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
20 use crate::RngCore;
21 use core::cmp::min;
22
23 /// Implement `next_u64` via `next_u32`, little-endian order.
next_u64_via_u32<R: RngCore + ?Sized>(rng: &mut R) -> u6424 pub fn next_u64_via_u32<R: RngCore + ?Sized>(rng: &mut R) -> u64 {
25 // Use LE; we explicitly generate one value before the next.
26 let x = u64::from(rng.next_u32());
27 let y = u64::from(rng.next_u32());
28 (y << 32) | x
29 }
30
31 /// Implement `fill_bytes` via `next_u64` and `next_u32`, little-endian order.
32 ///
33 /// The fastest way to fill a slice is usually to work as long as possible with
34 /// integers. That is why this method mostly uses `next_u64`, and only when
35 /// there are 4 or less bytes remaining at the end of the slice it uses
36 /// `next_u32` once.
fill_bytes_via_next<R: RngCore + ?Sized>(rng: &mut R, dest: &mut [u8])37 pub fn fill_bytes_via_next<R: RngCore + ?Sized>(rng: &mut R, dest: &mut [u8]) {
38 let mut left = dest;
39 while left.len() >= 8 {
40 let (l, r) = { left }.split_at_mut(8);
41 left = r;
42 let chunk: [u8; 8] = rng.next_u64().to_le_bytes();
43 l.copy_from_slice(&chunk);
44 }
45 let n = left.len();
46 if n > 4 {
47 let chunk: [u8; 8] = rng.next_u64().to_le_bytes();
48 left.copy_from_slice(&chunk[..n]);
49 } else if n > 0 {
50 let chunk: [u8; 4] = rng.next_u32().to_le_bytes();
51 left.copy_from_slice(&chunk[..n]);
52 }
53 }
54
55 macro_rules! fill_via_chunks {
56 ($src:expr, $dst:expr, $ty:ty) => {{
57 const SIZE: usize = core::mem::size_of::<$ty>();
58 let chunk_size_u8 = min($src.len() * SIZE, $dst.len());
59 let chunk_size = (chunk_size_u8 + SIZE - 1) / SIZE;
60
61 // The following can be replaced with safe code, but unfortunately it's
62 // ca. 8% slower.
63 if cfg!(target_endian = "little") {
64 unsafe {
65 core::ptr::copy_nonoverlapping(
66 $src.as_ptr() as *const u8,
67 $dst.as_mut_ptr(),
68 chunk_size_u8);
69 }
70 } else {
71 for (&n, chunk) in $src.iter().zip($dst.chunks_mut(SIZE)) {
72 let tmp = n.to_le();
73 let src_ptr = &tmp as *const $ty as *const u8;
74 unsafe {
75 core::ptr::copy_nonoverlapping(
76 src_ptr,
77 chunk.as_mut_ptr(),
78 chunk.len());
79 }
80 }
81 }
82
83 (chunk_size, chunk_size_u8)
84 }};
85 }
86
87 /// Implement `fill_bytes` by reading chunks from the output buffer of a block
88 /// based RNG.
89 ///
90 /// The return values are `(consumed_u32, filled_u8)`.
91 ///
92 /// `filled_u8` is the number of filled bytes in `dest`, which may be less than
93 /// the length of `dest`.
94 /// `consumed_u32` is the number of words consumed from `src`, which is the same
95 /// as `filled_u8 / 4` rounded up.
96 ///
97 /// # Example
98 /// (from `IsaacRng`)
99 ///
100 /// ```ignore
101 /// fn fill_bytes(&mut self, dest: &mut [u8]) {
102 /// let mut read_len = 0;
103 /// while read_len < dest.len() {
104 /// if self.index >= self.rsl.len() {
105 /// self.isaac();
106 /// }
107 ///
108 /// let (consumed_u32, filled_u8) =
109 /// impls::fill_via_u32_chunks(&mut self.rsl[self.index..],
110 /// &mut dest[read_len..]);
111 ///
112 /// self.index += consumed_u32;
113 /// read_len += filled_u8;
114 /// }
115 /// }
116 /// ```
fill_via_u32_chunks(src: &[u32], dest: &mut [u8]) -> (usize, usize)117 pub fn fill_via_u32_chunks(src: &[u32], dest: &mut [u8]) -> (usize, usize) {
118 fill_via_chunks!(src, dest, u32)
119 }
120
121 /// Implement `fill_bytes` by reading chunks from the output buffer of a block
122 /// based RNG.
