using LiteDB;
using LehrerApp.Sync.Models;

namespace LehrerApp.Api;

/// <summary>
/// Append-only Event-Log pro User. Server versteht Payload nicht.
/// </summary>
public class EventStore(string dataPath) : IDisposable
{
    private readonly Dictionary<string, LiteDatabase> _dbs = new();
    private readonly Lock _lock = new();

    public PushResponse Push(string userId, List<SyncEvent> events)
    {
        var col     = GetCol(userId);
        var seq     = LastSeq(col);
        var rejects = new List<Guid>();
        foreach (var e in events.OrderBy(e => e.Timestamp))
        {
            var recent = col.FindOne(x =>
                x.EntityType == e.EntityType && x.EntityId == e.EntityId &&
                x.DeviceId != e.DeviceId && x.Timestamp > e.Timestamp.AddSeconds(-30));
            if (recent is not null) { rejects.Add(e.EventId); continue; }
            col.Insert(new ServerEvent { EventId = e.EventId, DeviceId = e.DeviceId,
                DeviceType = e.DeviceType, Timestamp = e.Timestamp,
                ClientSeq = e.SequenceNr, ServerSeq = ++seq,
                EntityType = e.EntityType, EntityId = e.EntityId,
                Operation = e.Operation, Payload = e.Payload });
        }
        return new() { Success = true, ServerSequenceNr = seq, ConflictingEventIds = rejects };
    }

    public PullResponse Pull(string userId, long since, string requestingDeviceId)
    {
        var col    = GetCol(userId);
        var events = col.Find(e => e.ServerSeq > since && e.DeviceId != requestingDeviceId)
            .OrderBy(e => e.ServerSeq).Take(500)
            .Select(e => new SyncEvent { EventId = e.EventId, DeviceId = e.DeviceId,
                DeviceType = e.DeviceType, Timestamp = e.Timestamp,
                SequenceNr = e.ServerSeq, EntityType = e.EntityType,
                EntityId = e.EntityId, Operation = e.Operation, Payload = e.Payload })
            .ToList();
        return new() { Events = events, ServerSequenceNr = LastSeq(col) };
    }

    private ILiteCollection<ServerEvent> GetCol(string userId)
    {
        lock (_lock)
        {
            if (!_dbs.TryGetValue(userId, out var db))
            {
                var safe = string.Concat(userId.Where(c => char.IsLetterOrDigit(c) || c == '-'));
                db = new LiteDatabase(Path.Combine(dataPath, $"{safe}.db"));
                _dbs[userId] = db;
            }
            var col = db.GetCollection<ServerEvent>("events");
            col.EnsureIndex(x => x.ServerSeq);
            return col;
        }
    }

    private static long LastSeq(ILiteCollection<ServerEvent> col)
    {
        var last = col.FindOne(Query.All(nameof(ServerEvent.ServerSeq), Query.Descending));
        return last?.ServerSeq ?? 0;
    }

    public void Dispose() { foreach (var db in _dbs.Values) db.Dispose(); }
}

internal class ServerEvent
{
    public Guid EventId { get; set; }
    public string DeviceId { get; set; } = "";
    public DeviceType DeviceType { get; set; }
    public DateTime Timestamp { get; set; }
    public long ClientSeq { get; set; }
    public long ServerSeq { get; set; }
    public string EntityType { get; set; } = "";
    public string EntityId { get; set; } = "";
    public string Operation { get; set; } = "";
    public string Payload { get; set; } = "";
}