import '../map/map_provider_interface.dart';

/// Decodes Valhalla's precision-6 encoded polyline into standard AppLatLng coordinates.
class ValhallaPolylineDecoder {
  /// Decodes precision-6 polyline string into a list of [AppLatLng].
  ///
  /// Throws [FormatException] if the encoded string is truncated, contains
  /// invalid character codes (< 63), or resolves to coordinates outside valid
  /// geographic bounds (lat: [-90, 90], lng: [-180, 180]).
  static List<AppLatLng> decode(String encoded) {
    if (encoded.isEmpty) return const [];

    final List<AppLatLng> polyline = [];
    int index = 0;
    int lat = 0;
    int lng = 0;
    final int len = encoded.length;

    while (index < len) {
      // Decode latitude delta
      int b;
      int shift = 0;
      int result = 0;
      do {
        if (index >= len) {
          throw const FormatException('Truncated polyline data while reading latitude');
        }
        final code = encoded.codeUnitAt(index++);
        if (code < 63) {
          throw FormatException('Invalid polyline character at index ${index - 1}: code $code');
        }
        b = code - 63;
        result |= (b & 0x1f) << shift;
        shift += 5;
      } while (b >= 0x20);

      final int dlat = (result & 1) != 0 ? ~(result >> 1) : (result >> 1);
      lat += dlat;

      // Decode longitude delta
      shift = 0;
      result = 0;
      do {
        if (index >= len) {
          throw const FormatException('Truncated polyline data while reading longitude');
        }
        final code = encoded.codeUnitAt(index++);
        if (code < 63) {
          throw FormatException('Invalid polyline character at index ${index - 1}: code $code');
        }
        b = code - 63;
        result |= (b & 0x1f) << shift;
        shift += 5;
      } while (b >= 0x20);

      final int dlng = (result & 1) != 0 ? ~(result >> 1) : (result >> 1);
      lng += dlng;

      final latDeg = lat / 1e6;
      final lngDeg = lng / 1e6;

      if (latDeg < -90.0 || latDeg > 90.0 || lngDeg < -180.0 || lngDeg > 180.0) {
        throw FormatException('Decoded coordinate out of geographic bounds: ($latDeg, $lngDeg)');
      }

      polyline.add(AppLatLng(latDeg, lngDeg));
    }

    return polyline;
  }
}