| // 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 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 |