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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.

    //! Entropy generator, or wrapper around external generators

    use rand_core::{RngCore, CryptoRng, Error, ErrorKind, impls};
    #[allow(unused)]
    use rngs;

    /// An interface returning random data from external source(s), provided
    /// specifically for securely seeding algorithmic generators (PRNGs).
    ///
    /// Where possible, `EntropyRng` retrieves random data from the operating
    /// system's interface for random numbers ([`OsRng`]); if that fails it will
    /// fall back to the [`JitterRng`] entropy collector. In the latter case it will
    /// still try to use [`OsRng`] on the next usage.
    ///
    /// If no secure source of entropy is available `EntropyRng` will panic on use;
    /// i.e. it should never output predictable data.
    ///
    /// This is either a little slow ([`OsRng`] requires a system call) or extremely
    /// slow ([`JitterRng`] must use significant CPU time to generate sufficient
    /// jitter); for better performance it is common to seed a local PRNG from
    /// external entropy then primarily use the local PRNG ([`thread_rng`] is
    /// provided as a convenient, local, automatically-seeded CSPRNG).
    ///
    /// # Panics
    ///
    /// On most systems, like Windows, Linux, macOS and *BSD on common hardware, it
    /// is highly unlikely for both [`OsRng`] and [`JitterRng`] to fail. But on
    /// combinations like webassembly without Emscripten or stdweb both sources are
    /// unavailable. If both sources fail, only [`try_fill_bytes`] is able to
    /// report the error, and only the one from `OsRng`. The other [`RngCore`]
    /// methods will panic in case of an error.
    ///
    /// [`OsRng`]: rand_os::OsRng
    /// [`thread_rng`]: crate::thread_rng
    /// [`JitterRng`]: crate::rngs::JitterRng
    /// [`try_fill_bytes`]: RngCore::try_fill_bytes
    #[derive(Debug)]
    pub struct EntropyRng {
        source: Source,
    }

    #[derive(Debug)]
    enum Source {
        Os(Os),
        Custom(Custom),
        Jitter(Jitter),
        None,
    }

    impl EntropyRng {
        /// Create a new `EntropyRng`.
        ///
        /// This method will do no system calls or other initialization routines,
        /// those are done on first use. This is done to make `new` infallible,
        /// and `try_fill_bytes` the only place to report errors.
        pub fn new() -> Self {
            EntropyRng { source: Source::None }
        }
    }

    impl Default for EntropyRng {
        fn default() -> Self {
            EntropyRng::new()
        }
    }

    impl RngCore for EntropyRng {
        fn next_u32(&mut self) -> u32 {
            impls::next_u32_via_fill(self)
        }

        fn next_u64(&mut self) -> u64 {
            impls::next_u64_via_fill(self)
        }

        fn fill_bytes(&mut self, dest: &mut [u8]) {
            self.try_fill_bytes(dest).unwrap_or_else(|err|
                    panic!("all entropy sources failed; first error: {}", err))
        }

        fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Error> {
            let mut reported_error = None;

            if let Source::Os(ref mut os_rng) = self.source {
                match os_rng.fill(dest) {
                    Ok(()) => return Ok(()),
                    Err(err) => {
                        warn!("EntropyRng: OsRng failed \
                              [trying other entropy sources]: {}", err);
                        reported_error = Some(err);
                    },
                }
            } else if Os::is_supported() {
                match Os::new_and_fill(dest) {
                    Ok(os_rng) => {
                        debug!("EntropyRng: using OsRng");
                        self.source = Source::Os(os_rng);
                        return Ok(());
                    },
                    Err(err) => { reported_error = reported_error.or(Some(err)) },
                }
            }

            if let Source::Custom(ref mut rng) = self.source {
                match rng.fill(dest) {
                    Ok(()) => return Ok(()),
                    Err(err) => {
                        warn!("EntropyRng: custom entropy source failed \
                              [trying other entropy sources]: {}", err);
                        reported_error = Some(err);
                    },
                }
            } else if Custom::is_supported() {
                match Custom::new_and_fill(dest) {
                    Ok(custom) => {
                        debug!("EntropyRng: using custom entropy source");
                        self.source = Source::Custom(custom);
                        return Ok(());
                    },
                    Err(err) => { reported_error = reported_error.or(Some(err)) },
                }
            }

            if let Source::Jitter(ref mut jitter_rng) = self.source {
                match jitter_rng.fill(dest) {
                    Ok(()) => return Ok(()),
                    Err(err) => {
                        warn!("EntropyRng: JitterRng failed: {}", err);
                        reported_error = Some(err);
                    },
                }
            } else if Jitter::is_supported() {
                match Jitter::new_and_fill(dest) {
                    Ok(jitter_rng) => {
                        debug!("EntropyRng: using JitterRng");
                        self.source = Source::Jitter(jitter_rng);
                        return Ok(());
                    },
                    Err(err) => { reported_error = reported_error.or(Some(err)) },
                }
            }

            if let Some(err) = reported_error {
                Err(Error::with_cause(ErrorKind::Unavailable,
                                      "All entropy sources failed",
                                      err))
            } else {
                Err(Error::new(ErrorKind::Unavailable,
                               "No entropy sources available"))
            }
        }
    }

    impl CryptoRng for EntropyRng {}

    trait EntropySource {
        fn new_and_fill(dest: &mut [u8]) -> Result<Self, Error>
            where Self: Sized;

        fn fill(&mut self, dest: &mut [u8]) -> Result<(), Error>;

        fn is_supported() -> bool { true }
    }

    #[allow(unused)]
    #[derive(Clone, Debug)]
    struct NoSource;

    #[allow(unused)]
    impl EntropySource for NoSource {
        fn new_and_fill(dest: &mut [u8]) -> Result<Self, Error> {
            Err(Error::new(ErrorKind::Unavailable, "Source not supported"))
        }

        fn fill(&mut self, dest: &mut [u8]) -> Result<(), Error> {
            unreachable!()
        }

        fn is_supported() -> bool { false }
    }

    #[cfg(feature="rand_os")]
    #[derive(Clone, Debug)]
    pub struct Os(rngs::OsRng);

    #[cfg(feature="rand_os")]
    impl EntropySource for Os {
        fn new_and_fill(dest: &mut [u8]) -> Result<Self, Error> {
            let mut rng = rngs::OsRng::new()?;
            rng.try_fill_bytes(dest)?;
            Ok(Os(rng))
        }

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

    #[cfg(not(feature="std"))]
    type Os = NoSource;

    type Custom = NoSource;

    #[cfg(not(target_arch = "wasm32"))]
    #[derive(Clone, Debug)]
    pub struct Jitter(rngs::JitterRng);

    #[cfg(not(target_arch = "wasm32"))]
    impl EntropySource for Jitter {
        fn new_and_fill(dest: &mut [u8]) -> Result<Self, Error> {
            let mut rng = rngs::JitterRng::new()?;
            rng.try_fill_bytes(dest)?;
            Ok(Jitter(rng))
        }

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

    #[cfg(target_arch = "wasm32")]
    type Jitter = NoSource;

    #[cfg(test)]
    mod test {
        use super::*;

        #[test]
        fn test_entropy() {
            let mut rng = EntropyRng::new();
            let n = (rng.next_u32() ^ rng.next_u32()).count_ones();
            assert!(n >= 2);    // p(failure) approx 1e-7
        }
    }

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