Vintage VB6: Random number methods

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Vintage VB6: Random number methods

Linear Congruential Generator (LCG)

Assistant XenoEngineer (GPT)

LCGs are one of the oldest and simplest types of PRNG algorithms. They generate a sequence of pseudo-random numbers using the following formula:
X(n+1) = (a * Xn + c) mod m
  
X is the sequence of pseudo-random numbers,
Xn is the current number in the sequence,
a is the multiplier,
c is the increment,
m is the modulus,
X(n+1) is the next number in the sequence.

The values of a, c, and m are constants that determine the properties of the generated sequence, such as its period (how many numbers are generated before the sequence repeats).

Choosing an Algorithm for VB6

For most applications developed with VB6 that require random number generation, the built-in Rnd function suffices, especially when combined with the Randomize statement to seed the generator based on the system clock. This is generally adequate for applications like simulations, games, and basic statistical tasks.

Hand Coding a Custom PRNG in VB6

If you require a different kind of randomness than what Rnd offers or need a PRNG with specific characteristics (e.g., a longer period, better distribution, or cryptographic security), you might consider implementing a custom PRNG algorithm. However, VB6's standard capabilities and performance considerations make implementing more sophisticated algorithms (like Mersenne Twister or cryptographic PRNGs) challenging.

For a simple custom PRNG, sticking to algorithms that are well-documented and known to perform well in a wide range of applications (e.g., implementing a different kind of LCG with specific parameters) might be the best balance between complexity and functionality. Here’s an illustrative snippet for a basic custom LCG in VB6:

VB6 code

Linear Congruential Generator (LCG):
' Custom Linear Congruential Generator in VB6
Function CustomLCG() As Single
    Static seed As Long
    If seed = 0 Then seed = Timer ' Initialize seed based on system time if not already done
    
    ' Constants for the LCG (example values, choose based on specific needs)
    Const a As Long = 1664525
    Const c As Long = 1013904223
    Const m As Long = 2 ^ 32
    
    ' Generate the next number in the sequence
    seed = (a * seed + c) Mod m
    
    ' Normalize to [0, 1) range and return
    CustomLCG = seed / m
End Function

This function is a very basic example and would need adjustments for real-world applications, especially where the quality of randomness is crucial.

Linear Feedback Shift Register (LFSR)

Assistant XenoEngineer (Chat GPT 4.0)

For a VB6 function that aims to generate a sequence with a lack of repeat "blips" or noticeable patterns when looped, a simple yet effective approach could involve using a variation of the Linear Feedback Shift Register (LFSR) method. LFSRs are known for their good distribution and long periods before repetition, depending on the configuration. While not inherently built into VB6, an LFSR can be implemented with relative ease and can serve as a robust alternative to the built-in Rnd function for certain applications, particularly where the sequence's periodicity might be a concern.
 
Here’s how you could implement a basic LFSR in VB6:

Option Explicit

Private Const LFSR_SEED As Long = &HACE1 ' Example seed value
Private lfsr As Long

' Initialize the LFSR with a seed value
Private Sub Class_Initialize()
    lfsr = LFSR_SEED
    If lfsr = 0 Then lfsr = &HDEADBEEF ' Ensure the LFSR is never initialized to 0
End Sub

' Generates the next value in the sequence
Public Function NextValue() As Long
    Dim bit As Integer  ' Will hold the next bit to be fed back into the LFSR
    
    ' A simple 16-bit LFSR example: taps at positions 16,14,13,11 from the right
    bit = ((lfsr Xor (lfsr >> 2) Xor (lfsr >> 3) Xor (lfsr >> 5)) And 1)
    lfsr = (lfsr >> 1) Or (bit << 15)
    
    NextValue = lfsr
End Function

' Generate a pseudo-random Single in the range [0, 1)
Public Function RandomSingle() As Single
    RandomSingle = NextValue() / &HFFFF&
End Function

Chat GPT (continuing)

This example provides a basic LFSR implementation, using a 16-bit shift register for simplicity. LFSRs can be configured in various ways, with different "tap" positions (where the XOR operations are applied) to achieve different characteristics in terms of cycle length and randomness quality. The example uses a simple tap configuration that offers a decent balance for demonstration purposes.

To generate a sequence that is less likely to exhibit noticeable patterns upon repetition, you could utilize this LFSR-based generator. Keep in mind, for critical applications, especially those involving cryptography, more sophisticated approaches would be necessary, and the choice of tap positions should be carefully considered based on the specific requirements and the desired properties of the sequence.

This VB6 approach leverages bitwise operations to simulate the behavior of an LFSR, providing a method (NextValue) to generate the next number in the sequence and an example function (RandomSingle) to normalize the output to a Single in the range [0, 1), similar to the built-in Rnd function.