Simple Pseudo-Random Algorithm

Question

I'm need a pseudo-random generator which takes a number as input and returns another number witch is reproducible and seems to be random.

• Each input number should match to exactly one output number and vice versa
• same input numbers always result in same output numbers
• sequential input numbers that are close together (eg. 1 and 2) should produce completely different output numbers (eg. 1 => 9783526, 2 => 283)

It must not be perfect, it's just to create random but reproducible test data.

I use C#.

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I wrote this funny piece of code some time ago which produced something random.

  public static long Scramble(long number, long max) 
  {
    // some random values 
    long[] scramblers = { 3, 5, 7, 31, 343, 2348, 89897 };
    number += (max / 7) + 6;
    number %= max;
    // shuffle according to divisibility
    foreach (long scrambler in scramblers) 
    {
      if (scrambler >= max / 3) break;
      number = ((number * scrambler) % max) 
        + ((number * scrambler) / max);
    }

    return number % max;
  }

I would like to have something better, more reliable, working with any size of number (no max argument).

Could this probably be solved using a CRC algorithm? Or some bit shuffling stuff.

Answer 1

I remove the microsoft code from this answer, the GNU code file is a lot longer but basically it contains this from http://cs.uccs.edu/~cs591/bufferOverflow/glibc-2.2.4/stdlib/random_r.c :

int32_t val = state[0];
val = ((state[0] * 1103515245) + 12345) & 0x7fffffff;
state[0] = val;
*result = val;

for your purpose, the seed is state[0] so it would look more like

int getRand(int val)
{
    return ((val * 1103515245) + 12345) & 0x7fffffff;
}

Answer 2

You (maybe) can do this easily in C# using the Random class:

public int GetPseudoRandomNumber(int input)
{
    Random random = new Random(input);
    return random.Next();
}

Since you're explicitly seeding Random with the input, you will get the same output every time given the same input value.

Answer 3

A tausworthe generator is simple to implement and pretty fast. The following pseudocode implementation has full cycle (2**31 - 1, because zero is a fixed point):

def tausworthe(seed)
  seed ^= seed >> 13
  seed ^= seed << 18
  return seed & 0x7fffffff

I don't know C#, but I'm assuming it has XOR (^) and bit shift (<<, >>) operators as in C.

Set an initial seed value, and invoke with seed = tausworthe(seed).