removes awkward back-and-forth int -> double -> int conversion.
the last step also broke uniformity, though it likely doesn't
matter much in mpv's playlist shuffle context.
c365e2f7 added av_random_bytes() to avoid multiple
threads/processes getting the same seed. but the fallback case
still suffers from it.
also mix in a function pointer, which hopefully is randomized
per process due to ASLR (20+ years old technology) along with
the stack position which will be different on different threads.
splitmix was used since it's recommended by the official xoshiro
site [0] as a "radically different" PRNG to avoid correlation of
similar seeds. av_random_bytes() is cryptographically secure
and so we don't have to worry about that. so just use it to
initialize the whole state.
[0]: https://prng.di.unimi.it/
mutex itself was probably more costly than the PRNG itself. and
locking PRNGs is almost universally not recommended. simply add
a state parameter that the caller can hold to avoid locks.
When starting multiple processes of `mpv --shuffle` in parallel,
sometimes the random seed happens to be identical, so files are
played in the same random order.
mp_rand_seed(0) now uses a random seed provided by libavutil,
and only falls back to time in case of failure.
Fuzzing expect deterministic behavior. While we don't use this random
generator for anything significant, still good to set the seed, in case
of future usage.
Add xoshiro as a PRNG implementation instead of relying
on srand() and rand() from the C standard library. This,
in particular, lets us avoid platform-defined behavior with
respect to threading.
NRK