rand_xorshift reorganisation, courtesy of @newpavlov

This copies changes from the separate small-rngs repo

Author: dhardy

rand_xorshift/Cargo.toml

@@ -5,8 +5,8 @@ authors = ["The Rand Project Developers", "The Rust Project Developers"]
 license = "MIT/Apache-2.0"
 readme = "README.md"
 repository = "https://github.com/rust-random/rand"
-documentation = "https://docs.rs/rand_isaac"
-homepage = "https://crates.io/crates/rand_isaac"
+documentation = "https://docs.rs/rand_xorshift"
+homepage = "https://crates.io/crates/rand_xorshift"
 description = """
 Xorshift random number generator
 """

rand_xorshift/README.md

@@ -19,7 +19,7 @@ requirements, use a more secure one such as `StdRng` or `OsRng`.
 
 Documentation:
 [master branch](https://rust-random.github.io/rand/rand_xorshift/index.html),
-[by release](https://docs.rs/rand_isaac)
+[by release](https://docs.rs/rand_xorshift)
 
 [Changelog](CHANGELOG.md)
 

rand_xorshift/src/lib.rs

@@ -14,19 +14,110 @@
 
 #![deny(missing_docs)]
 #![deny(missing_debug_implementations)]
-#![doc(test(attr(allow(unused_variables), deny(warnings))))]
 
-#![cfg_attr(not(all(feature="serde1", test)), no_std)]
+#![no_std]
 
 extern crate rand_core;
 
 #[cfg(feature="serde1")] extern crate serde;
 #[cfg(feature="serde1")] #[macro_use] extern crate serde_derive;
 
-// To test serialization we need bincode and the standard library
-#[cfg(all(feature="serde1", test))] extern crate bincode;
-#[cfg(all(feature="serde1", test))] extern crate std as core;
+use core::num::Wrapping as w;
+use core::{fmt, slice};
+use rand_core::{RngCore, SeedableRng, Error, impls, le};
 
-mod xorshift;
+/// An Xorshift random number generator.
+///
+/// The Xorshift[^1] algorithm is not suitable for cryptographic purposes
+/// but is very fast. If you do not know for sure that it fits your
+/// requirements, use a more secure one such as `StdRng` or `OsRng`.
+///
+/// [^1]: Marsaglia, George (July 2003).
+///       ["Xorshift RNGs"](https://www.jstatsoft.org/v08/i14/paper).
+///       *Journal of Statistical Software*. Vol. 8 (Issue 14).
+#[derive(Clone)]
+#[cfg_attr(feature="serde1", derive(Serialize,Deserialize))]
+pub struct XorShiftRng {
+    x: w<u32>,
+    y: w<u32>,
+    z: w<u32>,
+    w: w<u32>,
+}
 
-pub use self::xorshift::XorShiftRng;
+// Custom Debug implementation that does not expose the internal state
+impl fmt::Debug for XorShiftRng {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "XorShiftRng {{}}")
+    }
+}
+
+impl RngCore for XorShiftRng {
+    #[inline]
+    fn next_u32(&mut self) -> u32 {
+        let x = self.x;
+        let t = x ^ (x << 11);
+        self.x = self.y;
+        self.y = self.z;
+        self.z = self.w;
+        let w_ = self.w;
+        self.w = w_ ^ (w_ >> 19) ^ (t ^ (t >> 8));
+        self.w.0
+    }
+
+    #[inline]
+    fn next_u64(&mut self) -> u64 {
+        impls::next_u64_via_u32(self)
+    }
+
+    #[inline]
+    fn fill_bytes(&mut self, dest: &mut [u8]) {
+        impls::fill_bytes_via_next(self, dest)
+    }
+
+    fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Error> {
+        Ok(self.fill_bytes(dest))
+    }
+}
+
+impl SeedableRng for XorShiftRng {
+    type Seed = [u8; 16];
+
+    fn from_seed(seed: Self::Seed) -> Self {
+        let mut seed_u32 = [0u32; 4];
+        le::read_u32_into(&seed, &mut seed_u32);
+
+        // Xorshift cannot be seeded with 0 and we cannot return an Error, but
+        // also do not wish to panic (because a random seed can legitimately be
+        // 0); our only option is therefore to use a preset value.
+        if seed_u32.iter().all(|&x| x == 0) {
+            seed_u32 = [0xBAD_5EED, 0xBAD_5EED, 0xBAD_5EED, 0xBAD_5EED];
+        }
+
+        XorShiftRng {
+            x: w(seed_u32[0]),
+            y: w(seed_u32[1]),
+            z: w(seed_u32[2]),
+            w: w(seed_u32[3]),
+        }
+    }
+
+    fn from_rng<R: RngCore>(mut rng: R) -> Result<Self, Error> {
+        let mut seed_u32 = [0u32; 4];
+        loop {
+            unsafe {
+                let ptr = seed_u32.as_mut_ptr() as *mut u8;
+
+                let slice = slice::from_raw_parts_mut(ptr, 4 * 4);
+                rng.try_fill_bytes(slice)?;
+            }
+            if !seed_u32.iter().all(|&x| x == 0) { break; }
+        }
+
+        Ok(XorShiftRng {
+            x: w(seed_u32[0]),
+            y: w(seed_u32[1]),
+            z: w(seed_u32[2]),
+            w: w(seed_u32[3]),
+        })
+    }
+}