Add support for jumping to event links (#103)

Fixes #78.

Reviewed-on: #103

lib/helpers/extensions/get_link_info.dart

@@ -0,0 +1,70 @@
+import "package:fast_immutable_collections/fast_immutable_collections.dart";
+
+/// A parsed Matrix link.
+///
+/// [identifier] is the room ID, room alias or user ID (e.g. "#room:matrix.org"),
+/// [eventId] is set for event permalinks (e.g. "$abc123"), and [via] holds any
+/// `via` servers from the link.
+typedef LinkInfo = ({String identifier, String? eventId, IList<String> via});
+
+extension GetLinkInfo on String {
+  /// Parses this string as a Matrix link.
+  ///
+  /// Supports:
+  /// - https://matrix.to/#/{id}[/{eventId}][?via=...]
+  /// - matrix:roomid/{id}[/e/{eventId}][?via=...]
+  /// - matrix:r/{alias}[/e/{eventId}][?via=...]
+  /// - matrix:u/{userId}
+  ///
+  /// Returns null if this is not a Matrix link.
+  LinkInfo? get linkInfo {
+    final uri = Uri.tryParse(trim());
+    if (uri == null) return null;
+
+    if ((uri.scheme == "http" || uri.scheme == "https") &&
+        uri.host == "matrix.to") {
+      // matrix.to keeps everything, including the query, in the fragment,
+      // so parse the fragment as its own URI.
+      final inner = Uri.tryParse(uri.fragment);
+      final segments = inner?.pathSegments.where((s) => s.isNotEmpty).toList();
+      if (inner == null || segments == null || segments.isEmpty) return null;
+
+      return (
+        identifier: segments.first,
+        eventId: segments.length > 1 && segments[1].startsWith(r"$")
+            ? segments[1]
+            : null,
+        via: .new(inner.queryParametersAll["via"] ?? []),
+      );
+    }
+
+    if (uri.scheme == "matrix") {
+      final segments = uri.pathSegments;
+      if (segments.length < 2 || segments[1].isEmpty) return null;
+
+      final sigil = switch (segments[0].toLowerCase()) {
+        "r" => "#",
+        "roomid" => "!",
+        "u" => "@",
+        _ => null,
+      };
+      if (sigil == null) return null;
+
+      // Event segment is "e/{eventId}" (without the "$" sigil) and is
+      // only valid on room links.
+      final hasEvent =
+          sigil != "@" &&
+          segments.length >= 4 &&
+          segments[2].toLowerCase() == "e" &&
+          segments[3].isNotEmpty;
+
+      return (
+        identifier: "$sigil${segments[1]}",
+        eventId: hasEvent ? "\$${segments[3]}" : null,
+        via: .new(uri.queryParametersAll["via"] ?? []),
+      );
+    }
+
+    return null;
+  }
+}