import { validateCoordinates, validateUnit, Unit } from './validator';

const RADIUS = {
    km: 6371,
    miles: 3958.8,
};

/**
 * Calculates the Haversine distance between two coordinates.
 * @param lat1 Latitude of point 1
 * @param lon1 Longitude of point 1
 * @param lat2 Latitude of point 2
 * @param lon2 Longitude of point 2
 * @param unit Unit of measurement: 'km' or 'miles' (default: 'km')
 * @returns Distance between the two coordinates
 */
export function haversineDistance(lat1: number, lon1: number, lat2: number, lon2: number, unit: Unit = 'km'): number {
    validateCoordinates(lat1, lon1, 'Point 1');
    validateCoordinates(lat2, lon2, 'Point 2');
    validateUnit(unit);

    const toRad = (value: number) => (value * Math.PI) / 180;
    const R = RADIUS[unit];

    const dLat = toRad(lat2 - lat1);
    const dLon = toRad(lon2 - lon1);
    const a = Math.sin(dLat / 2) ** 2 + Math.cos(toRad(lat1)) * Math.cos(toRad(lat2)) * Math.sin(dLon / 2) ** 2;

    return R * 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1 - a));
}