kd_tree.js

const kdbush = require('kdbush');
const geokdbush = require('geokdbush');
const haversine = require('haversine-distance');

// Example train track points as 2D vectors (longitude, latitude)
let track_points = [
    [longitude1, latitude1],
    [longitude2, latitude2],
    //...
];

// Function to convert degrees to radians 
function convert_to_radians(points) {
    return points.map(([longitude, latitude]) => [
        (longitude * Math.PI) / 180,
        (latitude * Math.PI) / 180,
    ]);
}

// Convert track points to radians
track_points = convert_to_radians(track_points);

// Build KDTree
let tree = kdbush(track_points);

// Function to find nearest track point for each person
function find_nearest_track_points(person_locations, track_tree) {
    person_locations = convert_to_radians(person_locations);  // Convert to radians
    let nearest_points = [];
    let distances = [];
    person_locations.forEach((location) => {
        const nearest = geokdbush.around(track_tree, location[0], location[1], 1);
        nearest_points.push(nearest[0]);
        distances.push(haversine(location, nearest[0]));  // Convert distance from radians to kilometers
    });
    return [nearest_points, distances];
}

// Example person locations as 2D vectors (longitude, latitude)
let person_locations = [
    [longitude_of_person1, latitude_of_person1],
    [longitude_of_person2, latitude_of_person2],
    //...
];

// Find nearest points and distances
let [nearest_points, distances] = find_nearest_track_points(person_locations, tree);

// Output results
nearest_points.forEach((point, i) => {
    console.log(`Person ${i+1} nearest track point is ${point} with distance ${distances[i].toFixed(2)} km.`);
});