using System.Threading.Channels;
using EngineeringSync.Domain.Constants;
using EngineeringSync.Domain.Entities;
using EngineeringSync.Infrastructure;
using EngineeringSync.Service.Hubs;
using EngineeringSync.Service.Models;
using Microsoft.AspNetCore.SignalR;
using Microsoft.EntityFrameworkCore;

namespace EngineeringSync.Service.Services;

/// <summary>
/// Verwaltet FileSystemWatcher-Instanzen für alle aktiven Projekte.
/// Nutzt ein Channel als Puffer zwischen dem schnellen FSW-Event-Thread
/// und dem langsamen DB-Schreib-Thread (Debouncing).
/// </summary>
public sealed class WatcherService(
    IDbContextFactory<AppDbContext> dbFactory,
    IHubContext<NotificationHub> hub,
    ILogger<WatcherService> logger) : BackgroundService
{
    // Unbounded Channel: FSW-Events kommen schnell, Verarbeitung ist langsam
    private readonly Channel<FileEvent> _channel =
        Channel.CreateUnbounded<FileEvent>(new UnboundedChannelOptions { SingleReader = true });

    // Watcher pro Projekt-ID – wird dynamisch verwaltet
    private readonly Dictionary<Guid, FileSystemWatcher[]> _watchers = [];
    private readonly SemaphoreSlim _watcherLock = new(1, 1);
    private readonly ILogger<WatcherService> _logger = logger;

    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
        // Alle aktiven Projekte beim Start laden
        List<ProjectConfig> projects;
        await using (var db = await dbFactory.CreateDbContextAsync(stoppingToken))
        {
            projects = await db.Projects.Where(p => p.IsActive).ToListAsync(stoppingToken);
            foreach (var project in projects)
                await StartWatchingAsync(project);
        }

        // Initialer Scan: Engineering- vs. Simulations-Ordner vergleichen
        foreach (var project in projects)
        {
            try { await ScanExistingFilesAsync(project, stoppingToken); }
            catch (Exception ex)
            {
                _logger.LogError(ex, "Initialer Scan fehlgeschlagen für '{Name}'", project.Name);
            }
        }

        // Channel-Consumer läuft, bis der Service gestoppt wird
        await ConsumeChannelAsync(stoppingToken);
    }

    /// <summary>Startet Watcher für ein Projekt. Idempotent – stoppt alten Watcher zuerst.</summary>
    public async Task StartWatchingAsync(ProjectConfig project)
    {
        await _watcherLock.WaitAsync();
        try
        {
            StopWatchingInternal(project.Id);
            if (!project.IsActive || !Directory.Exists(project.EngineeringPath))
                return;

            var extensions = project.GetExtensions().ToArray();

            // Pro Dateiendung einen eigenen Watcher (FSW unterstützt nur ein Filter-Pattern)
            var watchers = extensions.Select(ext => CreateWatcher(project, ext)).ToArray();
            _watchers[project.Id] = watchers;
            _logger.LogInformation("Watcher gestartet für Projekt '{Name}' ({Count} Extensions)",
                project.Name, watchers.Length);
        }
        finally
        {
            _watcherLock.Release();
        }
    }

    /// <summary>
    /// Initialer Scan: Vergleicht Engineering- und Simulations-Ordner.
    /// Erkennt Dateien, die im Engineering-Ordner vorhanden sind, aber im
    /// Simulations-Ordner fehlen oder sich unterscheiden.
    /// </summary>
    public async Task ScanExistingFilesAsync(ProjectConfig project, CancellationToken ct = default)
    {
        if (!Directory.Exists(project.EngineeringPath)) return;

        var extensions = project.GetExtensions().ToList();
        
