1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
// Copyright 2018 Developers of the Rand project.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

//! The `BlockRngCore` trait and implementation helpers
//!
//! The [`BlockRngCore`] trait exists to assist in the implementation of RNGs
//! which generate a block of data in a cache instead of returning generated
//! values directly.
//!
//! Usage of this trait is optional, but provides two advantages:
//! implementations only need to concern themselves with generation of the
//! block, not the various [`RngCore`] methods (especially [`fill_bytes`], where
//! the optimal implementations are not trivial), and this allows
//! `ReseedingRng` (see [`rand`](https://docs.rs/rand) crate) perform periodic
//! reseeding with very low overhead.
//!
//! # Example
//!
//! ```no_run
//! use rand_core::{RngCore, SeedableRng};
//! use rand_core::block::{BlockRngCore, BlockRng};
//!
//! struct MyRngCore;
//!
//! impl BlockRngCore for MyRngCore {
//!     type Item = u32;
//!     type Results = [u32; 16];
//!
//!     fn generate(&mut self, results: &mut Self::Results) {
//!         unimplemented!()
//!     }
//! }
//!
//! impl SeedableRng for MyRngCore {
//!     type Seed = [u8; 32];
//!     fn from_seed(seed: Self::Seed) -> Self {
//!         unimplemented!()
//!     }
//! }
//!
//! // optionally, also implement CryptoRng for MyRngCore
//!
//! // Final RNG.
//! let mut rng = BlockRng::<MyRngCore>::seed_from_u64(0);
//! println!("First value: {}", rng.next_u32());
//! ```
//!
//! [`BlockRngCore`]: crate::block::BlockRngCore
//! [`fill_bytes`]: RngCore::fill_bytes

use crate::impls::{fill_via_u32_chunks, fill_via_u64_chunks};
use crate::{CryptoRng, Error, RngCore, SeedableRng};
use core::convert::AsRef;
use core::fmt;
#[cfg(feature = "serde1")]
use serde::{Deserialize, Serialize};

/// A trait for RNGs which do not generate random numbers individually, but in
/// blocks (typically `[u32; N]`). This technique is commonly used by
/// cryptographic RNGs to improve performance.
///
/// See the [module][crate::block] documentation for details.
pub trait BlockRngCore {
    /// Results element type, e.g. `u32`.
    type Item;

    /// Results type. This is the 'block' an RNG implementing `BlockRngCore`
    /// generates, which will usually be an array like `[u32; 16]`.
    type Results: AsRef<[Self::Item]> + AsMut<[Self::Item]> + Default;

    /// Generate a new block of results.
    fn generate(&mut self, results: &mut Self::Results);
}

/// A wrapper type implementing [`RngCore`] for some type implementing
/// [`BlockRngCore`] with `u32` array buffer; i.e. this can be used to implement
/// a full RNG from just a `generate` function.
///
/// The `core` field may be accessed directly but the results buffer may not.
/// PRNG implementations can simply use a type alias
/// (`pub type MyRng = BlockRng<MyRngCore>;`) but might prefer to use a
/// wrapper type (`pub struct MyRng(BlockRng<MyRngCore>);`); the latter must
/// re-implement `RngCore` but hides the implementation details and allows
/// extra functionality to be defined on the RNG
/// (e.g. `impl MyRng { fn set_stream(...){...} }`).
///
/// `BlockRng` has heavily optimized implementations of the [`RngCore`] methods
/// reading values from the results buffer, as well as
/// calling [`BlockRngCore::generate`] directly on the output array when
/// [`fill_bytes`] / [`try_fill_bytes`] is called on a large array. These methods
/// also handle the bookkeeping of when to generate a new batch of values.
///
/// No whole generated `u32` values are thown away and all values are consumed
/// in-order. [`next_u32`] simply takes the next available `u32` value.
/// [`next_u64`] is implemented by combining two `u32` values, least
/// significant first. [`fill_bytes`] and [`try_fill_bytes`] consume a whole
/// number of `u32` values, converting each `u32` to a byte slice in
/// little-endian order. If the requested byte length is not a multiple of 4,
/// some bytes will be discarded.
///
/// See also [`BlockRng64`] which uses `u64` array buffers. Currently there is
/// no direct support for other buffer types.
///
/// For easy initialization `BlockRng` also implements [`SeedableRng`].
///
/// [`next_u32`]: RngCore::next_u32
/// [`next_u64`]: RngCore::next_u64
/// [`fill_bytes`]: RngCore::fill_bytes
/// [`try_fill_bytes`]: RngCore::try_fill_bytes
#[derive(Clone)]
#[cfg_attr(feature = "serde1", derive(Serialize, Deserialize))]
#[cfg_attr(
    feature = "serde1",
    serde(
        bound = "for<'x> R: Serialize + Deserialize<'x> + Sized, for<'x> R::Results: Serialize + Deserialize<'x>"
    )
)]
pub struct BlockRng<R: BlockRngCore + ?Sized> {
    results: R::Results,
    index: usize,
    /// The *core* part of the RNG, implementing the `generate` function.
    pub core: R,
}

