      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 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

# Help

## Keyboard Shortcuts

`?`
  Show this help dialog
`S`
  Focus the search field
`↑`
  Move up in search results
`↓`
  Move down in search results
`↹`
  Switch tab
`⏎`
  Go to active search result
`+`
  Expand all sections
`-`
  Collapse all sections

## Search Tricks

Prefix searches with a type followed by a colon (e.g., `fn:`) to restrict the search to a given type.

Accepted types are: `fn`, `mod`, `struct`, `enum`, `trait`, `type`, `macro`, and `const`.

Search functions by type signature (e.g., `vec -> usize` or `* -> vec`)

Search multiple things at once by splitting your query with comma (e.g., `str,u8` or `String,struct:Vec,test`)