<random>: Fast discard() for minstd_rand and minstd_rand0 via fast exponentiation

[mirounga]

discard() minstd_rand and minstd_rand0 now takes logarithmic time
due to fast exponentiation and avoiding constant divisions.

[StephanTLavavej]

Thanks! 😻 I cleaned up a mistakenly added file, added a benchmark, and got the following hilarious results on my 5950X:

Benchmark	Before	After	Speedup
BM_discard<std::minstd_rand>/0	1.68 ns	1.69 ns	0.99
BM_discard<std::minstd_rand>/1	3.32 ns	2.93 ns	1.13
BM_discard<std::minstd_rand>/8	14.8 ns	5.99 ns	2.47
BM_discard<std::minstd_rand>/64	107 ns	9.52 ns	11.24
BM_discard<std::minstd_rand>/512	843 ns	16.6 ns	50.78
BM_discard<std::minstd_rand>/4096	6805 ns	19.0 ns	358.16
BM_discard<std::minstd_rand>/32768	53885 ns	24.6 ns	2190.45
BM_discard<std::minstd_rand>/262144	430735 ns	29.1 ns	14801.89

Skipping an exponential amount of work is pretty awesome.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

Thanks for perhaps the biggest speedup in the STL ever! ⏱️ 😻 🐱