Here's a re-implementation of PICO-8's RNG in PICO-8. Of note, the last line of `rng.rnd`

is just returning `rng.hi%n`

but with some modulo arithmetic to account for within-PICO-8-overflow and be 1:1 with calling the built-in `rnd`

.

function init_rng(seed) local rng={} function rng.srand(seed) seed=seed or 0 rng.hi,rng.lo= seed==0 and 0x6000.9755 or seed^^0xbead.29ba, seed==0 and 0xdead.beef or seed for i=1,0x20 do rng.rnd() end end function rng.rnd(n) n=n or 1 rng.hi=(rng.hi<<0x10|rng.hi>>>0x10)+rng.lo rng.lo+=rng.hi return ((rng.hi<0 and 0x8000-n or 0)%n+(rng.hi&0x7fff.ffff)%n)%n end rng.srand(seed) return rng end |

While I'm not 100% certain, PICO-8 does not seem to be using Lua's in-built PRNG. Doing a test with Lua's math.random() function and PICO-8's rnd() function, both with the same seed, gives different results. I know that doesn't give you a complete answer, but at least eliminates one possibility. :)

thanks @Meep, very helpful!

a small correction: negative seeds don't behave properly; I messed with the code a bit and this seems to fix it:

`seed=(seed or 0)&0x7fff.ffff`

edit: hmm I made an automatic test case and found more discrepancies; there's some issue that occurs only sometimes when the `m`

in `prng.rnd(m)`

is negative. it doesn't break the prng's state, but something about the `return rng.hi%n`

replacement isn't working. here's a test cart:

and here's a minimal repro: (even after adding in my &0x7ff...)

cls() s,m = -2319,0xdc19.beb0 srand(s) prng=init_rng(s) assert(rnd(m)==prng.rnd(m)) |

I figured it out! here's a repro with nicer numbers:

cls() s,m = 1,-1 srand(s) prng = init_rng(s) r1 = rnd(m) r2 = prng.rnd(m) ?tostr(r1,1) ?tostr(r2,1) assert(r1==r2) |

the issue boils down to trying to do hi%n while treating hi and n as u32s, but pico8 represents them as i32s. for example, hi%n has issues when n=0xffff -- as a u32, n is almost certainly higher than hi, so the result should be hi. but as an i32, n is -1, and so hi%n discards the integer part of hi, returning hi&0x0.ffff instead.

more details:

here's the fixed code:

local function _rnd(n) n=n or 1 hi<<>=0x10 hi+=lo lo+=hi -- return hi%n, but treat hi and n as if they were u32 instead of i32 if n<0 then return n<=hi and hi<0 and hi-n or hi end -- hi := hi8 + hi7 -- hi%n = (hi8%n + hi7%n)%n return ((hi<0 and 0x7fff%n+1 or 0) + (hi&0x7fff.ffff)%n) % n end |

and here it is in the test cart:

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