rand_core/
block.rs

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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_u32_chunks, fill_via_u64_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) = fill_via_u32_chunks(
229                &mut self.results.as_mut()[self.index..],
230                &mut dest[read_len..],
231            );
232
233            self.index += consumed_u32;
234            read_len += filled_u8;
235        }
236    }
237}
238
239impl<R: BlockRngCore + SeedableRng> SeedableRng for BlockRng<R> {
240    type Seed = R::Seed;
241
242    #[inline(always)]
243    fn from_seed(seed: Self::Seed) -> Self {
244        Self::new(R::from_seed(seed))
245    }
246
247    #[inline(always)]
248    fn seed_from_u64(seed: u64) -> Self {
249        Self::new(R::seed_from_u64(seed))
250    }
251
252    #[inline(always)]
253    fn from_rng(rng: &mut impl RngCore) -> Self {
254        Self::new(R::from_rng(rng))
255    }
256
257    #[inline(always)]
258    fn try_from_rng<S: TryRngCore>(rng: &mut S) -> Result<Self, S::Error> {
259        R::try_from_rng(rng).map(Self::new)
260    }
261}
262
263impl<R: CryptoBlockRng + BlockRngCore<Item = u32>> CryptoRng for BlockRng<R> {}
264
265/// A wrapper type implementing [`RngCore`] for some type implementing
266/// [`BlockRngCore`] with `u64` array buffer; i.e. this can be used to implement
267/// a full RNG from just a `generate` function.
268///
269/// This is similar to [`BlockRng`], but specialized for algorithms that operate
270/// on `u64` values.
271///
272/// No whole generated `u64` values are thrown away and all values are consumed
273/// in-order. [`next_u64`] simply takes the next available `u64` value.
274/// [`next_u32`] is however a bit special: half of a `u64` is consumed, leaving
275/// the other half in the buffer. If the next function called is [`next_u32`]
276/// then the other half is then consumed, however both [`next_u64`] and
277/// [`fill_bytes`] discard the rest of any half-consumed `u64`s when called.
278///
279/// [`fill_bytes`] consumes a whole number of `u64` values. If the requested length
280/// is not a multiple of 8, some bytes will be discarded.
281///
282/// [`next_u32`]: RngCore::next_u32
283/// [`next_u64`]: RngCore::next_u64
284/// [`fill_bytes`]: RngCore::fill_bytes
285#[derive(Clone)]
286#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
287pub struct BlockRng64<R: BlockRngCore + ?Sized> {
288    results: R::Results,
289    index: usize,
290    half_used: bool, // true if only half of the previous result is used
291    /// The *core* part of the RNG, implementing the `generate` function.
292    pub core: R,
293}
294
295// Custom Debug implementation that does not expose the contents of `results`.
296impl<R: BlockRngCore + fmt::Debug> fmt::Debug for BlockRng64<R> {
297    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
298        fmt.debug_struct("BlockRng64")
299            .field("core", &self.core)
300            .field("result_len", &self.results.as_ref().len())
301            .field("index", &self.index)
302            .field("half_used", &self.half_used)
303            .finish()
304    }
305}
306
307impl<R: BlockRngCore> BlockRng64<R> {
308    /// Create a new `BlockRng` from an existing RNG implementing
309    /// `BlockRngCore`. Results will be generated on first use.
310    #[inline]
311    pub fn new(core: R) -> BlockRng64<R> {
312        let results_empty = R::Results::default();
313        BlockRng64 {
314            core,
315            index: results_empty.as_ref().len(),
316            half_used: false,
317            results: results_empty,
318        }
319    }
320
321    /// Get the index into the result buffer.
322    ///
323    /// If this is equal to or larger than the size of the result buffer then
324    /// the buffer is "empty" and `generate()` must be called to produce new
325    /// results.
326    #[inline(always)]
327    pub fn index(&self) -> usize {
328        self.index
329    }
330
331    /// Reset the number of available results.
332    /// This will force a new set of results to be generated on next use.
333    #[inline]
334    pub fn reset(&mut self) {
335        self.index = self.results.as_ref().len();
336        self.half_used = false;
337    }
338
339    /// Generate a new set of results immediately, setting the index to the
340    /// given value.
