I just posted a new pattern, “Synchronized Random Numbers” to the community patterns. You can copy and paste out of it the definition of `prng`

, a simple random number generator with a deterministic seed. This lets you have “randomness” synchronized across a fleet of Pixelblazes in case you still want them to be able to match. The implementation is very careful to avoid overflow and should be able to output values in [0, 2^16)

```
// ---- PRNG ---- //
// Modular addition which avoids overflow. Uses the identity
// (a + b) % mod = (a % mod) + (b % mod) % mod
function addmod(a, b, mod) {
var am = a % mod;
var bm = b % mod;
// If `mod`, `a`, and `b` are very large, just returning `(am + bm) % mod` could overflow
var diff = mod - bm;
if (diff <= am) { // aka if (am + bm >= mod)
// Since am < mod and bm < mod, am + bm < 2 * mod. In this specific case mod < am + bm < 2 * mod,
// so we have the identity am + bm % mod = am - mod + bm
return am - diff;
}
return am + bm;
}
// modular multiplication which avoids overflow. Takes O(log(a)) iterations.
function mulmod(a, b, mod) {
var res = 0;
if (b == 0) return 0;
while (a != 0) {
if (a & 1) {
res = addmod(res, b, mod);
}
a >>= 1;
b = addmod(b, b, mod);
}
return res;
}
// This is the BSD rand() from https://rosettacode.org/wiki/Linear_congruential_generator.
// The value of `a` has been expressed mod 2^16.
export var prng_state = 0;
var mod = pow(2, 16);
function prng(max) {
prng_state = addmod(mulmod(20077, prng_state, mod), 12345, mod);
return prng_state / mod * max;
}
// ---- END PRNG ---- //
```

Note that this version has some bugfixes and some cleanup from the one posted on the Patterns page. I’m not sure how to take down and re-post a pattern.