Fix GEORADIUS edge case with huge radius.

This commit closes issue redis#3698, at least for now, since the root cause
was not fixed: the bounding box function, for huge radiuses, does not
return a correct bounding box, there are points still within the radius
that are left outside.

So when using GEORADIUS queries with radiuses in the order of 5000 km or
more, it was possible to see, at the edge of the area, certain points
not correctly reported.

Because the bounding box for now was used just as an optimization, and
such huge radiuses are not common, for now the optimization is just
switched off when the radius is near such magnitude.

Three test cases found by the Continuous Integration test were added, so
that we can easily trigger the bug again, both for regression testing
and in order to properly fix it as some point in the future.

Author: antirez

src/geohash_helper.c

@@ -85,7 +85,21 @@ uint8_t geohashEstimateStepsByRadius(double range_meters, double lat) {
 /* Return the bounding box of the search area centered at latitude,longitude
  * having a radius of radius_meter. bounds[0] - bounds[2] is the minimum
  * and maxium longitude, while bounds[1] - bounds[3] is the minimum and
- * maximum latitude. */
+ * maximum latitude.
+ *
+ * This function does not behave correctly with very large radius values, for
+ * instance for the coordinates 81.634948934258375 30.561509253718668 and a
+ * radius of 7083 kilometers, it reports as bounding boxes:
+ *
+ * min_lon 7.680495, min_lat -33.119473, max_lon 155.589402, max_lat 94.242491
+ *
+ * However, for instance, a min_lon of 7.680495 is not correct, because the
+ * point -1.27579540014266968 61.33421815228281559 is at less than 7000
+ * kilometers away.
+ *
+ * Since this function is currently only used as an optimization, the
+ * optimization is not used for very big radiuses, however the function
+ * should be fixed. */
 int geohashBoundingBox(double longitude, double latitude, double radius_meters,
                        double *bounds) {
     if (!bounds) return 0;
@@ -154,25 +168,27 @@ GeoHashRadius geohashGetAreasByRadius(double longitude, double latitude, double
     }
 
     /* Exclude the search areas that are useless. */
-    if (area.latitude.min < min_lat) {
-        GZERO(neighbors.south);
-        GZERO(neighbors.south_west);
-        GZERO(neighbors.south_east);
-    }
-    if (area.latitude.max > max_lat) {
-        GZERO(neighbors.north);
-        GZERO(neighbors.north_east);
-        GZERO(neighbors.north_west);
-    }
-    if (area.longitude.min < min_lon) {
-        GZERO(neighbors.west);
-        GZERO(neighbors.south_west);
-        GZERO(neighbors.north_west);
-    }
-    if (area.longitude.max > max_lon) {
-        GZERO(neighbors.east);
-        GZERO(neighbors.south_east);
-        GZERO(neighbors.north_east);
+    if (steps >= 2) {
+        if (area.latitude.min < min_lat) {
+            GZERO(neighbors.south);
+            GZERO(neighbors.south_west);
+            GZERO(neighbors.south_east);
+        }
+        if (area.latitude.max > max_lat) {
+            GZERO(neighbors.north);
+            GZERO(neighbors.north_east);
+            GZERO(neighbors.north_west);
+        }
+        if (area.longitude.min < min_lon) {
+            GZERO(neighbors.west);
+            GZERO(neighbors.south_west);
+            GZERO(neighbors.north_west);
+        }
+        if (area.longitude.max > max_lon) {
+            GZERO(neighbors.east);
+            GZERO(neighbors.south_east);
+            GZERO(neighbors.north_east);
+        }
     }
     radius.hash = hash;
     radius.neighbors = neighbors;

tests/unit/geo.tcl

@@ -49,6 +49,9 @@ proc compare_lists {List1 List2} {
 #
 # The format is: seed km lon lat
 set regression_vectors {
+    {1482225976969 7083 81.634948934258375 30.561509253718668}
+    {1482340074151 5416 -70.863281847379767 -46.347003465679947}
+    {1499014685896 6064 -89.818768962202014 -40.463868561416803}
     {1412 156 149.29737817929004 15.95807862745508}
     {441574 143 59.235461856813856 66.269555127373678}
     {160645 187 -101.88575239939883 49.061997951502917}