import 'dart:typed_data';
import 'package:flutter/services.dart';

class GeoidGrid {
  final double southLat, westLon, latStep, lonStep;
  final int nRows, nCols;
  final Float32List grid; // big-endian, south-to-north

  GeoidGrid._(this.southLat, this.westLon, this.latStep, this.lonStep,
      this.nRows, this.nCols, this.grid);

  static Future<GeoidGrid> load(String assetPath) async {
    final bytes = (await rootBundle.load(assetPath)).buffer.asByteData();
    final southLat = bytes.getFloat64(0, Endian.big);
    final westLon = bytes.getFloat64(8, Endian.big);
    final latStep = bytes.getFloat64(16, Endian.big);
    final lonStep = bytes.getFloat64(24, Endian.big);
    final nRows = bytes.getInt32(32, Endian.big);
    final nCols = bytes.getInt32(36, Endian.big);

    final grid = Float32List(nRows * nCols);
    for (int i = 0; i < grid.length; i++) {
      grid[i] = bytes.getFloat32(40 + i * 4, Endian.big);
    }
    return GeoidGrid._(southLat, westLon, latStep, lonStep, nRows, nCols, grid);
  }

  /// Returns EHT2014 geoid undulation [m] at (lat, lon) via bilinear interpolation.
  /// Returns null if outside grid or in no-data area.
  double? getUndulation(double lat, double lon) {
    final rowF = (lat - southLat) / latStep;
    final colF = (lon - westLon) / lonStep;
    final r0 = rowF.floor();
    final c0 = colF.floor();
    if (r0 < 0 || r0 >= nRows - 1 || c0 < 0 || c0 >= nCols - 1) return null;

    final dr = rowF - r0;
    final dc = colF - c0;

    double v(int r, int c) => grid[r * nCols + c].toDouble();
    final noData = -80.0;
    final v00 = v(r0, c0);
    if (v00 < noData) return null;
    final v10 = v(r0 + 1, c0);
    if (v10 < noData) return null;
    final v01 = v(r0, c0 + 1);
    if (v01 < noData) return null;
    final v11 = v(r0 + 1, c0 + 1);
    if (v11 < noData) return null;

    return v00 * (1 - dr) * (1 - dc) +
        v10 * dr * (1 - dc) +
        v01 * (1 - dr) * dc +
        v11 * dr * dc;
  }

  /// Orthometric height in the Hungarian EHT2014 system.
  /// [nmea_ortho] = GGA field 9 (altitudeAboveMeanSeaLevel)
  /// [nmea_sep]   = GGA field 11 (geoidSeparation from receiver)
  double? toEovHeight(
      double lat, double lon, double nmeaOrtho, double nmeaSep) {
    final nEht2014 = getUndulation(lat, lon);
    if (nEht2014 == null) return null;
    final ellipsoidal = nmeaOrtho + nmeaSep; // h = H + N_receiver
    return ellipsoidal - nEht2014; // H_EOV = h - N_EHT2014
  }
}