SocialPub/PrivaPub/Api/Mastodon/Mappers/MastodonMapper.cs

using MongoDB.Entities;

using PrivaPub.Api.Mastodon.Entities;
using PrivaPub.Api.Mastodon.Infrastructure;
using PrivaPub.Domain.Media;
using PrivaPub.Federation.Actors;
using PrivaPub.Federation.Rendering;
using PrivaPub.Models.Federation;
using PrivaPub.Models.Group;
using PrivaPub.Domain.Privacy;
using PrivaPub.Federation.Objects;
using PrivaPub.Models.Post;
using PrivaPub.Domain.Statuses;
using PrivaPub.Models.Social;
using PrivaPub.Models.User;
using PrivaPub.StaticServices;

using System.Globalization;
using System.Net;

using GroupEntity = PrivaPub.Models.Group.Group;
using PostEntity = PrivaPub.Models.Post.Post;

namespace PrivaPub.Api.Mastodon.Mappers
{
	public class MastodonMapper
	{
		readonly DbEntities _dbEntities;
		readonly ILocalActorService _localActors;
		readonly IMediaProxy _proxy;

		public MastodonMapper(DbEntities dbEntities, ILocalActorService localActors, IMediaProxy proxy = default)
		{
			_dbEntities = dbEntities;
			_localActors = localActors;
			_proxy = proxy;
		}

		string Proxied(string url) => _proxy == default ? url : _proxy.Wrap(url);

		public string ProxiedUrl(string url) => Proxied(url);

		// notifications as the persona sees them: one whose account or post it can no longer see is left out
		public async Task<List<Entities.Notification>> Notifications(List<Models.Social.Notification> notifications, string viewerId, CancellationToken token)
		{
			var accounts = await Accounts(notifications.Select(n => n.FromAccountId), token);
			var postIds = notifications.Where(n => n.PostId != default).Select(n => n.PostId).Distinct().ToList();
			var posts = postIds.Count == 0
				? new List<PostEntity>()
				: await _dbEntities.Posts.Match(p => postIds.Contains(p.ID)).Match(VisibilityPolicy.IsShown).ExecuteAsync(token);
			var statuses = (await Statuses(posts, viewerId, token)).ToDictionary(s => s.Id);
			return notifications
				.Where(n => accounts.ContainsKey(n.FromAccountId ?? string.Empty) && (n.PostId == default || statuses.ContainsKey(n.PostId)))
				.Select(n => new Entities.Notification
				{
					Id = n.ID,
					Type = Controllers.NotificationsController.Names[n.Type],
					CreatedAt = MastodonJson.Time(n.CreatedAt),
					//the group it belongs to by default, so a notification told alone (v1, a stream) joins its group in a client
					GroupKey = Controllers.NotificationsController.GroupKey(n, Controllers.NotificationsController.DefaultGrouped),
					Account = accounts[n.FromAccountId],
					Status = n.PostId == default ? default : statuses[n.PostId],
					Emoji = n.Emoji == default ? default : n.EmojiURL == default ? n.Emoji : $":{n.Emoji}:",
					EmojiUrl = Proxied(n.EmojiURL)
				})
				.ToList();
		}

		string MissingAvatar => $"{_localActors.BaseAddress}/media/missing-avatar.png";
		string MissingHeader => $"{_localActors.BaseAddress}/media/missing-header.png";

		public async Task<Account> Local(LocalActor actor, bool withSource, CancellationToken token)
		{
			var followers = await DB.Default.CountAsync<Follower>(f => f.LocalActorId == actor.Id && f.LocalActorKind == actor.Kind && f.IsAccepted, token);
			var following = actor.Kind == LocalActorKind.Person
				? await DB.Default.CountAsync<Following>(f => f.AvatarId == actor.Id && f.State == FollowState.Accepted, token)
				: 0;
			var latest = await _dbEntities.Posts
				.Match(Counted.By(actor))
				.Sort(p => p.ID, Order.Descending)
				.Limit(1)
				.ExecuteFirstAsync(token);
			var statuses = latest == default ? 0 : await Counted.PostsBy(actor, token);

