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

using Microsoft.AspNetCore.Mvc;

using MongoDB.Entities;

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

namespace PrivaPub.Api.Mastodon.Controllers
{
	// Mastodon's lists (owner decision 2026-10-04, back from the cut list): a persona groups accounts it follows, reads them
	// as a timeline, and may keep an exclusive list's members out of its home. Nothing here leaves the server.
	public class ListsController : MastodonController
	{
		const int MaxLists = 50;
		const int MaxMembers = 500;

		readonly DbEntities _dbEntities;
		readonly MastodonMapper _mapper;

		public ListsController(DbEntities dbEntities, MastodonMapper mapper)
		{
			_dbEntities = dbEntities;
			_mapper = mapper;
		}

		static object View(PersonaList list) => new { id = list.ID, title = list.Title, replies_policy = list.RepliesPolicy, exclusive = list.Exclusive };

		Task<PersonaList> Mine(string id, CancellationToken token) =>
			DB.Default.Find<PersonaList>().Match(l => l.ID == id && l.AvatarId == MyId).ExecuteFirstAsync(token);

		// a list holds only accounts the persona still follows: whatever ended the follow (an unfollow, a Reject, a Block, the
		// account's deletion), its membership goes with it, pruned here on the next read
		async Task<List<string>> Members(PersonaList list, CancellationToken token)
		{
			var members = (await DB.Default.Find<PersonaListMember>().Match(m => m.ListId == list.ID).ExecuteAsync(token)).Select(m => m.AccountId).ToList();
			if (members.Count == 0)
				return members;
			var followed = (await _dbEntities.Followings.Match(f => f.AvatarId == MyId && f.State == FollowState.Accepted && members.Contains(f.TargetAccountId))
				.ExecuteAsync(token)).Select(f => f.TargetAccountId).ToHashSet();
			var gone = members.Where(m => !followed.Contains(m)).ToList();
			if (gone.Count > 0)
				await DB.Default.DeleteAsync<PersonaListMember>(m => m.ListId == list.ID && gone.Contains(m.AccountId));
			return members.Where(followed.Contains).ToList();
		}

		[HttpGet("/api/v1/lists"), Scope("read:lists")]
		public async Task<IActionResult> All(CancellationToken token) =>
			Json((await DB.Default.Find<PersonaList>().Match(l => l.AvatarId == MyId).Sort(l => l.Title, MongoDB.Entities.Order.Ascending)
				.ExecuteAsync(token)).Select(View).ToList());

		[HttpGet("/api/v1/lists/{id}"), Scope("read:lists")]
		public async Task<IActionResult> One(string id, CancellationToken token) =>
			await Mine(id, token) is { } list ? Json(View(list)) : NotFoundError();

		[HttpPost("/api/v1/lists"), Scope("write:lists")]
		public async Task<IActionResult> Create(CancellationToken token)
		{
			var title = Params.Get("title")?.Trim();
			var policy = Params.Get("replies_policy") ?? ListRepliesPolicy.List;
			if (string.IsNullOrEmpty(title) || title.Length > 200)
				return Error(StatusCodes.Status422UnprocessableEntity, "Validation failed: Title can't be blank");
			if (!ListRepliesPolicy.IsValid(policy))
				return Error(StatusCodes.Status422UnprocessableEntity, "Validation failed: Replies policy is not included in the list");
			if (await DB.Default.CountAsync<PersonaList>(l => l.AvatarId == MyId, token) >= MaxLists)
				return Error(StatusCodes.Status422UnprocessableEntity, "Validation failed: Too many lists");
			var list = new PersonaList { AvatarId = MyId, Title = title, RepliesPolicy = policy, Exclusive = Params.Bool("exclusive") == true };
			await DB.Default.SaveAsync(list, token);
			return Json(View(list));
		}

		[HttpPut("/api/v1/lists/{id}"), Scope("write:lists")]
		public async Task<IActionResult> Update(string id, CancellationToken token)
		{
			if (await Mine(id, token) is not { } list)
				return NotFoundError();
			var title = Params.Get("title")?.Trim();
			var policy = Params.Get("replies_policy");
			if (title != default)
			{
				if (title.Length == 0 || title.Length > 200)
					return Error(StatusCodes.Status422UnprocessableEntity, "Validation failed: Title can't be blank");
				list.Title = title;
			}
			if (policy != default)
			{
				if (!ListRepliesPolicy.IsValid(policy))
					return Error(StatusCodes.Status422UnprocessableEntity, "Validation failed: Replies policy is not included in the list");
				list.RepliesPolicy = policy;
			}
			if (Params.Bool("exclusive") is { } exclusive)
				list.Exclusive = exclusive;
			await DB.Default.SaveAsync(list, token);
			return Json(View(list));
		}

		[HttpDelete("/api/v1/lists/{id}"), Scope("write:lists")]
		public async Task<IActionResult> Delete(string id, CancellationToken token)
		{
			if (await Mine(id, token) is not { } list)
				return NotFoundError();
			await DB.Default.DeleteAsync<PersonaListMember>(m => m.ListId == list.ID);
			await DB.Default.DeleteAsync<PersonaList>(list.ID);
			return Json(new { });
		}

