String Repeat function in VBA

Question

VBA has built-in functions for repeating a single character:

Function String$(Number As Long, Character) As String

Function Space$(Number As Long) As String

But neither are of any use when you need to repeat a string that has more than one character.

You could repeat a string "abcde" 5 times by doing something crafty like:

?Join(Split(String$(5,"."),"."),"abcde")

But that is neither intuitive nor performant.

In Excel, there is also WorksheetFunction.Rept, but it is painfully slow, and only available in Excel.

So I made a custom function that builds the string, while minimizing the concatenations. In fact, it doesn't use any concatenation, but instead uses a buffer and CopyMemory to fill the buffer. And rather than filling the buffer one instance at a time, the code fills the buffer using a lookback that reduces the number of buffer writes exponentially:

Given a string "abcde" that repeats 5 times:

Create a buffer of 25 spaces

"                         "

1st buffer write - assign the string to the first buffer position

"abcde                    "
 [NEW]

2nd buffer write - copy the existing populated buffer (5 characters) into the next buffer position

"abcdeabcde               "
      [NEW]

3rd buffer write - copy the existing populated buffer (10 characters) into the next buffer position

"abcdeabcdeabcdeabcde     "
           [  NEW   ]`

4th buffer write - copy the lesser of the existing populated buffer (20 characters) and the remaining buffer (5 characters) into the next buffer position.

"abcdeabcdeabcdeabcdeabcde"
                     [NEW]

StringRepeat

Private Declare Sub CopyMemory Lib "kernel32.dll" Alias "RtlMoveMemory" (ByVal Destination As Long, ByVal source As Long, ByVal Length As Long)

Public Function StringRepeat(number As Long, expression As String) As String

  Dim copyBufferLength As Long
  copyBufferLength = LenB(expression)

  'Create a buffer
  StringRepeat = Space$(number * Len(expression))

  Dim bufferLengthBytes As Long
  bufferLengthBytes = LenB(StringRepeat)

  Dim bufferPointer As Long
  bufferPointer = StrPtr(StringRepeat)

  'Copy the original expression to the start of the buffer
  CopyMemory bufferPointer, StrPtr(expression), copyBufferLength

  Do While copyBufferLength < bufferLengthBytes
    Dim remainingByteCount As Long
    'Check we're not going to overflow the buffer
    remainingByteCount = bufferLengthBytes - copyBufferLength
    If copyBufferLength > remainingByteCount Then
      CopyMemory bufferPointer + copyBufferLength, bufferPointer, remainingByteCount
    Else
      CopyMemory bufferPointer + copyBufferLength, bufferPointer, copyBufferLength
    End If
    copyBufferLength = copyBufferLength * 2
  Loop

End Function

The performance varies by the number of repeats, and the number of characters in the string to be repeated. I tried handling special cases like repeating a string 1 time (just return the string), and/or repeating a single character (return the result of String$ instead), but while that speeds up the special cases, it slows down all other cases.

I'm not checking whether the number input is positive, and I'm not checking that the string to repeat is at least 1 character long, as for now, I'm focusing on performance.

In some instances (small values of number, short expression lengths), avoiding the Exponential lookback approach is not as fast as a straight-up loop and copy:

RepeatString Simple

Function StringRepeatSimple(number As Long, expression As String) As String

  Dim expressionLengthBytes As Long
  expressionLengthBytes = LenB(expression)

  'Create a buffer
  StringRepeatSimple= Space$(number * Len(expression))

  Dim bufferPointer As Long
  bufferPointer = StrPtr(StringRepeatSimple)

  Dim expressionPointer As Long
  expressionPointer = StrPtr(expression)

  Dim copyCounter As Long
  For copyCounter = 0 To number - 1
    CopyMemory bufferPointer + copyCounter * expressionLengthBytes, expressionPointer, expressionLengthBytes
  Next copyCounter 

End Function

Answer 1

FWIW, this might be a rare case where the Mid statement is apropos here. This doesn't require any API. This should yield similar performance characteristics since we only allocate the buffer once just as we do with the API version.

Public Function Replicate(RepeatString As String, NumOfTimes As Long)
    Dim s As String
    Dim c As Long
    Dim l As Long
    Dim i As Long

    l = Len(RepeatString)
    c = l * NumOfTimes
    s = Space$(c)

    For i = 1 To c Step l
        Mid(s, i, l) = RepeatString
    Next

    Replicate = s
End Function

Answer 2

I see you want to use pointers and so maybe you should use pointer safe methods -

Private Declare PtrSafe Sub CopyMemory Lib "kernel32.dll" Alias "RtlMoveMemory" (ByVal Destination As LongPtr, ByVal source As LongPtr, ByVal Length As LongPtr) 

All the Long should also be LongPtr.

That being said StrPtr is undocumented and the user will get a type mismatch (at least on 64bit) unless bufferPointer as LongPtr is declared.

Same goes for Dim expressionPointer As LongPtr.

Otherwise, I think this is pretty clever, kudos.