Added new export as FIT file format fixes #322

Actual FIT encoding logic created as a new repo "fit-file-writer"

js/format/Fit.js

@@ -0,0 +1,63 @@
+BR.Fit = {
+    format: function (geoJson, turnInstructionMode = 0) {
+        if (!geoJson?.features) return '';
+
+        function calcDistance(p1, p2) {
+            const [ilon1, ilat1] = btools.util.CheapRuler.toIntegerLngLat(p1);
+            const [ilon2, ilat2] = btools.util.CheapRuler.toIntegerLngLat(p2);
+            return btools.util.CheapRuler.distance(ilon1, ilat1, ilon2, ilat2);
+        }
+
+        const course = geoJson.features[0];
+        const track = course.geometry.coordinates;
+        const startPoint = track[0];
+        const endPoint = track[track.length - 1];
+        let trackStamps = course.properties.times || course.properties.coordTimes || [];
+        let last = 0;
+        trackStamps = trackStamps.map((stamp) => {
+            stamp = Math.round(stamp); // FIT only support seconds
+            // avoid strange & identical timestamps. This is required(?) for matching
+            // of turn instructions with the route.
+            if (stamp <= last) {
+                stamp = last + 1;
+            }
+            last = stamp;
+            // FIT epoch starts at 1989-12-31T00:00 - avoid wrapping
+            return (631065600 + stamp) * 1000;
+        });
+
+        const encoder = new FITCourseFile(
+            course.properties.name,
+            trackStamps[0],
+            course.properties['total-time'],
+            startPoint,
+            endPoint,
+            course.properties['filtered-ascend']
+        );
+        let distanceTotal = 0;
+        let distanceTillPoint = [0];
+        encoder.point(trackStamps[0], track[0], track[0][2], 0);
+        for (let i = 1; i < track.length; i++) {
+            distanceTotal += calcDistance(track[i - 1], track[i]);
+            distanceTillPoint[i] = distanceTotal;
+            encoder.point(trackStamps[i], track[i], track[i][2], distanceTotal);
+        }
+        // Note 1: Mixing points and hints is also legal should some devices
+        // require it - until then keep it simple.
+        // Note 2: The distance inside the FIT files seems somewhat redundant.
+        // Not sure if it is really required. May depend on the device?
+        // Note 3: Can't use hint.distance that is the value to the next turn.
+        const voiceHints = BR.voiceHints(geoJson, 2);
+        voiceHints._loopHints((hint, cmd, coord) => {
+            encoder.turn(
+                trackStamps[hint.indexInTrack],
+                coord,
+                cmd.fit,
+                cmd.fit === 'generic' ? cmd.message : undefined,
+                distanceTillPoint[hint.indexInTrack]
+            );
+        });
+
+        return encoder.finalize(trackStamps[trackStamps.length - 1]);
+    },
+};