src/librand/reseeding.rs

   1 // Copyright 2013 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 //! A wrapper around another RNG that reseeds it after it
  12 //! generates a certain number of random bytes.
  13
  14 use {Rng, SeedableRng};
  15
  16 /// How many bytes of entropy the underling RNG is allowed to generate
  17 /// before it is reseeded.
  18 const DEFAULT_GENERATION_THRESHOLD: usize = 32 * 1024;
  19
  20 /// A wrapper around any RNG which reseeds the underlying RNG after it
  21 /// has generated a certain number of random bytes.
  22 pub struct ReseedingRng<R, Rsdr> {
  23     rng: R,
  24     generation_threshold: usize,
  25     bytes_generated: usize,
  26     /// Controls the behaviour when reseeding the RNG.
  27     pub reseeder: Rsdr,
  28 }
  29
  30 impl<R: Rng, Rsdr: Reseeder<R>> ReseedingRng<R, Rsdr> {
  31     /// Create a new `ReseedingRng` with the given parameters.
  32     ///
  33     /// # Arguments
  34     ///
  35     /// * `rng`: the random number generator to use.
  36     /// * `generation_threshold`: the number of bytes of entropy at which to reseed the RNG.
  37     /// * `reseeder`: the reseeding object to use.
  38     pub fn new(rng: R, generation_threshold: usize, reseeder: Rsdr) -> ReseedingRng<R, Rsdr> {
  39         ReseedingRng {
  40             rng: rng,
  41             generation_threshold: generation_threshold,
  42             bytes_generated: 0,
  43             reseeder: reseeder,
  44         }
  45     }
  46
  47     /// Reseed the internal RNG if the number of bytes that have been
  48     /// generated exceed the threshold.
  49     pub fn reseed_if_necessary(&mut self) {
  50         if self.bytes_generated >= self.generation_threshold {
  51             self.reseeder.reseed(&mut self.rng);
  52             self.bytes_generated = 0;
  53         }
  54     }
  55 }
  56
  57
  58 impl<R: Rng, Rsdr: Reseeder<R>> Rng for ReseedingRng<R, Rsdr> {
  59     fn next_u32(&mut self) -> u32 {
  60         self.reseed_if_necessary();
  61         self.bytes_generated += 4;
  62         self.rng.next_u32()
  63     }
  64
  65     fn next_u64(&mut self) -> u64 {
  66         self.reseed_if_necessary();
  67         self.bytes_generated += 8;
  68         self.rng.next_u64()
  69     }
  70
  71     fn fill_bytes(&mut self, dest: &mut [u8]) {
  72         self.reseed_if_necessary();
  73         self.bytes_generated += dest.len();
  74         self.rng.fill_bytes(dest)
  75     }
  76 }
  77
  78 impl<S, R: SeedableRng<S>, Rsdr: Reseeder<R> + Default>
  79      SeedableRng<(Rsdr, S)> for ReseedingRng<R, Rsdr> {
  80     fn reseed(&mut self, (rsdr, seed): (Rsdr, S)) {
  81         self.rng.reseed(seed);
  82         self.reseeder = rsdr;
  83         self.bytes_generated = 0;
  84     }
  85
  86 /// Create a new `ReseedingRng` from the given reseeder and
  87 /// seed. This uses a default value for `generation_threshold`.
  88     fn from_seed((rsdr, seed): (Rsdr, S)) -> ReseedingRng<R, Rsdr> {
  89         ReseedingRng {
  90             rng: SeedableRng::from_seed(seed),
  91             generation_threshold: DEFAULT_GENERATION_THRESHOLD,
  92             bytes_generated: 0,
  93             reseeder: rsdr,
  94         }
  95     }
  96 }
  97
  98 /// Something that can be used to reseed an RNG via `ReseedingRng`.
  99 pub trait Reseeder<R> {
 100     /// Reseed the given RNG.
 101     fn reseed(&mut self, rng: &mut R);
 102 }
 103
 104 /// Reseed an RNG using a `Default` instance. This reseeds by
 105 /// replacing the RNG with the result of a `Default::default` call.
 106 #[derive(Copy, Clone)]
 107 pub struct ReseedWithDefault;
 108
 109 impl<R: Rng + Default> Reseeder<R> for ReseedWithDefault {
 110     fn reseed(&mut self, rng: &mut R) {
 111         *rng = Default::default();
 112     }
 113 }
 114 #[stable(feature = "rust1", since = "1.0.0")]
 115 impl Default for ReseedWithDefault {
 116     /// Creates an instance of `ReseedWithDefault`.
 117     fn default() -> ReseedWithDefault {
 118         ReseedWithDefault
 119     }
 120 }
 121
 122 #[cfg(test)]
 123 mod tests {
 124     use std::prelude::v1::*;
 125
 126     use super::{ReseedWithDefault, ReseedingRng};
 127     use {Rng, SeedableRng};
 128
 129     struct Counter {
 130         i: u32,
 131     }
 132
 133     impl Rng for Counter {
 134         fn next_u32(&mut self) -> u32 {
 135             self.i += 1;
 136             // very random
 137             self.i - 1
 138         }
 139     }
 140     impl Default for Counter {
 141         /// Constructs a `Counter` with initial value zero.
 142         fn default() -> Counter {
 143             Counter { i: 0 }
 144         }
 145     }
 146     impl SeedableRng<u32> for Counter {
 147         fn reseed(&mut self, seed: u32) {
 148             self.i = seed;
 149         }
 150         fn from_seed(seed: u32) -> Counter {
 151             Counter { i: seed }
 152         }
 153     }
 154     type MyRng = ReseedingRng<Counter, ReseedWithDefault>;
 155
 156     #[test]
 157     fn test_reseeding() {
 158         let mut rs = ReseedingRng::new(Counter { i: 0 }, 400, ReseedWithDefault);
 159
 160         let mut i = 0;
 161         for _ in 0..1000 {
 162             assert_eq!(rs.next_u32(), i % 100);
 163             i += 1;
 164         }
 165     }
 166
 167     #[test]
 168     fn test_rng_seeded() {
 169         let mut ra: MyRng = SeedableRng::from_seed((ReseedWithDefault, 2));
 170         let mut rb: MyRng = SeedableRng::from_seed((ReseedWithDefault, 2));
 171         assert!(ra.gen_ascii_chars()
 172             .take(100)
 173             .eq(rb.gen_ascii_chars().take(100)));
 174     }
 175
 176     #[test]
 177     fn test_rng_reseed() {
 178         let mut r: MyRng = SeedableRng::from_seed((ReseedWithDefault, 3));
 179         let string1: String = r.gen_ascii_chars().take(100).collect();
 180
 181         r.reseed((ReseedWithDefault, 3));
 182
 183         let string2: String = r.gen_ascii_chars().take(100).collect();
 184         assert_eq!(string1, string2);
 185     }
 186
 187     const FILL_BYTES_V_LEN: usize = 13579;
 188     #[test]
 189     fn test_rng_fill_bytes() {
 190         let mut v = vec![0; FILL_BYTES_V_LEN];
 191         ::test::rng().fill_bytes(&mut v);
 192
 193         // Sanity test: if we've gotten here, `fill_bytes` has not infinitely
 194         // recursed.
 195         assert_eq!(v.len(), FILL_BYTES_V_LEN);
 196
 197         // To test that `fill_bytes` actually did something, check that the
 198         // average of `v` is not 0.
 199         let mut sum = 0.0;
 200         for &x in &v {
 201             sum += x as f64;
 202         }
 203         assert!(sum / v.len() as f64 != 0.0);
 204     }
 205 }