Random Number

[dbasnett]

The following allows you to pick a group random numbers from a range of numbers like -19099 to 107,
or 1 to 52. It returns an array of integers, dim x() as Integer, as the result.
An example:
Dim cards As Integer() = generateRandomNumbers(1, 52, 4) 'returns four random numbers between 1 and 52
cards = generateRandomNumbers(1, 52, 4, False) 'get four more - note the Pool is not cleared

You can also check to see if a number has been selected by using picked:
If picked(an integer goes here) Then doSomething

The following test
generateRandomNumbers(1, 100000, 100000)
took about 100 ms. on my 1.8Ghz laptop.

Many thanks to kleinma for his insights.

Module RandomNumberGenerator
    '
    'The following Function's can be used to generate a set of
    'unique random numbers in a defined range.
    '
    'How it works:
    'An array of bits, one for each possible number, is created.  Initially all bits are set to 0
    'Then a loop is started that:
    '  generate a random number between low and high inclusive
    '  check bit at random number selected
    '   - if 1 then selected, skip. if 0 then set it to 1, add number to array, increment count.
    '  continue loop till requested count is satisfied 
    '
    'usage - generateRandomNumbers(low, high, howmany, new pool, exception)
    'where:
    '  low = lowest number to be generated - negatives allowed
    '  high = highest number to be generated - negatives allowed
    '  howmany = how many numbers to generate between low and high inclusive
    '  new pool = (true / false) start with no numbers selected
    '  exception =  (true / false) if true throw exception else return array of length 0
    '
    'the pool is all numbers from low to high inclusive e.g low = -3 high = 6
    '-3 -2 -1 0 1 2 3 4 5 6
    'size of the pool is 10
    '
    'the following are defined here intentionally
    Public theRandomNumbers As New BitArray(1)
    Public zeroPt As Integer = 0, genTime As Long, howManyAvail As Integer = 0
    '
    Public Function generateRandomNumbers _
            (ByVal RangeLow As Integer, _
             ByVal RangeHigh As Integer, _
             ByVal QtyToGenerate As Integer, _
             Optional ByVal NewPool As Boolean = True, _
             Optional ByVal ExceptionOnError As Boolean = True) As Integer()
        Dim LenPool As Integer
        Dim TheReturn As Integer() = {} 'zero length return
        Dim size As Integer = RangeHigh - RangeLow + 1 'size of pool
        Dim GenRndm As New Random, ctHits As Integer = 0, num2Check As Integer, rndSTPW As New Stopwatch
        If RangeLow >= RangeHigh Then 'is low greater or equal to high
            If ExceptionOnError Then
                Throw New System.Exception("Low must be less than High") 'low >= high not allowed
            Else
                Return TheReturn 'return array of length 0
            End If
        End If
        If QtyToGenerate <= 0 Then 'generate zero or less
            If ExceptionOnError Then
                Throw New System.Exception("Quantitiy must be greater than zero")
            Else
                Return TheReturn 'return array of length 0
            End If
        End If
        If size < QtyToGenerate Then
            If ExceptionOnError Then
                Throw New System.Exception("Quantitiy exceeds number available") 'low = 0 high = 2 gen =5????
            Else
                Return TheReturn 'return array of length 0
            End If
        End If
        rndSTPW.Reset() : rndSTPW.Start() 'start the clock
        ' determine pool size
        If Math.Abs(RangeLow) > Math.Abs(RangeHigh) Then LenPool = Math.Abs(RangeLow) Else LenPool = Math.Abs(RangeHigh)
        If LenPool < Math.Abs(RangeHigh - RangeLow) Then LenPool = Math.Abs(RangeHigh - RangeLow)
        theRandomNumbers.Length = LenPool + 1 'set size of random pool
        If NewPool Then 'is this new pool
            theRandomNumbers.SetAll(False) 'yes, set all to not selected
            howManyAvail = size 'set how many available
        Else 're-using the same pool
            If howManyAvail < QtyToGenerate Then 'generate less than available
                If ExceptionOnError Then 'yes
                    Throw New System.Exception("Quantitiy exceeds number available")
                Else
                    Array.Resize(TheReturn, 0) 'return array of length 0
                    Return TheReturn
                End If
            End If
        End If
        Array.Resize(TheReturn, QtyToGenerate) 'set return size
        'set zero point to handle any case   {-3 -2 -1 0 1 2 3}
        Select Case True
            Case RangeLow >= 0
                '0 1 2 3   low=0 high=3
                zeroPt = 0
            Case RangeHigh <= 0
                '-3 -2 -1 0   low=-3 high=0
                zeroPt = LenPool
            Case RangeLow < 0
                '-2 -1 0 1   low=-2 high=1
                zeroPt = Math.Abs(RangeLow)
        End Select
        'finally, generate the random numbers
        Do
            num2Check = GenRndm.Next(RangeLow, RangeHigh + 1) 'get a random number
            If Not theRandomNumbers(num2Check + zeroPt) Then 'picked previously
                theRandomNumbers.Set(num2Check + zeroPt, True) 'no, mark it picked
                TheReturn(ctHits) = num2Check 'add to return
                ctHits += 1 'increment count
                If ctHits >= QtyToGenerate Then Exit Do
            End If
        Loop
        rndSTPW.Stop() 'stop the clock
        howManyAvail -= ctHits
        genTime = rndSTPW.ElapsedMilliseconds 'store time to generate numbers
        Return TheReturn
    End Function
    Public Function Picked(ByVal number As Integer) As Boolean
        If number > theRandomNumbers.Length - 1 Then Return False
        If theRandomNumbers(number + zeroPt) Then Return True Else Return False
    End Function
End Module