From ce58f5f601f6cc3cb36997453c80a5edba04dbdf Mon Sep 17 00:00:00 2001
From: "Phyks (Lucas Verney)" <phyks@phyks.me>
Date: Wed, 21 Nov 2018 12:02:59 +0100
Subject: [PATCH] Use cheap-ruler rather than Haversine formula

---
 src/constants.js   |  6 ++----
 src/tools/index.js | 28 ++++++++++++++++------------
 2 files changed, 18 insertions(+), 16 deletions(-)

diff --git a/src/constants.js b/src/constants.js
index 5e373a0..bffeabd 100644
--- a/src/constants.js
+++ b/src/constants.js
@@ -31,10 +31,8 @@ export const UPDATE_REPORTS_DISTANCE_THRESHOLD = 500; // in meters
 // Minimal number of downvotes needed for a report to be masked
 export const REPORT_DOWNVOTES_THRESHOLD = 1;
 
-// Earth radius as recommended by the International Union of Geodesy and
-// Geophysics
-// https://rosettacode.org/wiki/Haversine_formula
-export const EARTH_RADIUS = 6371000; // in meters
+// Earth radius at equator
+export const EARTH_RADIUS = 6378137; // in meters
 
 // Keep reports only in a given radius around map center.
 export const KEEP_REPORTS_METERS_AROUND = 10000; // in meters
diff --git a/src/tools/index.js b/src/tools/index.js
index a1b1836..87ed4a7 100644
--- a/src/tools/index.js
+++ b/src/tools/index.js
@@ -1,6 +1,5 @@
 
 import {
-    EARTH_RADIUS,
     MOCK_LOCATION_GPX_PLAYBACK_SPEED,
     MOCK_LOCATION_UPDATE_INTERVAL,
     MOCK_LOCATION_USE_GPX,
@@ -15,20 +14,25 @@ if (process.env.NODE_ENV !== 'production') {
     mockGPX = require('mock_gpx.json'); // eslint-disable-line global-require
 }
 
+/**
+ * Cheap distance computation based on
+ * https://blog.mapbox.com/fast-geodesic-approximations-with-cheap-ruler-106f229ad016.
+ */
 export function distance(latLng1, latLng2) {
-    const lat1 = (latLng1[0] * Math.PI) / 180;
-    const lng1 = (latLng1[1] * Math.PI) / 180;
+    const cos = Math.cos((latLng1[0] + latLng2[0]) / 2 * Math.PI / 180);
+    const cos2 = 2 * cos * cos - 1;
+    const cos3 = 2 * cos * cos2 - cos;
+    const cos4 = 2 * cos * cos3 - cos2;
+    const cos5 = 2 * cos * cos4 - cos3;
 
-    const lat2 = (latLng2[0] * Math.PI) / 180;
-    const lng2 = (latLng2[1] * Math.PI) / 180;
+    // Multipliers for converting longitude and latitude degrees into distance
+    // (http://1.usa.gov/1Wb1bv7)
+    kx = 1000 * (111.41513 * cos - 0.09455 * cos3 + 0.00012 * cos5);
+    ky = 1000 * (111.13209 - 0.56605 * cos2 + 0.0012 * cos4);
 
-    const a = (
-        (Math.sin((lat2 - lat1) / 2.0) ** 2)
-        + (Math.cos(lat1) * Math.cos(lat2) * (Math.sin((lng2 - lng1) / 2.0) ** 2))
-    );
-    const c = 2.0 * Math.atan2(Math.sqrt(a), Math.sqrt(1 - a));
-
-    return EARTH_RADIUS * c;
+    const dx = (latLng1[1] - latLng2[1]) * kx;
+    const dy = (latLng1[0] - latLng2[0]) * ky;
+    return Math.sqrt(dx * dx + dy * dy);
 }
 
 export function mockLocationRandom() {