shoc-backend/Api.SeaHavenIndustries/Helper/ZipCodeDistance.cs

namespace Api.SeaHavenIndustries.Helper
{
    public class ZipCodeDistance
    {
        private readonly Dictionary<string, (double lat, double lng)> _coords = new();
        private bool _loaded;

        public void Load(string csvPath)
        {
            if (_loaded) return;
            foreach (var line in File.ReadLines(csvPath).Skip(1))
            {
                var parts = line.Split(',');
                if (parts.Length >= 3
                    && double.TryParse(parts[1].Trim(), out var lat)
                    && double.TryParse(parts[2].Trim(), out var lng))
                {
                    _coords[parts[0].Trim().PadLeft(5, '0')] = (lat, lng);
                }
            }
            _loaded = true;
        }

        public double? GetDistanceMiles(string? zip1, string? zip2)
        {
            if (string.IsNullOrWhiteSpace(zip1) || string.IsNullOrWhiteSpace(zip2)) return null;
            var z1 = zip1.Trim().PadLeft(5, '0');
            var z2 = zip2.Trim().PadLeft(5, '0');
            if (z1.Length > 5) z1 = z1.Substring(0, 5);
            if (z2.Length > 5) z2 = z2.Substring(0, 5);
            if (!_coords.TryGetValue(z1, out var c1) || !_coords.TryGetValue(z2, out var c2)) return null;
            return Haversine(c1.lat, c1.lng, c2.lat, c2.lng);
        }

        private static double Haversine(double lat1, double lon1, double lat2, double lon2)
        {
            const double R = 3958.8;
            var dLat = ToRad(lat2 - lat1);
            var dLon = ToRad(lon2 - lon1);
            var a = Math.Sin(dLat / 2) * Math.Sin(dLat / 2) +
                    Math.Cos(ToRad(lat1)) * Math.Cos(ToRad(lat2)) *
                    Math.Sin(dLon / 2) * Math.Sin(dLon / 2);
            return R * 2 * Math.Atan2(Math.Sqrt(a), Math.Sqrt(1 - a));
        }

        private static double ToRad(double deg) => deg * Math.PI / 180.0;
    }
}