Wichmann-Hill Pseudo Random Number Generator: an alternative for VB Rnd() function

[Merri]

Here is a suggestion for independent function. It doesn't work exactly the same, but I paid a lot attention to details on how it works.

Public Function RndM(Optional ByVal Number As Long) As Double
    Static lngX As Long, lngY As Long, lngZ As Long, blnInit As Boolean
    Dim dblRnd As Double
    ' if initialized and no input number given
    If blnInit And Number = 0 Then
        ' lngX, lngY and lngZ will never be 0
        lngX = (171 * lngX) Mod 30269
        lngY = (172 * lngY) Mod 30307
        lngZ = (170 * lngZ) Mod 30323
    Else
        ' if no initialization, use Timer, otherwise ensure positive Number
        If Number = 0 Then Number = Timer * 60 Else Number = Number And &H7FFFFFFF
        lngX = (Number Mod 30269)
        lngY = (Number Mod 30307)
        lngZ = (Number Mod 30323)
        ' lngX, lngY and lngZ must be bigger than 0
        If lngX > 0 Then Else lngX = 171
        If lngY > 0 Then Else lngY = 172
        If lngZ > 0 Then Else lngZ = 170
        ' mark initialization state
        blnInit = True
    End If
    ' generate a random number
    dblRnd = CDbl(lngX) / 30269# + CDbl(lngY) / 30307# + CDbl(lngZ) / 30323#
    ' return a value between 0 and 1
    RndM = dblRnd - Int(dblRnd)
End Function

Here are a few pointers:

• True conditions are faster than False conditions -> set code that is ran more often behind True condition.
• I minimized the amount of calculation when checking for initialized values. By using a local static variable we know that X, Y and Z are valid.
• I decided to use 0 as number to cause Timer to be used automatically for seeding: if someone wants repeatable random numbers, he knows to give a seed.
• ... which leads to this function having an advantage over Rnd as you don't need to call Randomize to get a different set of random numbers on each run of the application.
• And I used variable names that tell better what datatype they represent.