Xoshiro256++

What it is

xoshiro256++ is Blackman & Vigna’s all-purpose 64-bit generator: a small, extremely fast scrambled-linear engine with a period of 2²⁵⁶ − 1 that passes the full BigCrush / PractRand batteries. It is the default simdrng::Xoshiro.

When to use it

Reach for xoshiro256++ as the general-purpose default — simulation, sampling, games, procedural generation — anywhere you want maximum throughput of high-quality 64-bit values and do not need cryptographic guarantees or stateless counter-based seeking (use Philox for the latter).

Scalar, SIMD dispatch, and native

Three implementations share one interface; pick by include and type:

Type	Header	Use when
`XoshiroScalar`	`<simdrng/xoshiro_scalar.hpp>`	Portable, dependency-free, `constexpr`-friendly single-stream use.
`XoshiroSIMD`	`<simdrng/xoshiro_simd.hpp>`	One binary, many CPUs: the SIMD tier is chosen at runtime via xsimd dispatch (AVX-512 / AVX2 / SSE2 / NEON / …).
`XoshiroNative`	`<simdrng/xoshiro_simd.hpp>`	You build with `-march=native` and want the widest width inlined with no dispatch indirection.

simdrng::Xoshiro (from <simdrng/xoshiro.hpp>) aliases XoshiroSIMD when built with xsimd, and XoshiroScalar otherwise.

#include <simdrng/xoshiro.hpp>

simdrng::Xoshiro rng(42);          // seed (SplitMix64-expanded internally)
std::uint64_t x = rng();           // next 64-bit value
double u = rng.uniform();          // double in [0, 1)

The SIMD types refill an internal cache with wide batches, so an ordinary generate loop runs vectorised — there is no separate bulk-fill API.

Periods and streams

xoshiro256++ has a period of 2²⁵⁶ − 1. The seed is expanded through SplitMix64 (see SplitMix64) so even low-entropy seeds give sound state. For independent parallel streams, pass thread_id and cluster_id:

simdrng::Xoshiro rng(seed, thread_id, cluster_id);

Internally jump() carves out non-overlapping per-thread subsequences and long_jump() per-cluster starting points. See Generator properties for the exact jump spacing and the full period table.

References

xoshiro256++ and its SplitMix64 seeding follow D. Blackman and S. Vigna, Scrambled Linear Pseudorandom Number Generators (the design and the ++ scrambler) and S. Vigna’s https://prng.di.unimi.it/ (seeding and the shootout). See References.