1 files changed, 52 insertions, 0 deletions
diff --git a/tokio/src/executor/util/rand.rs b/tokio/src/executor/util/rand.rs
new file mode 100644
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--- /dev/null
+++ b/tokio/src/executor/util/rand.rs
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+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 {
+    /// Initialize 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)
+    }
+}