using MongoDB.Driver;
using MongoDB.Entities;

using PrivaPub.Models.Group;
using PrivaPub.Models.Social;

namespace PrivaPub.Domain.Statuses
{
	// A direct conversation's newest post (DmGroup.LastPostId, which pages the conversations list) and each persona's
	// read and removed marks on it (ConversationState). A conversation is unread for a persona when its newest post is
	// someone else's and newer than what the persona read; writing in it reads it.
	public static class ConversationStates
	{
		public static async Task Posted(string conversationId, string postId, string authorAvatarId, CancellationToken token)
		{
			await DB.Default.Update<DmGroup>().MatchID(conversationId)
				.Modify(g => g.UpdatedAt, DateTime.UtcNow)
				.Modify(g => g.LastPostId, postId)
				.ExecuteAsync(token);
			if (authorAvatarId != default)
				await Read(authorAvatarId, conversationId, postId, token);
		}

		public static Task Read(string avatarId, string conversationId, string postId, CancellationToken token) =>
			Mark(avatarId, conversationId, s => s.LastReadPostId, postId, token);

		public static Task Hide(string avatarId, string conversationId, string postId, CancellationToken token) =>
			Mark(avatarId, conversationId, s => s.HiddenUpTo, postId, token);

		public static bool IsUnread(ConversationState state, string lastPostId, string lastAuthorId, string avatarId) =>
			lastPostId != default && lastAuthorId != avatarId && string.CompareOrdinal(lastPostId, state?.LastReadPostId) > 0;

		public static bool IsHidden(ConversationState state, string lastPostId) =>
			state?.HiddenUpTo != default && string.CompareOrdinal(lastPostId, state.HiddenUpTo) <= 0;

		// the mark only moves forward; the first mark makes the state (two at once: the unique index keeps one, and the
		// loser marks again)
		static async Task Mark(string avatarId, string conversationId, System.Linq.Expressions.Expression<Func<ConversationState, string>> field,
			string postId, CancellationToken token)
		{
			if (postId == default)
				return;
			Task Write() => DB.Default.Update<ConversationState>()
				.Match(s => s.AvatarId == avatarId && s.ConversationId == conversationId)
				.Modify(b => b.Max(field, postId))
				.Modify(s => s.UpdatedAt, DateTime.UtcNow)
				.Option(o => o.IsUpsert = true)
				.ExecuteAsync(token);
			try
			{
				await Write();
			}
			catch (MongoWriteException ex) when (ex.WriteError?.Category == ServerErrorCategory.DuplicateKey)
			{
				await Write();
			}
		}
	}
}