		[HttpGet("/api/v1/lists/{id}/accounts"), Scope("read:lists")]
		public async Task<IActionResult> Accounts(string id, CancellationToken token)
		{
			if (await Mine(id, token) is not { } list)
				return NotFoundError();
			await Members(list, token);
			var members = await Page.From(Params, Limit(40, 80)).Fetch(DB.Default.Find<PersonaListMember>().Match(m => m.ListId == list.ID), m => m.ID, token);
			var accounts = await _mapper.Accounts(members.Select(m => m.AccountId), token);
			Link($"/api/v1/lists/{id}/accounts", members.LastOrDefault()?.ID, members.FirstOrDefault()?.ID);
			return Json(members.Where(m => accounts.ContainsKey(m.AccountId)).Select(m => accounts[m.AccountId]).ToList());
		}

		// only accounts the persona follows may join its list, as on Mastodon
		[HttpPost("/api/v1/lists/{id}/accounts"), Scope("write:lists")]
		public async Task<IActionResult> Add(string id, CancellationToken token)
		{
			if (await Mine(id, token) is not { } list)
				return NotFoundError();
			var ids = Params.List("account_ids").Distinct().ToList();
			if (ids.Count == 0)
				return Error(StatusCodes.Status422UnprocessableEntity, "Validation failed: Account ids can't be blank");
			var followed = (await _dbEntities.Followings.Match(f => f.AvatarId == MyId && f.State == FollowState.Accepted && ids.Contains(f.TargetAccountId))
				.ExecuteAsync(token)).Select(f => f.TargetAccountId).ToHashSet();
			if (ids.Any(a => !followed.Contains(a)))
				return Error(StatusCodes.Status422UnprocessableEntity, "Validation failed: You must follow this account to add it to a list");
			var present = (await DB.Default.Find<PersonaListMember>().Match(m => m.ListId == list.ID && ids.Contains(m.AccountId)).ExecuteAsync(token))
				.Select(m => m.AccountId).ToHashSet();
			if (await DB.Default.CountAsync<PersonaListMember>(m => m.ListId == list.ID, token) + ids.Count(a => !present.Contains(a)) > MaxMembers)
				return Error(StatusCodes.Status422UnprocessableEntity, "Validation failed: Too many accounts in the list");
			foreach (var account in ids.Where(a => !present.Contains(a)))
				await DB.Default.SaveAsync(new PersonaListMember { ListId = list.ID, AvatarId = MyId, AccountId = account }, token);
			return Json(new { });
		}

		[HttpDelete("/api/v1/lists/{id}/accounts"), Scope("write:lists")]
		public async Task<IActionResult> Remove(string id, CancellationToken token)
		{
			if (await Mine(id, token) is not { } list)
				return NotFoundError();
			var ids = Params.List("account_ids");
			await DB.Default.DeleteAsync<PersonaListMember>(m => m.ListId == list.ID && ids.Contains(m.AccountId));
			return Json(new { });
		}

		[HttpGet("/api/v1/accounts/{id}/lists"), Scope("read:lists")]
		public async Task<IActionResult> Containing(string id, CancellationToken token)
		{
			var listIds = (await DB.Default.Find<PersonaListMember>().Match(m => m.AvatarId == MyId && m.AccountId == id).ExecuteAsync(token))
				.Select(m => m.ListId).ToList();
			return Json((await DB.Default.Find<PersonaList>().Match(l => l.AvatarId == MyId && listIds.Contains(l.ID)).ExecuteAsync(token))
				.Select(View).ToList());
		}

		// the list's timeline: the persona's home entries by its members. Replies are kept as Mastodon keeps them: a member's
		// replies to itself and to the persona always, to others as the policy says (anyone followed, the list's members, no one)
		[HttpGet("/api/v1/timelines/list/{id}"), Scope("read:lists")]
		public async Task<IActionResult> Timeline(string id, CancellationToken token)
		{
			if (await Mine(id, token) is not { } list)
				return NotFoundError();
			var members = await Members(list, token);
			var entries = await Page.From(Params, Limit()).Fetch(
				_dbEntities.TimelineEntries.Match(e => e.AvatarId == MyId && members.Contains(e.AuthorAccountId)), e => e.PostId, token);
			var ids = entries.Select(e => e.PostId).ToList();
			var posts = (await _dbEntities.Posts.Match(p => ids.Contains(p.ID)).Match(VisibilityPolicy.IsShown).ExecuteAsync(token)).ToDictionary(p => p.ID);
			var answered = list.RepliesPolicy switch
			{
				ListRepliesPolicy.Followed => (await _dbEntities.Followings.Match(f => f.AvatarId == MyId && f.State == FollowState.Accepted).ExecuteAsync(token))
					.Select(f => f.TargetAccountId).Append(MyId).ToHashSet(),
				ListRepliesPolicy.List => members.Append(MyId).ToHashSet(),
				_ => new HashSet<string> { MyId }
			};
			var shown = ids.Where(posts.ContainsKey).Select(i => posts[i])
				.Where(p => p.ReblogOfPostId != default || string.IsNullOrEmpty(p.AnsweringToPostId) && string.IsNullOrEmpty(p.InReplyToURI)
					|| p.InReplyToAccountId != default && (p.InReplyToAccountId == p.AuthorAccountId || answered.Contains(p.InReplyToAccountId)))
				.ToList();
			Link($"/api/v1/timelines/list/{id}", entries.LastOrDefault()?.PostId, entries.FirstOrDefault()?.PostId);
			return Json(await _mapper.Statuses(shown, MyId, token));
		}
	}
}