const POLY: u32 = 0xedb88320;

static X2N_TABLE: [u32; 32] = [

    0x00800000, 0x00008000, 0xedb88320, 0xb1e6b092, 0xa06a2517, 0xed627dae, 0x88d14467, 0xd7bbfe6a,

    0xec447f11, 0x8e7ea170, 0x6427800e, 0x4d47bae0, 0x09fe548f, 0x83852d0f, 0x30362f1a, 0x7b5a9cc3,

    0x31fec169, 0x9fec022a, 0x6c8dedc4, 0x15d6874d, 0x5fde7a4e, 0xbad90e37, 0x2e4e5eef, 0x4eaba214,

    0xa8a472c0, 0x429a969e, 0x148d302a, 0xc40ba6d0, 0xc4e22c3c, 0x40000000, 0x20000000, 0x08000000,

];

// Calculates a(x) multiplied by b(x) modulo p(x), where p(x) is the CRC polynomial,

// reflected. For speed, this requires that a not be zero.

const fn multiply(a: u32, mut b: u32) -> u32 {

    let mut p = 0u32;

    let mut i = 0;

    while i < 32 {

        p ^= b & ((a >> (31 - i)) & 1).wrapping_neg();

        b = (b >> 1) ^ ((b & 1).wrapping_neg() & POLY);

        i += 1;

    }

    p

}

/// Returns x^(8 * len) modulo p(x), reflected. Multiplying a CRC state by it appends `len` zero

/// bytes.

const fn x8n(len: u64) -> u32 {

    let mut p = 0x8000_0000; // x^0

    let mut sq = 0x0080_0000; // x^8

    let mut n = len;

    while n != 0 {

        if n & 1 != 0 {

            p = multiply(sq, p);

        }

        sq = multiply(sq, sq);

        n >>= 1;

    }

    p

}

/// The constants that fold a 128-bit accumulator forward by `bits` bits with two carry-less

/// multiplies: `(x^(bits + 32), x^(bits - 32))` modulo p(x), reflected, as 33-bit values (the

/// `K1`..`K4` of `specialized/pclmulqdq.rs`, which a test checks).

#[cfg_attr(

    not(all(stable_arm_crc32_intrinsics, target_arch = "aarch64")),

    allow(dead_code)

)]

pub(crate) const fn fold_keys(bits: u64) -> [u64; 2] {

    [

        (x8n((bits + 32) / 8) as u64) << 1,

        (x8n((bits - 32) / 8) as u64) << 1,

    ]

}

/// Builds tables that append `len` zero bytes to a CRC state a byte at a time: the new state is

/// the XOR of `table[k][(state >> (8 * k)) & 0xFF]` for `k` in `0..4`.

#[cfg_attr(

    not(all(stable_arm_crc32_intrinsics, target_arch = "aarch64")),

    allow(dead_code)

)]

pub(crate) const fn shift_table(len: u64) -> [[u32; 256]; 4] {

    let x = x8n(len);

    let mut table = [[0u32; 256]; 4];

    let mut k = 0;

    while k < 4 {

        let mut v = 0;

        while v < 256 {

            table[k][v] = multiply(x, (v as u32) << (8 * k));

            v += 1;

        }

        k += 1;

    }

    table

}

/// Appends the zero bytes `table` was built for to the CRC state `state`.

#[cfg_attr(

    not(all(stable_arm_crc32_intrinsics, target_arch = "aarch64")),

    allow(dead_code)

)]

#[inline(always)]

pub(crate) fn shift(table: &[[u32; 256]; 4], state: u32) -> u32 {

    table[0][(state & 0xFF) as usize]

        ^ table[1][((state >> 8) & 0xFF) as usize]

        ^ table[2][((state >> 16) & 0xFF) as usize]

        ^ table[3][(state >> 24) as usize]

}

pub(crate) fn combine(crc1: u32, crc2: u32, len2: u64) -> u32 {

    // Special case: If the length of the second chunk is zero, return the hash

    // of the first chunk.

    if len2 == 0 {

        return crc1;

    }

    // We are padding the first checksum with len2-amount of zeroes. For efficiency,

    // this is done in powers-of-two via a lookup table rather than one by one.

    let mut p = crc1;

    let n = 64 - len2.leading_zeros();

    for i in 0..n {

        if (len2 >> i & 1) != 0 {

            p = multiply(X2N_TABLE[(i & 0x1F) as usize], p);

        }

    }

    p ^ crc2

}

#[test]

fn fold_keys_match_the_x86_constants() {

    assert_eq!(fold_keys(128), [0x1751997d0, 0x0ccaa009e]);

    assert_eq!(fold_keys(512), [0x154442bd4, 0x1c6e41596]);

    assert_eq!(fold_keys(1024), [0x1e88ef372, 0x14a7fe880]);

    assert_eq!(fold_keys(2048), [0x11542778a, 0x1322d1430]);

}

#[test]

fn shift_table_appends_zeros() {

    for &len in &[1u64, 7, 512, 32768] {

        let table = shift_table(len);

        let zeros = vec![0u8; len as usize];

        for &state in &[0u32, 1, 0x8000_0000, 0xDEAD_BEEF, 0xFFFF_FFFF, 0x1234_5678] {

            // `update_slow` takes and returns finalized CRCs, which are the inverted state.

            let expected = !crate::baseline::update_slow(!state, &zeros);

            assert_eq!(

                shift(&table, state),

                expected,

                "len={len} state={state:#010x}"

            );

        }

    }

}

#[test]

fn golden() {

    assert_eq!(combine(0x0, 0x1, 0x0), 0x0);

    assert_eq!(combine(0xc401f8c9, 0x00000000, 0x0), 0xc401f8c9);

    assert_eq!(combine(0x7cba3d5e, 0xe7466d39, 0xb), 0x76365c4f);

    assert_eq!(combine(0x576c62d6, 0x123256e1, 0x47), 0x579a636);

    assert_eq!(combine(0x4f626f9a, 0x9e5ccbf5, 0xa59d), 0x98d43168);

    assert_eq!(combine(0xa09b8a88, 0x815b0f48, 0x40f39511), 0xd7a5f79);

    assert_eq!(

        combine(0x7f6a4306, 0xbc929646, 0x828cde72b3e25301),

        0xef922dda

    );

}