SocialPub/PrivaPub/Api/Mastodon/Controllers/TrendsController.cs

using Microsoft.AspNetCore.Mvc;

using MongoDB.Driver;
using MongoDB.Entities;

using PrivaPub.Api.Mastodon.Infrastructure;
using PrivaPub.Api.Mastodon.Mappers;
using PrivaPub.Domain.Privacy;
using PrivaPub.Domain.Social;
using PrivaPub.Federation.Actors;
using PrivaPub.Models.User;
using PrivaPub.StaticServices;

namespace PrivaPub.Api.Mastodon.Controllers
{
	// Trends and the directory (owner decision 2026-10-04, back from the cut list), from what PrivaPub already holds
	// (Trends). The directory lists only accounts that let themselves be discovered.
	public class TrendsController : MastodonController
	{
		readonly Trends _trends;
		readonly MastodonMapper _mapper;
		readonly DbEntities _dbEntities;
		readonly ILocalActorService _localActors;

		public TrendsController(Trends trends, MastodonMapper mapper, DbEntities dbEntities, ILocalActorService localActors)
		{
			_trends = trends;
			_mapper = mapper;
			_dbEntities = dbEntities;
			_localActors = localActors;
		}

		static List<object> History(Trends.Trend trend) => trend.History.Select(h => (object)new
		{
			day = new DateTimeOffset(h.Day, TimeSpan.Zero).ToUnixTimeSeconds().ToString(),
			uses = h.Uses.ToString(),
			accounts = h.Accounts.ToString()
		}).ToList();

		IEnumerable<T> Window<T>(IEnumerable<T> items, int fallback, int max) =>
			items.Skip(Math.Max(0, Params.Int("offset") ?? 0)).Take(Limit(fallback, max));

		[HttpGet("/api/v1/trends"), HttpGet("/api/v1/trends/tags"), Scope("read", requiresUser: false), Microsoft.AspNetCore.Authorization.AllowAnonymous]
		public async Task<IActionResult> Tags(CancellationToken token)
		{
			var followed = MyId == default
				? new HashSet<string>()
				: (await DB.Default.Find<Models.Social.FollowedTag>().Match(t => t.AvatarId == MyId).ExecuteAsync(token)).Select(t => t.Name).ToHashSet();
			return Json(Window(await _trends.Tags(token), 10, 20).Select(t => new
			{
				name = t.Key,
				url = $"{_localActors.BaseAddress}/tags/{Uri.EscapeDataString(t.Key)}",
				history = History(t),
				following = MyId == default ? default(bool?) : followed.Contains(t.Key)
			}).ToList());
		}

		[HttpGet("/api/v1/trends/statuses"), Scope("read", requiresUser: false), Microsoft.AspNetCore.Authorization.AllowAnonymous]
		public async Task<IActionResult> Statuses(CancellationToken token)
		{
			var ids = Window(await _trends.Statuses(token), 20, 40).ToList();
			var posts = (await _dbEntities.Posts.Match(p => ids.Contains(p.ID)).Match(VisibilityPolicy.IsShown).ExecuteAsync(token)).ToDictionary(p => p.ID);
			return Json(await _mapper.Statuses(ids.Where(posts.ContainsKey).Select(id => posts[id]).ToList(), MyId, token));
		}

		[HttpGet("/api/v1/trends/links"), Scope("read", requiresUser: false), Microsoft.AspNetCore.Authorization.AllowAnonymous]
		public async Task<IActionResult> Links(CancellationToken token)
		{
			var trends = Window(await _trends.Links(token), 10, 20).ToList();
			var sampleIds = trends.Select(t => t.SamplePostId).ToList();
			var samples = (await _dbEntities.Posts.Match(p => sampleIds.Contains(p.ID)).ExecuteAsync(token)).ToDictionary(p => p.ID);
			return Json(trends.Where(t => samples.ContainsKey(t.SamplePostId)).Select(t =>
			{
				var link = samples[t.SamplePostId].Link;
				var card = _mapper.LinkCard(t.Key, link.Title, link.Description, link.ImageURL);
				return card == default ? default : (object)new
				{
					card.Url, card.Title, card.Description, card.Type, card.AuthorName, card.AuthorUrl, card.ProviderName, card.ProviderUrl,
					card.Html, card.Width, card.Height, card.Image, card.ImageDescription, card.EmbedUrl, card.Blurhash, card.Language,
					card.PublishedAt, card.Authors, History = History(t)
				};
			}).Where(c => c != default).ToList());
		}

