rand_core/
block.rs

1// Copyright 2018 Developers of the Rand project.
2//
3// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
4// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
5// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
6// option. This file may not be copied, modified, or distributed
7// except according to those terms.
8
9//! The `BlockRngCore` trait and implementation helpers
10//!
11//! The [`BlockRngCore`] trait exists to assist in the implementation of RNGs
12//! which generate a block of data in a cache instead of returning generated
13//! values directly.
14//!
15//! Usage of this trait is optional, but provides two advantages:
16//! implementations only need to concern themselves with generation of the
17//! block, not the various [`RngCore`] methods (especially [`fill_bytes`], where
18//! the optimal implementations are not trivial), and this allows
19//! `ReseedingRng` (see [`rand`](https://docs.rs/rand) crate) perform periodic
20//! reseeding with very low overhead.
21//!
22//! # Example
23//!
24//! ```no_run
25//! use rand_core::{RngCore, SeedableRng};
26//! use rand_core::block::{BlockRngCore, BlockRng};
27//!
28//! struct MyRngCore;
29//!
30//! impl BlockRngCore for MyRngCore {
31//!     type Item = u32;
32//!     type Results = [u32; 16];
33//!
34//!     fn generate(&mut self, results: &mut Self::Results) {
35//!         unimplemented!()
36//!     }
37//! }
38//!
39//! impl SeedableRng for MyRngCore {
40//!     type Seed = [u8; 32];
41//!     fn from_seed(seed: Self::Seed) -> Self {
42//!         unimplemented!()
43//!     }
44//! }
45//!
46//! // optionally, also implement CryptoBlockRng for MyRngCore
47//!
48//! // Final RNG.
49//! let mut rng = BlockRng::<MyRngCore>::seed_from_u64(0);
50//! println!("First value: {}", rng.next_u32());
51//! ```
52//!
53//! [`BlockRngCore`]: crate::block::BlockRngCore
54//! [`fill_bytes`]: RngCore::fill_bytes
55
56use crate::impls::fill_via_chunks;
57use crate::{CryptoRng, RngCore, SeedableRng, TryRngCore};
58use core::fmt;
59#[cfg(feature = "serde")]
60use serde::{Deserialize, Serialize};
61
62/// A trait for RNGs which do not generate random numbers individually, but in
63/// blocks (typically `[u32; N]`). This technique is commonly used by
64/// cryptographic RNGs to improve performance.
65///
66/// See the [module][crate::block] documentation for details.
67pub trait BlockRngCore {
68    /// Results element type, e.g. `u32`.
69    type Item;
70
71    /// Results type. This is the 'block' an RNG implementing `BlockRngCore`
72    /// generates, which will usually be an array like `[u32; 16]`.
73    type Results: AsRef<[Self::Item]> + AsMut<[Self::Item]> + Default;
74
75    /// Generate a new block of results.
76    fn generate(&mut self, results: &mut Self::Results);
77}
78
79/// A marker trait used to indicate that an [`RngCore`] implementation is
80/// supposed to be cryptographically secure.
81///
82/// See [`CryptoRng`] docs for more information.
83pub trait CryptoBlockRng: BlockRngCore {}
84
85/// A wrapper type implementing [`RngCore`] for some type implementing
86/// [`BlockRngCore`] with `u32` array buffer; i.e. this can be used to implement
87/// a full RNG from just a `generate` function.
88///
89/// The `core` field may be accessed directly but the results buffer may not.
90/// PRNG implementations can simply use a type alias
91/// (`pub type MyRng = BlockRng<MyRngCore>;`) but might prefer to use a
92/// wrapper type (`pub struct MyRng(BlockRng<MyRngCore>);`); the latter must
93/// re-implement `RngCore` but hides the implementation details and allows
94/// extra functionality to be defined on the RNG
95/// (e.g. `impl MyRng { fn set_stream(...){...} }`).
