Implement generic CRC-32

zlib-rs/Cargo.toml

@@ -18,3 +18,5 @@ libc.workspace = true
 [dev-dependencies]
 libloading = "0.8.1"
 libz-ng-sys = "1.1.12"
+crc32fast = "1.3.2"
+quickcheck = "1.0.3"

zlib-rs/src/crc32.rs

@@ -0,0 +1,169 @@
+// Several implementations of CRC-32:
+// * A naive byte-granularity approach
+// * A word-sized approach that processes a usize word at a time
+// * A "braid" implementation that processes a block of N words
+//   at a time, based on the algorithm in section 4.11 from
+//   https://github.com/zlib-ng/zlib-ng/blob/develop/doc/crc-doc.1.0.pdf.
+
+// The binary encoding of the CRC-32 polynomial.
+// We are assuming little-endianness so we process the input
+// LSB-first. We need to use the "reversed" value from e.g
+// https://en.wikipedia.org/wiki/Cyclic_redundancy_check#Polynomial_representations.
+const CRC32_LSB_POLY: usize = 0xedb8_8320usize;
+
+const W: usize = core::mem::size_of::<usize>();
+
+// The logic assumes that W >= sizeof(u32).
+// In Rust, this is generally true.
+const _: () = assert!(W >= core::mem::size_of::<u32>());
+
+// Pre-computed tables for the CRC32 algorithm.
+// CRC32_BYTE_TABLE corresponds to MulByXPowD from the paper.
+static CRC32_BYTE_TABLE: [[u32; 256]; 1] = build_crc32_table::<256, 1, 1>();
+// CRC32_WORD_TABLE is MulWordByXpowD.
+static CRC32_WORD_TABLE: [[u32; 256]; W] = build_crc32_table::<256, W, 1>();
+
+// Work-around for not being able to define generic consts or statics
+// Crc32BraidTable::<N>::TABLE is the generic table for any braid size N.
+struct Crc32BraidTable<const N: usize>;
+
+impl<const N: usize> Crc32BraidTable<N> {
+    const TABLE: [[u32; 256]; W] = build_crc32_table::<256, W, N>();
+}
+
+// Build the CRC32 tables using a more efficient and simpler approach
+// than the combination of Multiply and XpowN (which implement polynomial
+// multiplication and exponentiation, respectively) from the paper,
+// but with identical results. This function is const, so it should be
+// fully evaluated at compile time.
+const fn build_crc32_table<const A: usize, const W: usize, const N: usize>() -> [[u32; A]; W] {
+    let mut arr = [[0u32; A]; W];
+    let mut i = 0;
+    while i < W {
+        let mut j = 0;
+        while j < A {
+            let mut c = j;
+            let mut k = 0;
+            while k < 8 * (W * N - i) {
+                if c & 1 != 0 {
+                    c = CRC32_LSB_POLY ^ (c >> 1);
+                } else {
+                    c >>= 1;
+                }
+                k += 1;
+            }
+            arr[i][j] = c as u32;
+            j += 1;
+        }
+        i += 1;
+    }
+    arr
+}
+
+fn crc32_naive_inner(data: &[u8], start: u32) -> u32 {
+    data.iter().fold(start, |crc, val| {
+        let crc32_lsb = crc.to_le_bytes()[0];
+        CRC32_BYTE_TABLE[0][usize::from(crc32_lsb ^ *val)] ^ (crc >> 8)
+    })
+}
+
+fn crc32_words_inner(words: &[usize], start: u32, per_word_crcs: &[u32]) -> u32 {
+    words.iter().enumerate().fold(start, |crc, (i, word)| {
+        let value = *word ^ (crc ^ per_word_crcs.get(i).unwrap_or(&0)) as usize;
+        value
+            .to_le_bytes()
+            .into_iter()
+            .zip(CRC32_WORD_TABLE)
+            .fold(0u32, |crc, (b, tab)| crc ^ tab[usize::from(b)])
