Извлекаем факториал с абсолютной точностью.

Public Structure Ciberst
    ' Implements IFormattable, IComparable, IComparable(Of Ciberst), IEquatable(Of Ciberst)
    Friend _sign As Integer
    Friend _bits As UInt32()
    Private Shared ReadOnly s_bnMinInt As Ciberst
    Private Shared ReadOnly s_bnOneInt As Ciberst
    Private Shared ReadOnly s_bnZeroInt As Ciberst
    Private Shared ReadOnly s_bnMinusOneInt As Ciberst
    Private Const knMaskHighBit As Integer = -2147483648
    Private Const kuMaskHighBit As UInt32 = &H8000000
    Private Const kcbitUint As Integer = &H20
    Private Const kcbitUlong As Integer = &H40
    Private Const DecimalScaleFactorMask As Integer = &HFF0000
    Private Const DecimalSignMask As Integer = -2147483648
 
    Public Shared ReadOnly Property Zero() As Ciberst
 
        Get
            Return Ciberst.s_bnZeroInt
        End Get
    End Property
 
    Public Shared ReadOnly Property One() As Ciberst
 
        Get
            Return Ciberst.s_bnOneInt
        End Get
    End Property
 
    Public Shared ReadOnly Property MinusOne() As Ciberst
        Get
            Return Ciberst.s_bnMinusOneInt
        End Get
    End Property
 
    Public ReadOnly Property IsZero() As Boolean
        Get
            Return (Me._sign = 0)
        End Get
    End Property
 
    Public ReadOnly Property IsOne() As Boolean
        Get
            Return ((Me._sign = 1) AndAlso (Me._bits Is Nothing))
        End Get
    End Property
 
    Public ReadOnly Property IsEven() As Boolean
        Get
            If (Not Me._bits Is Nothing) Then
                Return ((Me._bits(0) And 1) = 0)
            End If
            Return ((Me._sign And 1) = 0)
        End Get
    End Property
 
    Public ReadOnly Property Sign() As Integer
        Get
            Return ((Me._sign >> &H1F) - (-Me._sign >> &H1F))
        End Get
    End Property
 
    Public Overloads Shared Operator +(ByVal left As Ciberst, ByVal right As Ciberst) As Ciberst
        If Right.IsZero Then
            Return left
        End If
        If Left.IsZero Then
            Return right
        End If
 
        Return New Ciberst(left._sign + right._sign)
    End Operator
    Public Overloads Shared Operator =(ByVal left As UInt64, ByVal right As Ciberst) As String
        Return right.Equals(left)
    End Operator
    Public Overloads Shared Operator <>(ByVal left As UInt64, ByVal right As Ciberst) As String
        Return Not right.Equals(left)
    End Operator
 
 
    Public Sub New(ByVal value As Long)
        If ((-2147483648 <= value) AndAlso (value <= &H7FFFFFFF)) Then
            If (value = -2147483648) Then
                'Me = Ciberst.s_bnMinInt
            Else
                Me._sign = CInt(value)
                Me._bits = Nothing
            End If
        Else
            Dim num As UInt64 = 0
            If (value < 0) Then
                num = CULng(-value)
                Me._sign = -1
            Else
                num = CULng(value)
                Me._sign = 1
            End If
            Me._bits = New UInt32() {DirectCast(CUInt(num), UInt32), DirectCast((CUInt(num) >> &H20), UInt32)}
        End If
    End Sub
 
End Structure