96///
97/// `BlockRng` has heavily optimized implementations of the [`RngCore`] methods
98/// reading values from the results buffer, as well as
99/// calling [`BlockRngCore::generate`] directly on the output array when
100/// [`fill_bytes`] is called on a large array. These methods also handle
101/// the bookkeeping of when to generate a new batch of values.
102///
103/// No whole generated `u32` values are thrown away and all values are consumed
104/// in-order. [`next_u32`] simply takes the next available `u32` value.
105/// [`next_u64`] is implemented by combining two `u32` values, least
106/// significant first. [`fill_bytes`] consume a whole number of `u32` values,
107/// converting each `u32` to a byte slice in little-endian order. If the requested byte
108/// length is not a multiple of 4, some bytes will be discarded.
109///
110/// See also [`BlockRng64`] which uses `u64` array buffers. Currently there is
111/// no direct support for other buffer types.
112///
113/// For easy initialization `BlockRng` also implements [`SeedableRng`].
114///
115/// [`next_u32`]: RngCore::next_u32
116/// [`next_u64`]: RngCore::next_u64
117/// [`fill_bytes`]: RngCore::fill_bytes
118#[derive(Clone)]
119#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
120#[cfg_attr(
121    feature = "serde",
122    serde(
123        bound = "for<'x> R: Serialize + Deserialize<'x>, for<'x> R::Results: Serialize + Deserialize<'x>"
124    )
125)]
126pub struct BlockRng<R: BlockRngCore> {
127    results: R::Results,
128    index: usize,
129    /// The *core* part of the RNG, implementing the `generate` function.
130    pub core: R,
131}
132
133// Custom Debug implementation that does not expose the contents of `results`.
134impl<R: BlockRngCore + fmt::Debug> fmt::Debug for BlockRng<R> {
135    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
136        fmt.debug_struct("BlockRng")
137            .field("core", &self.core)
138            .field("result_len", &self.results.as_ref().len())
139            .field("index", &self.index)
140            .finish()
141    }
142}
143
144impl<R: BlockRngCore> BlockRng<R> {
145    /// Create a new `BlockRng` from an existing RNG implementing
146    /// `BlockRngCore`. Results will be generated on first use.
147    #[inline]
148    pub fn new(core: R) -> BlockRng<R> {
149        let results_empty = R::Results::default();
150        BlockRng {
151            core,
152            index: results_empty.as_ref().len(),
153            results: results_empty,
154        }
155    }
156
157    /// Get the index into the result buffer.
158    ///
159    /// If this is equal to or larger than the size of the result buffer then
160    /// the buffer is "empty" and `generate()` must be called to produce new
161    /// results.
162    #[inline(always)]
163    pub fn index(&self) -> usize {
164        self.index
165    }
166
167    /// Reset the number of available results.
168    /// This will force a new set of results to be generated on next use.
169    #[inline]
170    pub fn reset(&mut self) {
171        self.index = self.results.as_ref().len();
172    }
173
174    /// Generate a new set of results immediately, setting the index to the
175    /// given value.
