From 0ff1b76450d01cddbca352b7bf9ec3bce3224ad9 Mon Sep 17 00:00:00 2001
From: Jan Tattermusch <jtattermusch@google.com>
Date: Thu, 22 Mar 2018 12:52:22 +0100
Subject: [PATCH] routeguide: reimplement distance calculation

---
 examples/route_guide/server/server.go | 24 +++++++++++-------------
 1 file changed, 11 insertions(+), 13 deletions(-)

diff --git a/examples/route_guide/server/server.go b/examples/route_guide/server/server.go
index cb452d21..ececfa7e 100644
--- a/examples/route_guide/server/server.go
+++ b/examples/route_guide/server/server.go
@@ -167,22 +167,20 @@ func toRadians(num float64) float64 {
 }
 
 // calcDistance calculates the distance between two points using the "haversine" formula.
-// This code was taken from http://www.movable-type.co.uk/scripts/latlong.html.
+// The formula is based on http://mathforum.org/library/drmath/view/51879.html.
 func calcDistance(p1 *pb.Point, p2 *pb.Point) int32 {
 	const CordFactor float64 = 1e7
-	const R float64 = float64(6371000) // metres
-	lat1 := float64(p1.Latitude) / CordFactor
-	lat2 := float64(p2.Latitude) / CordFactor
-	lng1 := float64(p1.Longitude) / CordFactor
-	lng2 := float64(p2.Longitude) / CordFactor
-	φ1 := toRadians(lat1)
-	φ2 := toRadians(lat2)
-	Δφ := toRadians(lat2 - lat1)
-	Δλ := toRadians(lng2 - lng1)
+	const R float64 = float64(6371000) // earth radius in metres
+	lat1 := toRadians(float64(p1.Latitude) / CordFactor)
+	lat2 := toRadians(float64(p2.Latitude) / CordFactor)
+	lng1 := toRadians(float64(p1.Longitude) / CordFactor)
+	lng2 := toRadians(float64(p2.Longitude) / CordFactor)
+	dlat := lat2 - lat1
+	dlng := lng2 - lng1
 
-	a := math.Sin(Δφ/2)*math.Sin(Δφ/2) +
-		math.Cos(φ1)*math.Cos(φ2)*
-			math.Sin(Δλ/2)*math.Sin(Δλ/2)
+	a := math.Sin(dlat/2)*math.Sin(dlat/2) +
+		math.Cos(lat1)*math.Cos(lat2)*
+			math.Sin(dlng/2)*math.Sin(dlng/2)
 	c := 2 * math.Atan2(math.Sqrt(a), math.Sqrt(1-a))
 
 	distance := R * c