Question

Maybe there is a very logic explanation for this, but I just can’t seem to understand why the seeds 0 and 2,147,483,647 produce the same “random” sequence, using .NET’s Random Class (System).

Quick code example:

var random1 = new Random(0);
var random2 = new Random(1);
var random3 = new Random(int.MaxValue); //2,147,483,647

var buffer1 = new byte[8];
var buffer2 = new byte[8];
var buffer3 = new byte[8];

random1.NextBytes(buffer1);
random2.NextBytes(buffer2);
random3.NextBytes(buffer3);

for (int i = 0; i < 8; i++)
{
    Console.WriteLine("{0}\t\t{1}\t\t{2}", buffer1[i], buffer2[i], buffer3[i]);
}

Output:

26      70      26
12      208     12
70      134     76
111     130     111
93      64      93
117     151     115
228     228     228
216     163     216

As you can see, the first and the third sequence are the same. Can someone please explain this to me?

EDIT: Apparently, as alro pointed out, these sequences are not the same. But they are very similar.

Answer 1

Well, the reason will be connected with whatever derivation function is used by the Random class to derive a pseudo-random sequence from the seed. The real answer, therefore, is mathematical (and beyond my ability).

Indeed – I don’t believe there’s any guarantee that two different seeds will necessarily produce different sequences anyway.

Edit Okay – I’m going to do what bitbonk has done – but explain why:

public Random(int Seed)
{
    int num = (Seed == -2147483648) ? 2147483647 : Math.Abs(Seed);
    int num2 = 161803398 - num;
    this.SeedArray[55] = num2;
    int num3 = 1;
    for (int i = 1; i < 55; i++)
    {
        int num4 = 21 * i % 55;
        this.SeedArray[num4] = num3;
        num3 = num2 - num3;
        if (num3 < 0)
        {
            num3 += 2147483647;
        }
        num2 = this.SeedArray[num4];
    }
    for (int j = 1; j < 5; j++)
    {
        for (int k = 1; k < 56; k++)
        {
            this.SeedArray[k] -= this.SeedArray[1 + (k + 30) % 55];
            if (this.SeedArray[k] < 0)
            {
                this.SeedArray[k] += 2147483647;
            }
        }
    }
    this.inext = 0;
    this.inextp = 21;
    Seed = 1;
} 

We don’t actually need to go too far into the code to see why – reading the code from top to bottom these are the values that will be stored by the above code when the seed is 0 and when the seed is 2147483647:

int num = (Seed == -2147483648) ? 2147483647 : Math.Abs(Seed);
  =>  num is 0 and 2147483647

int num2 = 161803398 - num;
  => num2 is 161803398 and -1985680249

this.SeedArray[55] = num2;
  => this.SeedArray is as above in both cases

int num3 = 1;
for (int i = 1; i < 55; i++)
{
  int num4 = 21 * i % 55
  this.SeedArray[num4] = num3;

  => num4 is 21, SeedArray[21] is 1

num3 = num2 - num3
  => num3 is 161803397 and -1985680250

if(num3 < 0)
  num3 += 2147483647

  => num3 is 161803397 and 161803397

After just the very first loop, algorithm has already converged for the two seed values.

Edit

As has been pointed out on the question – the sequences aren’t the same – but they are clearly very very similar – and here we can see the reason for that similarity.