From da7a0aa5d6602e49645819db7286cf74cfae702e Mon Sep 17 00:00:00 2001
From: Linus Groh <mail@linusgroh.de>
Date: Wed, 2 Oct 2019 19:23:45 +0100
Subject: [PATCH] Update distanceBetweenCoordinates

---
 src/util.js        | 36 +++++++++++++++++++++---------------
 tests/util.test.js |  6 +++---
 2 files changed, 24 insertions(+), 18 deletions(-)

diff --git a/src/util.js b/src/util.js
index c9b0747..d934cf1 100644
--- a/src/util.js
+++ b/src/util.js
@@ -35,27 +35,33 @@ export const isIsoDate = s => ISO_DATE_REGEXP.test(s);
 export const degreesToRadians = degrees => (degrees * Math.PI) / 180;
 
 /**
- * Calculate the distance between two coordinates.
- * https://stackoverflow.com/a/365853/5952681
+ * Calculate the distance between two coordinates. Uses the haversine formula,
+ * which is not 100% accurate - but that's not the goal here.
+ *
+ * https://en.wikipedia.org/wiki/Haversine_formula
  *
  * @param {Coordinate} c1 First coordinate
  * @param {Coordinate} c2 Second coordinate
  * @return {Number} Distance in meters
  */
 export const distanceBetweenCoordinates = (c1, c2) => {
-  const latDistanceInRad = degreesToRadians(c1.lat - c2.lat);
-  const lngDistanceInRad = degreesToRadians(c1.lng - c2.lng);
-  const lat1InRad = degreesToRadians(c1.lat);
-  const lat2InRad = degreesToRadians(c2.lat);
-  const a =
-    Math.sin(latDistanceInRad / 2) * Math.sin(latDistanceInRad / 2) +
-    Math.sin(lngDistanceInRad / 2) *
-      Math.sin(lngDistanceInRad / 2) *
-      Math.cos(lat1InRad) *
-      Math.cos(lat2InRad);
-  const c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1 - a));
-  // Return distance in meters
-  return EARTH_RADIUS_IN_KM * c * 1000;
+  const r = EARTH_RADIUS_IN_KM * 1000;
+  const phi1 = degreesToRadians(c1.lat);
+  const phi2 = degreesToRadians(c2.lat);
+  const lambda1 = degreesToRadians(c1.lng);
+  const lambda2 = degreesToRadians(c2.lng);
+  const d =
+    2 *
+    r *
+    Math.asin(
+      Math.sqrt(
+        Math.sin((phi2 - phi1) / 2) ** 2 +
+          Math.cos(phi1) *
+            Math.cos(phi2) *
+            Math.sin((lambda2 - lambda1) / 2) ** 2
+      )
+    );
+  return d;
 };
 
 /**
diff --git a/tests/util.test.js b/tests/util.test.js
index f897052..3fbf571 100644
--- a/tests/util.test.js
+++ b/tests/util.test.js
@@ -71,7 +71,7 @@ describe("distanceBetweenCoordinates", () => {
         { lat: 51.501752, lng: -0.1408258 }
       )
       // 3.74km according to Google Maps
-    ).toBe(3734.363267904623);
+    ).toBe(3734.3632679046705);
 
     // Gatwick Airport - Heathrow Airport
     expect(
@@ -80,7 +80,7 @@ describe("distanceBetweenCoordinates", () => {
         { lat: 51.4720694, lng: -0.4499871 }
       )
       // 40km according to Google Maps
-    ).toBe(40321.45758693094);
+    ).toBe(40321.457586930104);
 
     // Berlin - San Francisco
     expect(
@@ -89,6 +89,6 @@ describe("distanceBetweenCoordinates", () => {
         { lat: 37.7576948, lng: -122.4726193 }
       )
       // 9,102.73km according to Google Maps
-    ).toBe(9105627.810109459);
+    ).toBe(9105627.810109457);
   });
 });