// Custom Debug implementation that does not expose the contents of `results`.
impl<R: BlockRngCore + fmt::Debug> fmt::Debug for BlockRng<R> {
    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
        fmt.debug_struct("BlockRng")
            .field("core", &self.core)
            .field("result_len", &self.results.as_ref().len())
            .field("index", &self.index)
            .finish()
    }
}

impl<R: BlockRngCore> BlockRng<R> {
    /// Create a new `BlockRng` from an existing RNG implementing
    /// `BlockRngCore`. Results will be generated on first use.
    #[inline]
    pub fn new(core: R) -> BlockRng<R> {
        let results_empty = R::Results::default();
        BlockRng {
            core,
            index: results_empty.as_ref().len(),
            results: results_empty,
        }
    }

    /// Get the index into the result buffer.
    ///
    /// If this is equal to or larger than the size of the result buffer then
    /// the buffer is "empty" and `generate()` must be called to produce new
    /// results.
    #[inline(always)]
    pub fn index(&self) -> usize {
        self.index
    }

    /// Reset the number of available results.
    /// This will force a new set of results to be generated on next use.
    #[inline]
    pub fn reset(&mut self) {
        self.index = self.results.as_ref().len();
    }

    /// Generate a new set of results immediately, setting the index to the
    /// given value.
    #[inline]
    pub fn generate_and_set(&mut self, index: usize) {
        assert!(index < self.results.as_ref().len());
        self.core.generate(&mut self.results);
        self.index = index;
    }
}

impl<R: BlockRngCore<Item = u32>> RngCore for BlockRng<R>
where
    <R as BlockRngCore>::Results: AsRef<[u32]> + AsMut<[u32]>,
{
    #[inline]
    fn next_u32(&mut self) -> u32 {
        if self.index >= self.results.as_ref().len() {
            self.generate_and_set(0);
        }

        let value = self.results.as_ref()[self.index];
        self.index += 1;
        value
    }

    #[inline]
    fn next_u64(&mut self) -> u64 {
        let read_u64 = |results: &[u32], index| {
            let data = &results[index..=index + 1];
            u64::from(data[1]) << 32 | u64::from(data[0])
        };

        let len = self.results.as_ref().len();

        let index = self.index;
        if index < len - 1 {
            self.index += 2;
            // Read an u64 from the current index
            read_u64(self.results.as_ref(), index)
        } else if index >= len {
            self.generate_and_set(2);
            read_u64(self.results.as_ref(), 0)
        } else {
            let x = u64::from(self.results.as_ref()[len - 1]);
            self.generate_and_set(1);
            let y = u64::from(self.results.as_ref()[0]);
            (y << 32) | x
        }
    }

    #[inline]
    fn fill_bytes(&mut self, dest: &mut [u8]) {
        let mut read_len = 0;
        while read_len < dest.len() {
            if self.index >= self.results.as_ref().len() {
                self.generate_and_set(0);
            }
            let (consumed_u32, filled_u8) =
                fill_via_u32_chunks(&self.results.as_ref()[self.index..], &mut dest[read_len..]);

            self.index += consumed_u32;
            read_len += filled_u8;
        }
    }

    #[inline(always)]
    fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Error> {
        self.fill_bytes(dest);
        Ok(())
    }
}

impl<R: BlockRngCore + SeedableRng> SeedableRng for BlockRng<R> {
    type Seed = R::Seed;

    #[inline(always)]
    fn from_seed(seed: Self::Seed) -> Self {
        Self::new(R::from_seed(seed))
    }

    #[inline(always)]
    fn seed_from_u64(seed: u64) -> Self {
        Self::new(R::seed_from_u64(seed))
    }

    #[inline(always)]
    fn from_rng<S: RngCore>(rng: S) -> Result<Self, Error> {
        Ok(Self::new(R::from_rng(rng)?))
    }
}

/// A wrapper type implementing [`RngCore`] for some type implementing
/// [`BlockRngCore`] with `u64` array buffer; i.e. this can be used to implement
/// a full RNG from just a `generate` function.
///
/// This is similar to [`BlockRng`], but specialized for algorithms that operate
/// on `u64` values.
///
/// No whole generated `u64` values are thrown away and all values are consumed
/// in-order. [`next_u64`] simply takes the next available `u64` value.
/// [`next_u32`] is however a bit special: half of a `u64` is consumed, leaving
/// the other half in the buffer. If the next function called is [`next_u32`]
/// then the other half is then consumed, however both [`next_u64`] and
/// [`fill_bytes`] discard the rest of any half-consumed `u64`s when called.
///
/// [`fill_bytes`] and [`try_fill_bytes`] consume a whole number of `u64`
/// values. If the requested length is not a multiple of 8, some bytes will be
/// discarded.
///
/// [`next_u32`]: RngCore::next_u32
/// [`next_u64`]: RngCore::next_u64
/// [`fill_bytes`]: RngCore::fill_bytes
/// [`try_fill_bytes`]: RngCore::try_fill_bytes
#[derive(Clone)]
#[cfg_attr(feature = "serde1", derive(Serialize, Deserialize))]
pub struct BlockRng64<R: BlockRngCore + ?Sized> {
    results: R::Results,
    index: usize,
    half_used: bool, // true if only half of the previous result is used
    /// The *core* part of the RNG, implementing the `generate` function.
    pub core: R,
}

