SmallRng: Replace PCG algorithm with xoshiro{128,256}++

Due to close correlations of PCG streams (rust-random#907) and lack of right-state
propagation (rust-random#905), the `SmallRng` algorithm is switched to
xoshiro{128,256}++. The implementation is taken from the `rand_xoshiro`
crate and slightly simplified.

Fixes rust-random#910.

Cargo.toml

@@ -43,7 +43,7 @@ simd_support = ["packed_simd"]
 std_rng = ["rand_chacha", "rand_hc"]
 
 # Option: enable SmallRng
-small_rng = ["rand_pcg"]
+small_rng = []
 
 [workspace]
 members = [
@@ -56,7 +56,6 @@ members = [
 
 [dependencies]
 rand_core = { path = "rand_core", version = "0.5.1" }
-rand_pcg = { path = "rand_pcg", version = "0.2.1", optional = true }
 log = { version = "0.4.4", optional = true }
 serde = { version = "1.0.103", features = ["derive"], optional = true }
 

src/rngs/mod.rs

@@ -101,7 +101,13 @@
 
 pub mod mock; // Public so we don't export `StepRng` directly, making it a bit
               // more clear it is intended for testing.
+
+#[cfg(all(feature = "small_rng", target_pointer_width = "64"))]
+mod xoshiro256plusplus;
+#[cfg(all(feature = "small_rng", not(target_pointer_width = "64")))]
+mod xoshiro128plusplus;
 #[cfg(feature = "small_rng")] mod small;
+
 #[cfg(feature = "std_rng")] mod std;
 #[cfg(all(feature = "std", feature = "std_rng"))] pub(crate) mod thread;
 

src/rngs/small.rs

@@ -10,10 +10,10 @@
 
 use rand_core::{Error, RngCore, SeedableRng};
 
-#[cfg(all(not(target_os = "emscripten"), target_pointer_width = "64"))]
-type Rng = rand_pcg::Pcg64Mcg;
-#[cfg(not(all(not(target_os = "emscripten"), target_pointer_width = "64")))]
-type Rng = rand_pcg::Pcg32;
+#[cfg(target_pointer_width = "64")]
+type Rng = super::xoshiro256plusplus::Xoshiro256PlusPlus;
+#[cfg(not(target_pointer_width = "64"))]
+type Rng = super::xoshiro128plusplus::Xoshiro128PlusPlus;
 
 /// A small-state, fast non-crypto PRNG
 ///
@@ -25,15 +25,14 @@ type Rng = rand_pcg::Pcg32;
 /// The algorithm is deterministic but should not be considered reproducible
 /// due to dependence on platform and possible replacement in future
 /// library versions. For a reproducible generator, use a named PRNG from an
-/// external crate, e.g. [rand_pcg] or [rand_chacha].
+/// external crate, e.g. [rand_xoshiro] or [rand_chacha].
 /// Refer also to [The Book](https://rust-random.github.io/book/guide-rngs.html).
 ///
-/// The PRNG algorithm in `SmallRng` is chosen to be
-/// efficient on the current platform, without consideration for cryptography
-/// or security. The size of its state is much smaller than [`StdRng`].
-/// The current algorithm is [`Pcg64Mcg`](rand_pcg::Pcg64Mcg) on 64-bit
-/// platforms and [`Pcg32`](rand_pcg::Pcg32) on 32-bit platforms. Both are
-/// implemented by the [rand_pcg] crate.
+/// The PRNG algorithm in `SmallRng` is chosen to be efficient on the current
+/// platform, without consideration for cryptography or security. The size of
+/// its state is much smaller than [`StdRng`]. The current algorithm is
+/// `Xoshiro256PlusPlus` on 64-bit platforms and `Xoshiro128PlusPlus` on 32-bit
+/// platforms. Both are implemented by the [rand_xoshiro] crate.
 ///
 /// # Examples
 ///
@@ -69,7 +68,7 @@ type Rng = rand_pcg::Pcg32;
 /// [`StdRng`]: crate::rngs::StdRng
 /// [`thread_rng`]: crate::thread_rng
 /// [rand_chacha]: https://crates.io/crates/rand_chacha
-/// [rand_pcg]: https://crates.io/crates/rand_pcg
+/// [rand_xoshiro]: https://crates.io/crates/rand_pcg
 #[cfg_attr(doc_cfg, doc(cfg(feature = "small_rng")))]
 #[derive(Clone, Debug, PartialEq, Eq)]
 pub struct SmallRng(Rng);