reseeding.rs

// 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::fmt;
use {Rng, SeedableRng};

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

/// A wrapper around any RNG which reseeds the underlying RNG after it
/// has generated a certain number of random bytes.
pub struct ReseedingRng<R, Rsdr> {
    rng: R,
    generation_threshold: usize,
    bytes_generated: usize,
    /// 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: usize, 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();
        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,
        }
    }
}

impl<R: fmt::Debug, Rsdr: fmt::Debug> fmt::Debug for ReseedingRng<R, Rsdr> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_struct("ReseedingRng")
         .field("rng", &self.rng)
         .field("generation_threshold", &self.generation_threshold)
         .field("bytes_generated", &self.bytes_generated)
         .field("reseeder", &self.reseeder)
         .finish()
    }
}

/// Something that can be used to reseed an RNG via `ReseedingRng`.
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(Copy, Clone, Debug)]
pub struct ReseedWithDefault;

impl<R: Rng + Default> Reseeder<R> for ReseedWithDefault {
    fn reseed(&mut self, rng: &mut R) {
        *rng = Default::default();
    }
}
#[stable(feature = "rust1", since = "1.0.0")]
impl Default for ReseedWithDefault {
    /// Creates an instance of `ReseedWithDefault`.
    fn default() -> ReseedWithDefault {
        ReseedWithDefault
    }
}

#[cfg(test)]
mod tests {
    use std::prelude::v1::*;

    use super::{ReseedWithDefault, ReseedingRng};
    use {Rng, SeedableRng};

    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 {
        /// Constructs a `Counter` with initial value zero.
        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!(ra.gen_ascii_chars()
            .take(100)
            .eq(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 = vec![0; FILL_BYTES_V_LEN];
        ::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 {
            sum += x as f64;
        }
        assert!(sum / v.len() as f64 != 0.0);
    }
}