"How can I display 64 bit double number using VBScript on 32 Bit OS?" That was the question on StackOverflow back at the end of 2012. And nobody had answered it. What a challenge!

I assumed a pure-VBScript solution was needed, and set about finding and implementing a Very Large Integer class with which I could then implement a Hex64 to integer function. A workable candidate function was found on Rosetta Code in the Liberty BASIC solution to the Long Multiplication task.

The code for the class is as follows. It's pretty much the same as the original.
Option Explicit
Class VeryLongInteger
'http://rosettacode.org/wiki/Long_Multiplication#Liberty_BASIC
Public Function MULTIPLY(Str_A, Str_B)
Dim signA, signB, sResult, Str_Shift, i, d, Str_T
signA = 1
If Left(Str_A,1) = "-" Then
Str_A = Mid(Str_A,2)
signA = -1
End If
signB = 1
If Left(Str_B,1) = "-" Then
Str_B = Mid(Str_B,2)
signB = -1
End If
sResult = vbNullString
Str_T = vbNullString
Str_shift = vbNullString
For i = Len(Str_A) To 1 Step -1
d = CInt(Mid(Str_A,i,1))
Str_T = MULTBYDIGIT(Str_B, d)
sResult = ADD(sResult, Str_T & Str_shift)
Str_shift = Str_shift & "0"
'print d, Str_T, sResult
Next
If signA * signB < 0 Then sResult = "-" + sResult
'print sResult
MULTIPLY = sResult
End Function
Private Function MULTBYDIGIT(Str_A, d)
Dim sResult, carry, i, a, c
'multiply Str_A by digit d
sResult = vbNullString
carry = 0
For i = Len(Str_A) To 1 Step -1
a = CInt(Mid(Str_A,i,1))
c = a * d + carry
carry = c \ 10
c = c Mod 10
'print a, c
sResult = CStr(c) & sResult
Next
If carry > 0 Then sResult = CStr(carry) & sResult
'print sResult
MULTBYDIGIT = sResult
End Function
Public Function ADD(Str_A, Str_B)
Dim L, sResult, carry, i, a, b, c
'add Str_A + Str_B, for now only positive
l = MAX(Len(Str_A), Len(Str_B))
Str_A=PAD(Str_A,l)
Str_B=PAD(Str_B,l)
sResult = vbNullString 'result
carry = 0
For i = l To 1 Step -1
a = CInt(Mid(Str_A,i,1))
b = CInt(Mid(Str_B,i,1))
c = a + b + carry
carry = Int(c/10)
c = c Mod 10
'print a, b, c
sResult = CStr(c) & sResult
Next
If carry>0 Then sResult = CStr(carry) & sResult
'print sResult
ADD = sResult
End Function
Private Function Max(a,b)
If a > b Then
Max = a
Else
Max = b
End If
End Function
Private Function pad(a,n) 'pad from right with 0 to length n
Dim sResult
sResult = a
While Len(sResult) < n
sResult = "0" & sResult
Wend
pad = sResult
End Function
End Class

With that defined I have now all I need to implement a Hex64 function. This I did in two forms:

* A memoized version which precomputes all the relevant powers of 16
Function PrecomputedFromHex64(sHex)
Dim VLI
Set VLI = New VeryLongInteger
Dim Sixteen(16)
Sixteen(0) = "1"
Sixteen(1) = "16"
Sixteen(2) = VLI.MULTIPLY(Sixteen(1),"16")
Sixteen(3) = VLI.MULTIPLY(Sixteen(2),"16")
Sixteen(4) = VLI.MULTIPLY(Sixteen(3),"16")
Sixteen(5) = VLI.MULTIPLY(Sixteen(4),"16")
Sixteen(6) = VLI.MULTIPLY(Sixteen(5),"16")
Sixteen(7) = VLI.MULTIPLY(Sixteen(6),"16")
Sixteen(8) = VLI.MULTIPLY(Sixteen(7),"16")
Sixteen(9) = VLI.MULTIPLY(Sixteen(8),"16")
Sixteen(10) = VLI.MULTIPLY(Sixteen(9),"16")
Sixteen(11) = VLI.MULTIPLY(Sixteen(10),"16")
Sixteen(12) = VLI.MULTIPLY(Sixteen(11),"16")
Sixteen(13) = VLI.MULTIPLY(Sixteen(12),"16")
Sixteen(14) = VLI.MULTIPLY(Sixteen(13),"16")
Sixteen(15) = VLI.MULTIPLY(Sixteen(14),"16")
Dim theAnswer, i, theDigit, theMultiplier, thePower, aPower
theAnswer = "0"
aPower = 0
For i = Len(sHex) To 1 Step -1
theDigit = UCase(Mid(sHex,i,1))
theMultiplier = InStr("0123456789ABCDEF",theDigit)-1
thePower = Sixteen(aPower)
thePower = VLI.MULTIPLY(CStr(theMultiplier),thePower)
theAnswer = VLI.ADD(theAnswer,thePower )
aPower = aPower + 1
Next
PrecomputedFromHex64 = theAnswer
End Function

