C#/Java Number Randomization

Question

Is it possible, from .NET, to mimic the exact randomization that Java uses? I have a seed, and I would like to be able to recieve the same results in both C# and Java when creating a random number.

Answer 1

If you have the source code of the java.util.Random class for your Java implementation, you can easily port it to .NET.

If you require both applications (Java and .NET) to use a certain random number generator, you'd better implement one in both platforms and use it instead, as the system provided version might change its behavior as a result of an update.(Looks like the Java specification precisely describes the behavior of its PRNG.)

Answer 2

You don't need to read the source code. The formula is a one-liner and is given in the documentation for java.util.Random.

Here's a partial translation:

[Serializable]
public class Random
{
    public Random(UInt64 seed)
    {
        this.seed = (seed ^ 0x5DEECE66DUL) & ((1UL << 48) - 1);
    }

    public int NextInt(int n)
    {
        if (n <= 0) throw new ArgumentException("n must be positive");

        if ((n & -n) == n)  // i.e., n is a power of 2
            return (int)((n * (long)Next(31)) >> 31);

        long bits, val;
        do
        {
            bits = Next(31);
            val = bits % (UInt32) n;
        }
        while (bits - val + (n - 1) < 0);

        return (int) val;
    }

    protected UInt32 Next(int bits)
    {
        seed = (seed * 0x5DEECE66DL + 0xBL) & ((1L << 48) - 1);

        return (UInt32)(seed >> (48 - bits));
    }

    private UInt64 seed;
}

Example:

Random rnd = new Random(42);
Console.WriteLine(rnd.NextInt(10));
Console.WriteLine(rnd.NextInt(20));
Console.WriteLine(rnd.NextInt(30));
Console.WriteLine(rnd.NextInt(40));
Console.WriteLine(rnd.NextInt(50));

Output on both platforms is 0, 3, 18, 4, 20.