123 ///
124 /// The return values are `(consumed_u64, filled_u8)`.
125 /// `filled_u8` is the number of filled bytes in `dest`, which may be less than
126 /// the length of `dest`.
127 /// `consumed_u64` is the number of words consumed from `src`, which is the same
128 /// as `filled_u8 / 8` rounded up.
129 ///
130 /// See `fill_via_u32_chunks` for an example.
fill_via_u64_chunks(src: &[u64], dest: &mut [u8]) -> (usize, usize)131 pub fn fill_via_u64_chunks(src: &[u64], dest: &mut [u8]) -> (usize, usize) {
132 fill_via_chunks!(src, dest, u64)
133 }
134
135 /// Implement `next_u32` via `fill_bytes`, little-endian order.
next_u32_via_fill<R: RngCore + ?Sized>(rng: &mut R) -> u32136 pub fn next_u32_via_fill<R: RngCore + ?Sized>(rng: &mut R) -> u32 {
137 let mut buf = [0; 4];
138 rng.fill_bytes(&mut buf);
139 u32::from_le_bytes(buf)
140 }
141
142 /// Implement `next_u64` via `fill_bytes`, little-endian order.
next_u64_via_fill<R: RngCore + ?Sized>(rng: &mut R) -> u64143 pub fn next_u64_via_fill<R: RngCore + ?Sized>(rng: &mut R) -> u64 {
144 let mut buf = [0; 8];
145 rng.fill_bytes(&mut buf);
146 u64::from_le_bytes(buf)
147 }
148
149 #[cfg(test)]
150 mod test {
151 use super::*;
152
153 #[test]
test_fill_via_u32_chunks()154 fn test_fill_via_u32_chunks() {
155 let src = [1, 2, 3];
156 let mut dst = [0u8; 11];
157 assert_eq!(fill_via_u32_chunks(&src, &mut dst), (3, 11));
158 assert_eq!(dst, [1, 0, 0, 0, 2, 0, 0, 0, 3, 0, 0]);
159
160 let mut dst = [0u8; 13];
161 assert_eq!(fill_via_u32_chunks(&src, &mut dst), (3, 12));
162 assert_eq!(dst, [1, 0, 0, 0, 2, 0, 0, 0, 3, 0, 0, 0, 0]);
163
164 let mut dst = [0u8; 5];
165 assert_eq!(fill_via_u32_chunks(&src, &mut dst), (2, 5));
166 assert_eq!(dst, [1, 0, 0, 0, 2]);
167 }
168
169 #[test]
test_fill_via_u64_chunks()170 fn test_fill_via_u64_chunks() {
171 let src = [1, 2];
172 let mut dst = [0u8; 11];
173 assert_eq!(fill_via_u64_chunks(&src, &mut dst), (2, 11));
174 assert_eq!(dst, [1, 0, 0, 0, 0, 0, 0, 0, 2, 0, 0]);
175
176 let mut dst = [0u8; 17];
177 assert_eq!(fill_via_u64_chunks(&src, &mut dst), (2, 16));
178 assert_eq!(dst, [1, 0, 0, 0, 0, 0, 0, 0, 2, 0, 0, 0, 0, 0, 0, 0, 0]);
179
180 let mut dst = [0u8; 5];
181 assert_eq!(fill_via_u64_chunks(&src, &mut dst), (1, 5));
182 assert_eq!(dst, [1, 0, 0, 0, 0]);
183 }
184 }
185