			var account = new Account
			{
				Id = actor.Id,
				Username = actor.UserName,
				Acct = actor.UserName,
				DisplayName = actor.Name ?? actor.UserName,
				Locked = actor.ManuallyApprovesFollowers,
				Bot = actor.IsBot,
				Discoverable = actor.Discoverable,
				Indexable = actor.Indexable,
				Noindex = !actor.Indexable,
				HideCollections = true,//follower and following lists are never shown to others, only their counts
				Group = actor.Kind == LocalActorKind.Group,
				CreatedAt = MastodonJson.Day(actor.Published),
				Note = ActivityPubRenderer.Html(actor.Summary),
				Url = actor.HtmlUrl,
				Uri = actor.Uri,
				Avatar = actor.PictureURL ?? MissingAvatar,
				AvatarStatic = actor.PictureURL ?? MissingAvatar,
				Header = actor.ThumbnailURL ?? MissingHeader,
				HeaderStatic = actor.ThumbnailURL ?? MissingHeader,
				FollowersCount = (int)followers,
				FollowingCount = (int)following,
				StatusesCount = (int)statuses,
				LastStatusAt = latest == default ? default : latest.CreationDate.ToString("yyyy-MM-dd"),
				Fields = actor.Fields.Select(f => new Field { Name = f.Key, Value = ActivityPubRenderer.FieldValue(f.Value) }).ToList()
			};
			if (withSource)
				account.Source = new Source
				{
					Privacy = actor.Settings.DefaultVisibility ?? "public",
					Sensitive = actor.Settings.DefaultSensitive,
					Language = actor.Settings.DefaultLanguage,
					QuotePolicy = actor.Settings.QuotePolicy ?? QuotePolicies.Public,
					Note = actor.Summary ?? string.Empty,
					Fields = actor.Fields.Select(f => new Field { Name = f.Key, Value = f.Value }).ToList(),
					FollowRequestsCount = (int)await DB.Default.CountAsync<Follower>(f => f.LocalActorId == actor.Id && !f.IsAccepted, token)
				};
			return account;
		}

		public Account Foreign(ForeignAvatar foreign) => new()
		{
			Id = foreign.ID,
			Username = foreign.UserName,
			Acct = $"{foreign.UserName}@{foreign.Domain}",
			DisplayName = foreign.Name ?? foreign.UserName ?? string.Empty,
			Bot = foreign.AvatarType is AvatarType.Service or AvatarType.Application,
			Group = foreign.AvatarType == AvatarType.Group,
			Locked = foreign.IsLocked,
			Discoverable = foreign.IsDiscoverable,
			Indexable = foreign.IsIndexable,
			Memorial = foreign.IsMemorial ? true : default(bool?),
			CreatedAt = MastodonJson.Day(foreign.Published ?? foreign.CreatedAt),
			Emojis = Emojis(foreign.Emojis),
			Fields = foreign.Fields.Select(f => new Field { Name = f.Key, Value = f.Value }).ToList(),
			AvatarDescription = foreign.PictureDescription ?? string.Empty,
			HeaderDescription = foreign.HeaderDescription ?? string.Empty,
			Note = foreign.Biography ?? string.Empty,
			Url = foreign.Url ?? foreign.ActorURI,
			Uri = foreign.ActorURI,
			Avatar = Proxied(foreign.PictureURL) ?? MissingAvatar,
			AvatarStatic = Proxied(foreign.PictureURL) ?? MissingAvatar,
			Header = Proxied(foreign.ThumbnailURL) ?? MissingHeader,
			HeaderStatic = Proxied(foreign.ThumbnailURL) ?? MissingHeader,
			FollowersCount = foreign.FollowersCount ?? 0,//as its server publishes them (AccountCountsJob); 0 while unknown
			FollowingCount = foreign.FollowingCount ?? 0,
			StatusesCount = foreign.StatusesCount ?? 0
		};

		public async Task<Account> Account(string id, CancellationToken token) =>
			(await Accounts(new[] { id }, token)).GetValueOrDefault(id);

		public async Task<Dictionary<string, Account>> Accounts(IEnumerable<string> ids, CancellationToken token)
		{
			var wanted = ids.Where(id => !string.IsNullOrEmpty(id)).Distinct().ToList();
			var accounts = new Dictionary<string, Account>();
			if (wanted.Count == 0)
				return accounts;

			foreach (var avatar in await _dbEntities.Avatars.Match(a => wanted.Contains(a.ID) && !a.DeletionAt.HasValue).ExecuteAsync(token))
				accounts[avatar.ID] = await Local(_localActors.FromAvatar(avatar), false, token);
			foreach (var group in await _dbEntities.Groups.Match(g => wanted.Contains(g.ID) && !g.DeletionAt.HasValue && g.Kind == GroupKind.Community).ExecuteAsync(token))
				accounts[group.ID] = await Local(_localActors.FromGroup(group), false, token);
			var foreigns = await _dbEntities.ForeignAvatars.Match(f => wanted.Contains(f.ID) && !f.DeletionAt.HasValue).ExecuteAsync(token);
			foreach (var foreign in foreigns)
				accounts[foreign.ID] = Foreign(foreign);
			// an account that moved shows where to, once the new account names it as one of its own (as a Move is believed)
			var movedTo = foreigns.Where(f => !string.IsNullOrEmpty(f.MovedToURL)).Select(f => f.MovedToURL).Distinct().ToList();
			if (movedTo.Count > 0)
			{
				var targets = (await _dbEntities.ForeignAvatars.Match(f => movedTo.Contains(f.ActorURI) && !f.DeletionAt.HasValue).ExecuteAsync(token))
					.ToDictionary(f => f.ActorURI);
				foreach (var foreign in foreigns)
					if (foreign.MovedToURL != default && targets.TryGetValue(foreign.MovedToURL, out var target) && target.AlsoKnownAs.Contains(foreign.ActorURI))
						accounts[foreign.ID].Moved = Foreign(target);
			}
			return accounts;
		}

