Binaryworld - BinarySearch - Fast search in a sorted array ... [ VB.net -> Arrays ]

' Binary search in an array of any type
' Returns the index of the matching item, or -1 if the search fails
'
' The arrays *must* be sorted, in ascending or descending
' order (the routines finds out the sort direction).
' LASTEL is the index of the last item to be searched, and is
' useful if the array is only partially filled.
'
' Works with any kind of array, including objects if your are searching
' for their default property, and excluding UDTs and fixed-length strings.
' String are compared in case-sensitive mode.
'
' You can write faster procedures if you modify the first line
' to account for a specific data type, eg.
' Function BinarySearchL (arr() As Long, search As Long,
' Optional lastEl As Variant) As Long

Function BinarySearch(arr As Variant, search As Variant, _
  Optional lastEl As Variant) As Long
  Dim index As Long
  Dim first As Long
  Dim last As Long
  Dim middle As Long
  Dim inverseOrder As Boolean

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

  first = LBound(arr)
  last = lastEl

  ' deduct direction of sorting
  inverseOrder = (arr(first) > arr(last))

  ' assume searches failed
  BinarySearch = first - 1

  Do
    middle = (first + last) \ 2
    If arr(middle) = search Then
      BinarySearch = middle
      Exit Do
    ElseIf ((arr(middle) < search) Xor inverseOrder) Then
      first = middle + 1
    Else
      last = middle - 1
    End If
  Loop Until first > last
End Function