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Diffstat (limited to 'vendor/adler/src/algo.rs')
| -rw-r--r-- | vendor/adler/src/algo.rs | 146 |
1 files changed, 0 insertions, 146 deletions
diff --git a/vendor/adler/src/algo.rs b/vendor/adler/src/algo.rs deleted file mode 100644 index 650cffa..0000000 --- a/vendor/adler/src/algo.rs +++ /dev/null @@ -1,146 +0,0 @@ -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; - } - } -} |