        // Wenn keine Erweiterungen angegeben oder "*" → alle Dateien scannen
        var scanAllFiles = extensions.Count == 0 || (extensions.Count == 1 && extensions[0] == "*");
        
        var engFiles = scanAllFiles
            ? Directory.EnumerateFiles(project.EngineeringPath, "*", SearchOption.AllDirectories).ToList()
            : Directory.EnumerateFiles(project.EngineeringPath, "*", SearchOption.AllDirectories)
                .Where(f => extensions.Contains(Path.GetExtension(f), StringComparer.OrdinalIgnoreCase))
                .ToList();

        if (engFiles.Count == 0) return;

        await using var db = await dbFactory.CreateDbContextAsync(ct);
        var newChanges = 0;

        foreach (var engFile in engFiles)
        {
            var relativePath = Path.GetRelativePath(project.EngineeringPath, engFile);
            var simFile = Path.Combine(project.SimulationPath, relativePath);

            var engHash = await FileHasher.ComputeAsync(engFile, ct);
            var engInfo = new FileInfo(engFile);

            // FileRevision anlegen/aktualisieren
            var revision = await db.FileRevisions
                .FirstOrDefaultAsync(r => r.ProjectId == project.Id && r.RelativePath == relativePath, ct);

            if (revision is null)
            {
                db.FileRevisions.Add(new FileRevision
                {
                    ProjectId = project.Id,
                    RelativePath = relativePath,
                    FileHash = engHash,
                    Size = engInfo.Length,
                    LastModified = engInfo.LastWriteTimeUtc
                });
            }
            else
            {
                revision.FileHash = engHash;
                revision.Size = engInfo.Length;
                revision.LastModified = engInfo.LastWriteTimeUtc;
            }

            // Prüfen: Existiert die Datei im Simulations-Ordner und ist sie identisch?
            bool needsSync;
            if (!File.Exists(simFile))
            {
                needsSync = true;
            }
            else
            {
                var simHash = await FileHasher.ComputeAsync(simFile, ct);
                needsSync = engHash != simHash;
            }

            if (!needsSync) continue;

            // Nur anlegen, wenn nicht bereits ein offener PendingChange existiert
            var alreadyPending = await db.PendingChanges
                .AnyAsync(c => c.ProjectId == project.Id
                    && c.RelativePath == relativePath
                    && c.Status == ChangeStatus.Pending, ct);
            if (alreadyPending) continue;

            db.PendingChanges.Add(new PendingChange
            {
                ProjectId = project.Id,
                RelativePath = relativePath,
                ChangeType = File.Exists(simFile) ? ChangeType.Modified : ChangeType.Created
            });
            newChanges++;
        }

        if (newChanges > 0)
        {
            await db.SaveChangesAsync(ct);

            var totalPending = await db.PendingChanges
                .CountAsync(c => c.ProjectId == project.Id && c.Status == ChangeStatus.Pending, ct);

            await hub.Clients.All.SendAsync(HubMethodNames.ReceiveChangeNotification,
                project.Id, project.Name, totalPending, ct);

            _logger.LogInformation("Initialer Scan für '{Name}': {Count} Unterschied(e) erkannt",
                project.Name, newChanges);
        }
        else
        {
            _logger.LogInformation("Initialer Scan für '{Name}': Ordner sind synchron", project.Name);
        }
    }

    /// <summary>Stoppt und entfernt Watcher für ein Projekt.</summary>
    public async Task StopWatchingAsync(Guid projectId)
    {
        await _watcherLock.WaitAsync();
        try { StopWatchingInternal(projectId); }
        finally { _watcherLock.Release(); }
    }

    private void StopWatchingInternal(Guid projectId)
    {
        if (!_watchers.Remove(projectId, out var old)) return;
        foreach (var w in old) { w.EnableRaisingEvents = false; w.Dispose(); }
    }

    private FileSystemWatcher CreateWatcher(ProjectConfig project, string extension)
    {
        var watcher = new FileSystemWatcher(project.EngineeringPath)
        {
            Filter = extension == "*" ? "*.*" : $"*{extension}",
            IncludeSubdirectories = true,
            NotifyFilter = NotifyFilters.LastWrite | NotifyFilters.FileName | NotifyFilters.Size,
            InternalBufferSize = 65536 // 64KB – verhindert Buffer-Overflow bei vielen gleichzeitigen Events
        };