// Custom Debug implementation that does not expose the contents of `results`.
impl<R: BlockRngCore + fmt::Debug> fmt::Debug for BlockRng64<R> {
    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
        fmt.debug_struct("BlockRng64")
            .field("core", &self.core)
            .field("result_len", &self.results.as_ref().len())
            .field("index", &self.index)
            .field("half_used", &self.half_used)
            .finish()
    }
}

impl<R: BlockRngCore> BlockRng64<R> {
    /// Create a new `BlockRng` from an existing RNG implementing
    /// `BlockRngCore`. Results will be generated on first use.
    #[inline]
    pub fn new(core: R) -> BlockRng64<R> {
        let results_empty = R::Results::default();
        BlockRng64 {
            core,
            index: results_empty.as_ref().len(),
            half_used: false,
            results: results_empty,
        }
    }

    /// Get the index into the result buffer.
    ///
    /// If this is equal to or larger than the size of the result buffer then
    /// the buffer is "empty" and `generate()` must be called to produce new
    /// results.
    #[inline(always)]
    pub fn index(&self) -> usize {
        self.index
    }

    /// Reset the number of available results.
    /// This will force a new set of results to be generated on next use.
    #[inline]
    pub fn reset(&mut self) {
        self.index = self.results.as_ref().len();
        self.half_used = false;
    }

    /// Generate a new set of results immediately, setting the index to the
    /// given value.
    #[inline]
    pub fn generate_and_set(&mut self, index: usize) {
        assert!(index < self.results.as_ref().len());
        self.core.generate(&mut self.results);
        self.index = index;
        self.half_used = false;
    }
}

impl<R: BlockRngCore<Item = u64>> RngCore for BlockRng64<R>
where
    <R as BlockRngCore>::Results: AsRef<[u64]> + AsMut<[u64]>,
{
    #[inline]
    fn next_u32(&mut self) -> u32 {
        let mut index = self.index * 2 - self.half_used as usize;
        if index >= self.results.as_ref().len() * 2 {
            self.core.generate(&mut self.results);
            self.index = 0;
            // `self.half_used` is by definition `false`
            self.half_used = false;
            index = 0;
        }

        self.half_used = !self.half_used;
        self.index += self.half_used as usize;

        // Index as if this is a u32 slice.
        unsafe {
            let results = &*(self.results.as_ref() as *const [u64] as *const [u32]);
            if cfg!(target_endian = "little") {
                *results.get_unchecked(index)
            } else {
                *results.get_unchecked(index ^ 1)
            }
        }
    }

    #[inline]
    fn next_u64(&mut self) -> u64 {
        if self.index >= self.results.as_ref().len() {
            self.core.generate(&mut self.results);
            self.index = 0;
        }

        let value = self.results.as_ref()[self.index];
        self.index += 1;
        self.half_used = false;
        value
    }

    #[inline]
    fn fill_bytes(&mut self, dest: &mut [u8]) {
        let mut read_len = 0;
        self.half_used = false;
        while read_len < dest.len() {
            if self.index as usize >= self.results.as_ref().len() {
                self.core.generate(&mut self.results);
                self.index = 0;
            }

            let (consumed_u64, filled_u8) = fill_via_u64_chunks(
                &self.results.as_ref()[self.index as usize..],
                &mut dest[read_len..],
            );

            self.index += consumed_u64;
            read_len += filled_u8;
        }
    }

    #[inline(always)]
    fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Error> {
        self.fill_bytes(dest);
        Ok(())
    }
}

impl<R: BlockRngCore + SeedableRng> SeedableRng for BlockRng64<R> {
    type Seed = R::Seed;

    #[inline(always)]
    fn from_seed(seed: Self::Seed) -> Self {
        Self::new(R::from_seed(seed))
    }

    #[inline(always)]
    fn seed_from_u64(seed: u64) -> Self {
        Self::new(R::seed_from_u64(seed))
    }

    #[inline(always)]
    fn from_rng<S: RngCore>(rng: S) -> Result<Self, Error> {
        Ok(Self::new(R::from_rng(rng)?))
    }
}

impl<R: BlockRngCore + CryptoRng> CryptoRng for BlockRng<R> {}