Reviving a 2015 crc32 crate pushed its 17 KiB of lookup tables into build.rs

Slicing-by-16 needs 17 lookup tables: one 256-entry big-endian table and 16 256-entry little-endian tables at different offsets, which the post works out as 17,408 bytes that have to exist at runtime ^[7]. Those constants can be committed to git. They can also be built into a static at startup. crc32-v2 generates them during compilation instead: a proc-macro subcrate, crc32-codegen, is invoked from build.rs, generates all 17 tables from the CRC-32 polynomial arithmetic, writes them to $OUT_DIR/crc_tables.rs, and the library include!s the file, so nothing initializes at run time and the tables land in the binary at link time ^[8].

The whole of build.rs is one call to crc32_codegen::run() ^[9]. What makes it resolve is a path entry under [build-dependencies] pointing at the subcrate in the same repository ^[10]. 17,408 bytes is 17 KiB exactly ^[17]. I would take that trade in a checksum library: the work happens once per clean build, and a proc-macro in the build graph is fine until somebody has to vendor it.

The only throughput figure in the post attaches to the old code. The byte-at-a-time original was "Absolutely not capable of 1 GiB/s throughput", the author wrote ^{[6] [15]}. I would expect the wide table variants to show their gain on long buffers and to lose it on short messages, so measure your own input sizes before switching.

CRC-32 is in ZIP integrity checks, Ethernet frames, PNG and every zlib call that computes one internally, according to the post ^[14]. The author says people were shipping the 2015 crate in their dependency trees and did not know how to reach its maintainer ^[2]. The crate had no changelog and no CI, and a Cargo.toml older than the stable Rust syntax he uses daily ^[1].

The post describes publishing the rewrite under the name crc32-v2 and does not report acquiring the crates.io entry for crc32 ^[16]. A build that lists crc32 today still compiles the 2015 code ^[18]. Adoption is a deliberate line edit in Cargo.toml, made by a reader who decided a stranger's rewrite beat an unmaintained original.

The rewrite adds a streaming Digest, a crc32_combine() that squares GF(2) matrices in O(log n) ^[13], PyO3 bindings gated on the python and std features ^[11], and 40 integration tests in tests/byfour.rs ^[12]. It ships as no_std, and the crate root sets #![forbid(unsafe_code)] ^{[3] [4]}. The 2015 version was three files: a lookup table, a loop, a bitwise XOR ^[5]. "Because anything worth resurrecting is worth resurrecting properly," the author wrote ^[20].