#define DIGITS  "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"

Dim cadena(63) As String

Function mod2(a As Long, b As Integer) As Long
    Return a - (a \ b) * b
End Function

Function StrReverse(Byval text As String) As String
    Dim As String text2 = text
    Dim As Integer x, lt = Len(text)

    For x = 0 To lt Shr 1 - 1
        Swap text2[x], text2[lt - x - 1]
    Next x
    Return text2
End Function

Function EncodeNegativeBase(Byval n As Long, base_ As Integer) As String
    Dim As Long Puntero, idx, rem_
    Dim result As String

    If base_ > -1 Or base_ < -62 Then
        Return result
    Else
        If n = 0 Then
            Return "0"
        Else
            Puntero = 0
            Do While n <> 0
                rem_ = mod2(n, base_)
                n \= base_
                If rem_ < 0 Then
                    n += 1
                    rem_ = rem_ - base_
                End If
                result &= Mid(DIGITS, rem_ + 1, 1)
            Loop
        End If
    End If
    Return StrReverse(result)
End Function

Function DecodeNegativeBase(ns As String, base_ As Integer) As Long
    Dim As Long total, bb
    Dim As Integer i, j

    If base_ < -62 Or base_ > -1 Then Return 0
    If Mid(ns, 1, 1) = "0" Or (Mid(ns, 1, 1) = "0" And Mid(ns, 2, 1) = "0") Then Return 0
    i = Len(ns)
    total = 0
    bb = 1
    Do While i >= 1
        j = Instr(DIGITS, Mid(ns, i, 1)) - 1
        total += j * bb
        bb *= base_
        i -= 1
    Loop
    Return total
End Function

Sub Driver(n As Long, b As Integer)
    Dim As String ns = EncodeNegativeBase(n, b)
    Print Str(n); " encoded in base "; b; " = "; ns
    Dim As Long p = DecodeNegativeBase(ns, b)
    Print ns; " decoded in base "; b; " ="; p
    Print
End Sub

Driver 10, -2
Driver 146, -3
Driver 15, -10
Driver 118492, -62

Sleep