ring/
bits.rs

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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 AUTHOR DISCLAIMS ALL WARRANTIES
8// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
9// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR 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
15//! Bit lengths.
16
17use crate::{error::InputTooLongError, polyfill};
18
19/// The length of something, in bits.
20///
21/// This can represent a bit length that isn't a whole number of bytes.
22#[derive(Clone, Copy, Debug, Eq, PartialEq, PartialOrd)]
23#[repr(transparent)]
24pub struct BitLength<T = usize>(T);
25
26pub(crate) trait FromByteLen<T>: Sized {
27    /// Constructs a `BitLength` from the given length in bytes.
28    ///
29    /// Fails if `bytes * 8` is too large for a `T`.
30    fn from_byte_len(bytes: T) -> Result<Self, InputTooLongError<T>>;
31}
32
33impl FromByteLen<usize> for BitLength<usize> {
34    #[inline]
35    fn from_byte_len(bytes: usize) -> Result<Self, InputTooLongError> {
36        match bytes.checked_mul(8) {
37            Some(bits) => Ok(Self(bits)),
38            None => Err(InputTooLongError::new(bytes)),
39        }
40    }
41}
42
43impl FromByteLen<u64> for BitLength<u64> {
44    #[inline]
45    fn from_byte_len(bytes: u64) -> Result<Self, InputTooLongError<u64>> {
46        match bytes.checked_mul(8) {
47            Some(bits) => Ok(Self(bits)),
48            None => Err(InputTooLongError::new(bytes)),
49        }
50    }
51}
52
53impl FromByteLen<usize> for BitLength<u64> {
54    #[inline]
55    fn from_byte_len(bytes: usize) -> Result<Self, InputTooLongError<usize>> {
56        match polyfill::u64_from_usize(bytes).checked_mul(8) {
57            Some(bits) => Ok(Self(bits)),
58            None => Err(InputTooLongError::new(bytes)),
59        }
60    }
61}
62
63impl<T> BitLength<T> {
64    /// Constructs a `BitLength` from the given length in bits.
65    #[inline]
66    pub const fn from_bits(bits: T) -> Self {
67        Self(bits)
68    }
69}
70
71impl<T: Copy> BitLength<T> {
72    /// The number of bits this bit length represents, as the underlying type.
73    #[inline]
74    pub fn as_bits(self) -> T {
75        self.0
76    }
77}
78
79// Lengths measured in bits, where all arithmetic is guaranteed not to
80// overflow.
81impl BitLength<usize> {
82    #[cfg(feature = "alloc")]
83    #[inline]
84    pub(crate) fn half_rounded_up(&self) -> Self {
85        let round_up = self.0 & 1;
86        Self((self.0 / 2) + round_up)
87    }
88
89    /// The bit length, rounded up to a whole number of bytes.
90    #[inline]
91    pub const fn as_usize_bytes_rounded_up(&self) -> usize {
92        // Equivalent to (self.0 + 7) / 8, except with no potential for
93        // overflow and without branches.
94
95        // Branchless round_up = if self.0 & 0b111 != 0 { 1 } else { 0 };
96        let round_up = ((self.0 >> 2) | (self.0 >> 1) | self.0) & 1;
97
98        (self.0 / 8) + round_up
99    }
100
101    #[cfg(feature = "alloc")]
102    #[inline]
103    pub(crate) fn try_sub_1(self) -> Result<Self, crate::error::Unspecified> {
104        let sum = self.0.checked_sub(1).ok_or(crate::error::Unspecified)?;
105        Ok(Self(sum))
106    }
107}
108
109impl BitLength<u64> {
110    pub fn to_be_bytes(self) -> [u8; 8] {
111        self.0.to_be_bytes()
112    }
113}
114
115#[cfg(any(target_pointer_width = "32", target_pointer_width = "64"))]
116impl From<BitLength<usize>> for BitLength<u64> {
117    fn from(BitLength(value): BitLength<usize>) -> Self {
118        BitLength(polyfill::u64_from_usize(value))
119    }
120}
121
122impl TryFrom<BitLength<u64>> for BitLength<core::num::NonZeroU64> {
123    type Error = <core::num::NonZeroU64 as TryFrom<u64>>::Error;
124
125    fn try_from(BitLength(value): BitLength<u64>) -> Result<Self, Self::Error> {
126        value.try_into().map(BitLength)
127    }
128}
129
130const _TEST_AS_USIZE_BYTES_ROUNDED_UP_EVEN: () =
131    assert!(BitLength::from_bits(8192).as_usize_bytes_rounded_up() == 8192 / 8);
132const _TEST_AS_USIZE_BYTES_ROUNDED_UP_ONE_BIT_HIGH: () =
133    assert!(BitLength::from_bits(8192 + 1).as_usize_bytes_rounded_up() == (8192 / 8) + 1);
134const _TEST_AS_USIZE_BYTES_ROUNDED_UP_SEVEN_BITS_HIGH: () =
135    assert!(BitLength::from_bits(8192 + 7).as_usize_bytes_rounded_up() == (8192 / 8) + 1);
ring/bits.rs

ring/
bits.rs