176    #[inline]
177    pub fn generate_and_set(&mut self, index: usize) {
178        assert!(index < self.results.as_ref().len());
179        self.core.generate(&mut self.results);
180        self.index = index;
181    }
182}
183
184impl<R: BlockRngCore<Item = u32>> RngCore for BlockRng<R> {
185    #[inline]
186    fn next_u32(&mut self) -> u32 {
187        if self.index >= self.results.as_ref().len() {
188            self.generate_and_set(0);
189        }
190
191        let value = self.results.as_ref()[self.index];
192        self.index += 1;
193        value
194    }
195
196    #[inline]
197    fn next_u64(&mut self) -> u64 {
198        let read_u64 = |results: &[u32], index| {
199            let data = &results[index..=index + 1];
200            (u64::from(data[1]) << 32) | u64::from(data[0])
201        };
202
203        let len = self.results.as_ref().len();
204
205        let index = self.index;
206        if index < len - 1 {
207            self.index += 2;
208            // Read an u64 from the current index
209            read_u64(self.results.as_ref(), index)
210        } else if index >= len {
211            self.generate_and_set(2);
212            read_u64(self.results.as_ref(), 0)
213        } else {
214            let x = u64::from(self.results.as_ref()[len - 1]);
215            self.generate_and_set(1);
216            let y = u64::from(self.results.as_ref()[0]);
217            (y << 32) | x
218        }
219    }
220
221    #[inline]
222    fn fill_bytes(&mut self, dest: &mut [u8]) {
223        let mut read_len = 0;
224        while read_len < dest.len() {
225            if self.index >= self.results.as_ref().len() {
226                self.generate_and_set(0);
227            }
228            let (consumed_u32, filled_u8) =
229                fill_via_chunks(&self.results.as_mut()[self.index..], &mut dest[read_len..]);
230
231            self.index += consumed_u32;
232            read_len += filled_u8;
233        }
234    }
235}
236
237impl<R: BlockRngCore + SeedableRng> SeedableRng for BlockRng<R> {
238    type Seed = R::Seed;
239
240    #[inline(always)]
241    fn from_seed(seed: Self::Seed) -> Self {
242        Self::new(R::from_seed(seed))
243    }
244
245    #[inline(always)]
246    fn seed_from_u64(seed: u64) -> Self {
247        Self::new(R::seed_from_u64(seed))
248    }
249
250    #[inline(always)]
251    fn from_rng(rng: &mut impl RngCore) -> Self {
252        Self::new(R::from_rng(rng))
253    }
254
255    #[inline(always)]
256    fn try_from_rng<S: TryRngCore>(rng: &mut S) -> Result<Self, S::Error> {
257        R::try_from_rng(rng).map(Self::new)
258    }
259}
260
261impl<R: CryptoBlockRng + BlockRngCore<Item = u32>> CryptoRng for BlockRng<R> {}
262
263/// A wrapper type implementing [`RngCore`] for some type implementing
264/// [`BlockRngCore`] with `u64` array buffer; i.e. this can be used to implement
265/// a full RNG from just a `generate` function.
266///
267/// This is similar to [`BlockRng`], but specialized for algorithms that operate
268/// on `u64` values.
269///
270/// No whole generated `u64` values are thrown away and all values are consumed
271/// in-order. [`next_u64`] simply takes the next available `u64` value.
272/// [`next_u32`] is however a bit special: half of a `u64` is consumed, leaving
273/// the other half in the buffer. If the next function called is [`next_u32`]
274/// then the other half is then consumed, however both [`next_u64`] and
275/// [`fill_bytes`] discard the rest of any half-consumed `u64`s when called.
276///
277/// [`fill_bytes`] consumes a whole number of `u64` values. If the requested length
278/// is not a multiple of 8, some bytes will be discarded.
279///
280/// [`next_u32`]: RngCore::next_u32
281/// [`next_u64`]: RngCore::next_u64
282/// [`fill_bytes`]: RngCore::fill_bytes
283#[derive(Clone)]
284#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
285pub struct BlockRng64<R: BlockRngCore + ?Sized> {
286    results: R::Results,
287    index: usize,
288    half_used: bool, // true if only half of the previous result is used
289    /// The *core* part of the RNG, implementing the `generate` function.
290    pub core: R,
291}
292
293// Custom Debug implementation that does not expose the contents of `results`.
294impl<R: BlockRngCore + fmt::Debug> fmt::Debug for BlockRng64<R> {
295    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
296        fmt.debug_struct("BlockRng64")
297            .field("core", &self.core)
298            .field("result_len", &self.results.as_ref().len())
299            .field("index", &self.index)
300            .field("half_used", &self.half_used)
301            .finish()
302    }
303}
304
305impl<R: BlockRngCore> BlockRng64<R> {
306    /// Create a new `BlockRng` from an existing RNG implementing
307    /// `BlockRngCore`. Results will be generated on first use.