		// order=active: by their latest public post PrivaPub holds; order=new: by when they say they joined, which for a
		// persona is its published day (never its creation, so personas made together are not told apart by their order)
		[HttpGet("/api/v1/directory"), Scope("read", requiresUser: false), Microsoft.AspNetCore.Authorization.AllowAnonymous]
		public async Task<IActionResult> Directory(CancellationToken token)
		{
			var offset = Math.Max(0, Params.Int("offset") ?? 0);
			var limit = Limit(40, 80);
			var localOnly = Params.Bool("local") == true;
			var barred = await Counted.BarredPersonas(token);
			var wanted = offset + limit;
			var ordered = new List<string>();
			if (Params.Get("order") == "new")
			{
				// many share a day: the first `wanted` are read with every other of the last one's day, so each page is cut the same way
				var personas = await _dbEntities.Avatars.Match(a => a.Settings.IsDiscoverable && !a.DeletionAt.HasValue && !barred.Contains(a.ID))
					.Sort(a => a.PublishedOn, Order.Descending).Limit(wanted).ExecuteAsync(token);
				if (personas.Count == wanted)
				{
					var edge = personas[^1].PublishedOn;
					var known = personas.Select(a => a.ID).ToList();
					personas.AddRange(await _dbEntities.Avatars.Match(a => a.Settings.IsDiscoverable && !a.DeletionAt.HasValue && !barred.Contains(a.ID)
						&& a.PublishedOn == edge && !known.Contains(a.ID)).ExecuteAsync(token));
				}
				var remote = localOnly
					? new List<ForeignAvatar>()
					: await _dbEntities.ForeignAvatars.Match(f => f.IsDiscoverable && !f.DeletionAt.HasValue && !f.SuspendedAt.HasValue && f.Published != null)
						.Sort(f => f.Published, Order.Descending).Limit(wanted).ExecuteAsync(token);
				if (remote.Count == wanted)
				{
					var edge = remote[^1].Published;
					var known = remote.Select(f => f.ID).ToList();
					remote.AddRange(await _dbEntities.ForeignAvatars.Match(f => f.IsDiscoverable && !f.DeletionAt.HasValue && !f.SuspendedAt.HasValue
						&& f.Published == edge && !known.Contains(f.ID)).ExecuteAsync(token));
				}
				ordered = personas.Select(a => (a.ID, Joined: a.PublishedOn, Tie: Tie(a.UserName)))
					.Concat(remote.Select(f => (f.ID, Joined: f.Published.Value, Tie: Tie(f.ActorURI))))
					.OrderByDescending(a => a.Joined).ThenBy(a => a.Tie, StringComparer.Ordinal)
					.Select(a => a.ID).Skip(offset).Take(limit).ToList();
			}
			else
			{
				var discoverableLocal = (await _dbEntities.Avatars.Match(a => a.Settings.IsDiscoverable && !a.DeletionAt.HasValue && !barred.Contains(a.ID))
					.Project(a => a.Include(x => x.ID)).ExecuteAsync(token)).Select(a => a.ID).ToHashSet();
				var seen = new HashSet<string>();
				var maxId = default(string);
				for (var round = 0; round < 20 && ordered.Count < wanted; round++)
				{
					var page = _dbEntities.Posts.Match(p => p.Visibility == Models.Post.PostVisibility.Public && !p.DeletedAt.HasValue && p.ReblogOfPostId == null);
					if (maxId != default)
						page.Match(f => f.Lt(p => p.ID, maxId));
					if (localOnly)
						page.Match(p => !p.IsFederatedCopy);
					var batch = await page.Sort(p => p.ID, Order.Descending).Limit(500)
						.Project(p => p.Include(x => x.ID).Include(x => x.AuthorAccountId).Include(x => x.GroupUserId).Include(x => x.IsFederatedCopy))
						.ExecuteAsync(token);
					if (batch.Count == 0)
						break;
					maxId = batch[^1].ID;
					var fresh = batch.Select(p => (Id: p.AuthorAccountId ?? p.GroupUserId, p.IsFederatedCopy)).Where(a => a.Id != default && seen.Add(a.Id)).ToList();
					var remoteIds = fresh.Where(a => a.IsFederatedCopy).Select(a => a.Id).ToList();
					var discoverableRemote = remoteIds.Count == 0
						? new HashSet<string>()
						: (await _dbEntities.ForeignAvatars.Match(f => remoteIds.Contains(f.ID) && f.IsDiscoverable && !f.DeletionAt.HasValue && !f.SuspendedAt.HasValue)
							.Project(f => f.Include(x => x.ID)).ExecuteAsync(token)).Select(f => f.ID).ToHashSet();
					ordered.AddRange(fresh.Where(a => a.IsFederatedCopy ? discoverableRemote.Contains(a.Id) : discoverableLocal.Contains(a.Id)).Select(a => a.Id));
				}
				ordered = ordered.Skip(offset).Take(limit).ToList();
			}
			var accounts = await _mapper.Accounts(ordered, token);
			return Json(ordered.Where(accounts.ContainsKey).Select(id => accounts[id]).ToList());
		}

		static string Tie(string name) =>
			Convert.ToHexString(System.Security.Cryptography.SHA256.HashData(System.Text.Encoding.UTF8.GetBytes(name ?? string.Empty)));
	}
}