		public async Task<Status> Status(PostEntity post, string viewerId, CancellationToken token) =>
			(await Statuses(new[] { post }, viewerId, nested: false, token, hideFromViewer: false)).FirstOrDefault();

		public Task<List<Status>> Statuses(IReadOnlyCollection<PostEntity> posts, string viewerId, CancellationToken token) =>
			Statuses(posts, viewerId, nested: false, token);

		// Lists leave out what the viewer blocked or muted; a status asked for by its id is still shown, as Mastodon shows it
		// (only its author's block of the viewer hides it, and that is decided before mapping)
		async Task<List<Status>> Statuses(IReadOnlyCollection<PostEntity> posts, string viewerId, bool nested, CancellationToken token,
			bool hideFromViewer = true)
		{
			var originalIds = posts.Where(p => p.ReblogOfPostId != default).Select(p => p.ReblogOfPostId).Distinct().ToList();
			var originals = originalIds.Count == 0
				? new Dictionary<string, PostEntity>()
				: (await _dbEntities.Posts.Match(p => originalIds.Contains(p.ID)).Match(VisibilityPolicy.IsShown).ExecuteAsync(token)).ToDictionary(p => p.ID);
			var all = posts.Concat(originals.Values).ToList();

			var mentionUris = all.SelectMany(p => p.Mentions).Where(m => !m.IsLocal).Select(m => m.ActorURI).Distinct().ToList();
			var mentionAccounts = mentionUris.Count == 0
				? new Dictionary<string, ForeignAvatar>()
				: (await _dbEntities.ForeignAvatars.Match(f => mentionUris.Contains(f.ActorURI)).ExecuteAsync(token)).ToDictionary(f => f.ActorURI);
			var accounts = await Accounts(all.Select(AuthorOf), token);

			var ids = all.Select(p => p.ID).ToList();
			var favourited = viewerId == default
				? new HashSet<string>()
				: (await _dbEntities.Favourites.Match(f => f.AccountId == viewerId && ids.Contains(f.PostId)).ExecuteAsync(token)).Select(f => f.PostId).ToHashSet();
			var bookmarked = viewerId == default
				? new HashSet<string>()
				: (await DB.Default.Find<Bookmark>().Match(b => b.AvatarId == viewerId && ids.Contains(b.PostId)).ExecuteAsync(token)).Select(b => b.PostId).ToHashSet();
			var pinned = (await DB.Default.Find<Pin>().Match(p => ids.Contains(p.PostId)).ExecuteAsync(token)).Select(p => p.PostId).ToHashSet();
			var reactions = (await DB.Default.Find<Reaction>().Match(r => ids.Contains(r.PostId)).ExecuteAsync(token))
				.GroupBy(r => r.PostId)
				.ToDictionary(g => g.Key, g => g.GroupBy(r => r.Emoji)
					.Select(e => new EmojiReactionEntity
					{
						Name = e.Key,
						Count = e.Count(),
						Me = viewerId != default && e.Any(r => r.AccountId == viewerId),
						Url = Proxied(e.First().EmojiURL),
						StaticUrl = Proxied(e.First().EmojiURL)
					})
					.OrderByDescending(e => e.Count)
					.ToList());
			var eventIds = all.Where(p => p.Event != default && p.IsFederatedCopy).Select(p => p.ID).ToList();
			var participations = viewerId == default || eventIds.Count == 0
				? new Dictionary<string, Models.Social.ParticipationState>()
				: (await DB.Default.Find<Models.Social.Participation>().Match(p => p.AvatarId == viewerId && eventIds.Contains(p.PostId)).ExecuteAsync(token))
					.ToDictionary(p => p.PostId, p => p.State);
			var pollIds = all.Where(p => p.Poll != default).Select(p => p.ID).ToList();
			var ownVotes = viewerId == default || pollIds.Count == 0
				? new Dictionary<string, List<int>>()
				: (await DB.Default.Find<PollVote>().Match(v => pollIds.Contains(v.PostId) && v.VoterAccountId == viewerId && v.IsLocalVoter).ExecuteAsync(token))
					.GroupBy(v => v.PostId).ToDictionary(g => g.Key, g => g.Select(v => v.Choice).ToList());
			var hidden = viewerId == default || !hideFromViewer
				? new HashSet<string>()
				: await Domain.Relationships.Hidden.AuthorsHiddenFrom(viewerId, all.Select(p => p.ActorURI), forNotifications: false, token);
			var filters = await Domain.Social.Filters.Of(viewerId, token);
			var reblogged = viewerId == default
				? new HashSet<string>()
				: (await _dbEntities.Posts.Match(p => p.AuthorAccountId == viewerId && ids.Contains(p.ReblogOfPostId) && !p.DeletedAt.HasValue).ExecuteAsync(token))
					.Select(p => p.ReblogOfPostId).ToHashSet();

