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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.

    //! The standard RNG

    use crate::{RngCore, CryptoRng, Error, SeedableRng};

    #[cfg(target_os = "emscripten")] pub(crate) use rand_hc::Hc128Core as Core;
    #[cfg(not(target_os = "emscripten"))] pub(crate) use rand_chacha::ChaCha20Core as Core;
    #[cfg(target_os = "emscripten")] use rand_hc::Hc128Rng as Rng;
    #[cfg(not(target_os = "emscripten"))] use rand_chacha::ChaCha20Rng as Rng;

    /// The standard RNG. The PRNG algorithm in `StdRng` is chosen to be efficient
    /// on the current platform, to be statistically strong and unpredictable
    /// (meaning a cryptographically secure PRNG).
    ///
    /// The current algorithm used is the ChaCha block cipher with either 20 or 12
    /// rounds (see the `stdrng_*` feature flags, documented in the README).
    /// This may change as new evidence of cipher security and performance
    /// becomes available.
    ///
    /// The algorithm is deterministic but should not be considered reproducible
    /// due to dependence on configuration and possible replacement in future
    /// library versions. For a secure reproducible generator, we recommend use of
    /// the [rand_chacha] crate directly.
    ///
    /// [rand_chacha]: https://crates.io/crates/rand_chacha
    #[derive(Clone, Debug)]
    pub struct StdRng(Rng);

    impl RngCore for StdRng {
        #[inline(always)]
        fn next_u32(&mut self) -> u32 {
            self.0.next_u32()
        }

        #[inline(always)]
        fn next_u64(&mut self) -> u64 {
            self.0.next_u64()
        }

        #[inline(always)]
        fn fill_bytes(&mut self, dest: &mut [u8]) {
            self.0.fill_bytes(dest);
        }

        #[inline(always)]
        fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Error> {
            self.0.try_fill_bytes(dest)
        }
    }

    impl SeedableRng for StdRng {
        type Seed = <Rng as SeedableRng>::Seed;

        #[inline(always)]
        fn from_seed(seed: Self::Seed) -> Self {
            StdRng(Rng::from_seed(seed))
        }

        #[inline(always)]
        fn from_rng<R: RngCore>(rng: R) -> Result<Self, Error> {
            Rng::from_rng(rng).map(StdRng)
        }
    }

    impl CryptoRng for StdRng {}

    #[cfg(test)]
    mod test {
        use crate::{RngCore, SeedableRng};
        use crate::rngs::StdRng;

        #[test]
        fn test_stdrng_construction() {
            // Test value-stability of StdRng. This is expected to break any time
            // the algorithm is changed.
            let seed = [1,0,0,0, 23,0,0,0, 200,1,0,0, 210,30,0,0,
                        0,0,0,0, 0,0,0,0, 0,0,0,0, 0,0,0,0];

            #[cfg(any(feature="stdrng_strong", not(feature="stdrng_fast")))]
            let target = [3950704604716924505, 5573172343717151650];
            #[cfg(all(not(feature="stdrng_strong"), feature="stdrng_fast"))]
            let target = [10719222850664546238, 14064965282130556830];

            let mut rng0 = StdRng::from_seed(seed);
            let x0 = rng0.next_u64();

            let mut rng1 = StdRng::from_rng(rng0).unwrap();
            let x1 = rng1.next_u64();

            assert_eq!([x0, x1], target);
        }
    }

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