using System.Numerics;

namespace TheLivingWorld.Core.Geo;

/// <summary>
/// Flattens WGS84 coordinates onto a local metric plane centred on an origin: X grows east, Y grows north,
/// both in metres. This is an equirectangular projection with the scale factor frozen at the origin latitude,
/// which stays sub-metre accurate across the 5-20 km worlds this game generates and — unlike Web Mercator —
/// keeps distances usable directly as game units.
/// </summary>
public sealed class LocalProjection
{
    public const double EarthRadiusMeters = 6378137.0;
    public const double MetersPerDegreeLatitude = EarthRadiusMeters * Math.PI / 180.0;

    private readonly double _metersPerDegreeLongitude;

    public LocalProjection(GeoPoint origin)
    {
        if (!origin.IsValid)
            throw new ArgumentOutOfRangeException(nameof(origin), origin, "Origin is not a valid WGS84 coordinate.");

        Origin = origin;
        var cosLat = Math.Cos(origin.Latitude * Math.PI / 180.0);
        _metersPerDegreeLongitude = MetersPerDegreeLatitude * Math.Max(cosLat, 1e-6);
    }

    public GeoPoint Origin { get; }

    public Vector2 Project(GeoPoint point) => Project(point.Latitude, point.Longitude);

    public Vector2 Project(double latitude, double longitude) => new(
        (float)((longitude - Origin.Longitude) * _metersPerDegreeLongitude),
        (float)((latitude - Origin.Latitude) * MetersPerDegreeLatitude));

    public GeoPoint Unproject(Vector2 local) => new(
        Origin.Latitude + local.Y / MetersPerDegreeLatitude,
        Origin.Longitude + local.X / _metersPerDegreeLongitude);
}