From f9f8ad78cc7e0093345c065a29c197242979d796 Mon Sep 17 00:00:00 2001
From: Justin Hwang <justinhwang@uber.com>
Date: Tue, 27 Oct 2026 12:46:36 -0400
Subject: [PATCH] h3: clamp the haversine term so antipodal distances are not
 NaN

For some antipodal pairs rounding pushes the haversine term just above
its mathematical maximum of 1, and sqrt(1 - h) is then NaN, so
GreatCircleDistanceRads and the kilometre and metre forms returned NaN
instead of pi times the radius. Clamping the term at 1 affects only that
overshoot. This is upstream uber/h3#1267. The conformance suite is
unaffected: its distance sampler already redraws far targets within 1e-3
radians of antipodal, where the haversine is ill-conditioned.

The test helper assertRelClose compared with >, and every comparison
against NaN is false, so a NaN result passed; it now rejects NaN, and the
new antipodal case fails without the clamp.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
---
 area_test.go   |  4 +++-
 latlng.go      |  4 ++++
 latlng_test.go | 13 +++++++++++++
 3 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/area_test.go b/area_test.go
index 80749fc..48d2be7 100644
--- a/area_test.go
+++ b/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)
 	}
 }
diff --git a/latlng.go b/latlng.go
index ee7c59f..0bd1649 100644
--- a/latlng.go
+++ b/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))
 }
 
diff --git a/latlng_test.go b/latlng_test.go
index 70750bd..690b8d7 100644
--- a/latlng_test.go
+++ b/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()