//! Specialized checksum code for the x86 CPU architecture, based on the efficient algorithm described

//! in the following whitepaper:

//!

//! Gopal, V., Ozturk, E., Guilford, J., Wolrich, G., Feghali, W., Dixon, M., & Karakoyunlu, D. (2009).

//! _Fast CRC computation for generic polynomials using PCLMULQDQ instruction_. Intel.

//! (Mirror link: <https://fossies.org/linux/zlib-ng/doc/crc-pclmulqdq.pdf>, accessed 2024-05-20)

//!

//! Throughout the code, this work is referred to as "the paper".

#[cfg(target_arch = "x86")]

use core::arch::x86 as arch;

#[cfg(target_arch = "x86_64")]

use core::arch::x86_64 as arch;

#[derive(Clone)]

pub struct State {

    state: u32,

}

impl State {

    #[cfg(not(feature = "std"))]

    pub fn new(state: u32) -> Option<Self> {

        if cfg!(target_feature = "pclmulqdq")

            && cfg!(target_feature = "sse2")

            && cfg!(target_feature = "sse4.1")

        {

            // SAFETY: The conditions above ensure that all

            //         required instructions are supported by the CPU.

            Some(Self { state })

        } else {

            None

        }

    }

    #[cfg(feature = "std")]

    pub fn new(state: u32) -> Option<Self> {

        if is_x86_feature_detected!("pclmulqdq")

            && is_x86_feature_detected!("sse2")

            && is_x86_feature_detected!("sse4.1")

        {

            // SAFETY: The conditions above ensure that all

            //         required instructions are supported by the CPU.

            Some(Self { state })

        } else {

            None

        }

    }

    pub fn update(&mut self, buf: &[u8]) {

        // SAFETY: The `State::new` constructor ensures that all

        //         required instructions are supported by the CPU.

        self.state = unsafe { calculate(self.state, buf) }

    }

    pub fn finalize(self) -> u32 {

        self.state

    }

    pub fn reset(&mut self) {

        self.state = 0;

    }

    pub fn combine(&mut self, other: u32, amount: u64) {

        self.state = crate::combine::combine(self.state, other, amount);

    }

}

const K1: i64 = 0x154442bd4;

const K2: i64 = 0x1c6e41596;

const K3: i64 = 0x1751997d0;

const K4: i64 = 0x0ccaa009e;

const K5: i64 = 0x163cd6124;

const P_X: i64 = 0x1DB710641;

const U_PRIME: i64 = 0x1F7011641;

#[target_feature(enable = "pclmulqdq", enable = "sse2", enable = "sse4.1")]

unsafe fn calculate(crc: u32, mut data: &[u8]) -> u32 {

    // In theory we can accelerate smaller chunks too, but for now just rely on

    // the fallback implementation as it's too much hassle and doesn't seem too

    // beneficial.

    if data.len() < 128 {

        return crate::baseline::update_fast_16(crc, data);

    }

    // Step 1: fold by 4 loop

    let mut x3 = get(&mut data);

    let mut x2 = get(&mut data);

    let mut x1 = get(&mut data);

    let mut x0 = get(&mut data);

    // fold in our initial value, part of the incremental crc checksum

    x3 = arch::_mm_xor_si128(x3, arch::_mm_cvtsi32_si128(!crc as i32));

    let k1k2 = arch::_mm_set_epi64x(K2, K1);

    while data.len() >= 64 {

        x3 = reduce128(x3, get(&mut data), k1k2);

        x2 = reduce128(x2, get(&mut data), k1k2);

        x1 = reduce128(x1, get(&mut data), k1k2);

        x0 = reduce128(x0, get(&mut data), k1k2);

    }

    let k3k4 = arch::_mm_set_epi64x(K4, K3);

    let mut x = reduce128(x3, x2, k3k4);

    x = reduce128(x, x1, k3k4);

    x = reduce128(x, x0, k3k4);

    // Step 2: fold by 1 loop

    while data.len() >= 16 {

        x = reduce128(x, get(&mut data), k3k4);

    }

    // Perform step 3, reduction from 128 bits to 64 bits. This is

    // significantly different from the paper and basically doesn't follow it

    // at all. It's not really clear why, but implementations of this algorithm

    // in Chrome/Linux diverge in the same way. It is beyond me why this is

    // different than the paper, maybe the paper has like errata or something?

