use crate::table::CRC32_TABLE;

#[derive(Clone)]

pub struct State {

    state: u32,

}

impl State {

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

        State { state }

    }

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

        self.state = update_fast_16(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);

    }

}

/// Folds the first 16 bytes of `block` into `crc`.

///

/// `block` must be at least 16 bytes long. The decode is explicitly

/// little-endian, so the result is identical on big-endian targets.

#[inline(always)]

fn fold_16(crc: u32, block: &[u8]) -> u32 {

    let w0 = u32::from_le_bytes([block[0], block[1], block[2], block[3]]) ^ crc;

    CRC32_TABLE[0x0][block[0xf] as usize]

        ^ CRC32_TABLE[0x1][block[0xe] as usize]

        ^ CRC32_TABLE[0x2][block[0xd] as usize]

        ^ CRC32_TABLE[0x3][block[0xc] as usize]

        ^ CRC32_TABLE[0x4][block[0xb] as usize]

        ^ CRC32_TABLE[0x5][block[0xa] as usize]

        ^ CRC32_TABLE[0x6][block[0x9] as usize]

        ^ CRC32_TABLE[0x7][block[0x8] as usize]

        ^ CRC32_TABLE[0x8][block[0x7] as usize]

        ^ CRC32_TABLE[0x9][block[0x6] as usize]

        ^ CRC32_TABLE[0xa][block[0x5] as usize]

        ^ CRC32_TABLE[0xb][block[0x4] as usize]

        ^ CRC32_TABLE[0xc][(w0 >> 24) as usize]

        ^ CRC32_TABLE[0xd][((w0 >> 16) & 0xFF) as usize]

        ^ CRC32_TABLE[0xe][((w0 >> 8) & 0xFF) as usize]

        ^ CRC32_TABLE[0xf][(w0 & 0xFF) as usize]

}

pub(crate) fn update_fast_16(prev: u32, mut buf: &[u8]) -> u32 {

    const UNROLL: usize = 4;

    const BYTES_AT_ONCE: usize = 16 * UNROLL;

    let mut crc = !prev;

    while buf.len() >= BYTES_AT_ONCE {

        for _ in 0..UNROLL {

            crc = fold_16(crc, buf);

            buf = &buf[16..];

        }

    }

    // Fold whatever whole 16-byte blocks remain rather than handing them to the

    // byte-at-a-time loop.

    while buf.len() >= 16 {

        crc = fold_16(crc, buf);

        buf = &buf[16..];

    }

    update_slow(!crc, buf)

}

pub(crate) fn update_slow(prev: u32, buf: &[u8]) -> u32 {

    let mut crc = !prev;

    for &byte in buf.iter() {

        crc = CRC32_TABLE[0][((crc as u8) ^ byte) as usize] ^ (crc >> 8);

    }

    !crc

}

#[cfg(test)]

mod test {

    #[test]

    fn slow() {

        assert_eq!(super::update_slow(0, b""), 0);

        // test vectors from the iPXE project (input and output are bitwise negated)

        assert_eq!(super::update_slow(!0x12345678, b""), !0x12345678);

        assert_eq!(super::update_slow(!0xffffffff, b"hello world"), !0xf2b5ee7a);

        assert_eq!(super::update_slow(!0xffffffff, b"hello"), !0xc9ef5979);

        assert_eq!(super::update_slow(!0xc9ef5979, b" world"), !0xf2b5ee7a);

        // Some vectors found on Rosetta code

        assert_eq!(super::update_slow(0, b"\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00"), 0x190A55AD);

        assert_eq!(super::update_slow(0, b"\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF"), 0xFF6CAB0B);

        assert_eq!(super::update_slow(0, b"\x00\x01\x02\x03\x04\x05\x06\x07\x08\x09\x0A\x0B\x0C\x0D\x0E\x0F\x10\x11\x12\x13\x14\x15\x16\x17\x18\x19\x1A\x1B\x1C\x1D\x1E\x1F"), 0x91267E8A);

    }

    quickcheck::quickcheck! {

        fn fast_16_is_the_same_as_slow(crc: u32, bytes: Vec<u8>) -> bool {

            super::update_fast_16(crc, &bytes) == super::update_slow(crc, &bytes)

        }

    }

}