ring/
bits.rs

1// Copyright 2016 Brian Smith.
2//
3// Permission to use, copy, modify, and/or distribute this software for any
4// purpose with or without fee is hereby granted, provided that the above
5// copyright notice and this permission notice appear in all copies.
6//
7// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHORS DISCLAIM ALL WARRANTIES
8// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
9// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY
10// SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
11// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
12// OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
13// CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
14
15use crate::error;
16
17#[derive(Clone, Copy, Debug, Eq, PartialEq, PartialOrd)]
18pub struct BitLength(usize);
19
20// Lengths measured in bits, where all arithmetic is guaranteed not to
21// overflow.
22impl BitLength {
23    #[inline]
24    pub const fn from_usize_bits(bits: usize) -> Self {
25        Self(bits)
26    }
27
28    #[inline]
29    pub fn from_usize_bytes(bytes: usize) -> Result<Self, error::Unspecified> {
30        let bits = bytes.checked_mul(8).ok_or(error::Unspecified)?;
31        Ok(Self::from_usize_bits(bits))
32    }
33
34    #[cfg(feature = "alloc")]
35    #[inline]
36    pub fn half_rounded_up(&self) -> Self {
37        let round_up = self.0 & 1;
38        Self((self.0 / 2) + round_up)
39    }
40
41    #[inline]
42    pub fn as_usize_bits(&self) -> usize {
43        self.0
44    }
45
46    #[cfg(feature = "alloc")]
47    #[inline]
48    pub fn as_usize_bytes_rounded_up(&self) -> usize {
49        // Equivalent to (self.0 + 7) / 8, except with no potential for
50        // overflow and without branches.
51
52        // Branchless round_up = if self.0 & 0b111 != 0 { 1 } else { 0 };
53        let round_up = ((self.0 >> 2) | (self.0 >> 1) | self.0) & 1;
54
55        (self.0 / 8) + round_up
56    }
57
58    #[cfg(feature = "alloc")]
59    #[inline]
60    pub fn try_sub_1(self) -> Result<BitLength, error::Unspecified> {
61        let sum = self.0.checked_sub(1).ok_or(error::Unspecified)?;
62        Ok(BitLength(sum))
63    }
64}
ring/bits.rs

ring/
bits.rs