    // Unclear.

    //

    // It's also not clear to me what's actually happening here and/or why, but

    // algebraically what's happening is:

    //

    // x = (x[0:63] • K4) ^ x[64:127]           // 96 bit result

    // x = ((x[0:31] as u64) • K5) ^ x[32:95]   // 64 bit result

    //

    // It's... not clear to me what's going on here. The paper itself is pretty

    // vague on this part but definitely uses different constants at least.

    // It's not clear to me, reading the paper, where the xor operations are

    // happening or why things are shifting around. This implementation...

    // appears to work though!

    let x = arch::_mm_xor_si128(

        arch::_mm_clmulepi64_si128(x, k3k4, 0x10),

        arch::_mm_srli_si128(x, 8),

    );

    let x = arch::_mm_xor_si128(

        arch::_mm_clmulepi64_si128(

            arch::_mm_and_si128(x, arch::_mm_set_epi32(0, 0, 0, !0)),

            arch::_mm_set_epi64x(0, K5),

            0x00,

        ),

        arch::_mm_srli_si128(x, 4),

    );

    // Perform a Barrett reduction from our now 64 bits to 32 bits. The

    // algorithm for this is described at the end of the paper, and note that

    // this also implements the "bit reflected input" variant.

    let pu = arch::_mm_set_epi64x(U_PRIME, P_X);

    // T1(x) = ⌊(R(x) % x^32)⌋ • μ

    let t1 = arch::_mm_clmulepi64_si128(

        arch::_mm_and_si128(x, arch::_mm_set_epi32(0, 0, 0, !0)),

        pu,

        0x10,

    );

    // T2(x) = ⌊(T1(x) % x^32)⌋ • P(x)

    let t2 = arch::_mm_clmulepi64_si128(

        arch::_mm_and_si128(t1, arch::_mm_set_epi32(0, 0, 0, !0)),

        pu,

        0x00,

    );

    // We're doing the bit-reflected variant, so get the upper 32-bits of the

    // 64-bit result instead of the lower 32-bits.

    //

    // C(x) = R(x) ^ T2(x) / x^32

    let c = arch::_mm_extract_epi32(arch::_mm_xor_si128(x, t2), 1) as u32;

    if !data.is_empty() {

        crate::baseline::update_fast_16(!c, data)

    } else {

        !c

    }

}

unsafe fn reduce128(a: arch::__m128i, b: arch::__m128i, keys: arch::__m128i) -> arch::__m128i {

    let t1 = arch::_mm_clmulepi64_si128(a, keys, 0x00);

    let t2 = arch::_mm_clmulepi64_si128(a, keys, 0x11);

    arch::_mm_xor_si128(arch::_mm_xor_si128(b, t1), t2)

}

unsafe fn get(a: &mut &[u8]) -> arch::__m128i {

    debug_assert!(a.len() >= 16);

    let r = arch::_mm_loadu_si128(a.as_ptr() as *const arch::__m128i);

    *a = &a[16..];

    r

}

#[cfg(test)]

mod test {

    quickcheck::quickcheck! {

        fn check_against_baseline(init: u32, chunks: Vec<(Vec<u8>, usize)>) -> bool {

            let mut baseline = super::super::super::baseline::State::new(init);

            let mut pclmulqdq = super::State::new(init).expect("not supported");

            for (chunk, mut offset) in chunks {

                // simulate random alignments by offsetting the slice by up to 15 bytes

                offset &= 0xF;

                if chunk.len() <= offset {

                    baseline.update(&chunk);

                    pclmulqdq.update(&chunk);

                } else {

                    baseline.update(&chunk[offset..]);

                    pclmulqdq.update(&chunk[offset..]);

                }

            }

            pclmulqdq.finalize() == baseline.finalize()

        }

    }

}