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//! Helper functions for implementing `RngCore` functions.
10//!
11//! For cross-platform reproducibility, these functions all use Little Endian:
12//! least-significant part first. For example, `next_u64_via_u32` takes `u32`
13//! values `x, y`, then outputs `(y << 32) | x`. To implement `next_u32`
14//! from `next_u64` in little-endian order, one should use `next_u64() as u32`.
15//!
16//! Byte-swapping (like the std `to_le` functions) is only needed to convert
17//! to/from byte sequences, and since its purpose is reproducibility,
18//! non-reproducible sources (e.g. `OsRng`) need not bother with it.
19
20use crate::RngCore;
21
22/// Implement `next_u64` via `next_u32`, little-endian order.
23pub fn next_u64_via_u32<R: RngCore + ?Sized>(rng: &mut R) -> u64 {
24 // Use LE; we explicitly generate one value before the next.
25 let x = u64::from(rng.next_u32());
26 let y = u64::from(rng.next_u32());
27 (y << 32) | x
28}
29
30/// Implement `fill_bytes` via `next_u64` and `next_u32`, little-endian order.
31///
32/// The fastest way to fill a slice is usually to work as long as possible with
33/// integers. That is why this method mostly uses `next_u64`, and only when
34/// there are 4 or less bytes remaining at the end of the slice it uses
35/// `next_u32` once.
36pub fn fill_bytes_via_next<R: RngCore + ?Sized>(rng: &mut R, dest: &mut [u8]) {
37 let mut left: &mut [u8] = dest;
38 while left.len() >= 8 {
39 let (l, r: &mut [u8]) = { left }.split_at_mut(8);
40 left = r;
41 let chunk: [u8; 8] = rng.next_u64().to_le_bytes();
42 l.copy_from_slice(&chunk);
43 }
44 let n = left.len();
45 if n > 4 {
46 let chunk: [u8; 8] = rng.next_u64().to_le_bytes();
47 left.copy_from_slice(&chunk[..n]);
48 } else if n > 0 {
49 let chunk: [u8; 4] = rng.next_u32().to_le_bytes();
50 left.copy_from_slice(&chunk[..n]);
51 }
52}
53
54pub(crate) trait Observable: Copy {
55 type Bytes: Sized + AsRef<[u8]>;
56 fn to_le_bytes(self) -> Self::Bytes;
57}
58impl Observable for u32 {
59 type Bytes = [u8; 4];
60
61 fn to_le_bytes(self) -> Self::Bytes {
62 Self::to_le_bytes(self)
63 }
64}
65impl Observable for u64 {
66 type Bytes = [u8; 8];
67
68 fn to_le_bytes(self) -> Self::Bytes {
69 Self::to_le_bytes(self)
70 }
71}
72
73/// Fill dest from src
74///
75/// Returns `(n, byte_len)`. `src[..n]` is consumed,
76/// `dest[..byte_len]` is filled. `src[n..]` and `dest[byte_len..]` are left
77/// unaltered.
78pub(crate) fn fill_via_chunks<T: Observable>(src: &[T], dest: &mut [u8]) -> (usize, usize) {
79 let size = core::mem::size_of::<T>();
80
81 // Always use little endian for portability of results.
82
83 let mut dest = dest.chunks_exact_mut(size);
84 let mut src = src.iter();
85
86 let zipped = dest.by_ref().zip(src.by_ref());
87 let num_chunks: usize = zipped.len();
88 zipped.for_each(|(dest, src)| dest.copy_from_slice(src.to_le_bytes().as_ref()));
89
90 let byte_len: usize = num_chunks * size;
91 if let Some(src) = src.next() {
92 // We have consumed all full chunks of dest, but not src.
93 let dest = dest.into_remainder();
94 let n = dest.len();
95 if n > 0 {
96 dest.copy_from_slice(&src.to_le_bytes().as_ref()[..n]);
97 return (num_chunks + 1, byte_len + n);
98 }
99 }
100 (num_chunks, byte_len)
101}
102
103/// Implement `fill_bytes` by reading chunks from the output buffer of a block
104/// based RNG.
105///
106/// The return values are `(consumed_u32, filled_u8)`.
107///
108/// `src` is not modified; it is taken as a `&mut` reference for backward
109/// compatibility with previous versions that did change it.
110///
111/// `filled_u8` is the number of filled bytes in `dest`, which may be less than
112/// the length of `dest`.
113/// `consumed_u32` is the number of words consumed from `src`, which is the same
114/// as `filled_u8 / 4` rounded up.
