h3: clamp the haversine term so antipodal distances are not NaN

area_test.go

@@ -308,8 +308,10 @@ func rotateHalfTurn(lng float64) float64 {
 func assertRelClose(t *testing.T, got, want float64, label string) {
 	t.Helper()
 
+	// Every comparison against NaN is false, so test for it explicitly or a
+	// NaN result would pass.
 	const tol = 1e-9
-	if math.Abs(got-want) > tol*math.Max(1, math.Abs(want)) {
+	if math.IsNaN(got) || math.Abs(got-want) > tol*math.Max(1, math.Abs(want)) {
 		t.Fatalf("%s = %.17g, want %.17g", label, got, want)
 	}
 }

latlng.go

@@ -166,6 +166,10 @@ func GreatCircleDistanceRads(a, b LatLng) float64 {
 
 	h := sinLat*sinLat + math.Cos(aLat)*math.Cos(bLat)*sinLng*sinLng
 
+	// Rounding can push h slightly above its mathematical maximum of 1 for
+	// antipodal points, and sqrt(1 - h) would then be NaN.
+	h = math.Min(h, 1)
+
 	return 2 * math.Atan2(math.Sqrt(h), math.Sqrt(1-h))
 }
 

latlng_test.go

@@ -250,6 +250,19 @@ func TestGreatCircleDistance(t *testing.T) {
 		}
 	})
 
+	t.Run("antipodal", func(t *testing.T) {
+		t.Parallel()
+
+		// Rounding pushes the haversine term just above 1 for this pair, so an
+		// unclamped sqrt(1 - h) is NaN.
+		point := LatLng{Lat: 10.123456789, Lng: 20.987654321}
+		antipode := LatLng{Lat: -point.Lat, Lng: point.Lng - piDeg}
+
+		assertRelClose(t, GreatCircleDistanceRads(point, antipode), math.Pi, "rads")
+		assertRelClose(t, GreatCircleDistanceKm(point, antipode), math.Pi*earthRadiusKm, "km")
+		assertRelClose(t, GreatCircleDistanceM(point, antipode), math.Pi*earthRadiusKm*1000, "m")
+	})
+
 	t.Run("symmetric", func(t *testing.T) {
 		t.Parallel()