I need to generate a deterministic (i.e. repeatable) sequence of pseudo-random numbers given an initial seed and select the nth item from that sequence.
If JavaScript's random function was seedable, I could just do:
function randomNth(seed, seq)
{
var r;
Math.randomSeed(seed);
for (var i = 0; i++ < seq; i++)
{
r = Math.random();
}
return r;
}
However, it's not, and alternative, seedable PRNGs look to be a little slow; asking for the 250th number would be expensive.
I think a hash is what I want here, perhaps something like md5(seed + seq) % max
but JavaScript doesn't have md5() and if I'm doing it in code there's probably a better choice of hash.
I'd like a function where
x = randomNth(seed, seq, maxVal) // x is int && x >= 0 && x < maxVal
or, ideally
x = randomNth(seed, seq) // x >= 0 && x < 1, same as Math.random()
Other requirements:
There are some good int -> int hashing functions on this page you can use one of them.
function hash(a)
{
a = (a+0x7ed55d16) + (a<<12);
a = (a^0xc761c23c) ^ (a>>19);
a = (a+0x165667b1) + (a<<5);
a = (a+0xd3a2646c) ^ (a<<9);
a = (a+0xfd7046c5) + (a<<3);
a = (a^0xb55a4f09) ^ (a>>16);
if( a < 0 ) a = 0xffffffff + a;
return a;
}
var seed = 26254;
var index = 250;
alert( hash( seed + index ) );