, 10.0}, IsSuperset},
+		{Geofence{36.1699, -115.1398, 10.0}, Geofence{36.1699, -115.1398, 1000.0}, IsSubset},
+		{Geofence{36.1699, -115.1398, 10.0}, Geofence{37.7749, -122.4194, 1000.0}, IsDisjoint},
+		{Geofence{36.1699, -115.1398, 10.0}, Geofence{36.1699, -115.1398, 10.0}, IsSubset | IsSuperset},
+		{Geofence{36.1699, -115.13983, 100.0}, Geofence{36.1699, -115.1398, 100.0}, 0},
+	}
+
+	for _, test := range tests {
+		got := test.Mi.Intersection(&test.Tu)
+		want := test.Result
+		if want != got {
+			t.Errorf("With %s and %s: expected intersection code %b, got %b",
+				BlacklistRule{Geofence: &test.Mi}, BlacklistRule{Geofence: &test.Tu}, want, got)
+		}
+	}
+}