maps-saas/packages/tactical_app/lib/services/dem_tile_elevation_service.dart

import 'dart:io';
import 'dart:math' as math;
import 'dart:ui' as ui;
import 'package:flutter/foundation.dart';
import 'package:http/http.dart' as http;
import 'package:intaleq_maps/intaleq_maps.dart';
import 'package:path_provider/path_provider.dart';
import 'offline_los_engine.dart';

class DecodedDemTile {
  final int zoom;
  final int x;
  final int y;
  final ByteData rgbaBytes;
  final int width;
  final int height;

  DecodedDemTile({
    required this.zoom,
    required this.x,
    required this.y,
    required this.rgbaBytes,
    this.width = 256,
    this.height = 256,
  });

  /// Sample sub-pixel elevation from Terrarium DEM tile using Bilinear Interpolation
  double sampleElevation(double subX, double subY) {
    final clampedX = subX.clamp(0.0, 254.99);
    final clampedY = subY.clamp(0.0, 254.99);

    final x0 = clampedX.floor();
    final x1 = x0 + 1;
    final y0 = clampedY.floor();
    final y1 = y0 + 1;

    final fx = clampedX - x0;
    final fy = clampedY - y0;

    double decodePixel(int px, int py) {
      final idx = (py * width + px) * 4;
      if (idx + 3 >= rgbaBytes.lengthInBytes) return 750.0;
      final r = rgbaBytes.getUint8(idx);
      final g = rgbaBytes.getUint8(idx + 1);
      final b = rgbaBytes.getUint8(idx + 2);
      // Terrarium formula: (R * 256 + G + B / 256) - 32768
      return (r * 256.0 + g.toDouble() + b.toDouble() / 256.0) - 32768.0;
    }

    final z00 = decodePixel(x0, y0);
    final z10 = decodePixel(x1, y0);
    final z01 = decodePixel(x0, y1);
    final z11 = decodePixel(x1, y1);

    final zTop = z00 * (1.0 - fx) + z10 * fx;
    final zBottom = z01 * (1.0 - fx) + z11 * fx;
    return (zTop * (1.0 - fy) + zBottom * fy).roundToDouble();
  }
}

/// Sovereign Satellite DEM Elevation Service with On-Device Disk Caching
class DemTileElevationService {
  DemTileElevationService._();

  static final Map<String, DecodedDemTile> _memoryCache = {};
  static Directory? _cacheDir;

  static Future<Directory> _getCacheDirectory() async {
    if (_cacheDir != null) return _cacheDir!;
    final appDir = await getApplicationDocumentsDirectory();
    final demDir = Directory('${appDir.path}/dem_tiles');
    if (!await demDir.exists()) {
      await demDir.create(recursive: true);
    }
    _cacheDir = demDir;
    return _cacheDir!;
  }

  /// Convert Lat/Lng to tile X, Y and Sub-pixel offsets at specified zoom level
  static ({int tileX, int tileY, double subX, double subY}) _coordToTile(
      double lat, double lng, int zoom) {
    final n = 1 << zoom;
    final xVal = ((lng + 180.0) / 360.0) * n;
    final latRad = lat * (math.pi / 180.0);
    final yVal = (1.0 -
            math.log(math.tan(latRad) + 1.0 / math.cos(latRad)) / math.pi) /
        2.0 *
        n;

    final tileX = xVal.floor();
    final tileY = yVal.floor();

    final subX = (xVal - tileX) * 256.0;
    final subY = (yVal - tileY) * 256.0;

    return (tileX: tileX, tileY: tileY, subX: subX, subY: subY);
  }

  /// Get or fetch a single DEM tile with disk persistence
  static Future<DecodedDemTile?> getTile(int zoom, int x, int y) async {
    final key = '$zoom/$x/$y';
    if (_memoryCache.containsKey(key)) {
      return _memoryCache[key];
    }

    try {
      final cacheDir = await _getCacheDirectory();
      final localFile = File('${cacheDir.path}/${zoom}_${x}_$y.png');

      Uint8List bytes;
      if (await localFile.exists()) {
        bytes = await localFile.readAsBytes();
      } else {
        final url = Uri.parse(
            'https://s3.amazonaws.com/elevation-tiles-prod/terrarium/$zoom/$x/$y.png');
        final resp = await http.get(url).timeout(const Duration(seconds: 4));
        if (resp.statusCode == 200) {
          bytes = resp.bodyBytes;
          await localFile.writeAsBytes(bytes);
        } else {
          return null;
        }
      }

      final codec = await ui.instantiateImageCodec(bytes);
      final frame = await codec.getNextFrame();
      final image = frame.image;
      final byteData =
          await image.toByteData(format: ui.ImageByteFormat.rawRgba);

      if (byteData != null) {
        final decoded = DecodedDemTile(
          zoom: zoom,
          x: x,
          y: y,
          rgbaBytes: byteData,
          width: image.width,
          height: image.height,
        );
        _memoryCache[key] = decoded;
        return decoded;
      }
    } catch (e) {
      debugPrint('DemTileElevationService: Failed to load tile $key: $e');
    }
    return null;
  }

  /// Pre-fetch all DEM tiles needed for a bounding box or list of coordinates
  static Future<void> prefetchTilesForCoords(List<LatLng> coords,
      {int zoom = 12}) async {
    final uniqueTiles = <String, ({int z, int x, int y})>{};
    for (final pt in coords) {
      final t = _coordToTile(pt.latitude, pt.longitude, zoom);
      final k = '$zoom/${t.tileX}/${t.tileY}';
      uniqueTiles[k] = (z: zoom, x: t.tileX, y: t.tileY);
    }

    await Future.wait(
      uniqueTiles.values.map((t) => getTile(t.z, t.x, t.y)),
    );
  }

  /// Synchronously get elevation if tile is cached in memory, otherwise fallback to Jordan DEM Surface
  static double getElevationSync(double lat, double lng, {int zoom = 12}) {
    final t = _coordToTile(lat, lng, zoom);
    final key = '$zoom/${t.tileX}/${t.tileY}';
    final tile = _memoryCache[key];
    if (tile != null) {
      return tile.sampleElevation(t.subX, t.subY);
    }
    return JordanDemSurface.elevationAt(lat, lng);
  }

  /// Asynchronously get real satellite elevation with on-demand tile loading
  static Future<double> getElevationAsync(double lat, double lng,
      {int zoom = 12}) async {
    final t = _coordToTile(lat, lng, zoom);
    final tile = await getTile(zoom, t.tileX, t.tileY);
    if (tile != null) {
      return tile.sampleElevation(t.subX, t.subY);
    }
    return JordanDemSurface.elevationAt(lat, lng);
  }

  /// Standard alias for asynchronous elevation query
  static Future<double> getElevation(double lat, double lng, {int zoom = 12}) {
    return getElevationAsync(lat, lng, zoom: zoom);
  }
}