use crate::Adler32;

use std::ops::{AddAssign, MulAssign, RemAssign};

impl Adler32 {

    pub(crate) fn compute(&mut self, bytes: &[u8]) {

        // The basic algorithm is, for every byte:

        //   a = (a + byte) % MOD

        //   b = (b + a) % MOD

        // where MOD = 65521.

        //

        // For efficiency, we can defer the `% MOD` operations as long as neither a nor b overflows:

        // - Between calls to `write`, we ensure that a and b are always in range 0..MOD.

        // - We use 32-bit arithmetic in this function.

        // - Therefore, a and b must not increase by more than 2^32-MOD without performing a `% MOD`

        //   operation.

        //

        // According to Wikipedia, b is calculated as follows for non-incremental checksumming:

        //   b = n×D1 + (n−1)×D2 + (n−2)×D3 + ... + Dn + n*1 (mod 65521)

        // Where n is the number of bytes and Di is the i-th Byte. We need to change this to account

        // for the previous values of a and b, as well as treat every input Byte as being 255:

        //   b_inc = n×255 + (n-1)×255 + ... + 255 + n*65520

        // Or in other words:

        //   b_inc = n*65520 + n(n+1)/2*255

        // The max chunk size is thus the largest value of n so that b_inc <= 2^32-65521.

        //   2^32-65521 = n*65520 + n(n+1)/2*255

        // Plugging this into an equation solver since I can't math gives n = 5552.18..., so 5552.

        //

        // On top of the optimization outlined above, the algorithm can also be parallelized with a

        // bit more work:

        //

        // Note that b is a linear combination of a vector of input bytes (D1, ..., Dn).

        //

        // If we fix some value k<N and rewrite indices 1, ..., N as

        //

        //   1_1, 1_2, ..., 1_k, 2_1, ..., 2_k, ..., (N/k)_k,

        //

        // then we can express a and b in terms of sums of smaller sequences kb and ka:

        //

        //   ka(j) := D1_j + D2_j + ... + D(N/k)_j where j <= k

        //   kb(j) := (N/k)*D1_j + (N/k-1)*D2_j + ... + D(N/k)_j where j <= k

        //

        //  a = ka(1) + ka(2) + ... + ka(k) + 1

        //  b = k*(kb(1) + kb(2) + ... + kb(k)) - 1*ka(2) - ...  - (k-1)*ka(k) + N

        //

        // We use this insight to unroll the main loop and process k=4 bytes at a time.

        // The resulting code is highly amenable to SIMD acceleration, although the immediate speedups

        // stem from increased pipeline parallelism rather than auto-vectorization.

        //

        // This technique is described in-depth (here:)[https://software.intel.com/content/www/us/\

        // en/develop/articles/fast-computation-of-fletcher-checksums.html]

        const MOD: u32 = 65521;

        const CHUNK_SIZE: usize = 5552 * 4;

        let mut a = u32::from(self.a);

        let mut b = u32::from(self.b);

        let mut a_vec = U32X4([0; 4]);

        let mut b_vec = a_vec;

        let (bytes, remainder) = bytes.split_at(bytes.len() - bytes.len() % 4);

        // iterate over 4 bytes at a time

        let chunk_iter = bytes.chunks_exact(CHUNK_SIZE);

        let remainder_chunk = chunk_iter.remainder();

        for chunk in chunk_iter {

            for byte_vec in chunk.chunks_exact(4) {

                let val = U32X4::from(byte_vec);

                a_vec += val;

                b_vec += a_vec;

            }

            b += CHUNK_SIZE as u32 * a;

            a_vec %= MOD;

            b_vec %= MOD;

            b %= MOD;

        }

        // special-case the final chunk because it may be shorter than the rest

        for byte_vec in remainder_chunk.chunks_exact(4) {

            let val = U32X4::from(byte_vec);

            a_vec += val;

            b_vec += a_vec;

        }

        b += remainder_chunk.len() as u32 * a;

        a_vec %= MOD;

        b_vec %= MOD;

        b %= MOD;

        // combine the sub-sum results into the main sum

        b_vec *= 4;

        b_vec.0[1] += MOD - a_vec.0[1];

        b_vec.0[2] += (MOD - a_vec.0[2]) * 2;

        b_vec.0[3] += (MOD - a_vec.0[3]) * 3;

        for &av in a_vec.0.iter() {

            a += av;

        }

        for &bv in b_vec.0.iter() {

            b += bv;

        }

        // iterate over the remaining few bytes in serial

        for &byte in remainder.iter() {

            a += u32::from(byte);

            b += a;

        }

        self.a = (a % MOD) as u16;

        self.b = (b % MOD) as u16;

    }

}

#[derive(Copy, Clone)]

struct U32X4([u32; 4]);

impl U32X4 {

    fn from(bytes: &[u8]) -> Self {

        U32X4([

            u32::from(bytes[0]),

            u32::from(bytes[1]),

            u32::from(bytes[2]),

            u32::from(bytes[3]),

        ])

    }

}

impl AddAssign<Self> for U32X4 {

    fn add_assign(&mut self, other: Self) {

        for (s, o) in self.0.iter_mut().zip(other.0.iter()) {

            *s += o;

        }

    }

}

impl RemAssign<u32> for U32X4 {

    fn rem_assign(&mut self, quotient: u32) {

        for s in self.0.iter_mut() {

            *s %= quotient;

        }

    }

}

impl MulAssign<u32> for U32X4 {

    fn mul_assign(&mut self, rhs: u32) {

        for s in self.0.iter_mut() {

            *s *= rhs;

        }

    }

}