			Status Map(PostEntity post)
			{
				if (AuthorOf(post) is not { } author || !accounts.TryGetValue(author, out var account))
					return default;//a post whose author we cannot show is left out, never a 500
				var status = new Status
				{
					Id = post.ID,
					Uri = post.ObjectURI,
					Url = post.Url ?? post.ObjectURI,
					Account = account,
					InReplyToId = post.AnsweringToPostId,
					InReplyToAccountId = post.InReplyToAccountId,
					Content = Content(post),
					CreatedAt = MastodonJson.Time(post.CreationDate),
					EditedAt = post.EditedAt.HasValue ? MastodonJson.Time(post.EditedAt.Value) : default,
					RepliesCount = post.RepliesCount,
					ReblogsCount = post.ReblogsCount,
					FavouritesCount = post.FavouritesCount,
					Favourited = favourited.Contains(post.ID),
					Reblogged = reblogged.Contains(post.ID),
					Bookmarked = bookmarked.Contains(post.ID),
					Pinned = pinned.Contains(post.ID),
					Sensitive = post.HasContentWarning,
					//a remote post keeps its own summary: sensitive with none (Akkoma's sensitive media, Lemmy's NSFW) hides the
					//media, never the words behind an invented warning; a local post warned without words gets the default one
					SpoilerText = post.SpoilerText ?? (post.HasContentWarning && !post.IsFederatedCopy ? post.Title ?? ActivityPubRenderer.ContentWarning : string.Empty),
					Visibility = Visibility(post.Visibility),
					Language = post.Language,
					MediaAttachments = post.Media.Count > 0 ? post.Media.Select(Media).ToList() : Playable(post),
					Card = Card(post),
					Poll = Poll(post, viewerId, ownVotes.GetValueOrDefault(post.ID)),
					Emojis = Emojis(post.Emojis),
					Privapub = Extension(post),
					EmojiReactions = reactions.GetValueOrDefault(post.ID) ?? new(),
					Pleroma = new PleromaStatus { EmojiReactions = reactions.GetValueOrDefault(post.ID) ?? new() },
					Mentions = post.Mentions.Where(m => !m.Silent).Select(m => Mention(m, mentionAccounts)).Where(m => m != default).ToList(),
					Tags = post.Tags.Select(t => new StatusTag { Name = t, Url = ActivityPubRenderer.TagUrl(_localActors.BaseAddress, t) }).ToList(),
					Filtered = filters.Of(post)
				};
				return status;
			}

			var localAuthors = all.Where(p => !p.IsFederatedCopy && p.GroupUserId != default).Select(p => p.GroupUserId).Distinct().ToList();
			var followsAuthor = viewerId == default || localAuthors.Count == 0
				? new HashSet<string>()
				: await FollowedLocalAuthors(viewerId, localAuthors, token);
			var quotedIds = nested ? new List<string>() : all.Where(p => p.QuoteState == QuoteState.Accepted && p.QuotedPostId != default).Select(p => p.QuotedPostId).Distinct().ToList();
			var quotedPosts = new List<PostEntity>();
			foreach (var quotedPost in quotedIds.Count == 0 ? new List<PostEntity>() : await _dbEntities.Posts.Match(p => quotedIds.Contains(p.ID)).Match(VisibilityPolicy.IsShown).ExecuteAsync(token))
				if (quotedPost.Visibility != PostVisibility.LocalGeo && await VisibilityPolicy.CanSee(quotedPost, viewerId, token))
					quotedPosts.Add(quotedPost);
			var quotedStatuses = quotedPosts.Count == 0
				? new Dictionary<string, Status>()
				: (await Statuses(quotedPosts, viewerId, nested: true, token)).ToDictionary(q => q.Id);

