poly1305 0.9.0

The Poly1305 universal hash function and message authentication code
Documentation 
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//! Autodetection support for AVX2 CPU intrinsics on x86 CPUs, with fallback
//! to the "soft" backend when it's unavailable.

use universal_hash::{UhfClosure, UniversalHash, common::BlockSizeUser, consts::U16};

use crate::{Block, Key, Tag, backend};
use core::mem::ManuallyDrop;

cpufeatures::new!(avx2_cpuid, "avx2");

pub(crate) struct State {
    inner: Inner,
    token: avx2_cpuid::InitToken,
}

union Inner {
    avx2: ManuallyDrop<backend::avx2::State>,
    soft: ManuallyDrop<backend::soft::State>,
}

impl BlockSizeUser for State {
    type BlockSize = U16;
}

impl State {
    /// Initialize Poly1305 [`State`] with the given key
    #[inline]
    pub(crate) fn new(key: &Key) -> State {
        let (token, avx2_present) = avx2_cpuid::init_get();

        let inner = if avx2_present {
            Inner {
                avx2: ManuallyDrop::new(backend::avx2::State::new(key)),
            }
        } else {
            Inner {
                soft: ManuallyDrop::new(backend::soft::State::new(key)),
            }
        };

        Self { inner, token }
    }

    /// Compute a Poly1305 block
    #[inline]
    pub(crate) fn compute_block(&mut self, block: &Block, partial: bool) {
        if self.token.get() {
            unsafe { (*self.inner.avx2).compute_block(block, partial) }
        } else {
            unsafe { (*self.inner.soft).compute_block(block, partial) }
        }
    }
}

impl UniversalHash for State {
    fn update_with_backend(&mut self, f: impl UhfClosure<BlockSize = Self::BlockSize>) {
        if self.token.get() {
            unsafe { f.call(&mut *self.inner.avx2) }
        } else {
            unsafe { f.call(&mut *self.inner.soft) }
        }
    }

    /// Finalize output producing a [`Tag`]
    #[inline]
    fn finalize(mut self) -> Tag {
        if self.token.get() {
            unsafe { (*self.inner.avx2).finalize() }
        } else {
            unsafe { (*self.inner.soft).finalize_mut() }
        }
    }
}

impl Clone for State {
    fn clone(&self) -> Self {
        let inner = if self.token.get() {
            Inner {
                avx2: ManuallyDrop::new(unsafe { (*self.inner.avx2).clone() }),
            }
        } else {
            Inner {
                soft: ManuallyDrop::new(unsafe { (*self.inner.soft).clone() }),
            }
        };

        Self {
            inner,
            token: self.token,
        }
    }
}

#[cfg(feature = "zeroize")]
impl Drop for State {
    fn drop(&mut self) {
        use zeroize::Zeroize;
        const SIZE: usize = size_of::<State>();
        let state = unsafe { &mut *core::ptr::from_mut::<State>(self).cast::<[u8; SIZE]>() };
        state.zeroize();
    }
}