vendor/rand_core-0.5.1/src/impls.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 //! 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 core::ptr::copy_nonoverlapping;
  21 use core::slice;
  22 use core::cmp::min;
  23 use core::mem::size_of;
  24 use crate::RngCore;
  25
  26
  27 /// Implement `next_u64` via `next_u32`, little-endian order.
  28 pub fn next_u64_via_u32<R: RngCore + ?Sized>(rng: &mut R) -> u64 {
  29     // Use LE; we explicitly generate one value before the next.
  30     let x = u64::from(rng.next_u32());
  31     let y = u64::from(rng.next_u32());
  32     (y << 32) | x
  33 }
  34
  35 /// Implement `fill_bytes` via `next_u64` and `next_u32`, little-endian order.
  36 ///
  37 /// The fastest way to fill a slice is usually to work as long as possible with
  38 /// integers. That is why this method mostly uses `next_u64`, and only when
  39 /// there are 4 or less bytes remaining at the end of the slice it uses
  40 /// `next_u32` once.
  41 pub fn fill_bytes_via_next<R: RngCore + ?Sized>(rng: &mut R, dest: &mut [u8]) {
  42     let mut left = dest;
  43     while left.len() >= 8 {
  44         let (l, r) = {left}.split_at_mut(8);
  45         left = r;
  46         let chunk: [u8; 8] = rng.next_u64().to_le_bytes();
  47         l.copy_from_slice(&chunk);
  48     }
  49     let n = left.len();
  50     if n > 4 {
  51         let chunk: [u8; 8] = rng.next_u64().to_le_bytes();
  52         left.copy_from_slice(&chunk[..n]);
  53     } else if n > 0 {
  54         let chunk: [u8; 4] = rng.next_u32().to_le_bytes();
  55         left.copy_from_slice(&chunk[..n]);
  56     }
  57 }
  58
  59 macro_rules! impl_uint_from_fill {
  60     ($rng:expr, $ty:ty, $N:expr) => ({
  61         debug_assert!($N == size_of::<$ty>());
  62
  63         let mut int: $ty = 0;
  64         unsafe {
  65             let ptr = &mut int as *mut $ty as *mut u8;
  66             let slice = slice::from_raw_parts_mut(ptr, $N);
  67             $rng.fill_bytes(slice);
  68         }
  69         int
  70     });
  71 }
  72
  73 macro_rules! fill_via_chunks {
  74     ($src:expr, $dst:expr, $ty:ty, $size:expr) => ({
  75         let chunk_size_u8 = min($src.len() * $size, $dst.len());
  76         let chunk_size = (chunk_size_u8 + $size - 1) / $size;
  77         if cfg!(target_endian="little") {
  78             unsafe {
  79                 copy_nonoverlapping(
  80                     $src.as_ptr() as *const u8,
  81                     $dst.as_mut_ptr(),
  82                     chunk_size_u8);
  83             }
  84         } else {
  85             for (&n, chunk) in $src.iter().zip($dst.chunks_mut($size)) {
  86                 let tmp = n.to_le();
  87                 let src_ptr = &tmp as *const $ty as *const u8;
  88                 unsafe {
  89                     copy_nonoverlapping(src_ptr,
  90                                         chunk.as_mut_ptr(),
  91                                         chunk.len());
  92                 }
  93             }
  94         }
  95
  96         (chunk_size, chunk_size_u8)
  97     });
  98 }
  99
 100 /// Implement `fill_bytes` by reading chunks from the output buffer of a block
 101 /// based RNG.
 102 ///
 103 /// The return values are `(consumed_u32, filled_u8)`.
 104 ///
 105 /// `filled_u8` is the number of filled bytes in `dest`, which may be less than
 106 /// the length of `dest`.
 107 /// `consumed_u32` is the number of words consumed from `src`, which is the same
 108 /// as `filled_u8 / 4` rounded up.
 109 ///
 110 /// # Example
 111 /// (from `IsaacRng`)
 112 ///
 113 /// ```ignore
 114 /// fn fill_bytes(&mut self, dest: &mut [u8]) {
 115 ///     let mut read_len = 0;
 116 ///     while read_len < dest.len() {
 117 ///         if self.index >= self.rsl.len() {
 118 ///             self.isaac();
 119 ///         }
 120 ///
 121 ///         let (consumed_u32, filled_u8) =
 122 ///             impls::fill_via_u32_chunks(&mut self.rsl[self.index..],
 123 ///                                        &mut dest[read_len..]);
 124 ///
 125 ///         self.index += consumed_u32;
 126 ///         read_len += filled_u8;
 127 ///     }
 128 /// }
 129 /// ```
 130 pub fn fill_via_u32_chunks(src: &[u32], dest: &mut [u8]) -> (usize, usize) {
 131     fill_via_chunks!(src, dest, u32, 4)
 132 }
 133
 134 /// Implement `fill_bytes` by reading chunks from the output buffer of a block
 135 /// based RNG.
 136 ///
 137 /// The return values are `(consumed_u64, filled_u8)`.
 138 /// `filled_u8` is the number of filled bytes in `dest`, which may be less than
 139 /// the length of `dest`.
 140 /// `consumed_u64` is the number of words consumed from `src`, which is the same
 141 /// as `filled_u8 / 8` rounded up.
 142 ///
 143 /// See `fill_via_u32_chunks` for an example.
 144 pub fn fill_via_u64_chunks(src: &[u64], dest: &mut [u8]) -> (usize, usize) {
 145     fill_via_chunks!(src, dest, u64, 8)
 146 }
 147
 148 /// Implement `next_u32` via `fill_bytes`, little-endian order.
 149 pub fn next_u32_via_fill<R: RngCore + ?Sized>(rng: &mut R) -> u32 {
 150     impl_uint_from_fill!(rng, u32, 4)
 151 }
 152
 153 /// Implement `next_u64` via `fill_bytes`, little-endian order.
 154 pub fn next_u64_via_fill<R: RngCore + ?Sized>(rng: &mut R) -> u64 {
 155     impl_uint_from_fill!(rng, u64, 8)
 156 }
 157
 158 // TODO: implement tests for the above