GEOSEARCH BYBOX: Reduce wastefull computation on geohashGetDistanceIf…

…InRectangle and geohashGetDistance (redis#11535)

Optimize geohashGetDistanceIfInRectangle when there are many misses.
It calls 3x geohashGetDistance. The first 2 times we call them to produce intermediate results.
This PR focus on optimizing for those 2 intermediate results.

1 Reduce expensive computation on intermediate geohashGetDistance with same long
2 Avoid expensive lon_distance calculation if lat_distance fails beforehand

Co-authored-by: Oran Agra <oran@redislabs.com>

src/geohash_helper.c

@@ -225,8 +225,12 @@ double geohashGetDistance(double lon1d, double lat1d, double lon2d, double lat2d
     lon2r = deg_rad(lon2d);
     u = sin((lat2r - lat1r) / 2);
     v = sin((lon2r - lon1r) / 2);
-    return 2.0 * EARTH_RADIUS_IN_METERS *
-           asin(sqrt(u * u + cos(lat1r) * cos(lat2r) * v * v));
+    double a = u * u;
+    /* if v == 0 we can avoid doing expensive math */
+    if (v != 0.0){
+        a += cos(lat1r) * cos(lat2r) * v * v;
+    }
+    return 2.0 * EARTH_RADIUS_IN_METERS * asin(sqrt(a));
 }
 
 int geohashGetDistanceIfInRadius(double x1, double y1,
@@ -253,9 +257,14 @@ int geohashGetDistanceIfInRadiusWGS84(double x1, double y1, double x2,
  */
 int geohashGetDistanceIfInRectangle(double width_m, double height_m, double x1, double y1,
                                     double x2, double y2, double *distance) {
-    double lon_distance = geohashGetDistance(x2, y2, x1, y2);
+    /* latitude distance is less expensive to compute than longitude distance
+     * so we check first for the latitude condition */
     double lat_distance = geohashGetDistance(x2, y2, x2, y1);
-    if (lon_distance > width_m/2 || lat_distance > height_m/2) {
+    if (lat_distance > height_m/2) {
+        return 0;
+    }
+    double lon_distance = geohashGetDistance(x2, y2, x1, y2);
+    if (lon_distance > width_m/2) {
         return 0;
     }
     *distance = geohashGetDistance(x1, y1, x2, y2);