341    #[inline]
342    pub fn generate_and_set(&mut self, index: usize) {
343        assert!(index < self.results.as_ref().len());
344        self.core.generate(&mut self.results);
345        self.index = index;
346        self.half_used = false;
347    }
348}
349
350impl<R: BlockRngCore<Item = u64>> RngCore for BlockRng64<R> {
351    #[inline]
352    fn next_u32(&mut self) -> u32 {
353        let mut index = self.index - self.half_used as usize;
354        if index >= self.results.as_ref().len() {
355            self.core.generate(&mut self.results);
356            self.index = 0;
357            index = 0;
358            // `self.half_used` is by definition `false`
359            self.half_used = false;
360        }
361
362        let shift = 32 * (self.half_used as usize);
363
364        self.half_used = !self.half_used;
365        self.index += self.half_used as usize;
366
367        (self.results.as_ref()[index] >> shift) as u32
368    }
369
370    #[inline]
371    fn next_u64(&mut self) -> u64 {
372        if self.index >= self.results.as_ref().len() {
373            self.core.generate(&mut self.results);
374            self.index = 0;
375        }
376
377        let value = self.results.as_ref()[self.index];
378        self.index += 1;
379        self.half_used = false;
380        value
381    }
382
383    #[inline]
384    fn fill_bytes(&mut self, dest: &mut [u8]) {
385        let mut read_len = 0;
386        self.half_used = false;
387        while read_len < dest.len() {
388            if self.index >= self.results.as_ref().len() {
389                self.core.generate(&mut self.results);
390                self.index = 0;
391            }
392
393            let (consumed_u64, filled_u8) = fill_via_u64_chunks(
394                &mut self.results.as_mut()[self.index..],
395                &mut dest[read_len..],
396            );
397
398            self.index += consumed_u64;
399            read_len += filled_u8;
400        }
401    }
402}
403
404impl<R: BlockRngCore + SeedableRng> SeedableRng for BlockRng64<R> {
405    type Seed = R::Seed;
406
407    #[inline(always)]
408    fn from_seed(seed: Self::Seed) -> Self {
409        Self::new(R::from_seed(seed))
410    }
411
412    #[inline(always)]
413    fn seed_from_u64(seed: u64) -> Self {
414        Self::new(R::seed_from_u64(seed))
415    }
416
417    #[inline(always)]
418    fn from_rng(rng: &mut impl RngCore) -> Self {
419        Self::new(R::from_rng(rng))
420    }
421
422    #[inline(always)]
423    fn try_from_rng<S: TryRngCore>(rng: &mut S) -> Result<Self, S::Error> {
424        R::try_from_rng(rng).map(Self::new)
425    }
426}
427
428impl<R: CryptoBlockRng + BlockRngCore<Item = u64>> CryptoRng for BlockRng64<R> {}
429
430#[cfg(test)]
431mod test {
432    use crate::block::{BlockRng, BlockRng64, BlockRngCore};
433    use crate::{RngCore, SeedableRng};
434
435    #[derive(Debug, Clone)]
436    struct DummyRng {
437        counter: u32,
438    }
439
440    impl BlockRngCore for DummyRng {
441        type Item = u32;
442        type Results = [u32; 16];
443
444        fn generate(&mut self, results: &mut Self::Results) {
445            for r in results {
446                *r = self.counter;
447                self.counter = self.counter.wrapping_add(3511615421);
448            }
449        }
450    }
451
452    impl SeedableRng for DummyRng {
453        type Seed = [u8; 4];
454
455        fn from_seed(seed: Self::Seed) -> Self {
456            DummyRng {
457                counter: u32::from_le_bytes(seed),
458            }
459        }
460    }
461
462    #[test]
463    fn blockrng_next_u32_vs_next_u64() {
464        let mut rng1 = BlockRng::<DummyRng>::from_seed([1, 2, 3, 4]);
465        let mut rng2 = rng1.clone();
466        let mut rng3 = rng1.clone();
467
468        let mut a = [0; 16];
469        a[..4].copy_from_slice(&rng1.next_u32().to_le_bytes());
470        a[4..12].copy_from_slice(&rng1.next_u64().to_le_bytes());
471        a[12..].copy_from_slice(&rng1.next_u32().to_le_bytes());
472
473        let mut b = [0; 16];
474        b[..4].copy_from_slice(&rng2.next_u32().to_le_bytes());
475        b[4..8].copy_from_slice(&rng2.next_u32().to_le_bytes());
476        b[8..].copy_from_slice(&rng2.next_u64().to_le_bytes());
477        assert_eq!(a, b);
478
479        let mut c = [0; 16];
480        c[..8].copy_from_slice(&rng3.next_u64().to_le_bytes());
481        c[8..12].copy_from_slice(&rng3.next_u32().to_le_bytes());
482        c[12..].copy_from_slice(&rng3.next_u32().to_le_bytes());
483        assert_eq!(a, c);
484    }
485
486    #[derive(Debug, Clone)]
487    struct DummyRng64 {
488        counter: u64,
489    }
490
491    impl BlockRngCore for DummyRng64 {
492        type Item = u64;
493        type Results = [u64; 8];
494
495        fn generate(&mut self, results: &mut Self::Results) {
496            for r in results {
497                *r = self.counter;
498                self.counter = self.counter.wrapping_add(2781463553396133981);
499            }
500        }
501    }
502
503    impl SeedableRng for DummyRng64 {
504        type Seed = [u8; 8];
505
506        fn from_seed(seed: Self::Seed) -> Self {
507            DummyRng64 {
508                counter: u64::from_le_bytes(seed),
509            }
510        }
511    }
512
513    #[test]
514    fn blockrng64_next_u32_vs_next_u64() {
515        let mut rng1 = BlockRng64::<DummyRng64>::from_seed([1, 2, 3, 4, 5, 6, 7, 8]);
516        let mut rng2 = rng1.clone();
517        let mut rng3 = rng1.clone();
518
519        let mut a = [0; 16];
520        a[..4].copy_from_slice(&rng1.next_u32().to_le_bytes());
521        a[4..12].copy_from_slice(&rng1.next_u64().to_le_bytes());
522        a[12..].copy_from_slice(&rng1.next_u32().to_le_bytes());
523
524        let mut b = [0; 16];
525        b[..4].copy_from_slice(&rng2.next_u32().to_le_bytes());
526        b[4..8].copy_from_slice(&rng2.next_u32().to_le_bytes());
527        b[8..].copy_from_slice(&rng2.next_u64().to_le_bytes());
528        assert_ne!(a, b);
529        assert_eq!(&a[..4], &b[..4]);
530        assert_eq!(&a[4..12], &b[8..]);
531
532        let mut c = [0; 16];
533        c[..8].copy_from_slice(&rng3.next_u64().to_le_bytes());
534        c[8..12].copy_from_slice(&rng3.next_u32().to_le_bytes());
535        c[12..].copy_from_slice(&rng3.next_u32().to_le_bytes());
536        assert_eq!(b, c);
537    }
538}
rand_core/block.rs

rand_core/
block.rs
