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diff --git a/rand/src/rngs/thread.rs b/rand/src/rngs/thread.rs
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+// Copyright 2018 Developers of the Rand project.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! Thread-local random number generator
+
+use std::cell::UnsafeCell;
+
+use {RngCore, CryptoRng, SeedableRng, Error};
+use rngs::adapter::ReseedingRng;
+use rngs::EntropyRng;
+use rand_hc::Hc128Core;
+
+// Rationale for using `UnsafeCell` in `ThreadRng`:
+//
+// Previously we used a `RefCell`, with an overhead of ~15%. There will only
+// ever be one mutable reference to the interior of the `UnsafeCell`, because
+// we only have such a reference inside `next_u32`, `next_u64`, etc. Within a
+// single thread (which is the definition of `ThreadRng`), there will only ever
+// be one of these methods active at a time.
+//
+// A possible scenario where there could be multiple mutable references is if
+// `ThreadRng` is used inside `next_u32` and co. But the implementation is
+// completely under our control. We just have to ensure none of them use
+// `ThreadRng` internally, which is nonsensical anyway. We should also never run
+// `ThreadRng` in destructors of its implementation, which is also nonsensical.
+//
+// The additional `Rc` is not strictly neccesary, and could be removed. For now
+// it ensures `ThreadRng` stays `!Send` and `!Sync`, and implements `Clone`.
+
+
+// Number of generated bytes after which to reseed `TreadRng`.
+//
+// The time it takes to reseed HC-128 is roughly equivalent to generating 7 KiB.
+// We pick a treshold here that is large enough to not reduce the average
+// performance too much, but also small enough to not make reseeding something
+// that basically never happens.
+const THREAD_RNG_RESEED_THRESHOLD: u64 = 32*1024*1024; // 32 MiB
+
+/// The type returned by [`thread_rng`], essentially just a reference to the
+/// PRNG in thread-local memory.
+///
+/// `ThreadRng` uses [`ReseedingRng`] wrapping the same PRNG as [`StdRng`],
+/// which is reseeded after generating 32 MiB of random data. A single instance
+/// is cached per thread and the returned `ThreadRng` is a reference to this
+/// instance — hence `ThreadRng` is neither `Send` nor `Sync` but is safe to use
+/// within a single thread. This RNG is seeded and reseeded via [`EntropyRng`]
+/// as required.
+///
+/// Note that the reseeding is done as an extra precaution against entropy
+/// leaks and is in theory unnecessary — to predict `ThreadRng`'s output, an
+/// attacker would have to either determine most of the RNG's seed or internal
+/// state, or crack the algorithm used.
+///
+/// Like [`StdRng`], `ThreadRng` is a cryptographically secure PRNG. The current
+/// algorithm used is [HC-128], which is an array-based PRNG that trades memory
+/// usage for better performance. This makes it similar to ISAAC, the algorithm
+/// used in `ThreadRng` before rand 0.5.
+///
+/// Cloning this handle just produces a new reference to the same thread-local
+/// generator.
+/// 
+/// [`thread_rng`]: ../fn.thread_rng.html
+/// [`ReseedingRng`]: adapter/struct.ReseedingRng.html
+/// [`StdRng`]: struct.StdRng.html
+/// [`EntropyRng`]: struct.EntropyRng.html
+/// [HC-128]: ../../rand_hc/struct.Hc128Rng.html
+#[derive(Clone, Debug)]
+pub struct ThreadRng {
+    // use of raw pointer implies type is neither Send nor Sync
+    rng: *mut ReseedingRng<Hc128Core, EntropyRng>,
+}
+
+thread_local!(
+    static THREAD_RNG_KEY: UnsafeCell<ReseedingRng<Hc128Core, EntropyRng>> = {
+        let mut entropy_source = EntropyRng::new();
+        let r = Hc128Core::from_rng(&mut entropy_source).unwrap_or_else(|err|
+                panic!("could not initialize thread_rng: {}", err));
+        let rng = ReseedingRng::new(r,
+                                    THREAD_RNG_RESEED_THRESHOLD,
+                                    entropy_source);
+        UnsafeCell::new(rng)
+    }
+);
+
+/// Retrieve the lazily-initialized thread-local random number
+/// generator, seeded by the system. Intended to be used in method
+/// chaining style, e.g. `thread_rng().gen::<i32>()`, or cached locally, e.g.
+/// `let mut rng = thread_rng();`.
+///
+/// For more information see [`ThreadRng`].
+///
+/// [`ThreadRng`]: rngs/struct.ThreadRng.html
+pub fn thread_rng() -> ThreadRng {
+    ThreadRng { rng: THREAD_RNG_KEY.with(|t| t.get()) }
+}
+
+impl RngCore for ThreadRng {
+    #[inline(always)]
+    fn next_u32(&mut self) -> u32 {
+        unsafe { (*self.rng).next_u32() }
+    }
+
+    #[inline(always)]
+    fn next_u64(&mut self) -> u64 {
+        unsafe { (*self.rng).next_u64() }
+    }
+
+    fn fill_bytes(&mut self, dest: &mut [u8]) {
+        unsafe { (*self.rng).fill_bytes(dest) }
+    }
+
+    fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Error> {
+        unsafe { (*self.rng).try_fill_bytes(dest) }
+    }
+}
+
+impl CryptoRng for ThreadRng {}
+
+
+#[cfg(test)]
+mod test {
+    #[test]
+    #[cfg(not(feature="stdweb"))]
+    fn test_thread_rng() {
+        use Rng;
+        let mut r = ::thread_rng();
+        r.gen::<i32>();
+        assert_eq!(r.gen_range(0, 1), 0);
+    }
+}