The Numerical Recipes book supplies a specific example of a hash-style generator that is up to scratch as a PRNG. There is a pointer here: http://www.nr.com/forum/showthread.php?t=1653. The principle is simple: you only need a PRNG that is so thorough that the values generated by seed,seed+1,seed+2,.. are acceptably random. You can view the code (for free) in section 7.1.4 of the online version of the 3rd edition (via http://www.nr.com), and more importantly the discussion about what qualifies as "up to scratch". The code amounts to the following:

```
#include <stdio.h>
#include <inttypes.h>
/* I used http://stackoverflow.com/questions/2844/
* how-do-you-printf-an-unsigned-long-long-int
* for advice on uint64_t PRIu64 and inttypes.h */
uint64_t
ranhash(uint64_t v) {
v *= 3935559000370003845LL;
v += 2691343689449507681LL;
v ^= v >> 21; v ^= v << 37; v ^= v >> 4;
v *= 4768777513237032717LL;
v ^= v << 20; v ^= v >> 41; v ^= v << 5;
return v;
}
double
ranhashdoub(uint64_t v) {
return 5.42101086242752217E-20 * ranhash(v);
}
int
main(int argc, char*argv[])
{
uint64_t seed = 0; /* same results every time */
long j;
for(j=0; j<10; j++)
printf("%"PRIu64"\n",ranhash(seed++));
return 0;
}
```

afterexisting should take N computations. – John Rivers Aug 17 '12 at 14:57