308    #[inline]
309    pub fn new(core: R) -> BlockRng64<R> {
310        let results_empty = R::Results::default();
311        BlockRng64 {
312            core,
313            index: results_empty.as_ref().len(),
314            half_used: false,
315            results: results_empty,
316        }
317    }
318
319    /// Get the index into the result buffer.
320    ///
321    /// If this is equal to or larger than the size of the result buffer then
322    /// the buffer is "empty" and `generate()` must be called to produce new
323    /// results.
324    #[inline(always)]
325    pub fn index(&self) -> usize {
326        self.index
327    }
328
329    /// Reset the number of available results.
330    /// This will force a new set of results to be generated on next use.
331    #[inline]
332    pub fn reset(&mut self) {
333        self.index = self.results.as_ref().len();
334        self.half_used = false;
335    }
336
337    /// Generate a new set of results immediately, setting the index to the
338    /// given value.
339    #[inline]
340    pub fn generate_and_set(&mut self, index: usize) {
341        assert!(index < self.results.as_ref().len());
342        self.core.generate(&mut self.results);
343        self.index = index;
344        self.half_used = false;
345    }
346}
347
348impl<R: BlockRngCore<Item = u64>> RngCore for BlockRng64<R> {
349    #[inline]
350    fn next_u32(&mut self) -> u32 {
351        let mut index = self.index - self.half_used as usize;
352        if index >= self.results.as_ref().len() {
353            self.core.generate(&mut self.results);
354            self.index = 0;
355            index = 0;
356            // `self.half_used` is by definition `false`
357            self.half_used = false;
358        }
359
360        let shift = 32 * (self.half_used as usize);
361
362        self.half_used = !self.half_used;
363        self.index += self.half_used as usize;
364
365        (self.results.as_ref()[index] >> shift) as u32
366    }
367
368    #[inline]
369    fn next_u64(&mut self) -> u64 {
370        if self.index >= self.results.as_ref().len() {
371            self.core.generate(&mut self.results);
372            self.index = 0;
373        }
374
375        let value = self.results.as_ref()[self.index];
376        self.index += 1;
377        self.half_used = false;
378        value
379    }
380
381    #[inline]
382    fn fill_bytes(&mut self, dest: &mut [u8]) {
383        let mut read_len = 0;
384        self.half_used = false;
385        while read_len < dest.len() {
386            if self.index >= self.results.as_ref().len() {
387                self.core.generate(&mut self.results);
388                self.index = 0;
389            }
390
391            let (consumed_u64, filled_u8) =
392                fill_via_chunks(&self.results.as_mut()[self.index..], &mut dest[read_len..]);
393
394            self.index += consumed_u64;
395            read_len += filled_u8;
396        }
397    }
398}
399
400impl<R: BlockRngCore + SeedableRng> SeedableRng for BlockRng64<R> {
401    type Seed = R::Seed;
402
403    #[inline(always)]
404    fn from_seed(seed: Self::Seed) -> Self {
405        Self::new(R::from_seed(seed))
406    }
407
408    #[inline(always)]
409    fn seed_from_u64(seed: u64) -> Self {
410        Self::new(R::seed_from_u64(seed))
411    }
412
413    #[inline(always)]
414    fn from_rng(rng: &mut impl RngCore) -> Self {
415        Self::new(R::from_rng(rng))
416    }
417
418    #[inline(always)]
419    fn try_from_rng<S: TryRngCore>(rng: &mut S) -> Result<Self, S::Error> {
420        R::try_from_rng(rng).map(Self::new)
421    }
422}
423
424impl<R: CryptoBlockRng + BlockRngCore<Item = u64>> CryptoRng for BlockRng64<R> {}
