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    // Copyright 2013 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    //! A wrapper around another RNG that reseeds it after it
    //! generates a certain number of random bytes.

    use core::default::Default;

    use {Rng, SeedableRng};

    /// How many bytes of entropy the underling RNG is allowed to generate
    /// before it is reseeded
    const DEFAULT_GENERATION_THRESHOLD: u64 = 32 * 1024;

    /// A wrapper around any RNG which reseeds the underlying RNG after it
    /// has generated a certain number of random bytes.
    #[derive(Debug)]
    pub struct ReseedingRng<R, Rsdr> {
        rng: R,
        generation_threshold: u64,
        bytes_generated: u64,
        /// Controls the behaviour when reseeding the RNG.
        pub reseeder: Rsdr,
    }

    impl<R: Rng, Rsdr: Reseeder<R>> ReseedingRng<R, Rsdr> {
        /// Create a new `ReseedingRng` with the given parameters.
        ///
        /// # Arguments
        ///
        /// * `rng`: the random number generator to use.
        /// * `generation_threshold`: the number of bytes of entropy at which to reseed the RNG.
        /// * `reseeder`: the reseeding object to use.
        pub fn new(rng: R, generation_threshold: u64, reseeder: Rsdr) -> ReseedingRng<R,Rsdr> {
            ReseedingRng {
                rng: rng,
                generation_threshold: generation_threshold,
                bytes_generated: 0,
                reseeder: reseeder
            }
        }

        /// Reseed the internal RNG if the number of bytes that have been
        /// generated exceed the threshold.
        pub fn reseed_if_necessary(&mut self) {
            if self.bytes_generated >= self.generation_threshold {
                self.reseeder.reseed(&mut self.rng);
                self.bytes_generated = 0;
            }
        }
    }

    impl<R: Rng, Rsdr: Reseeder<R>> Rng for ReseedingRng<R, Rsdr> {
        fn next_u32(&mut self) -> u32 {
            self.reseed_if_necessary();
            self.bytes_generated += 4;
            self.rng.next_u32()
        }

        fn next_u64(&mut self) -> u64 {
            self.reseed_if_necessary();
            self.bytes_generated += 8;
            self.rng.next_u64()
        }

        fn fill_bytes(&mut self, dest: &mut [u8]) {
            self.reseed_if_necessary();
            self.bytes_generated += dest.len() as u64;
            self.rng.fill_bytes(dest)
        }
    }

    impl<S, R: SeedableRng<S>, Rsdr: Reseeder<R> + Default>
         SeedableRng<(Rsdr, S)> for ReseedingRng<R, Rsdr> {
        fn reseed(&mut self, (rsdr, seed): (Rsdr, S)) {
            self.rng.reseed(seed);
            self.reseeder = rsdr;
            self.bytes_generated = 0;
        }

        /// Create a new `ReseedingRng` from the given reseeder and
        /// seed. This uses a default value for `generation_threshold`.
        fn from_seed((rsdr, seed): (Rsdr, S)) -> ReseedingRng<R, Rsdr> {
            ReseedingRng {
                rng: SeedableRng::from_seed(seed),
                generation_threshold: DEFAULT_GENERATION_THRESHOLD,
                bytes_generated: 0,
                reseeder: rsdr
            }
        }
    }

    /// Something that can be used to reseed an RNG via `ReseedingRng`.
    ///
    /// # Example
    ///
    /// ```rust
    /// use rand::{Rng, SeedableRng, StdRng};
    /// use rand::reseeding::{Reseeder, ReseedingRng};
    ///
    /// struct TickTockReseeder { tick: bool }
    /// impl Reseeder<StdRng> for TickTockReseeder {
    ///     fn reseed(&mut self, rng: &mut StdRng) {
    ///         let val = if self.tick {0} else {1};
    ///         rng.reseed(&[val]);
    ///         self.tick = !self.tick;
    ///     }
    /// }
    /// fn main() {
    ///     let rsdr = TickTockReseeder { tick: true };
    ///
    ///     let inner = StdRng::new().unwrap();
    ///     let mut rng = ReseedingRng::new(inner, 10, rsdr);
    ///
    ///     // this will repeat, because it gets reseeded very regularly.
    ///     let s: String = rng.gen_ascii_chars().take(100).collect();
    ///     println!("{}", s);
    /// }
    ///
    /// ```
    pub trait Reseeder<R> {
        /// Reseed the given RNG.
        fn reseed(&mut self, rng: &mut R);
    }

    /// Reseed an RNG using a `Default` instance. This reseeds by
    /// replacing the RNG with the result of a `Default::default` call.
    #[derive(Clone, Copy, Debug)]
    pub struct ReseedWithDefault;

    impl<R: Rng + Default> Reseeder<R> for ReseedWithDefault {
        fn reseed(&mut self, rng: &mut R) {
            *rng = Default::default();
        }
    }
    impl Default for ReseedWithDefault {
        fn default() -> ReseedWithDefault { ReseedWithDefault }
    }

    #[cfg(test)]
    mod test {
        use std::default::Default;
        use std::iter::repeat;
        use super::{ReseedingRng, ReseedWithDefault};
        use {SeedableRng, Rng};

        struct Counter {
            i: u32
        }

        impl Rng for Counter {
            fn next_u32(&mut self) -> u32 {
                self.i += 1;
                // very random
                self.i - 1
            }
        }
        impl Default for Counter {
            fn default() -> Counter {
                Counter { i: 0 }
            }
        }
        impl SeedableRng<u32> for Counter {
            fn reseed(&mut self, seed: u32) {
                self.i = seed;
            }
            fn from_seed(seed: u32) -> Counter {
                Counter { i: seed }
            }
        }
        type MyRng = ReseedingRng<Counter, ReseedWithDefault>;

        #[test]
        fn test_reseeding() {
            let mut rs = ReseedingRng::new(Counter {i:0}, 400, ReseedWithDefault);

            let mut i = 0;
            for _ in 0..1000 {
                assert_eq!(rs.next_u32(), i % 100);
                i += 1;
            }
        }

        #[test]
        fn test_rng_seeded() {
            let mut ra: MyRng = SeedableRng::from_seed((ReseedWithDefault, 2));
            let mut rb: MyRng = SeedableRng::from_seed((ReseedWithDefault, 2));
            assert!(::test::iter_eq(ra.gen_ascii_chars().take(100),
                                    rb.gen_ascii_chars().take(100)));
        }

        #[test]
        fn test_rng_reseed() {
            let mut r: MyRng = SeedableRng::from_seed((ReseedWithDefault, 3));
            let string1: String = r.gen_ascii_chars().take(100).collect();

            r.reseed((ReseedWithDefault, 3));

            let string2: String = r.gen_ascii_chars().take(100).collect();
            assert_eq!(string1, string2);
        }

        const FILL_BYTES_V_LEN: usize = 13579;
        #[test]
        fn test_rng_fill_bytes() {
            let mut v = repeat(0u8).take(FILL_BYTES_V_LEN).collect::<Vec<_>>();
            ::test::rng().fill_bytes(&mut v);

            // Sanity test: if we've gotten here, `fill_bytes` has not infinitely
            // recursed.
            assert_eq!(v.len(), FILL_BYTES_V_LEN);

            // To test that `fill_bytes` actually did something, check that the
            // average of `v` is not 0.
            let mut sum = 0.0;
            for &x in v.iter() {
                sum += x as f64;
            }
            assert!(sum / v.len() as f64 != 0.0);
        }
    }

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