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
// 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.

//! Helper functions for implementing `RngCore` functions.
//!
//! For cross-platform reproducibility, these functions all use Little Endian:
//! least-significant part first. For example, `next_u64_via_u32` takes `u32`
//! values `x, y`, then outputs `(y << 32) | x`. To implement `next_u32`
//! from `next_u64` in little-endian order, one should use `next_u64() as u32`.
//!
//! Byte-swapping (like the std `to_le` functions) is only needed to convert
//! to/from byte sequences, and since its purpose is reproducibility,
//! non-reproducible sources (e.g. `OsRng`) need not bother with it.

use core::intrinsics::transmute;
use core::ptr::copy_nonoverlapping;
use core::slice;
use core::cmp::min;
use core::mem::size_of;
use crate::RngCore;


/// Implement `next_u64` via `next_u32`, little-endian order.
pub fn next_u64_via_u32<R: RngCore + ?Sized>(rng: &mut R) -> u64 {
    // Use LE; we explicitly generate one value before the next.
    let x = u64::from(rng.next_u32());
    let y = u64::from(rng.next_u32());
    (y << 32) | x
}

/// Implement `fill_bytes` via `next_u64` and `next_u32`, little-endian order.
///
/// The fastest way to fill a slice is usually to work as long as possible with
/// integers. That is why this method mostly uses `next_u64`, and only when
/// there are 4 or less bytes remaining at the end of the slice it uses
/// `next_u32` once.
pub fn fill_bytes_via_next<R: RngCore + ?Sized>(rng: &mut R, dest: &mut [u8]) {
    let mut left = dest;
    while left.len() >= 8 {
        let (l, r) = {left}.split_at_mut(8);
        left = r;
        let chunk: [u8; 8] = unsafe {
            transmute(rng.next_u64().to_le())
        };
        l.copy_from_slice(&chunk);
    }
    let n = left.len();
    if n > 4 {
        let chunk: [u8; 8] = unsafe {
            transmute(rng.next_u64().to_le())
        };
        left.copy_from_slice(&chunk[..n]);
    } else if n > 0 {
        let chunk: [u8; 4] = unsafe {
            transmute(rng.next_u32().to_le())
        };
        left.copy_from_slice(&chunk[..n]);
    }
}

macro_rules! impl_uint_from_fill {
    ($rng:expr, $ty:ty, $N:expr) => ({
        debug_assert!($N == size_of::<$ty>());

        let mut int: $ty = 0;
        unsafe {
            let ptr = &mut int as *mut $ty as *mut u8;
            let slice = slice::from_raw_parts_mut(ptr, $N);
            $rng.fill_bytes(slice);
        }
        int
    });
}

macro_rules! fill_via_chunks {
    ($src:expr, $dst:expr, $ty:ty, $size:expr) => ({
        let chunk_size_u8 = min($src.len() * $size, $dst.len());
        let chunk_size = (chunk_size_u8 + $size - 1) / $size;
        if cfg!(target_endian="little") {
            unsafe {
                copy_nonoverlapping(
                    $src.as_ptr() as *const u8,
                    $dst.as_mut_ptr(),
                    chunk_size_u8);
            }
        } else {
            for (&n, chunk) in $src.iter().zip($dst.chunks_mut($size)) {
                let tmp = n.to_le();
                let src_ptr = &tmp as *const $ty as *const u8;
                unsafe {
                    copy_nonoverlapping(src_ptr,
                                        chunk.as_mut_ptr(),
                                        chunk.len());
                }
            }
        }

        (chunk_size, chunk_size_u8)
    });
}

/// Implement `fill_bytes` by reading chunks from the output buffer of a block
/// based RNG.
///
/// The return values are `(consumed_u32, filled_u8)`.
///
/// `filled_u8` is the number of filled bytes in `dest`, which may be less than
/// the length of `dest`.
/// `consumed_u32` is the number of words consumed from `src`, which is the same
/// as `filled_u8 / 4` rounded up.
///
/// # Example
/// (from `IsaacRng`)
///
/// ```ignore
/// fn fill_bytes(&mut self, dest: &mut [u8]) {
///     let mut read_len = 0;
///     while read_len < dest.len() {
///         if self.index >= self.rsl.len() {
///             self.isaac();
///         }
///
///         let (consumed_u32, filled_u8) =
///             impls::fill_via_u32_chunks(&mut self.rsl[self.index..],
///                                        &mut dest[read_len..]);
///
///         self.index += consumed_u32;
///         read_len += filled_u8;
///     }
/// }
/// ```
pub fn fill_via_u32_chunks(src: &[u32], dest: &mut [u8]) -> (usize, usize) {
    fill_via_chunks!(src, dest, u32, 4)
}

/// Implement `fill_bytes` by reading chunks from the output buffer of a block
/// based RNG.
///
/// The return values are `(consumed_u64, filled_u8)`.
/// `filled_u8` is the number of filled bytes in `dest`, which may be less than
/// the length of `dest`.
/// `consumed_u64` is the number of words consumed from `src`, which is the same
/// as `filled_u8 / 8` rounded up.
///
/// See `fill_via_u32_chunks` for an example.
pub fn fill_via_u64_chunks(src: &[u64], dest: &mut [u8]) -> (usize, usize) {
    fill_via_chunks!(src, dest, u64, 8)
}

/// Implement `next_u32` via `fill_bytes`, little-endian order.
pub fn next_u32_via_fill<R: RngCore + ?Sized>(rng: &mut R) -> u32 {
    impl_uint_from_fill!(rng, u32, 4)
}

/// Implement `next_u64` via `fill_bytes`, little-endian order.
pub fn next_u64_via_fill<R: RngCore + ?Sized>(rng: &mut R) -> u64 {
    impl_uint_from_fill!(rng, u64, 8)
}

// TODO: implement tests for the above