			void Quote(Status status, PostEntity post)
			{
				status.QuotesCount = post.QuotesCount;
				status.QuoteApproval = post.IsFederatedCopy ? Approval(post, viewerId) : LocalApproval(post, viewerId, followsAuthor.Contains(post.GroupUserId));
				status.InteractionPolicy = Policies(post);
				if (status.Privapub?.Event != default && participations.TryGetValue(post.ID, out var participation))
					status.Privapub.Event.Participation = participation.ToString().ToLowerInvariant();
				if (post.QuoteState == QuoteState.None)
					return;
				var state = post.QuoteState.ToString().ToLowerInvariant();
				var quoted = post.QuotedPostId != default && quotedStatuses.TryGetValue(post.QuotedPostId, out var shown) ? shown : default;
				status.Quote = nested
					? new QuoteEntity { State = state, QuotedStatusId = post.QuotedPostId }
					: new QuoteEntity { State = state, QuotedStatus = post.QuoteState == QuoteState.Accepted ? quoted : default, QuotedStatusId = post.QuotedPostId };
				if (!nested && post.QuoteState == QuoteState.Accepted && quoted != default)
					status.Content = WithoutQuoteFallback(status.Content);
			}

			var mapped = new List<Status>();
			foreach (var post in posts)
			{
				if (!VisibilityPolicy.Shown(post) || hidden.Contains(post.ActorURI) || post.ReblogOfPostId != default && originals.TryGetValue(post.ReblogOfPostId, out var hiddenOriginal)
					&& hidden.Contains(hiddenOriginal.ActorURI))
					continue;
				var status = Map(post);
				if (status == default)
					continue;
				Quote(status, post);
				if (post.ReblogOfPostId != default)
				{
					if (!originals.TryGetValue(post.ReblogOfPostId, out var original) || Map(original) is not { } inner)
						continue;
					Quote(inner, original);
					status.Reblog = inner;
					status.Filtered = inner.Filtered;//a boost is filtered as what it boosts
					status.Url = inner.Url;//a boost has no page of its own; /grunts would answer JSON to a browser
					status.Content = string.Empty;
					status.Reblogged = reblogged.Contains(original.ID);
				}
				mapped.Add(status);
			}
			return mapped;
		}

		public static string AuthorOf(PostEntity post) => post.AuthorAccountId ?? post.GroupUserId;

		static readonly AngleSharp.Html.Parser.HtmlParser Fragments = new();

		public static string WithoutQuoteFallback(string html)
		{
			if (string.IsNullOrEmpty(html) || !html.Contains("quote-inline", StringComparison.Ordinal))
				return html;
			var document = Fragments.ParseDocument($"<body>{html}</body>");
			foreach (var fallback in document.QuerySelectorAll(".quote-inline").ToList())
				fallback.Remove();
			return document.Body!.InnerHtml;
		}

		async Task<HashSet<string>> FollowedLocalAuthors(string viewerId, List<string> authorIds, CancellationToken token)
		{
			var uris = new Dictionary<string, string>();
			foreach (var avatar in await _dbEntities.Avatars.Match(a => authorIds.Contains(a.ID)).ExecuteAsync(token))
				uris[_localActors.FromAvatar(avatar).Uri] = avatar.ID;
			var targets = uris.Keys.ToList();
			return (await _dbEntities.Followings.Match(f => f.AvatarId == viewerId && targets.Contains(f.TargetActorURI) && f.State == FollowState.Accepted).ExecuteAsync(token))
				.Select(f => uris[f.TargetActorURI]).ToHashSet();
		}

		static QuoteApprovalEntity LocalApproval(PostEntity post, string viewerId, bool viewerFollows)
		{
			var policy = ActivityPubRenderer.QuotableBy(post);
			var everyone = policy switch { QuotePolicies.Public => new List<string> { "public" }, QuotePolicies.Followers => new List<string> { "followers" }, _ => new List<string>() };
			var current = viewerId == default ? "unknown"
				: viewerId == post.GroupUserId && post.Visibility is PostVisibility.Public or PostVisibility.Unlisted ? "automatic"
				: policy == QuotePolicies.Public || policy == QuotePolicies.Followers && viewerFollows ? "automatic"
				: "denied";
			return new QuoteApprovalEntity { Automatic = everyone, CurrentUser = current };
		}