+    })
+}
+
+pub fn crc32_naive(data: &[u8], start: u32) -> u32 {
+    let crc = !start;
+    let crc = crc32_naive_inner(data, crc);
+    !crc
+}
+
+pub fn crc32_words(data: &[u8], start: u32) -> u32 {
+    // Get a word-aligned sub-slice of the input data
+    let (prefix, words, suffix) = unsafe { data.align_to::<usize>() };
+    let crc = !start;
+    let crc = crc32_naive_inner(prefix, crc);
+    let crc = crc32_words_inner(words, crc, &[]);
+    let crc = crc32_naive_inner(suffix, crc);
+    !crc
+}
+
+pub fn crc32_braid<const N: usize>(data: &[u8], start: u32) -> u32 {
+    // Get a word-aligned sub-slice of the input data
+    let (prefix, words, suffix) = unsafe { data.align_to::<usize>() };
+    let crc = !start;
+    let crc = crc32_naive_inner(prefix, crc);
+
+    let mut crcs = [0u32; N];
+    crcs[0] = crc;
+
+    // TODO: this would normally use words.chunks_exact(N), but
+    // we need to pass the last full block to crc32_words_inner
+    // because we accumulate partial crcs in the array and we
+    // need to roll those into the final value. The last call to
+    // crc32_words_inner does that for us with its per_word_crcs
+    // argument.
+    let blocks = words.len() / N;
+    let blocks = blocks.saturating_sub(1);
+    for i in 0..blocks {
+        // Load the next N words.
+        let mut buffer: [usize; N] =
+            core::array::from_fn(|j| usize::to_le(words[i * N + j] ^ (crcs[j] as usize)));
+
+        crcs.fill(0);
+        for j in 0..W {
+            for k in 0..N {
+                crcs[k] ^= Crc32BraidTable::<N>::TABLE[j][buffer[k] & 0xff];
+                buffer[k] >>= 8;
+            }
+        }
+    }
+
+    let crc = core::mem::take(&mut crcs[0]);
+    let crc = crc32_words_inner(&words[blocks * N..], crc, &crcs);
+    let crc = crc32_naive_inner(suffix, crc);
+    !crc
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    quickcheck::quickcheck! {
+        fn naive_is_crc32fast(v: Vec<u8>, start: u32) -> bool {
+            let mut h = crc32fast::Hasher::new_with_initial(start);
+            h.update(&v[..]);
+            crc32_naive(&v[..], start) == h.finalize()
+        }
+
+        fn words_is_crc32fast(v: Vec<u8>, start: u32) -> bool {
+            let mut h = crc32fast::Hasher::new_with_initial(start);
+            h.update(&v[..]);
+            crc32_words(&v[..], start) == h.finalize()
+        }
+
+        fn braid_4_is_crc32fast(v: Vec<u8>, start: u32) -> bool {
+            let mut h = crc32fast::Hasher::new_with_initial(start);
+            h.update(&v[..]);
+            crc32_braid::<4>(&v[..], start) == h.finalize()
+        }
+
+        fn braid_5_is_crc32fast(v: Vec<u8>, start: u32) -> bool {
+            let mut h = crc32fast::Hasher::new_with_initial(start);
+            h.update(&v[..]);
+            crc32_braid::<5>(&v[..], start) == h.finalize()
+        }
+
+        fn braid_6_is_crc32fast(v: Vec<u8>, start: u32) -> bool {
+            let mut h = crc32fast::Hasher::new_with_initial(start);
+            h.update(&v[..]);
+            crc32_braid::<6>(&v[..], start) == h.finalize()
+        }
+    }
+}

zlib-rs/src/lib.rs

@@ -1,6 +1,7 @@
 mod adler32;
 pub mod allocate;
 pub mod c_api;
+mod crc32;
 pub mod deflate;
 pub mod inflate;