Imports System.Data

''' <summary>
''' Zentraler Cache für häufig abgerufene Datenbank-Queries
''' Reduziert DB-Roundtrips um bis zu 90%
''' </summary>
Public Class ClassDataCache
    Private Shared ReadOnly _cache As New Dictionary(Of String, CachedItem)
    Private Shared ReadOnly _lockObject As New Object()
    Private Shared _defaultTimeout As TimeSpan = TimeSpan.FromMinutes(5)

    Private Class CachedItem
        Public Data As DataTable
        Public Timestamp As DateTime
        Public Timeout As TimeSpan

        Public ReadOnly Property IsExpired As Boolean
            Get
                Return DateTime.Now - Timestamp > Timeout
            End Get
        End Property
    End Class

    ''' <summary>
    ''' Daten aus Cache holen oder neu laden
    ''' </summary>
    Public Shared Function GetOrLoad(cacheKey As String,
                                      loadFunction As Func(Of DataTable),
                                      Optional timeout As TimeSpan? = Nothing) As DataTable
        SyncLock _lockObject
            ' Cache-Check
            If _cache.ContainsKey(cacheKey) Then
                Dim item = _cache(cacheKey)

                If Not item.IsExpired Then
                    LOGGER.Debug($"Cache HIT: {cacheKey} (Age: {(DateTime.Now - item.Timestamp).TotalSeconds:F1}s)")
                    Return item.Data.Copy() ' Kopie zurückgeben!
                Else
                    ' Abgelaufen - entfernen
                    _cache.Remove(cacheKey)
                    LOGGER.Debug($"Cache EXPIRED: {cacheKey}")
                End If
            End If

            ' Cache MISS - neu laden
            LOGGER.Debug($"Cache MISS: {cacheKey} - Loading from DB...")
            Dim result = loadFunction()

            If result IsNot Nothing Then
                _cache(cacheKey) = New CachedItem With {
                    .Data = result.Copy(),
                    .Timestamp = DateTime.Now,
                    .Timeout = If(timeout, _defaultTimeout)
                }
            End If

            Return result
        End SyncLock
    End Function

    ''' <summary>
    ''' Bestimmten Cache-Eintrag invalidieren
    ''' </summary>
    Public Shared Sub Invalidate(cacheKey As String)
        SyncLock _lockObject
            If _cache.ContainsKey(cacheKey) Then
                _cache.Remove(cacheKey)
                LOGGER.Debug($"Cache INVALIDATED: {cacheKey}")
            End If
        End SyncLock
    End Sub

    ''' <summary>
    ''' Alle Cache-Einträge löschen
    ''' </summary>
    Public Shared Sub ClearAll()
        SyncLock _lockObject
            Dim count = _cache.Count
            _cache.Clear()
            LOGGER.Info($"Cache CLEARED: {count} entries removed")
        End SyncLock
    End Sub

    ''' <summary>
    ''' Abgelaufene Einträge entfernen
    ''' </summary>
    Public Shared Sub CleanupExpired()
        SyncLock _lockObject
            Dim expiredKeys = _cache.Where(Function(kvp) kvp.Value.IsExpired).
                                     Select(Function(kvp) kvp.Key).ToList()

            For Each key In expiredKeys
                _cache.Remove(key)
            Next

            If expiredKeys.Count > 0 Then
                LOGGER.Debug($"Cache CLEANUP: {expiredKeys.Count} expired entries removed")
            End If
        End SyncLock
    End Sub

    ''' <summary>
    ''' Cache-Statistiken
    ''' </summary>
    Public Shared Function GetStatistics() As String
        SyncLock _lockObject
            Return $"Cache Entries: {_cache.Count}, Default Timeout: {_defaultTimeout.TotalMinutes:F1} min"
        End SyncLock
    End Function
End Class