C# Random(Long)

Question

I'm trying to generate a number based on a seed in C#. The only problem is that the seed is too big to be an int32. Is there a way I can use a long as the seed?

And yes, the seed MUST be a long.

Answer 1

Here's a C# version of Java.Util.Random that I ported from the Java Specification.

The best thing to do is to write a Java program to generate a load of numbers and check that this C# version generates the same numbers.

public sealed class JavaRng
{
    public JavaRng(long seed)
    {
        _seed = (seed ^ LARGE_PRIME) & ((1L << 48) - 1);
    }

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

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

        int bits, val;

        do
        {
            bits = next(31);
            val = bits % n;
        } while (bits - val + (n-1) < 0);
        return val;
    }

    private int next(int bits)
    {
        _seed = (_seed*LARGE_PRIME + SMALL_PRIME) & ((1L << 48) - 1);
        return (int) (((uint)_seed) >> (48 - bits));
    }

    private long _seed;

    private const long LARGE_PRIME = 0x5DEECE66DL;
    private const long SMALL_PRIME = 0xBL;
}

Answer 2

For anyone seeing this question today, .NET 6 and upwards provides Random.NextInt64, which has the following overloads:

• NextInt64()

  • Returns a non-negative random integer.

• NextInt64(Int64)

  • Returns a non-negative random integer that is less than the specified maximum.

• NextInt64(Int64, Int64)

  • Returns a random integer that is within a specified range.

Answer 3

I'd go for the answer provided here by @Dyppl: Random number in long range, is this the way?

Put this function where it's accessible to the code that needs to generate the random number:

long LongRandom(long min, long max, Random rand) 
{
    byte[] buf = new byte[8];
    rand.NextBytes(buf);
    long longRand = BitConverter.ToInt64(buf, 0);
    return (Math.Abs(longRand % (max - min)) + min);
}

Then call the function like this:

long r = LongRandom(100000000000000000, 100000000000000050, new Random());