		// a remote post's canLike, canReply and canAnnounce, in GoToSocial's client terms; null when it states none
		static InteractionPolicyEntity Policies(PostEntity post)
		{
			if (!post.IsFederatedCopy || post.LikePolicy == default && post.ReplyPolicy == default && post.AnnouncePolicy == default)
				return default;
			var collections = post.ActorURI?.TrimEnd('/');
			string Named(string who) => Addressing.IsPublic(who) ? "public"
				: who == post.ActorURI ? "author"
				: who.EndsWith("/followers", StringComparison.Ordinal) && who.StartsWith(collections, StringComparison.Ordinal) ? "followers"
				: who.EndsWith("/following", StringComparison.Ordinal) && who.StartsWith(collections, StringComparison.Ordinal) ? "following"
				: who;
			InteractionRuleEntity Rule(InteractionRule rule) => rule == default
				? new InteractionRuleEntity { AutomaticApproval = new List<string> { "public" } }
				: new InteractionRuleEntity { AutomaticApproval = rule.Automatic.Select(Named).Distinct().ToList(), ManualApproval = rule.Manual.Select(Named).Distinct().ToList() };
			return new InteractionPolicyEntity { CanFavourite = Rule(post.LikePolicy), CanReply = Rule(post.ReplyPolicy), CanReblog = Rule(post.AnnouncePolicy) };
		}

		static QuoteApprovalEntity Approval(PostEntity post, string viewerId)
		{
			if (!post.IsFederatedCopy || post.Visibility is not (PostVisibility.Public or PostVisibility.Unlisted))
				return new QuoteApprovalEntity();
			if (post.QuotePolicy is not { } policy)
				return new QuoteApprovalEntity { Automatic = new List<string> { "public" }, CurrentUser = viewerId == default ? "unknown" : "automatic" };
			static List<string> Named(IEnumerable<string> who) => who.Select(w => Addressing.IsPublic(w) ? "public" : w.EndsWith("/followers") ? "followers" : "unsupported_policy").Distinct().ToList();
			return new QuoteApprovalEntity
			{
				Automatic = Named(policy.Automatic),
				Manual = Named(policy.Manual),
				CurrentUser = viewerId == default ? "unknown" : policy.Automatic.Any(Addressing.IsPublic) ? "automatic" : policy.Manual.Any(Addressing.IsPublic) ? "manual" : "denied"
			};
		}

		public static string Content(PostEntity post)
		{
			var content = post.ContentHtml ?? ActivityPubRenderer.Html(post.Text);
			if (post.IsFederatedCopy && string.IsNullOrWhiteSpace(content) && !string.IsNullOrEmpty(post.Excerpt))
				content = $"<p>{WebUtility.HtmlEncode(post.Excerpt)}</p>";
			if (post.Event is { } happening && EventLine(happening) is { } line)
				content = $"<p>{WebUtility.HtmlEncode(line)}</p>{content}";
			var titled = !string.IsNullOrEmpty(post.Title) && (!post.IsFederatedCopy || post.ObjectType is not (null or "Note" or "Question"));
			return titled
				? $"<p><strong>{WebUtility.HtmlEncode(post.Title)}</strong></p>{content}"
				: content ?? string.Empty;
		}

		public static string EventLine(EventDetails happening)
		{
			if (happening.Start is not { } start)
				return default;
			var zone = Zone(happening.FloatingTime ? default : happening.TimeZone);
			var local = zone == default ? start : TimeZoneInfo.ConvertTimeFromUtc(start, zone);
			var when = local.ToString("yyyy-MM-dd HH:mm", CultureInfo.InvariantCulture);
			if (happening.End is { } end && end > start)
			{
				var localEnd = zone == default ? end : TimeZoneInfo.ConvertTimeFromUtc(end, zone);
				when += " – " + localEnd.ToString(localEnd.Date == local.Date ? "HH:mm" : "yyyy-MM-dd HH:mm", CultureInfo.InvariantCulture);
			}
			when += happening.FloatingTime ? " (local time)" : zone == default ? " UTC" : $" ({happening.TimeZone})";
			var where = happening.Places.Select(p => p.Name ?? p.Address).FirstOrDefault(p => p != default)
				?? (happening.OnlineURL != default ? "online" : default);
			return where == default ? when : $"{when} · {where}";
		}

		static TimeZoneInfo Zone(string id)
		{
			if (string.IsNullOrWhiteSpace(id))
				return default;
			try
			{
				return TimeZoneInfo.FindSystemTimeZoneById(id);
			}
			catch (Exception ex) when (ex is TimeZoneNotFoundException or InvalidTimeZoneException)
			{
				return default;
			}
		}

		public PollEntity Poll(PostEntity post, string viewerId, IReadOnlyCollection<int> ownVotes)
		{
			if (post.Poll is not { } poll)
				return default;
			var expired = PollService.Ended(poll);
			var hidden = poll.HideTotals && !expired && post.GroupUserId != viewerId;
			return new PollEntity
			{
				Id = post.ID,
				ExpiresAt = poll.ExpiresAt is { } ends ? MastodonJson.Time(ends) : default,
				Expired = expired,
				Multiple = poll.Multiple,
				VotesCount = poll.Options.Sum(o => o.Votes),
				VotersCount = poll.Multiple ? poll.VotersCount : poll.VotersCount ?? poll.Options.Sum(o => o.Votes),
				Voted = viewerId == default ? default(bool?) : ownVotes?.Count > 0 || post.GroupUserId == viewerId,
				OwnVotes = viewerId == default ? default : ownVotes?.ToList() ?? new List<int>(),
				Options = poll.Options.Select(o => new PollOptionEntity { Title = o.Title, VotesCount = hidden ? default(int?) : o.Votes }).ToList(),
				Emojis = Emojis(post.Emojis.Where(e => poll.Options.Any(o => o.Title.Contains($":{e.Shortcode}:"))))
			};
		}

