LehrerApp/LehrerApp.Sync/SyncEngine.cs

using System.Net.Http.Json;
using LehrerApp.Sync.Models;

namespace LehrerApp.Sync;

/// <summary>
/// Orchestriert Push, Pull, EventApply und Snapshot-Export.
/// Automatisch alle N Minuten + manuell auslösbar.
/// </summary>
public class SyncEngine : IDisposable
{
    private readonly EventQueue _queue;
    private readonly ConflictResolver _resolver;
    private readonly EventApplier? _applier;
    private readonly ReadableSnapshotService? _snapshotExport;
    private readonly HttpClient _http;
    private readonly SyncConfig _config;
    private readonly Timer _timer;

    public SyncStatus Status { get; private set; } = new();
    public event Action<SyncStatus>? StatusChanged;

    public SyncEngine(
        EventQueue queue,
        ConflictResolver resolver,
        HttpClient http,
        SyncConfig config,
        EventApplier? applier = null,
        ReadableSnapshotService? snapshotExport = null)
    {
        _queue          = queue;
        _resolver       = resolver;
        _http           = http;
        _config         = config;
        _applier        = applier;
        _snapshotExport = snapshotExport;

        _timer = new Timer(
            async _ => await SyncNowAsync(isAutomatic: true),
            null,
            TimeSpan.FromMinutes(config.AutoSyncIntervalMinutes),
            TimeSpan.FromMinutes(config.AutoSyncIntervalMinutes));

        UpdateStatus();
    }

    // ── Öffentliche API ───────────────────────────────────────────────────────

    public async Task<SyncResult> SyncNowAsync(bool isAutomatic = false)
    {
        if (Status.State == SyncState.Syncing)
            return new SyncResult { Skipped = true, Reason = "Sync bereits aktiv" };

        SetState(SyncState.Syncing);

        try
        {
            // 1. Ausstehende Events pushen
            var pushResult = await PushAsync();

            // 2. Neue Events vom Server holen
            var pullResult = await PullAsync();

            // 3. Geholte Events auf LiteDB anwenden
            if (_applier is not null && pullResult.Events.Count > 0)
                _applier.ApplyAll(pullResult.Events);

            // 4. Nach erfolgreichem Sync: lesbaren Snapshot exportieren
            //    (nur bei automatischem Sync oder explizit – nicht bei jedem
            //     manuellen Push um Traffic zu sparen)
            if (_snapshotExport is not null && (isAutomatic || pushResult.Pushed > 0))
                await _snapshotExport.ExportAndPushAsync();

            _queue.SetLastSyncAt(DateTime.UtcNow);
            SetState(SyncState.Idle);

            return new SyncResult
            {
                Success       = true,
                EventsPushed  = pushResult.Pushed,
                EventsPulled  = pullResult.Events.Count,
                Conflicts     = pullResult.Conflicts,
            };
        }
        catch (HttpRequestException)
        {
            SetState(SyncState.Offline);
            return new SyncResult { Success = false, Reason = "Server nicht erreichbar" };
        }
        catch (Exception ex)
        {
            SetState(SyncState.Error, ex.Message);
            return new SyncResult { Success = false, Reason = ex.Message };
        }
    }

    // ── Push ──────────────────────────────────────────────────────────────────

    private async Task<(int Pushed, int Conflicts)> PushAsync()
    {
        var pending = _queue.GetPending(maxBatch: 200);
        if (pending.Count == 0) return (0, 0);

        var response = await _http.PostAsJsonAsync("/api/sync/push", pending);
        response.EnsureSuccessStatusCode();

        var result = await response.Content.ReadFromJsonAsync<PushResponse>();
        if (result is null) return (0, 0);

        var sentIds = pending
            .Where(e => !result.ConflictingEventIds.Contains(e.EventId))
            .Select(e => e.EventId);
        _queue.Acknowledge(sentIds);
        _queue.SetLastServerSequenceNr(result.ServerSequenceNr);

        return (pending.Count - result.ConflictingEventIds.Count,
                result.ConflictingEventIds.Count);
    }

    // ── Pull ──────────────────────────────────────────────────────────────────

    private async Task<(List<SyncEvent> Events, int Conflicts)> PullAsync()
    {
        var since = _queue.GetLastServerSequenceNr();
        var response = await _http.GetFromJsonAsync<PullResponse>(
            $"/api/sync/pull?since={since}&deviceId={_config.DeviceId}");

        if (response is null || response.Events.Count == 0)
            return ([], 0);

        var conflicts = 0;
        foreach (var remoteEvent in response.Events)
        {
            var conflict = _resolver.TryResolve(remoteEvent, _config.DeviceId);
            if (conflict is not null)
            {
                _queue.AddConflict(conflict);
                conflicts++;
            }
        }

        _queue.SetLastServerSequenceNr(response.ServerSequenceNr);
        return (response.Events, conflicts);
    }

    // ── Hilfsmethoden ─────────────────────────────────────────────────────────

    private void SetState(SyncState state, string? error = null)
    {
        Status = new SyncStatus
        {
            State         = state,
            LastSyncAt    = _queue.GetLastSyncAt(),
            PendingEvents = _queue.PendingCount(),
            ConflictCount = _queue.ConflictCount(),
            ErrorMessage  = error,
        };
        StatusChanged?.Invoke(Status);
    }

    private void UpdateStatus() => SetState(Status.State);

    public void Dispose()
    {
        _timer.Dispose();
        _queue.Dispose();
    }
}

public class SyncConfig
{
    public string ServerUrl { get; set; } = "";
    public string DeviceId { get; set; } = "";
    public DeviceType DeviceType { get; set; } = DeviceType.Desktop;
    public int AutoSyncIntervalMinutes { get; set; } = 5;
}

public class SyncResult
{
    public bool Success { get; set; }
    public bool Skipped { get; set; }
    public string? Reason { get; set; }
    public int EventsPushed { get; set; }
    public int EventsPulled { get; set; }
    public int Conflicts { get; set; }
}