bring back random

Author: dignifiedquire

src/utils.rs

@@ -20,6 +20,40 @@ pub fn abort_on_panic<T>(f: impl FnOnce() -> T) -> T {
     t
 }
 
+/// Generates a random number in `0..n`.
+#[cfg(feature = "unstable")]
+pub fn random(n: u32) -> u32 {
+    use std::cell::Cell;
+    use std::num::Wrapping;
+
+    thread_local! {
+        static RNG: Cell<Wrapping<u32>> = {
+            // Take the address of a local value as seed.
+            let mut x = 0i32;
+            let r = &mut x;
+            let addr = r as *mut i32 as usize;
+            Cell::new(Wrapping(addr as u32))
+        }
+    }
+
+    RNG.with(|rng| {
+        // This is the 32-bit variant of Xorshift.
+        //
+        // Source: https://en.wikipedia.org/wiki/Xorshift
+        let mut x = rng.get();
+        x ^= x << 13;
+        x ^= x >> 17;
+        x ^= x << 5;
+        rng.set(x);
+
+        // This is a fast alternative to `x % n`.
+        //
+        // Author: Daniel Lemire
+        // Source: https://lemire.me/blog/2016/06/27/a-fast-alternative-to-the-modulo-reduction/
+        ((u64::from(x.0)).wrapping_mul(u64::from(n)) >> 32) as u32
+    })
+}
+
 /// Add additional context to errors
 pub(crate) trait Context {
     fn context(self, message: impl Fn() -> String) -> Self;