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Imports System
Imports System.Text
Imports System.Security.Cryptography
Namespace gfoidl.HashFunktionen
Public Class CRC32
Inherits HashAlgorithm
Public Shared Function CRC32OfString(ByVal s As String) As String
Dim original As Byte()
Dim encoded As Byte()
Dim crc32 As New CRC32()
original = Encoding.[Default].GetBytes(s)
encoded = crc32.ComputeHash(original)
Dim sbEncoded As New StringBuilder()
For i As Integer = 0 To encoded.Length - 1
sbEncoded.Append(encoded(i).ToString("x2"))
Next
Return sbEncoded.ToString()
End Function
#Region "Konstanten"
Public Const DefaultPolynomial As UInt32 = 3988292384
Public Const DefaultSeed As UInt32 = 4294967295
#End Region
#Region "Private Variablen"
Private _hash As UInt32
Private _seed As UInt32
Private _table As UInt32()
#End Region
#Region "Kontruktoren"
Public Sub New()
_table = InitializeTable(DefaultPolynomial)
_seed = DefaultSeed
Initialize()
End Sub
Public Sub New(ByVal polynomial As UInt32, ByVal seed As UInt32)
_table = InitializeTable(polynomial)
Me._seed = seed
Initialize()
End Sub
#End Region
#Region "Überschreibungen"
Public Overloads Overrides Sub Initialize()
_hash = _seed
End Sub
Protected Overloads Overrides Sub HashCore(ByVal buffer As Byte(), ByVal start As Integer, ByVal length As Integer)
_hash = CalculateHash(_table, _hash, buffer, start, length)
End Sub
Protected Overloads Overrides Function HashFinal() As Byte()
Dim hashBuffer As Byte() = UInt32ToBigEndianBytes(_hash)
Me.HashValue = hashBuffer
Return hashBuffer
End Function
Public Overloads Overrides ReadOnly Property HashSize() As Integer
Get
Return 32
End Get
End Property
#End Region
#Region "Implementierung"
Public Shared Function Compute(ByVal polynomial As UInt32, ByVal seed As UInt32, ByVal buffer As Byte()) As UInt32
Return CalculateHash(InitializeTable(polynomial), seed, buffer, 0, buffer.Length)
End Function
Private Shared Function InitializeTable(ByVal polynomial As UInt32) As UInt32()
Dim createTable As UInt32() = New UInt32(255) {}
For i As Integer = 0 To 255
Dim entry As UInt32 = i
For j As Integer = 0 To 7
If (entry And 1) = 1 Then
entry = (entry >> 1) Xor polynomial
Else
entry = entry >> 1
End If
Next
createTable(i) = entry
Next
Return createTable
End Function
Private Shared Function CalculateHash(ByVal table As UInt32(), ByVal seed As UInt32, ByVal buffer As Byte(), ByVal start As Integer, ByVal size As Integer) As UInt32
Dim crc As UInt32 = seed
For i As Integer = start To size - 1
crc = (crc >> 8) Xor table(buffer(i) Xor crc And 255)
Next
Return Not crc
End Function
Private Function UInt32ToBigEndianBytes(ByVal x As UInt32) As Byte()
Return New Byte() {CByte(((x >> 24) And 255)), CByte(((x >> 16) And 255)), CByte(((x >> 8) And 255)), CByte((x And 255))}
End Function
#End Region
End Class
End Namespace
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