Europa/Nuvolari/app/lib/features/map/map_style.dart

import 'dart:convert';

import '../../core/config/env.dart';
import '../../core/region/region_config.dart';

/// Which base map the app is showing, and therefore which credits are owed.
enum BaseMapKind {
  /// The default: free OpenStreetMap vector tiles from OpenFreeMap. No API key,
  /// no registration, no request limits. Owes the OpenMapTiles and
  /// OpenStreetMap credits.
  openFreeMap,

  /// A style URL supplied through `MAP_STYLE_URL`. Assumed to be OSM-derived,
  /// since every practical alternative is, so it owes the same credits.
  custom,

  /// The bundled fallback: a flat background and the region outline, drawn from
  /// the region config. Contains no third-party map data, so crediting
  /// OpenStreetMap here would be a false attribution.
  offlineFallback,
}

/// Resolves the MapLibre style the app should load.
class MapStyle {
  const MapStyle({required this.kind, required this.styleString});

  /// OpenFreeMap's Positron style.
  ///
  /// Chosen over Liberty and Bright because a radar overlay has to be the
  /// loudest thing on screen. Positron is a desaturated grey base designed for
  /// exactly this — the precipitation colours read against it instead of
  /// competing with road casings and landuse fills.
  ///
  /// Free, unmetered and key-less: <https://openfreemap.org>.
  static const String openFreeMapStyleUrl =
      'https://tiles.openfreemap.org/styles/positron';

  factory MapStyle.forRegion(RegionConfig region) {
    if (Env.wantsOfflineStyle) {
      return MapStyle(
        kind: BaseMapKind.offlineFallback,
        styleString: buildFallbackStyle(region),
      );
    }
    if (Env.hasCustomMapStyle) {
      return MapStyle(kind: BaseMapKind.custom, styleString: Env.mapStyleUrl);
    }
    return const MapStyle(
      kind: BaseMapKind.openFreeMap,
      styleString: openFreeMapStyleUrl,
    );
  }

  final BaseMapKind kind;

  /// Either a style URL or an inline MapLibre style document; the map widget
  /// accepts both.
  final String styleString;

  /// Attribution ids owed by the base map itself, as they appear in the region
  /// config. The radar layer adds its own on top of these.
  ///
  /// OpenFreeMap's own credit is optional by their terms, so it lives on the
  /// Sources screen rather than in the always-visible bar; the OpenMapTiles and
  /// OpenStreetMap credits are mandatory and go in the bar.
  Set<String> get attributionIds => switch (kind) {
    BaseMapKind.openFreeMap ||
    BaseMapKind.custom => const <String>{'openmaptiles', 'osm'},
    BaseMapKind.offlineFallback => const <String>{},
  };

  /// Builds a self-contained MapLibre style: a flat background plus the region
  /// bounding box, with no network sources at all.
  ///
  /// The outline is deliberately the *bounding box*, not the administrative
  /// boundary. A real boundary would need a third-party dataset with its own
  /// licence, and would make the fallback look like a finished map when it is
  /// a placeholder.
  static String buildFallbackStyle(RegionConfig region) {
    final b = region.bounds;
    final ring = <List<double>>[
      <double>[b.west, b.south],
      <double>[b.east, b.south],
      <double>[b.east, b.north],
      <double>[b.west, b.north],
      <double>[b.west, b.south],
    ];

    final style = <String, Object?>{
      'version': 8,
      'name': 'Nuvolari offline fallback',
      'sources': <String, Object?>{
        'region-extent': <String, Object?>{
          'type': 'geojson',
          'data': <String, Object?>{
            'type': 'Feature',
            'properties': <String, Object?>{'id': region.id},
            'geometry': <String, Object?>{
              'type': 'Polygon',
              'coordinates': <Object?>[ring],
            },
          },
        },
      },
      'layers': <Object?>[
        <String, Object?>{
          'id': 'background',
          'type': 'background',
          'paint': <String, Object?>{'background-color': '#0B1720'},
        },
        <String, Object?>{
          'id': 'region-extent-fill',
          'type': 'fill',
          'source': 'region-extent',
          'paint': <String, Object?>{
            'fill-color': '#13303F',
            'fill-opacity': 0.7,
          },
        },
        <String, Object?>{
          'id': 'region-extent-outline',
          'type': 'line',
          'source': 'region-extent',
          'paint': <String, Object?>{
            'line-color': '#4FA3D1',
            'line-width': 1.5,
          },
        },
      ],
    };

    return jsonEncode(style);
  }
}