P7: community flairs, from Lemmy and PieFed

- A post's flairs are read in both dialects: Lemmy 1.0's CommunityPostTag (id, slug, name, description, colour slot)
  and PieFed's lemmy:CommunityTag (display name, text and background colours, whether to blur images). Colours are
  kept only as a colour slot or a hex value.
- A community's own list comes from its `tag` and PieFed's older `lemmy:tagsForPosts`, and fills in a post that names
  only a flair's id and slug, as Lemmy's posts do. Edits and refreshes keep them current.
- Clients get them as `privapub.flairs`.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012CzABvBkbcFqoHdmi8b9WB

PrivaPub/Federation/Objects/NoteParser.cs

@@ -40,6 +40,7 @@ namespace PrivaPub.Federation.Objects
 		public IReadOnlyList<PostMedia> Attachments { get; init; } = Array.Empty<PostMedia>();
 		public PostSource Source { get; init; }
 		public PostPoll Poll { get; init; }
+		public List<PostFlair> Flairs { get; init; } = new();
 		public InteractionRule QuotePolicy { get; init; }
 		public InteractionRule ReplyPolicy { get; init; }
 		public InteractionRule LikePolicy { get; init; }
@@ -119,6 +120,7 @@ namespace PrivaPub.Federation.Objects
 				Attachments = Attachments(note),
 				Source = ObjectShapes.Source(note),
 				Poll = ObjectShapes.Poll(note),
+				Flairs = ObjectShapes.Flairs(note["tag"]),
 				QuotePolicy = ObjectShapes.QuotePolicy(note),
 				ReplyPolicy = ObjectShapes.Policy(note, "canReply"),
 				LikePolicy = ObjectShapes.Policy(note, "canLike"),

PrivaPub/Federation/Objects/ObjectShapes.cs

@@ -98,6 +98,48 @@ namespace PrivaPub.Federation.Objects
 				.Take(MaxEmojis)
 				.ToList();
 
+		const int MaxFlairs = 20;
+
+		// a post's flairs, or a community's (its `tag`, and PieFed's older `lemmy:tagsForPosts`), in both dialects
+		public static List<PostFlair> Flairs(params JsonNode[] lists) =>
+			lists.SelectMany(Objects)
+				.Where(t => Value(t, "type") is "CommunityPostTag" or "lemmy:CommunityTag" or "CommunityTag")
+				.Select(t => new PostFlair
+				{
+					Id = Id(t) is { } id && Origin.Of(id) != default ? id : default,
+					Name = Text(Value(t, "name") ?? Value(t, "display_name") ?? Value(t, "preferredUsername"), 100),
+					Slug = Text(Value(t, "preferredUsername") ?? Value(t, "display_name"), 100),
+					Description = Text(Value(t, "content"), 500),
+					Color = Value(t, "color") is { } slot && FlairSlot().IsMatch(slot) ? slot : default,
+					TextColor = Hex(Value(t, "textColor") ?? Value(t, "text_color")),
+					BackgroundColor = Hex(Value(t, "backgroundColor") ?? Value(t, "background_color")),
+					BlurImages = Bool(t, "blurImages") ?? Bool(t, "blur_images") ?? false
+				})
+				.Where(f => f.Name != default)
+				.DistinctBy(f => f.Id ?? f.Slug)
+				.Take(MaxFlairs)
+				.ToList();
+
+		// a post's flairs as its community describes them: Lemmy names only the id and slug on the post
+		public static List<PostFlair> Described(List<PostFlair> onPost, List<PostFlair> ofCommunity)
+		{
+			if (onPost == default || onPost.Count == 0)
+				return default;
+			foreach (var flair in onPost)
+				if (ofCommunity?.FirstOrDefault(c => c.Id != default && c.Id == flair.Id || c.Slug == flair.Slug) is { } known)
+				{
+					flair.Name = flair.Name == flair.Slug ? known.Name ?? flair.Name : flair.Name;
+					flair.Description ??= known.Description;
+					flair.Color ??= known.Color;
+					flair.TextColor ??= known.TextColor;
+					flair.BackgroundColor ??= known.BackgroundColor;
+					flair.BlurImages |= known.BlurImages;
+				}
+			return onPost;
+		}
+
+		static string Hex(string value) => value != default && HexColour().IsMatch(value) ? value : default;
+
 		public static InteractionRule QuotePolicy(JsonObject note) => Policy(note, "canQuote");
 
 		// one rule of a post's interactionPolicy (GoToSocial, FEP-044f): who may, automatically or once asked; the old names
@@ -399,6 +441,12 @@ namespace PrivaPub.Federation.Objects
 		[GeneratedRegex(@"^[\w+\-.@]+$")]
 		private static partial Regex Shortcode();
 
+		[GeneratedRegex(@"^color\d{2}$")]
+		private static partial Regex FlairSlot();
+
+		[GeneratedRegex(@"^#[0-9a-fA-F]{3,8}$")]
+		private static partial Regex HexColour();
+
 		[GeneratedRegex("<[^>]*>")]
 		private static partial Regex Tags();