ratio for calculation of bbox area should always be > 1 (#708)

src/apps/testapps/testBBox.c

@@ -246,6 +246,26 @@ SUITE(BBox) {
                  "bboxHexEstimate of invalid resolution fails");
     }
 
+    TEST(bboxHexEstimate_ratio) {
+        BBox bbox1 = {0.82294, 0.82273, 0.131671, 0.131668};
+        BBox bbox2 = {0.131671, 0.131668, 0.82294, 0.82273};
+        int64_t numHexagons1;
+        int64_t numHexagons2;
+
+        t_assertSuccess(bboxHexEstimate(&bbox1, 15, &numHexagons1));
+        t_assertSuccess(bboxHexEstimate(&bbox2, 15, &numHexagons2));
+
+        double diffPercentage = fabs(1.0 - numHexagons1 / (double)numHexagons2);
+
+        // numHexagons1 and numHexagons2 cannot be exactly equal because the
+        // diameter of the two bboxes is not exactly the same (it's calculated
+        // using greatCircleDistanceKm)
+        t_assert(
+            diffPercentage < 0.03,
+            "Should be true for bounding boxes with (almost) the same diameter "
+            "and side ratio");
+    }
+
     TEST(lineHexEstimate_invalidRes) {
         int64_t numHexagons;
         LatLng origin = {0.0, 0.0};

src/h3lib/lib/bbox.c

@@ -121,14 +121,17 @@ H3Error bboxHexEstimate(const BBox *bbox, int res, int64_t *out) {
     p2.lat = bbox->south;
     p2.lng = bbox->west;
     double d = H3_EXPORT(greatCircleDistanceKm)(&p1, &p2);
-    // Derived constant based on: https://math.stackexchange.com/a/1921940
-    // Clamped to 3 as higher values tend to rapidly drag the estimate to zero.
-    double lngDiff = p1.lng - p2.lng;
-    double latDiff = p1.lat - p2.lat;
+    double lngDiff = fabs(p1.lng - p2.lng);
+    double latDiff = fabs(p1.lat - p2.lat);
     if (lngDiff == 0 || latDiff == 0) {
         return E_FAILED;
     }
-    double a = d * d / fmin(3.0, fabs(lngDiff / latDiff));
+    double length = fmax(lngDiff, latDiff);
+    double width = fmin(lngDiff, latDiff);
+    double ratio = length / width;
+    // Derived constant based on: https://math.stackexchange.com/a/1921940
+    // Clamped to 3 as higher values tend to rapidly drag the estimate to zero.
+    double a = d * d / fmin(3.0, ratio);
 
     // Divide the two to get an estimate of the number of hexagons needed
     double estimateDouble = ceil(a / pentagonAreaKm2);