Question

I’m looking for a random number that always generates the same “random” number for a given seed. The seed is defined by x + (y << 16), where x and y are positions on a heightmap.

I could create a new instance of System.Random every time with my seed, but thats a lot of GC pressure. Especially since this will be called a lot of times.

EDIT:
“A lot” means half a million times.

Thanks to everyone that answered! I know I was unclear, but I learned here that a hash function is exactly what I want.

Answer 1

Since a hash function is apparently closer to what you want, consider a variation of the following:

int Hash(int n) {
    const int prime = 1031;
    return (((n & 0xFFFF) * prime % 0xFFFF)) ^ (n >> 16);
}

This XORs the least significant two bytes with the most significant two bytes of a four-byte number after shuffling the least significant two byte around a little bit by multiplication with a prime number. The result is thus in the range 0 < 0x10000 (i.e. it fits in an Int16).

This should “shuffle” the input number a bit, reliably produces the same value for the same input and looks “random”. Now, I haven’t done a stochastic analysis of the distribution and if ever a statistician was to look at it, he would probably go straight into anaphylactic shock. (In fact, I have really written this implementation off the top of my head.)

If you require something less half-baked, consider using an established check sum (such as CRC32).