reimplement distance calculation in routeguide

1 files changed, 10 insertions, 11 deletions

diff --git a/examples/ruby/route_guide/route_guide_server.rb b/examples/ruby/route_guide/route_guide_server.rb
index 8ea07a21c5..5eb268b533 100755
--- a/examples/ruby/route_guide/route_guide_server.rb
+++ b/examples/ruby/route_guide/route_guide_server.rb
@@ -32,19 +32,18 @@ COORD_FACTOR = 1e7
 RADIUS = 637_100
 
 # Determines the distance between two points.
+# The formula is based on http://mathforum.org/library/drmath/view/51879.html.
 def calculate_distance(point_a, point_b)
   to_radians = proc { |x| x * Math::PI / 180 }
-  lat_a = point_a.latitude / COORD_FACTOR
-  lat_b = point_b.latitude / COORD_FACTOR
-  long_a = point_a.longitude / COORD_FACTOR
-  long_b = point_b.longitude / COORD_FACTOR
-  φ1 = to_radians.call(lat_a)
-  φ2 = to_radians.call(lat_b)
-  Δφ = to_radians.call(lat_a - lat_b)
-  Δλ = to_radians.call(long_a - long_b)
-  a = Math.sin(Δφ / 2)**2 +
-      Math.cos(φ1) * Math.cos(φ2) +
-      Math.sin(Δλ / 2)**2
+  lat_a = to_radians.call(point_a.latitude / COORD_FACTOR)
+  lat_b = to_radians.call(point_b.latitude / COORD_FACTOR)
+  lon_a = to_radians.call(point_a.longitude / COORD_FACTOR)
+  lon_b = to_radians.call(point_b.longitude / COORD_FACTOR)
+  delta_lat = lat_a - lat_b
+  delta_lon = lon_a - lon_b
+  a = Math.sin(delta_lat / 2)**2 +
+      Math.cos(lat_a) * Math.cos(lat_b) +
+      Math.sin(delta_lon / 2)**2
   (2 * RADIUS *  Math.atan2(Math.sqrt(a), Math.sqrt(1 - a))).to_i
 end