		List<CustomEmojiEntity> Emojis(IEnumerable<CustomEmoji> emojis) =>
			emojis?.Select(e => new CustomEmojiEntity { Shortcode = e.Shortcode, Url = Proxied(e.URL), StaticUrl = Proxied(e.URL) }).ToList() ?? new();

		PrivaPubStatus Extension(PostEntity post) => new()
		{
			ObjectType = post.ObjectType,
			Locked = post.LockedAt.HasValue,
			Approval = post.Approval is ApprovalState.Pending or ApprovalState.Rejected ? post.Approval.ToString().ToLowerInvariant() : default,
			Title = post.IsFederatedCopy ? post.Title : default,
			Excerpt = post.Excerpt,
			Cover = Proxied(post.CoverURL),
			Source = post.Source == default ? default : new PrivaPubSource { Content = post.Source.Content, MediaType = post.Source.MediaType },
			Link = post.Link == default ? default : new PrivaPubLink
			{
				Url = post.Link.Href, Title = post.Link.Title, Description = post.Link.Description, Image = Proxied(post.Link.ImageURL), FromPublisher = post.Link.FromPublisher
			},
			Video = post.Video == default ? default : new PrivaPubVideo
			{
				Duration = post.Video.DurationSeconds,
				Variants = post.Video.Variants.Select(Variant).ToList(),
				IsLive = post.Video.IsLive,
				State = post.Video.State,
				Views = post.Video.Views,
				CommentsPolicy = post.Video.CommentsPolicy,
				DownloadEnabled = post.Video.DownloadEnabled,
				Licence = post.Video.Licence,
				Category = post.Video.Category,
				Channel = post.Video.ChannelURI,
				Captions = post.Video.Captions.Select(c => new PrivaPubCaption { Language = c.Language, Url = Proxied(c.Url) }).ToList()
			},
			Audio = post.Audio == default ? default : new PrivaPubAudio
			{
				Duration = post.Audio.DurationSeconds,
				Variants = post.Audio.Variants.Select(Variant).ToList(),
				Album = post.Audio.Album,
				Position = post.Audio.Position,
				Licence = post.Audio.Licence
			},
			Event = post.Event == default ? default : new PrivaPubEvent
			{
				Start = post.Event.Start is { } start ? MastodonJson.Time(start) : default,
				End = post.Event.End is { } end ? MastodonJson.Time(end) : default,
				TimeZone = post.Event.TimeZone,
				Floating = post.Event.FloatingTime,
				Places = post.Event.Places.Select(p => new PrivaPubPlace { Name = p.Name, Address = p.Address, Latitude = p.Latitude, Longitude = p.Longitude, Url = p.Url }).ToList(),
				OnlineUrl = post.Event.OnlineURL,
				JoinMode = post.Event.JoinMode,
				Participants = post.Event.ParticipantCount,
				Capacity = post.Event.Capacity,
				Status = post.Event.Status,
				Category = post.Event.Category
			},
			Place = post.Place == default ? default : new PrivaPubPlace
			{
				Name = post.Place.Name, Latitude = post.Place.Latitude, Longitude = post.Place.Longitude, Country = post.Place.Country
			},
			Votes = new PrivaPubVotes { Up = post.FavouritesCount, Down = post.DownvotesCount }
		};

		PrivaPubVariant Variant(MediaVariant variant) => new()
		{
			Url = Proxied(variant.Url),
			MediaType = variant.MediaType,
			Width = variant.Width,
			Height = variant.Height,
			Fps = variant.Fps,
			Size = variant.Size,
			Streams = variant.Streams
		};

