Share map theme profiles as profile codes

lib/providers/user_preferences_provider.dart

@@ -336,9 +336,12 @@ class MapThemeProfilesProvider extends Notifier<MapThemeProfilesState> {
         .toList(growable: false);
     final builtInProfileIds =
         builtInProfiles.map((profile) => profile.id).toSet();
+    // Oldest first, so a new or imported profile lands at the end of the
+    // list. Box order follows the random profile ids.
     final customProfiles = allProfiles
         .where((profile) => !builtInProfileIds.contains(profile.id))
-        .toList(growable: false);
+        .toList()
+      ..sort((a, b) => a.createdAt.compareTo(b.createdAt));
     final sortedProfiles = [
       ...builtInProfiles,
       ...customProfiles,

lib/services/map_theme_profile_code.dart

@@ -0,0 +1,130 @@
+import 'dart:convert';
+
+/// A map theme profile as one line of text, so players can paste their map
+/// colors into Discord and a teammate can import them.
+///
+/// The code is `icarus-theme:` followed by base64url (unpadded) of a
+/// versioned JSON object:
+///
+///     {"v":1,"name":"Lotus Moss","base":"#18221A","detail":"#7FA36B","highlight":"#E3C567"}
+///
+/// Colors are `#RRGGBB` strings, the same shape `MapThemePalette.toJson`
+/// writes. The codec depends on nothing but `dart:convert`, so any client or
+/// server can read the format.
+///
+/// Parsing finds the code anywhere in the pasted text, so a code copied
+/// together with the message around it still imports.
+class MapThemeProfileCode {
+  MapThemeProfileCode._();
+
+  static const String prefix = 'icarus-theme:';
+  static const int version = 1;
+  static const int maxNameLength = 40;
+
+  static final RegExp _codePattern = RegExp(r'icarus-theme:([A-Za-z0-9_-]+)');
+  static final RegExp _hexColor = RegExp(r'^#[0-9a-fA-F]{6}$');
+
+  /// [colors] holds `base`, `detail`, and `highlight` as `#RRGGBB`.
+  static String encode({
+    required String name,
+    required Map<String, dynamic> colors,
+  }) {
+    final json = jsonEncode({
+      'v': version,
+      'name': name,
+      'base': colors['base'],
+      'detail': colors['detail'],
+      'highlight': colors['highlight'],
+    });
+    final encoded = base64Url.encode(utf8.encode(json)).replaceAll('=', '');
+    return '$prefix$encoded';
+  }
+
+  /// The first profile code inside [text], or null when it holds none.
+  static String? find(String text) => _codePattern.firstMatch(text)?[0];
+
+  static MapThemeProfileCodeResult parse(String text) {
+    final match = _codePattern.firstMatch(text);
+    if (match == null) {
+      return text.trim().isEmpty
+          ? const MapThemeProfileCodeEmpty()
+          : const MapThemeProfileCodeInvalid();
+    }
+
+    final Object? decoded;
+    try {
+      decoded = jsonDecode(utf8.decode(base64Url.decode(_padded(match[1]!))));
+    } on FormatException {
+      return const MapThemeProfileCodeIncomplete();
+    }
+    if (decoded is! Map<String, dynamic>) {
+      return const MapThemeProfileCodeInvalid();
+    }
+
+    final codeVersion = decoded['v'];
+    if (codeVersion is! int) return const MapThemeProfileCodeInvalid();
+    if (codeVersion > version) return const MapThemeProfileCodeNewerVersion();
+    if (codeVersion != version) return const MapThemeProfileCodeInvalid();
+
+    final base = decoded['base'];
+    final detail = decoded['detail'];
+    final highlight = decoded['highlight'];
+    if (![base, detail, highlight]
+        .every((c) => c is String && _hexColor.hasMatch(c))) {
+      return const MapThemeProfileCodeIncomplete();
+    }
+
+    final rawName = decoded['name'];
+    final trimmedName = rawName is String ? rawName.trim() : '';
+    // Cap by code point so an emoji at the limit is never cut in half.
+    final name = trimmedName.isEmpty
+        ? 'Shared profile'
+        : String.fromCharCodes(trimmedName.runes.take(maxNameLength));
+
+    return MapThemeProfileCodeValid(
+      name: name,
+      colors: {
+        'base': (base as String).toUpperCase(),
+        'detail': (detail as String).toUpperCase(),
+        'highlight': (highlight as String).toUpperCase(),
+      },
+    );
+  }
+
+  static String _padded(String value) =>
+      value.padRight(value.length + (4 - value.length % 4) % 4, '=');
+}
+
+sealed class MapThemeProfileCodeResult {
+  const MapThemeProfileCodeResult();
+}
+
+class MapThemeProfileCodeValid extends MapThemeProfileCodeResult {
+  const MapThemeProfileCodeValid({required this.name, required this.colors});
+
+  final String name;
+
+  /// `base`, `detail`, and `highlight` as `#RRGGBB`, ready for
+  /// `MapThemePalette.fromJson`.
+  final Map<String, String> colors;
+}
+
+/// Nothing pasted yet.
+class MapThemeProfileCodeEmpty extends MapThemeProfileCodeResult {
+  const MapThemeProfileCodeEmpty();
+}
+
+/// Text with no profile code in it.
+class MapThemeProfileCodeInvalid extends MapThemeProfileCodeResult {
+  const MapThemeProfileCodeInvalid();
+}
+
+/// A code that was cut short or damaged on the way.
+class MapThemeProfileCodeIncomplete extends MapThemeProfileCodeResult {
+  const MapThemeProfileCodeIncomplete();
+}
+
+/// A code written by a newer Icarus than this one.
+class MapThemeProfileCodeNewerVersion extends MapThemeProfileCodeResult {
+  const MapThemeProfileCodeNewerVersion();
+}