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 15 //! Bit lengths. 16 17 use crate::error; 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 pub struct BitLength(usize); 24 25 // Lengths measured in bits, where all arithmetic is guaranteed not to 26 // overflow. 27 impl BitLength { 28 /// Constructs a `BitLength` from the given length in bits. 29 #[inline] from_usize_bits(bits: usize) -> Self30 pub const fn from_usize_bits(bits: usize) -> Self { 31 Self(bits) 32 } 33 34 /// Constructs a `BitLength` from the given length in bytes. 35 /// 36 /// Fails if `bytes * 8` is too large for a `usize`. 37 #[inline] from_usize_bytes(bytes: usize) -> Result<Self, error::Unspecified>38 pub fn from_usize_bytes(bytes: usize) -> Result<Self, error::Unspecified> { 39 let bits = bytes.checked_mul(8).ok_or(error::Unspecified)?; 40 Ok(Self::from_usize_bits(bits)) 41 } 42 43 #[cfg(feature = "alloc")] 44 #[inline] half_rounded_up(&self) -> Self45 pub(crate) fn half_rounded_up(&self) -> Self { 46 let round_up = self.0 & 1; 47 Self((self.0 / 2) + round_up) 48 } 49 50 /// The number of bits this bit length represents, as a `usize`. 51 #[inline] as_usize_bits(&self) -> usize52 pub fn as_usize_bits(&self) -> usize { 53 self.0 54 } 55 56 /// The bit length, rounded up to a whole number of bytes. 57 #[cfg(feature = "alloc")] 58 #[inline] as_usize_bytes_rounded_up(&self) -> usize59 pub fn as_usize_bytes_rounded_up(&self) -> usize { 60 // Equivalent to (self.0 + 7) / 8, except with no potential for 61 // overflow and without branches. 62 63 // Branchless round_up = if self.0 & 0b111 != 0 { 1 } else { 0 }; 64 let round_up = ((self.0 >> 2) | (self.0 >> 1) | self.0) & 1; 65 66 (self.0 / 8) + round_up 67 } 68 69 #[cfg(feature = "alloc")] 70 #[inline] try_sub_1(self) -> Result<Self, error::Unspecified>71 pub(crate) fn try_sub_1(self) -> Result<Self, error::Unspecified> { 72 let sum = self.0.checked_sub(1).ok_or(error::Unspecified)?; 73 Ok(Self(sum)) 74 } 75 } 76