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 CryptoRng 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, Error, RngCore, SeedableRng};
58use core::convert::AsRef;
59use core::fmt;
60#[cfg(feature = "serde1")]
61use serde::{Deserialize, Serialize};
62
63/// A trait for RNGs which do not generate random numbers individually, but in
64/// blocks (typically `[u32; N]`). This technique is commonly used by
65/// cryptographic RNGs to improve performance.
66///
67/// See the [module][crate::block] documentation for details.
68pub trait BlockRngCore {
69    /// Results element type, e.g. `u32`.
70    type Item;
71
72    /// Results type. This is the 'block' an RNG implementing `BlockRngCore`
73    /// generates, which will usually be an array like `[u32; 16]`.
74    type Results: AsRef<[Self::Item]> + AsMut<[Self::Item]> + Default;
75
76    /// Generate a new block of results.
77    fn generate(&mut self, results: &mut Self::Results);
78}
79
80/// A wrapper type implementing [`RngCore`] for some type implementing
81/// [`BlockRngCore`] with `u32` array buffer; i.e. this can be used to implement
82/// a full RNG from just a `generate` function.
83///
84/// The `core` field may be accessed directly but the results buffer may not.
85/// PRNG implementations can simply use a type alias
86/// (`pub type MyRng = BlockRng<MyRngCore>;`) but might prefer to use a
87/// wrapper type (`pub struct MyRng(BlockRng<MyRngCore>);`); the latter must
88/// re-implement `RngCore` but hides the implementation details and allows
89/// extra functionality to be defined on the RNG
90/// (e.g. `impl MyRng { fn set_stream(...){...} }`).
91///
92/// `BlockRng` has heavily optimized implementations of the [`RngCore`] methods
93/// reading values from the results buffer, as well as
94/// calling [`BlockRngCore::generate`] directly on the output array when
95/// [`fill_bytes`] / [`try_fill_bytes`] is called on a large array. These methods
96/// also handle the bookkeeping of when to generate a new batch of values.
97///
98/// No whole generated `u32` values are thown away and all values are consumed
99/// in-order. [`next_u32`] simply takes the next available `u32` value.
100/// [`next_u64`] is implemented by combining two `u32` values, least
101/// significant first. [`fill_bytes`] and [`try_fill_bytes`] consume a whole
102/// number of `u32` values, converting each `u32` to a byte slice in
103/// little-endian order. If the requested byte length is not a multiple of 4,
104/// some bytes will be discarded.
105///
106/// See also [`BlockRng64`] which uses `u64` array buffers. Currently there is
107/// no direct support for other buffer types.
108///
109/// For easy initialization `BlockRng` also implements [`SeedableRng`].
110///
111/// [`next_u32`]: RngCore::next_u32
112/// [`next_u64`]: RngCore::next_u64
113/// [`fill_bytes`]: RngCore::fill_bytes
114/// [`try_fill_bytes`]: RngCore::try_fill_bytes
115#[derive(Clone)]
116#[cfg_attr(feature = "serde1", derive(Serialize, Deserialize))]
117#[cfg_attr(
118    feature = "serde1",
119    serde(
120        bound = "for<'x> R: Serialize + Deserialize<'x> + Sized, for<'x> R::Results: Serialize + Deserialize<'x>"
121    )
122)]
123pub struct BlockRng<R: BlockRngCore + ?Sized> {
124    results: R::Results,
125    index: usize,
126    /// The *core* part of the RNG, implementing the `generate` function.
127    pub core: R,
128}
129
130// Custom Debug implementation that does not expose the contents of `results`.
131impl<R: BlockRngCore + fmt::Debug> fmt::Debug for BlockRng<R> {
132    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
133        fmt.debug_struct("BlockRng")
134            .field("core", &self.core)
135            .field("result_len", &self.results.as_ref().len())
136            .field("index", &self.index)
137            .finish()
138    }
139}
140
141impl<R: BlockRngCore> BlockRng<R> {
142    /// Create a new `BlockRng` from an existing RNG implementing
143    /// `BlockRngCore`. Results will be generated on first use.
