1 files changed, 64 insertions, 0 deletions
diff --git a/third_party/rust/tokio/src/util/rand.rs b/third_party/rust/tokio/src/util/rand.rs
new file mode 100644
index 0000000000..6b19c8be95
--- /dev/null
+++ b/third_party/rust/tokio/src/util/rand.rs
@@ -0,0 +1,64 @@
+use std::cell::Cell;
+
+/// Fast random number generate.
+///
+/// Implement xorshift64+: 2 32-bit xorshift sequences added together.
+/// Shift triplet `[17,7,16]` was calculated as indicated in Marsaglia's
+/// Xorshift paper: <https://www.jstatsoft.org/article/view/v008i14/xorshift.pdf>
+/// This generator passes the SmallCrush suite, part of TestU01 framework:
+/// <http://simul.iro.umontreal.ca/testu01/tu01.html>
+#[derive(Debug)]
+pub(crate) struct FastRand {
+    one: Cell<u32>,
+    two: Cell<u32>,
+}
+
+impl FastRand {
+    /// Initializes a new, thread-local, fast random number generator.
+    pub(crate) fn new(seed: u64) -> FastRand {
+        let one = (seed >> 32) as u32;
+        let mut two = seed as u32;
+
+        if two == 0 {
+            // This value cannot be zero
+            two = 1;
+        }
+
+        FastRand {
+            one: Cell::new(one),
+            two: Cell::new(two),
+        }
+    }
+
+    pub(crate) fn fastrand_n(&self, n: u32) -> u32 {
+        // This is similar to fastrand() % n, but faster.
+        // See https://lemire.me/blog/2016/06/27/a-fast-alternative-to-the-modulo-reduction/
+        let mul = (self.fastrand() as u64).wrapping_mul(n as u64);
+        (mul >> 32) as u32
+    }
+
+    fn fastrand(&self) -> u32 {
+        let mut s1 = self.one.get();
+        let s0 = self.two.get();
+
+        s1 ^= s1 << 17;
+        s1 = s1 ^ s0 ^ s1 >> 7 ^ s0 >> 16;
+
+        self.one.set(s0);
+        self.two.set(s1);
+
+        s0.wrapping_add(s1)
+    }
+}
+
+// Used by the select macro and `StreamMap`
+#[cfg(any(feature = "macros"))]
+#[doc(hidden)]
+#[cfg_attr(not(feature = "macros"), allow(unreachable_pub))]
+pub fn thread_rng_n(n: u32) -> u32 {
+    thread_local! {
+        static THREAD_RNG: FastRand = FastRand::new(crate::loom::rand::seed());
+    }
+
+    THREAD_RNG.with(|rng| rng.fastrand_n(n))
+}