• Home
 · 
• Math/Dates
 · 
• Arithmetic operations on memory address pointers

Arithmetic operations on memory address pointers

Function to enable valid addition and subtraction of unsigned long integers. Treats the passed value as an unsigned long and returns an unsigned long. Allows safe arithmetic operations on memory address pointers. Assumes valid pointer and pointer offset. Rather fast too, has very small performance hit compared to unsafe in-line calculation, even in intensive loops. The code for addition came from VBC (LukeH) but I needed subtraction as well, thought I'd share. Confirmation that this produces valid results would be appreciated.

Rate Arithmetic operations on memory address pointers

• 1
• 2
• 3
• 4
• 5

(4(4 Vote))

  

' Used for unsigned arithmetic

Private Const DW_MSB = &H80000000 ' DWord Most Significant Bit

  


 ' + Sum Unsigned Long ++++++++++++++++++++++

  

 ' Enables valid addition and subtraction of unsigned long integers.

 ' Treats lPtr as an unsigned long and returns an unsigned long.

 ' Allows safe arithmetic operations on memory address pointers.

 ' Assumes valid pointer and pointer offset.

  


Private Function SumUnsignedLong(ByVal lPtr As Long, ByVal lOffset As Long) As Long

  

  If lOffset > 0& Then

     If lPtr And DW_MSB Then             ' if ptr < 0

        SumUnsignedLong = lPtr + lOffset ' ignors > unsigned max (see assumption)

  

     ElseIf (lPtr Or DW_MSB) < -lOffset Then

        SumUnsignedLong = lPtr + lOffset ' result is below signed int max

  

     Else                                ' result wraps to min signed int

        SumUnsignedLong = (lPtr + DW_MSB) + (lOffset + DW_MSB)

     End If

  

  ElseIf lOffset = 0& Then

     SumUnsignedLong = lPtr

  

  Else 'If lOffset < 0 Then

     If (lPtr And DW_MSB) = 0& Then      ' if ptr > 0

        SumUnsignedLong = lPtr + lOffset ' ignors unsigned < zero (see assumption)

  

     ElseIf (lPtr - DW_MSB) >= -lOffset Then

        SumUnsignedLong = lPtr + lOffset ' result is above signed int min

  

     Else                                ' result wraps to max signed int

        SumUnsignedLong = (lOffset - DW_MSB) + (lPtr - DW_MSB)

     End If

  End If

End Function

  


 ' ++++++++++++++++++++++++++++++++++++++++++

  

 ' Extract from the classic ShellSort algorithm

  


  Dim s1 As String, lpStr1 As Long

  Dim s2 As String, lpStr2 As Long

  


  lpStr1 = VarPtr(s1)


  lpStr2 = VarPtr(s2)

  


  lpArr = VarPtr(sArr(lb))

  


 'lpLast = lpArr + ((ub - lb) * 4&)




 lpLast = SumUnsignedLong(lpArr, (ub - lb) * 4&)


 '...

  


   CopyMemByV lpStr1, lpLast, 4&

  


   'CopyMemByV lpStr2, lpLast - lRange, 4&




   CopyMemByV lpStr2, SumUnsignedLong(lpLast, -lRange), 4&

  


   If StrComp(s2, s1, eMethod) = eComp Then


   '...