using LehrerApp.Core.Interfaces;
using LehrerApp.Core.Models;
using LehrerApp.Core.Services;

namespace LehrerApp.Desktop.Services;

public sealed record AnnualPlanPollResult(int EventCount, int Added, int Updated, int Deleted)
{
    public int ChangeCount => Added + Updated + Deleted;
}

/// <summary>
/// Periodischer Vollabgleich des ClassyPlan-Jahresplans. Dieser Dienst hat bewusst keinerlei
/// Abhängigkeit zu Stundenplan-, Vertretungs- oder Lerngruppen-Repositories.
/// </summary>
public sealed class AnnualPlanSyncService : IDisposable
{
    private static readonly TimeSpan PollInterval = TimeSpan.FromHours(6);

    private readonly HttpClient _http;
    private readonly AnnualPlanSettingsService _settings;
    private readonly IAnnualPlanEventRepository _events;
    private readonly AppLogger? _logger;
    private readonly SemaphoreSlim _gate = new(1, 1);
    private readonly Timer _timer;

    public event Action? DataChanged;

    public AnnualPlanSyncService(HttpClient http, AnnualPlanSettingsService settings,
        IAnnualPlanEventRepository events, AppLogger? logger = null)
    {
        _http = http;
        _settings = settings;
        _events = events;
        _logger = logger;
        _timer = new Timer(async _ => await PollAsync(), null, PollInterval, PollInterval);
    }

    public async Task PollAsync(CancellationToken cancellationToken = default)
    {
        if (!await _gate.WaitAsync(0).ConfigureAwait(false)) return;
        try
        {
            var url = _settings.GetIcalUrl();
            if (string.IsNullOrWhiteSpace(url)) return;

            string icsText;
            try
            {
                icsText = await _http.GetStringAsync(url, cancellationToken).ConfigureAwait(false);
            }
            catch (Exception ex)
            {
                _logger?.Error("Jahresplan: Abruf fehlgeschlagen", ex);
                _settings.SetLastSync(DateTime.UtcNow, $"Fehler beim Abruf: {ex.Message}");
                return;
            }

            AnnualPlanPollResult result;
            try
            {
                result = await Task.Run(() => ProcessIcsText(icsText)).ConfigureAwait(false);
            }
            catch (Exception ex)
            {
                _logger?.Error("Jahresplan: Verarbeitung fehlgeschlagen", ex);
                _settings.SetLastSync(DateTime.UtcNow, $"Fehler bei der Verarbeitung: {ex.Message}");
                return;
            }

            _settings.SetLastSync(DateTime.UtcNow,
                $"{result.EventCount} Termine — {result.Added} neu, {result.Updated} geändert, {result.Deleted} entfernt.");
            _logger?.Info($"Jahresplan: {result.EventCount} Termine, {result.ChangeCount} Änderung(en).");
            if (result.ChangeCount > 0) DataChanged?.Invoke();
        }
        finally
        {
            _gate.Release();
        }
    }

    /// <summary>HTTP-freier, vollständig testbarer Snapshot-Abgleich.</summary>
    public AnnualPlanPollResult ProcessIcsText(string icsText)
    {
        // Parse muss vollständig erfolgreich sein, bevor das Repository verändert wird. Der
        // strikte Parser verhindert so Teilimporte und fälschliches Löschen des Restbestands.
        var imported = AnnualPlanIcsParser.Parse(icsText);
        var existing = _events.GetAll();
        var existingByExternalId = existing.ToDictionary(e => e.ExternalId, StringComparer.Ordinal);
        var importedIds = imported.Select(e => e.ExternalId).ToHashSet(StringComparer.Ordinal);
        var added = 0;
        var updated = 0;

        foreach (var entry in imported)
        {
            if (!existingByExternalId.TryGetValue(entry.ExternalId, out var previous))
            {
                _events.Save(entry);
                added++;
                continue;
            }

            entry.Id = previous.Id;
            if (SameContent(previous, entry)) continue;
            _events.Save(entry);
            updated++;
        }

        var stale = existing.Where(e => !importedIds.Contains(e.ExternalId)).ToList();
        foreach (var entry in stale) _events.Delete(entry.Id);

        return new AnnualPlanPollResult(imported.Count, added, updated, stale.Count);
    }

    private static bool SameContent(AnnualPlanEvent left, AnnualPlanEvent right) =>
        left.ExternalId == right.ExternalId &&
        left.Title == right.Title &&
        left.Description == right.Description &&
        left.Location == right.Location &&
        left.StartDate == right.StartDate &&
        left.EndDate == right.EndDate &&
        left.StartTime == right.StartTime &&
        left.EndTime == right.EndTime &&
        left.IsAllDay == right.IsAllDay &&
        left.CalendarGroup == right.CalendarGroup &&
        left.Color == right.Color &&
        left.Status == right.Status &&
        left.Sequence == right.Sequence &&
        SameInstant(left.SourceLastModifiedUtc, right.SourceLastModifiedUtc);

    private static bool SameInstant(DateTime? left, DateTime? right) =>
        left is null && right is null ||
        left is not null && right is not null && left.Value.ToUniversalTime() == right.Value.ToUniversalTime();

    public void Clear()
    {
        var existing = _events.GetAll();
        foreach (var entry in existing) _events.Delete(entry.Id);
        if (existing.Count > 0) DataChanged?.Invoke();
    }

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