classic-mceliece-rust 3.0.0

//! This file is for loading/storing data in a little-endian fashion and a `bitrev` function

use crate::{gf::Gf, params::GFMASK};

/// Store Gf element `a` in array `dest`
#[inline(always)]
pub(crate) fn store_gf(dest: &mut [u8; 2], gf: Gf) {
    *dest = gf.to_le_bytes();
}

/// Interpret 2 bytes from `src` as integer and return it as Gf element
#[inline(always)]
pub(crate) fn load_gf(src: &[u8; 2]) -> Gf {
    u16::from_le_bytes(*src) & (GFMASK as u16)
}

/// Reverse the bits of Gf element `a`. The LSB becomes the MSB.
/// The 2nd LSB becomes the 2nd MSB. etc …
pub(crate) fn bitrev(mut a: Gf) -> Gf {
    a = ((a & 0x00FF) << 8) | ((a & 0xFF00) >> 8);
    a = ((a & 0x0F0F) << 4) | ((a & 0xF0F0) >> 4);
    a = ((a & 0x3333) << 2) | ((a & 0xCCCC) >> 2);
    a = ((a & 0x5555) << 1) | ((a & 0xAAAA) >> 1);

    #[cfg(any(feature = "mceliece348864", feature = "mceliece348864f"))]
    {
        a >> 4
    }
    #[cfg(not(any(feature = "mceliece348864", feature = "mceliece348864f")))]
    {
        a >> 3
    }
}

/// Ugly hack to allocate a `Box<[u8; _]>` directly on the heap without first
/// putting the array on the stack
#[cfg(feature = "alloc")]
#[inline(always)]
pub fn alloc_boxed_array<const SIZE: usize>() -> alloc::boxed::Box<[u8; SIZE]> {
    alloc::boxed::Box::<[u8; SIZE]>::try_from(alloc::vec![0u8; SIZE].into_boxed_slice()).unwrap()
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_load_gf() {
        assert_eq!(load_gf(&[0xAB, 0x42]), 0x02AB);
    }

    #[test]
    #[cfg(any(feature = "mceliece348864", feature = "mceliece348864f"))]
    fn test_bitrev() {
        assert_eq!(bitrev(0b1011_0111_0111_1011), 0b0000_1101_1110_1110);
        assert_eq!(bitrev(0b0110_1010_0101_1011), 0b0000_1101_1010_0101);
    }

    #[test]
    #[cfg(not(any(feature = "mceliece348864", feature = "mceliece348864f")))]
    fn test_bitrev() {
        assert_eq!(bitrev(0b1011_0111_0111_1011), 0b0001_1011_1101_1101);
        assert_eq!(bitrev(0b0110_1010_0101_1011), 0b0001_1011_0100_1010);
    }
}