        // Lokale Kopie für Lambda-Capture
        var projectId = project.Id;
        var basePath = project.EngineeringPath;

        watcher.Created += (_, e) => EnqueueEvent(projectId, basePath, e.FullPath, FileEventType.CreatedOrChanged);
        watcher.Changed += (_, e) => EnqueueEvent(projectId, basePath, e.FullPath, FileEventType.CreatedOrChanged);
        watcher.Deleted += (_, e) => EnqueueEvent(projectId, basePath, e.FullPath, FileEventType.Deleted);
        watcher.Renamed += (_, e) => EnqueueRenamedEvent(projectId, basePath, e);
        watcher.Error += (_, args) =>
        {
            _logger.LogWarning(args.GetException(), "FSW Buffer-Overflow für Projekt {Id} – starte Re-Scan", projectId);
            _ = Task.Run(async () =>
            {
                try
                {
                    await ScanExistingFilesAsync(project);
                }
                catch (Exception ex)
                {
                    _logger.LogError(ex, "Re-Scan nach Buffer-Overflow fehlgeschlagen für Projekt {Id}", projectId);
                }
            });
        };

        watcher.EnableRaisingEvents = true;
        return watcher;
    }

    private void EnqueueEvent(Guid projectId, string basePath, string fullPath, FileEventType type)
    {
        var rel = Path.GetRelativePath(basePath, fullPath);
        _channel.Writer.TryWrite(new FileEvent(projectId, fullPath, rel, type));
    }

    private void EnqueueRenamedEvent(Guid projectId, string basePath, RenamedEventArgs e)
    {
        var rel = Path.GetRelativePath(basePath, e.FullPath);
        var oldRel = Path.GetRelativePath(basePath, e.OldFullPath);
        _channel.Writer.TryWrite(new FileEvent(projectId, e.FullPath, rel, FileEventType.Renamed, oldRel));
    }

    /// <summary>
    /// Debouncing-Consumer: Gruppiert Events nach (ProjectId, RelativePath) innerhalb
    /// eines 2000ms-Fensters. Verhindert, dass eine Datei 10× verarbeitet wird.
    /// </summary>
    private async Task ConsumeChannelAsync(CancellationToken ct)
    {
        // Dictionary: Key=(ProjectId,RelativePath) → letztes Event
        var pending = new Dictionary<(Guid, string), FileEvent>();

        while (!ct.IsCancellationRequested)
        {
            // Warte auf erstes Event (blockierend)
            if (!await _channel.Reader.WaitToReadAsync(ct)) break;

            // Sammle alle sofort verfügbaren Events (nicht blockierend)
            while (_channel.Reader.TryRead(out var evt))
                pending[(evt.ProjectId, evt.RelativePath)] = evt; // neuester gewinnt

            // 2s warten – kommen in dieser Zeit weitere Events, werden sie im nächsten Batch verarbeitet
            await Task.Delay(2000, ct);

            // Noch mehr eingelaufene Events sammeln
            while (_channel.Reader.TryRead(out var evt))
                pending[(evt.ProjectId, evt.RelativePath)] = evt;

            if (pending.Count == 0) continue;

            var batch = pending.Values.ToList();
            pending.Clear();

            foreach (var evt in batch)
            {
                try { await ProcessEventAsync(evt, ct); }
                catch (Exception ex)
                {
                    _logger.LogError(ex, "Fehler beim Verarbeiten von {Path}", evt.RelativePath);
                }
            }
        }
    }

    private async Task ProcessEventAsync(FileEvent evt, CancellationToken ct)
    {
        await using var db = await dbFactory.CreateDbContextAsync(ct);

        var project = await db.Projects.FindAsync([evt.ProjectId], ct);
        if (project is null) return;

        // Existierende Revision lesen
        var revision = await db.FileRevisions
            .FirstOrDefaultAsync(r => r.ProjectId == evt.ProjectId && r.RelativePath == evt.RelativePath, ct);

        if (evt.EventType == FileEventType.Deleted)
        {
            await HandleDeleteAsync(db, project, revision, evt, ct);
            return;
        }

        if (!File.Exists(evt.FullPath)) return; // Race condition: Datei schon weg

        // Hash berechnen mit Retry-Logik für gesperrte Dateien
        string newHash;
        try
        {
            newHash = await ComputeHashWithRetryAsync(evt.FullPath, ct);
        }
        catch (IOException ex)
        {
            _logger.LogWarning(ex, "Hash-Berechnung für {Path} fehlgeschlagen nach Retrys – überspringe Event", evt.RelativePath);
            return;
        }

        if (revision is not null && revision.FileHash == newHash) return; // Keine echte Änderung

        var info = new FileInfo(evt.FullPath);