425
426#[cfg(test)]
427mod test {
428    use crate::block::{BlockRng, BlockRng64, BlockRngCore};
429    use crate::{RngCore, SeedableRng};
430
431    #[derive(Debug, Clone)]
432    struct DummyRng {
433        counter: u32,
434    }
435
436    impl BlockRngCore for DummyRng {
437        type Item = u32;
438        type Results = [u32; 16];
439
440        fn generate(&mut self, results: &mut Self::Results) {
441            for r in results {
442                *r = self.counter;
443                self.counter = self.counter.wrapping_add(3511615421);
444            }
445        }
446    }
447
448    impl SeedableRng for DummyRng {
449        type Seed = [u8; 4];
450
451        fn from_seed(seed: Self::Seed) -> Self {
452            DummyRng {
453                counter: u32::from_le_bytes(seed),
454            }
455        }
456    }
457
458    #[test]
459    fn blockrng_next_u32_vs_next_u64() {
460        let mut rng1 = BlockRng::<DummyRng>::from_seed([1, 2, 3, 4]);
461        let mut rng2 = rng1.clone();
462        let mut rng3 = rng1.clone();
463
464        let mut a = [0; 16];
465        a[..4].copy_from_slice(&rng1.next_u32().to_le_bytes());
466        a[4..12].copy_from_slice(&rng1.next_u64().to_le_bytes());
467        a[12..].copy_from_slice(&rng1.next_u32().to_le_bytes());
468
469        let mut b = [0; 16];
470        b[..4].copy_from_slice(&rng2.next_u32().to_le_bytes());
471        b[4..8].copy_from_slice(&rng2.next_u32().to_le_bytes());
472        b[8..].copy_from_slice(&rng2.next_u64().to_le_bytes());
473        assert_eq!(a, b);
474
475        let mut c = [0; 16];
476        c[..8].copy_from_slice(&rng3.next_u64().to_le_bytes());
477        c[8..12].copy_from_slice(&rng3.next_u32().to_le_bytes());
478        c[12..].copy_from_slice(&rng3.next_u32().to_le_bytes());
479        assert_eq!(a, c);
480    }
481
482    #[derive(Debug, Clone)]
483    struct DummyRng64 {
484        counter: u64,
485    }
486
487    impl BlockRngCore for DummyRng64 {
488        type Item = u64;
489        type Results = [u64; 8];
490
491        fn generate(&mut self, results: &mut Self::Results) {
492            for r in results {
493                *r = self.counter;
494                self.counter = self.counter.wrapping_add(2781463553396133981);
495            }
496        }
497    }
498
499    impl SeedableRng for DummyRng64 {
500        type Seed = [u8; 8];
501
502        fn from_seed(seed: Self::Seed) -> Self {
503            DummyRng64 {
504                counter: u64::from_le_bytes(seed),
505            }
506        }
507    }
508
509    #[test]
510    fn blockrng64_next_u32_vs_next_u64() {
511        let mut rng1 = BlockRng64::<DummyRng64>::from_seed([1, 2, 3, 4, 5, 6, 7, 8]);
512        let mut rng2 = rng1.clone();
513        let mut rng3 = rng1.clone();
514
515        let mut a = [0; 16];
516        a[..4].copy_from_slice(&rng1.next_u32().to_le_bytes());
517        a[4..12].copy_from_slice(&rng1.next_u64().to_le_bytes());
518        a[12..].copy_from_slice(&rng1.next_u32().to_le_bytes());
519
520        let mut b = [0; 16];
521        b[..4].copy_from_slice(&rng2.next_u32().to_le_bytes());
522        b[4..8].copy_from_slice(&rng2.next_u32().to_le_bytes());
523        b[8..].copy_from_slice(&rng2.next_u64().to_le_bytes());
524        assert_ne!(a, b);
525        assert_eq!(&a[..4], &b[..4]);
526        assert_eq!(&a[4..12], &b[8..]);
527
528        let mut c = [0; 16];
529        c[..8].copy_from_slice(&rng3.next_u64().to_le_bytes());
530        c[8..12].copy_from_slice(&rng3.next_u32().to_le_bytes());
531        c[12..].copy_from_slice(&rng3.next_u32().to_le_bytes());
532        assert_eq!(b, c);
533    }
534}
rand_core/block.rs

rand_core/
block.rs