144    #[inline]
145    pub fn new(core: R) -> BlockRng<R> {
146        let results_empty = R::Results::default();
147        BlockRng {
148            core,
149            index: results_empty.as_ref().len(),
150            results: results_empty,
151        }
152    }
153
154    /// Get the index into the result buffer.
155    ///
156    /// If this is equal to or larger than the size of the result buffer then
157    /// the buffer is "empty" and `generate()` must be called to produce new
158    /// results.
159    #[inline(always)]
160    pub fn index(&self) -> usize {
161        self.index
162    }
163
164    /// Reset the number of available results.
165    /// This will force a new set of results to be generated on next use.
166    #[inline]
167    pub fn reset(&mut self) {
168        self.index = self.results.as_ref().len();
169    }
170
171    /// Generate a new set of results immediately, setting the index to the
172    /// given value.
173    #[inline]
174    pub fn generate_and_set(&mut self, index: usize) {
175        assert!(index < self.results.as_ref().len());
176        self.core.generate(&mut self.results);
177        self.index = index;
178    }
179}
180
181impl<R: BlockRngCore<Item = u32>> RngCore for BlockRng<R>
182where
183    <R as BlockRngCore>::Results: AsRef<[u32]> + AsMut<[u32]>,
184{
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_u32_chunks(&self.results.as_ref()[self.index..], &mut dest[read_len..]);
230
231            self.index += consumed_u32;
232            read_len += filled_u8;
233        }
234    }
235
236    #[inline(always)]
237    fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Error> {
238        self.fill_bytes(dest);
239        Ok(())
240    }
241}
242
243impl<R: BlockRngCore + SeedableRng> SeedableRng for BlockRng<R> {
244    type Seed = R::Seed;
245
246    #[inline(always)]
247    fn from_seed(seed: Self::Seed) -> Self {
248        Self::new(R::from_seed(seed))
249    }
250
251    #[inline(always)]
252    fn seed_from_u64(seed: u64) -> Self {
253        Self::new(R::seed_from_u64(seed))
254    }
255
256    #[inline(always)]
257    fn from_rng<S: RngCore>(rng: S) -> Result<Self, Error> {
258        Ok(Self::new(R::from_rng(rng)?))
259    }
260}
261
262/// A wrapper type implementing [`RngCore`] for some type implementing
263/// [`BlockRngCore`] with `u64` array buffer; i.e. this can be used to implement
264/// a full RNG from just a `generate` function.
265///
266/// This is similar to [`BlockRng`], but specialized for algorithms that operate
267/// on `u64` values.
268///
269/// No whole generated `u64` values are thrown away and all values are consumed
270/// in-order. [`next_u64`] simply takes the next available `u64` value.
271/// [`next_u32`] is however a bit special: half of a `u64` is consumed, leaving
272/// the other half in the buffer. If the next function called is [`next_u32`]
273/// then the other half is then consumed, however both [`next_u64`] and
274/// [`fill_bytes`] discard the rest of any half-consumed `u64`s when called.
275///
276/// [`fill_bytes`] and [`try_fill_bytes`] consume a whole number of `u64`
277/// values. If the requested length is not a multiple of 8, some bytes will be
278/// discarded.
279///
280/// [`next_u32`]: RngCore::next_u32
281/// [`next_u64`]: RngCore::next_u64
282/// [`fill_bytes`]: RngCore::fill_bytes
283/// [`try_fill_bytes`]: RngCore::try_fill_bytes
284#[derive(Clone)]
285#[cfg_attr(feature = "serde1", derive(Serialize, Deserialize))]
286pub struct BlockRng64<R: BlockRngCore + ?Sized> {
287    results: R::Results,
288    index: usize,
289    half_used: bool, // true if only half of the previous result is used
290    /// The *core* part of the RNG, implementing the `generate` function.
291    pub core: R,
292}
293
294// Custom Debug implementation that does not expose the contents of `results`.
295impl<R: BlockRngCore + fmt::Debug> fmt::Debug for BlockRng64<R> {
296    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
297        fmt.debug_struct("BlockRng64")
298            .field("core", &self.core)
299            .field("result_len", &self.results.as_ref().len())
300            .field("index", &self.index)
301            .field("half_used", &self.half_used)
302            .finish()
303    }
304}
305
306impl<R: BlockRngCore> BlockRng64<R> {
307    /// Create a new `BlockRng` from an existing RNG implementing
308    /// `BlockRngCore`. Results will be generated on first use.