		List<MediaAttachment> Playable(PostEntity post)
		{
			var (variant, kind, duration) = post.Video is { } video ? (BestVideo(video.Variants), "video", video.DurationSeconds)
				: post.Audio is { } audio ? (audio.Variants.FirstOrDefault(), "audio", audio.DurationSeconds)
				: default;
			if (variant == default)
				return new List<MediaAttachment>();
			return new List<MediaAttachment>
			{
				Media(new PostMedia
				{
					Id = new Guid(System.Security.Cryptography.MD5.HashData(System.Text.Encoding.UTF8.GetBytes(variant.Url))),
					RemoteURL = variant.Url,
					ContentType = variant.MediaType ?? kind + "/*",
					RemotePreviewURL = post.CoverURL,
					Width = variant.Width,
					Height = variant.Height,
					DurationSeconds = duration,
					Description = post.Title
				})
			};
		}

		public static MediaVariant BestVideo(IReadOnlyCollection<MediaVariant> variants)
		{
			var playable = variants.Where(v => v.MediaType?.StartsWith("video/") == true && (v.Streams.Count == 0 || v.Streams.Contains("audio") && v.Streams.Contains("video"))).ToList();
			return playable.Where(v => v.Height <= 720).OrderByDescending(v => v.Height).FirstOrDefault()
				?? playable.OrderBy(v => v.Height ?? int.MaxValue).FirstOrDefault();
		}

		PreviewCard Card(PostEntity post)
		{
			if (post.Link is { } link)
				return Card(link.Href, link.Title ?? post.Title, link.Description ?? (post.Media.Count == 0 ? post.Excerpt : default), link.ImageURL);
			if (post.IsFederatedCopy && post.ObjectType is "Article" or "Event" && post.Url != default && post.Media.Count == 0
				|| post.IsFederatedCopy && post.ObjectType is "Video" or "Audio" && post.Url != default && post.Media.Count == 0 && Playable(post).Count == 0)
				return Card(post.Url, post.Title, post.Excerpt, post.CoverURL);
			return default;
		}

		public PreviewCard LinkCard(string url, string title, string description, string image) => Card(url, title, description, image);

		PreviewCard Card(string url, string title, string description, string image)
		{
			if (!Uri.TryCreate(url, UriKind.Absolute, out var uri))
				return default;
			return new PreviewCard
			{
				Url = url,
				Title = title ?? uri.Host,
				Description = description ?? string.Empty,
				ProviderName = uri.Host,
				ProviderUrl = uri.GetLeftPart(UriPartial.Authority),
				Image = Proxied(image)
			};
		}

		public static string Visibility(PostVisibility visibility) => visibility switch
		{
			PostVisibility.Unlisted => "unlisted",
			PostVisibility.FollowersOnly or PostVisibility.Circle => "private",
			PostVisibility.Direct => "direct",
			_ => "public"
		};

		StatusMention Mention(PostMention mention, IReadOnlyDictionary<string, ForeignAvatar> foreign)
		{
			if (mention.IsLocal)
			{
				var handle = mention.Handle?.TrimStart('@').Split('@')[0];
				return new StatusMention { Id = mention.AccountId, Username = handle, Acct = handle, Url = mention.ActorURI };
			}
			if (!foreign.TryGetValue(mention.ActorURI, out var account))
				return default;
			return new StatusMention { Id = account.ID, Username = account.UserName, Acct = $"{account.UserName}@{account.Domain}", Url = account.Url ?? account.ActorURI };
		}

		static Dictionary<string, object> Meta(PostMedia media)
		{
			var original = new Dictionary<string, object>();
			if (media.Width > 0 && media.Height > 0)
			{
				original["width"] = media.Width;
				original["height"] = media.Height;
				original["size"] = $"{media.Width}x{media.Height}";
				original["aspect"] = (double)media.Width.Value / media.Height.Value;
			}
			if (media.DurationSeconds is { } seconds)
				original["duration"] = seconds;
			var meta = new Dictionary<string, object>();
			if (original.Count > 0)
				meta["original"] = original;
			if (media.DurationSeconds is { } length)
				meta["length"] = TimeSpan.FromSeconds(length).ToString(@"h\:mm\:ss\.ff", CultureInfo.InvariantCulture);
			if (media.Focus is { Length: 2 })
				meta["focus"] = new { x = media.Focus[0], y = media.Focus[1] };
			return meta;
		}

		MediaAttachment Media(PostMedia media) => new()
		{
			Id = media.AttachmentId ?? media.Id.ToString("N"),
			Type = media.ContentType switch
			{
				{ } type when type.StartsWith("image/gif") => "gifv",
				{ } type when type.StartsWith("image/") => "image",
				{ } type when type.StartsWith("video/") => "video",
				{ } type when type.StartsWith("audio/") => "audio",
				_ => "unknown"
			},
			Url = media.URL ?? Proxied(media.RemoteURL),
			PreviewUrl = media.PreviewURL ?? Proxied(media.RemotePreviewURL)
				?? (media.ContentType?.StartsWith("image/") == true || media.ContentType == default ? media.URL ?? Proxied(media.RemoteURL) : default),
			RemoteUrl = media.RemoteURL,
			Description = media.Description,
			Blurhash = media.Blurhash,
			Meta = Meta(media)
		};
	}
}