* A non-memoized version which computes the relevant power when it is needed.
Function FromHex64(sHex)
Dim VLI
Set VLI = New VeryLongInteger
Dim theAnswer, i, theDigit, theMultiplier, thePower, aPower
theAnswer = "0"
thePower = "1"
For i = Len(sHex) To 1 Step -1
theDigit = UCase(Mid(sHex,i,1))
theMultiplier = InStr("0123456789ABCDEF",theDigit)-1
theAnswer = VLI.ADD(theAnswer,VLI.MULTIPLY(thePower,theMultiplier))
thePower = VLI.MULTIPLY(thePower,"16")
Next
FromHex64 = theAnswer
End Function

The memoized version is slightly faster than the non-memoized, despite the overhead of the memoization. If the class/function pair were to be used a lot in a script, one might consider making both the VLI instantiation and the Sixteen() array global and precomputing the array at the beginning of the script rather than on each invocation.

A rough test of the code follows:
Const test = "FFFFFFFFFFFFFFFF" '"41417724EBA8953E"
Dim t, I, S
t=Timer
For I = 1 To 100
S = FromHex64(test)
Next
WScript.Echo "No memoization", Timer-t
t=Timer
For I = 1 To 100
S = PrecomputedFromHex64(test)
Next
WScript.Echo "Memoized each time",Timer-t
Function GlobalMemoFromHex64(sHex)
Dim theAnswer, i, theDigit, theMultiplier, thePower, aPower
theAnswer = "0"
aPower = 0
For i = Len(sHex) To 1 Step -1
theDigit = UCase(Mid(sHex,i,1))
theMultiplier = InStr("0123456789ABCDEF",theDigit)-1
thePower = Sixteen(aPower)
thePower = VLI.MULTIPLY(CStr(theMultiplier),thePower)
theAnswer = VLI.ADD(theAnswer,thePower )
aPower = aPower + 1
Next
GlobalMemoFromHex64 = theAnswer
End Function
Dim VLI
Set VLI = New VeryLongInteger
Dim Sixteen(16)
Sixteen(0) = "1"
Sixteen(1) = "16"
Sixteen(2) = VLI.MULTIPLY(Sixteen(1),"16")
Sixteen(3) = VLI.MULTIPLY(Sixteen(2),"16")
Sixteen(4) = VLI.MULTIPLY(Sixteen(3),"16")
Sixteen(5) = VLI.MULTIPLY(Sixteen(4),"16")
Sixteen(6) = VLI.MULTIPLY(Sixteen(5),"16")
Sixteen(7) = VLI.MULTIPLY(Sixteen(6),"16")
Sixteen(8) = VLI.MULTIPLY(Sixteen(7),"16")
Sixteen(9) = VLI.MULTIPLY(Sixteen(8),"16")
Sixteen(10) = VLI.MULTIPLY(Sixteen(9),"16")
Sixteen(11) = VLI.MULTIPLY(Sixteen(10),"16")
Sixteen(12) = VLI.MULTIPLY(Sixteen(11),"16")
Sixteen(13) = VLI.MULTIPLY(Sixteen(12),"16")
Sixteen(14) = VLI.MULTIPLY(Sixteen(13),"16")
Sixteen(15) = VLI.MULTIPLY(Sixteen(14),"16")
t=Timer
For I = 1 To 100
S = GlobalMemoFromHex64(test)
Next
WScript.Echo "Global memo",Timer-t

Running the above in VBSEdit a few times under varying conditions gave the following results:
No memoization 2.632813
Memoized each time 1.023438
Global memo 0.328125
No memoization 1.667969
Memoized each time 0.8867188
Global memo 0.2695313
No memoization 1.488281
Memoized each time 0.9335938
Global memo 0.3046875

The global precomputation of the memo array is fastest technique, but if you're only using the function once you'll get by with the non-memo version.

Further optimisations are possible. Compilation, of course, would make it even faster.

Enjoy!

*© Copyright Bruce M. Axtens, 2014*