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public/js/distance.js

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+(function (ns) {
+  'use strict';
+  const radians = value => value * Math.PI / 180;
+  ns.Distance = {
+    meters(a, b) {
+      const dLat = radians(b.lat - a.lat); const dLon = radians(b.lon - a.lon);
+      const h = Math.sin(dLat / 2) ** 2 + Math.cos(radians(a.lat)) * Math.cos(radians(b.lat)) * Math.sin(dLon / 2) ** 2;
+      return 2 * 6371000 * Math.asin(Math.sqrt(h));
+    },
+    format(meters) { return meters < 1000 ? `${Math.round(meters)} m` : `${(meters / 1000).toFixed(1)} km`; }
+  };
+}(window.Wegwichtel));