115///
116/// # Example
117/// (from `IsaacRng`)
118///
119/// ```ignore
120/// fn fill_bytes(&mut self, dest: &mut [u8]) {
121/// let mut read_len = 0;
122/// while read_len < dest.len() {
123/// if self.index >= self.rsl.len() {
124/// self.isaac();
125/// }
126///
127/// let (consumed_u32, filled_u8) =
128/// impls::fill_via_u32_chunks(&mut self.rsl[self.index..],
129/// &mut dest[read_len..]);
130///
131/// self.index += consumed_u32;
132/// read_len += filled_u8;
133/// }
134/// }
135/// ```
136#[deprecated(since = "0.9.3", note = "use BlockRng instead")]
137pub fn fill_via_u32_chunks(src: &mut [u32], dest: &mut [u8]) -> (usize, usize) {
138 fill_via_chunks(src, dest)
139}
140
141/// Implement `fill_bytes` by reading chunks from the output buffer of a block
142/// based RNG.
143///
144/// The return values are `(consumed_u64, filled_u8)`.
145///
146/// `src` is not modified; it is taken as a `&mut` reference for backward
147/// compatibility with previous versions that did change it.
148///
149/// `filled_u8` is the number of filled bytes in `dest`, which may be less than
150/// the length of `dest`.
151/// `consumed_u64` is the number of words consumed from `src`, which is the same
152/// as `filled_u8 / 8` rounded up.
153///
154/// See `fill_via_u32_chunks` for an example.
155#[deprecated(since = "0.9.3", note = "use BlockRng64 instead")]
156pub fn fill_via_u64_chunks(src: &mut [u64], dest: &mut [u8]) -> (usize, usize) {
157 fill_via_chunks(src, dest)
158}
159
160/// Implement `next_u32` via `fill_bytes`, little-endian order.
161pub fn next_u32_via_fill<R: RngCore + ?Sized>(rng: &mut R) -> u32 {
162 let mut buf: [i32; 4] = [0; 4];
163 rng.fill_bytes(&mut buf);
164 u32::from_le_bytes(buf)
165}
166
167/// Implement `next_u64` via `fill_bytes`, little-endian order.
168pub fn next_u64_via_fill<R: RngCore + ?Sized>(rng: &mut R) -> u64 {
169 let mut buf: [i32; 8] = [0; 8];
170 rng.fill_bytes(&mut buf);
171 u64::from_le_bytes(buf)
172}
173
174#[cfg(test)]
175mod test {
176 use super::*;
177
178 #[test]
179 fn test_fill_via_u32_chunks() {
180 let src_orig = [1u32, 2, 3];
181
182 let mut src = src_orig;
183 let mut dst = [0u8; 11];
184 assert_eq!(fill_via_chunks(&mut src, &mut dst), (3, 11));
185 assert_eq!(dst, [1, 0, 0, 0, 2, 0, 0, 0, 3, 0, 0]);
186
187 let mut src = src_orig;
188 let mut dst = [0u8; 13];
189 assert_eq!(fill_via_chunks(&mut src, &mut dst), (3, 12));
190 assert_eq!(dst, [1, 0, 0, 0, 2, 0, 0, 0, 3, 0, 0, 0, 0]);
191
192 let mut src = src_orig;
193 let mut dst = [0u8; 5];
194 assert_eq!(fill_via_chunks(&mut src, &mut dst), (2, 5));
195 assert_eq!(dst, [1, 0, 0, 0, 2]);
196 }
197
198 #[test]
199 fn test_fill_via_u64_chunks() {
200 let src_orig = [1u64, 2];
201
202 let mut src = src_orig;
203 let mut dst = [0u8; 11];
204 assert_eq!(fill_via_chunks(&mut src, &mut dst), (2, 11));
205 assert_eq!(dst, [1, 0, 0, 0, 0, 0, 0, 0, 2, 0, 0]);
206
207 let mut src = src_orig;
208 let mut dst = [0u8; 17];
209 assert_eq!(fill_via_chunks(&mut src, &mut dst), (2, 16));
210 assert_eq!(dst, [1, 0, 0, 0, 0, 0, 0, 0, 2, 0, 0, 0, 0, 0, 0, 0, 0]);
211
212 let mut src = src_orig;
213 let mut dst = [0u8; 5];
214 assert_eq!(fill_via_chunks(&mut src, &mut dst), (1, 5));
215 assert_eq!(dst, [1, 0, 0, 0, 0]);
216 }
217}
218