309    #[inline]
310    pub fn new(core: R) -> BlockRng64<R> {
311        let results_empty = R::Results::default();
312        BlockRng64 {
313            core,
314            index: results_empty.as_ref().len(),
315            half_used: false,
316            results: results_empty,
317        }
318    }
319
320    /// Get the index into the result buffer.
321    ///
322    /// If this is equal to or larger than the size of the result buffer then
323    /// the buffer is "empty" and `generate()` must be called to produce new
324    /// results.
325    #[inline(always)]
326    pub fn index(&self) -> usize {
327        self.index
328    }
329
330    /// Reset the number of available results.
331    /// This will force a new set of results to be generated on next use.
332    #[inline]
333    pub fn reset(&mut self) {
334        self.index = self.results.as_ref().len();
335        self.half_used = false;
336    }
337
338    /// Generate a new set of results immediately, setting the index to the
339    /// given value.
340    #[inline]
341    pub fn generate_and_set(&mut self, index: usize) {
342        assert!(index < self.results.as_ref().len());
343        self.core.generate(&mut self.results);
344        self.index = index;
345        self.half_used = false;
346    }
347}
348
349impl<R: BlockRngCore<Item = u64>> RngCore for BlockRng64<R>
350where
351    <R as BlockRngCore>::Results: AsRef<[u64]> + AsMut<[u64]>,
352{
353    #[inline]
354    fn next_u32(&mut self) -> u32 {
355        let mut index = self.index * 2 - self.half_used as usize;
356        if index >= self.results.as_ref().len() * 2 {
357            self.core.generate(&mut self.results);
358            self.index = 0;
359            // `self.half_used` is by definition `false`
360            self.half_used = false;
361            index = 0;
362        }
363
364        self.half_used = !self.half_used;
365        self.index += self.half_used as usize;
366
367        // Index as if this is a u32 slice.
368        unsafe {
369            let results = &*(self.results.as_ref() as *const [u64] as *const [u32]);
370            if cfg!(target_endian = "little") {
371                *results.get_unchecked(index)
372            } else {
373                *results.get_unchecked(index ^ 1)
374            }
375        }
376    }
377
378    #[inline]
379    fn next_u64(&mut self) -> u64 {
380        if self.index >= self.results.as_ref().len() {
381            self.core.generate(&mut self.results);
382            self.index = 0;
383        }
384
385        let value = self.results.as_ref()[self.index];
386        self.index += 1;
387        self.half_used = false;
388        value
389    }
390
391    #[inline]
392    fn fill_bytes(&mut self, dest: &mut [u8]) {
393        let mut read_len = 0;
394        self.half_used = false;
395        while read_len < dest.len() {
396            if self.index as usize >= self.results.as_ref().len() {
397                self.core.generate(&mut self.results);
398                self.index = 0;
399            }
400
401            let (consumed_u64, filled_u8) = fill_via_u64_chunks(
402                &self.results.as_ref()[self.index as usize..],
403                &mut dest[read_len..],
404            );
405
406            self.index += consumed_u64;
407            read_len += filled_u8;
408        }
409    }
410
411    #[inline(always)]
412    fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Error> {
413        self.fill_bytes(dest);
414        Ok(())
415    }
416}
417
418impl<R: BlockRngCore + SeedableRng> SeedableRng for BlockRng64<R> {
419    type Seed = R::Seed;
420
421    #[inline(always)]
422    fn from_seed(seed: Self::Seed) -> Self {
423        Self::new(R::from_seed(seed))
424    }
425
426    #[inline(always)]
427    fn seed_from_u64(seed: u64) -> Self {
428        Self::new(R::seed_from_u64(seed))
429    }
430
431    #[inline(always)]
432    fn from_rng<S: RngCore>(rng: S) -> Result<Self, Error> {
433        Ok(Self::new(R::from_rng(rng)?))
434    }
435}
436
437impl<R: BlockRngCore + CryptoRng> CryptoRng for BlockRng<R> {}
rand_core/block.rs

rand_core/
block.rs