        // FileRevision anlegen oder aktualisieren
        if (revision is null)
        {
            db.FileRevisions.Add(new FileRevision
            {
                ProjectId = evt.ProjectId,
                RelativePath = evt.RelativePath,
                FileHash = newHash,
                Size = info.Length,
                LastModified = info.LastWriteTimeUtc
            });
        }
        else
        {
            revision.FileHash = newHash;
            revision.Size = info.Length;
            revision.LastModified = info.LastWriteTimeUtc;
            if (evt.EventType == FileEventType.Renamed)
                revision.RelativePath = evt.RelativePath;
        }

        // PendingChange schreiben
        var changeType = evt.EventType switch
        {
            FileEventType.Renamed => ChangeType.Renamed,
            _ when revision is null => ChangeType.Created,
            _ => ChangeType.Modified
        };

        db.PendingChanges.Add(new PendingChange
        {
            ProjectId = evt.ProjectId,
            RelativePath = evt.RelativePath,
            ChangeType = changeType,
            OldRelativePath = evt.OldRelativePath
        });

        await db.SaveChangesAsync(ct);

        // Alle SignalR-Clients benachrichtigen
        var count = await db.PendingChanges
            .CountAsync(c => c.ProjectId == evt.ProjectId && c.Status == ChangeStatus.Pending, ct);

        await hub.Clients.All.SendAsync(HubMethodNames.ReceiveChangeNotification,
            evt.ProjectId, project.Name, count, ct);

        _logger.LogInformation("[{Type}] {Path} in Projekt '{Name}'",
            changeType, evt.RelativePath, project.Name);
    }

    private async Task HandleDeleteAsync(AppDbContext db, ProjectConfig project,
        FileRevision? revision, FileEvent evt, CancellationToken ct)
    {
        if (revision is not null)
            db.FileRevisions.Remove(revision);

        db.PendingChanges.Add(new PendingChange
        {
            ProjectId = evt.ProjectId,
            RelativePath = evt.RelativePath,
            ChangeType = ChangeType.Deleted
        });

        await db.SaveChangesAsync(ct);

        var count = await db.PendingChanges
            .CountAsync(c => c.ProjectId == evt.ProjectId && c.Status == ChangeStatus.Pending, ct);

        await hub.Clients.All.SendAsync(HubMethodNames.ReceiveChangeNotification,
            evt.ProjectId, project.Name, count, ct);
    }

    /// <summary>
    /// Berechnet den Hash einer Datei mit Retry-Logik.
    /// Versucht bis zu 3 Mal, mit exponentieller Backoff-Wartezeit.
    /// Wirft IOException, wenn alle Versuche scheitern.
    /// </summary>
    private async Task<string> ComputeHashWithRetryAsync(string fullPath, CancellationToken ct)
    {
        const int maxAttempts = 3;
        for (int attempt = 0; attempt < maxAttempts; attempt++)
        {
            try
            {
                return await FileHasher.ComputeAsync(fullPath, ct);
            }
            catch (IOException) when (attempt < maxAttempts - 1)
            {
                var delaySeconds = 2 * (attempt + 1);
                _logger.LogDebug("Hash-Berechnung für {Path} fehlgeschlagen (Versuch {Attempt}), warte {Seconds}s...",
                    fullPath, attempt + 1, delaySeconds);
                await Task.Delay(TimeSpan.FromSeconds(delaySeconds), ct);
            }
        }
        
        // Wenn alle Versuche fehlschlagen, IOException werfen
        throw new IOException($"Hash-Berechnung für {fullPath} fehlgeschlagen nach {maxAttempts} Versuchen");
    }

    public override void Dispose()
    {
        // Channel-Writer schließen, um ConsumeChannelAsync zum Beenden zu bringen
        _channel.Writer.TryComplete();
        
        foreach (var watchers in _watchers.Values)
            foreach (var w in watchers) w.Dispose();
        _watcherLock.Dispose();
        base.Dispose();
    }
}