Converting VERY large number to a hex string - vb6

Question

Public Function MyMod(a As Double, b As Double) As Double
MyMod = a - Int(a / b) * b
End Function
This code doesn't work as it doesn't correctly show the remainder do be able to then calculate HEX.
Correct : 10009335357561071 / 16 = 62558345984756.69
VB6 MyMod returns 0 instead of a valid remainder.
I have been unable to figure out how to convert such a large value into a hex string?

Answer 1

I was able to code it myself. Because of the vb6 limitations of the size of a number, I had to go about it in different ways. I needed this to be able to covert VERY LARGE WHOLE numbers to Binary and Hexadecimal.
This this code, there are three functions you can use.
1) Decimal 2 Hex
2) Binary to Hex
3) Decimal 2 Binary
The code works and gives me CORRECT returns for the VERY large numbers.
Public Function Dec2Hex(Dec As String) As String
Dec2Hex = Binary2Hex(Dec2Bin(Dec))
End Function
Public Function Binary2Hex(Binary As String, Optional Pos As Long = 0) As String
Dim tic As Long
Dim Sz As Long
Dim x As Long
Dim z As Long
Dim AT As Long
Dim Hx As Long
Dim HxB As String
Dim xstart As Long
Dim xstop As Long
HxB = vbNullString
If InStrB(Binary, " ") <> 0 Then Binary = Replace(Binary, " ", "")
Sz = Len(Binary)
xstart = Sz
xstop = xstart - 3
Do
AT = 0
Hx = 0
If xstop < 1 Then xstop = 1
For x = xstart To xstop Step -1
AT = AT + 1
If AscB(Mid$(Binary, x, 1)) = 49 Then
Select Case AT
Case 1: Hx = Hx + 1
Case 2: Hx = Hx + 2
Case 3: Hx = Hx + 4
Case 4: Hx = Hx + 8
End Select
End If
Next x
HxB = Digit2Hex(CStr(Hx)) + HxB
If x <= 1 Then Exit Do
xstart = x
xstop = xstart - 3
Loop
Binary2Hex = HxB
End Function
Private Function Digit2Hex(digit As String) As String
Select Case digit
Case "0": Digit2Hex = "0"
Case "1": Digit2Hex = "1"
Case "2": Digit2Hex = "2"
Case "3": Digit2Hex = "3"
Case "4": Digit2Hex = "4"
Case "5": Digit2Hex = "5"
Case "6": Digit2Hex = "6"
Case "7": Digit2Hex = "7"
Case "8": Digit2Hex = "8"
Case "9": Digit2Hex = "9"
Case "10": Digit2Hex = "A"
Case "11": Digit2Hex = "B"
Case "12": Digit2Hex = "C"
Case "13": Digit2Hex = "D"
Case "14": Digit2Hex = "E"
Case "15": Digit2Hex = "F"
Case Else: Digit2Hex = vbNullString
End Select
End Function
Public Function Dec2Bin(Dec As String) As String
Dim Bin As String
Dim Var As Variant
Dim p As Long
Dim Tmp As String
Bin = vbNullString
Tmp = Dec
Do
Bin = IIf(isEven(Tmp), "0", "1") + Bin
Var = CDec(Tmp)
Var = Var / 2
Tmp = CStr(Var)
p = InStr(Tmp, ".")
If p > 0 Then Tmp = Mid(Tmp, 1, p - 1)
If Len(Tmp) = 1 Then
If CLng(Tmp) = 0 Then Exit Do
End If
Loop
Dec2Bin = Bin
End Function
Public Function isEven(Dec As String) As Boolean
Dim OE As Long
Dim myDec As Variant
OE = CLng(Right$(CStr(Dec), 1))
isEven = (OE = 0 Or OE = 2 Or OE = 4 Or OE = 6 Or OE = 8)
End Function

Answer 2

The only convenient data type in VB6 that can accurately represent 10009335357561071 is the Variant's Decimal subtype. Both Double and Currency native types lack the precision required.
There is also the matter of handling signed values and for that matter how many bytes of precision are desired, whether leading zeros should be suppressed, and probably others.
It is very hard to conceive of a need for this in a real application.
Even if we presume you are doing something "especially special" or if some instructor has given you this problem as an aid to general understanding...
... there just isn't much you can do with this without a BigNum library of some sort, or possibly using Decimal with some care though it only gains you a few more digits of precision.

Answer 3

Here is a working sample (using Fix), that is not mine, credit to http://visualbasic.ittoolbox.com/groups/technical-functional/visualbasic-l/vb60-hex-function-overflow-error-2744358.
Private Function MyHex(ByVal TempDec As Double) As String
Dim TNo As Integer
MyHex = ""
Do
TNo = TempDec - (Fix(TempDec / 16) * 16)
If TNo > 9 Then
MyHex = Chr(55 + TNo) & MyHex
Else
MyHex = TNo & MyHex
End If
TempDec = Fix(TempDec / 16)
Loop Until (TempDec = 0)
End Function

Answer 4

Private Declare Sub CopyMemory Lib "kernel32" Alias "RtlMoveMemory" (Destination As Any, Source As Any, ByVal Length As Long)
Private Function Dec2Hex(ByVal strDec As Variant) As String
Dim mybyte(0 To 19) As Byte
Dim lp As Long
CopyMemory mybyte(0), ByVal VarPtr(CDec(strDec)), 16
' Quick reorganise so we can then just step through the entire thing in one loop
For lp = 7 To 4 Step -1
mybyte(12 + lp) = mybyte(lp)
Next
' Build the hex string
For lp = 19 To 8 Step -1
If (Not Len(Dec2Hex) And mybyte(lp) <> 0) Or Len(Dec2Hex) Then
'Dec2Hex = Dec2Hex & Format(hex(mybyte(lp)), IIf(Len(Dec2Hex), "00", "0"))
Dec2Hex = Dec2Hex & IIf(Len(Dec2Hex), Right$("0" & hex(mybyte(lp)), 2), hex(mybyte(lp)))
End If
Next
End Function