Algorithm	Polynomial	Init	Reflect	XOR Out	Check (`"123456789"`)	Standard
CRC-16-CCITT	0x1021	0xFFFF	No	None	0x29B1	ITU-T V.41, CCSDS FECF
CRC-16-X.25	0x1021	0xFFFF	Yes	0xFFFF	0x906E	ITU-T X.25, RFC 9171
CRC-32	0xEDB88320	0xFFFFFFFF	Yes	0xFFFFFFFF	0xCBF43926	ISO 3309, ITU-T V.42
CRC-32C	0x82F63B78	0xFFFFFFFF	Yes	0xFFFFFFFF	0xE3069283	RFC 3720 (iSCSI)

Performance#

Three implementation tiers, selected automatically:

Tier	Strategy	CRC-32C throughput	When
Hardware	SSE4.2 / ARM CRC intrinsics	~15-30 GB/s	x86_64 with SSE4.2, aarch64
Slicing-by-8	8 x 256-entry tables	~1-4 GB/s	Software fallback (native, jsoo, wasm)
Byte-at-a-time	256-entry table	~300 MB/s	CRC-16 variants

Hardware detection follows the mirage-crypto pattern: compile-time flag gating via dune-configurator + runtime CPUID check. On x86_64, SSE4.2 accelerates CRC-32C; on ARM, both CRC-32 and CRC-32C use hardware instructions. The software slicing-by-8 path is always available as fallback and is the default on js_of_ocaml and wasm_of_ocaml targets.

# Crc.has_hardware_crc;;
- : bool = true

Usage#

let () =
  let data = "Hello, world!" in
  Printf.printf "CRC-16-CCITT: 0x%04X\n" (Crc.crc16_ccitt data);
  Printf.printf "CRC-16-X.25:  0x%04X\n" (Crc.crc16_x25 data);
  Printf.printf "CRC-32:       0x%08X\n" (Crc.crc32 data);
  Printf.printf "CRC-32C:      0x%08X\n" (Crc.crc32c data)

Sub-range variants for bytes buffers:

let () =
  let buf = Bytes.of_string "Hello, world!" in
  Printf.printf "Hello:  0x%04X\n" (Crc.crc16_ccitt_bytes buf 0 5);
  Printf.printf "world!: 0x%08X\n" (Crc.crc32c_bytes buf 7 6)

Installation#

Install with opam:

$ opam install nox-crc

If opam cannot find the package, it may not yet be released in the public opam-repository. Add the overlay repository, then install it:

$ opam repo add samoht https://tangled.org/gazagnaire.org/opam-overlay.git
$ opam update
$ opam install nox-crc

Portability#

All CRC-32 variants use Int32 arithmetic internally, ensuring correct results on both 64-bit native OCaml and 32-bit targets (js_of_ocaml, wasm_of_ocaml). The C stubs for hardware acceleration are optional: without them, the pure OCaml slicing-by-8 path is used automatically.

Test vectors#

All check values are verified against standard test vectors from ITU-T V.41, ISO 3309, and RFC 3720 Section 12.1. Property-based fuzz tests (Crowbar/AFL) verify string/bytes agreement, output ranges, self-check residues, and determinism.

License#

ISC

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