Binaryworld - ShellSort - Sort Arrays using the ShellSort Algorithm ... [ VB -> Arrays ]

' ShellSort an array of any type
'
' ShellSort behaves pretty well with arrays of any size, even
' if the array is already "nearly-sorted", even though in
' particular cases BubbleSort or QuickSort can be more efficient.
'
' LASTEL is the index of the last item to be sorted, and is
' useful if the array is only partially filled. This updated version accounts
' for arrays whose LBound is 0 and 1 (or whatever)
'
' Works with any kind of array, except UDTs and fixed-length
' strings, and including objects if your are sorting on their
' default property. String are sorted in case-sensitive mode.
'
' You can write faster procedures if you modify the first two lines
' to account for a specific data type, eg.
' Sub ShellSortS(arr() As Single, Optional lastEl As Variant,
' Optional descending As Boolean)
' Dim value As Single

Sub ShellSort(arr As Variant, Optional lastEl As Variant, _
  Optional descending As Boolean)
  Dim value As Variant
  Dim index As Long, index2 As Long
  Dim firstEl As Long
  Dim distance As Long
  Dim numEls As Long

  ' account for optional arguments
  If IsMissing(lastEl) Then lastEl = UBound(arr)
  firstEl = LBound(arr)

  numEls = lastEl - firstEl + 1
  ' find the best value for distance
  Do
    distance = distance * 3 + 1
  Loop Until distance > numEls

  Do
    distance = distance \ 3
    For index = distance + firstEl To lastEl
      value = arr(index)
      index2 = index
      Do While (arr(index2 - distance) > value) Xor descending
        arr(index2) = arr(index2 - distance)
        index2 = index2 - distance
        If index2 - distance < firstEl Then Exit Do
      Loop
      arr(index2) = value
    Next
  Loop Until distance = 1
End Sub