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Crc32.Compute

Computes a CRC-32 checksum over a binary value.

Syntax​

Crc32.Compute(
    data as binary,
    optional variant as text,
    optional options as record
) as number

Remarks​

M's standard library has no hashing at all, yet binary formats are full of CRC-32 fields: gzip trailers, zip central directories, PNG chunks, Avro snappy blocks (a plain CRC-32 of the uncompressed data) and snappy framing-format chunks (a masked CRC-32C). Without this, those checksums simply go unverified.

The implementation is table-driven — a buffered 256-entry lookup table, one lookup and three bitwise operations per byte. Every intermediate stays below 232 by construction, so correctness never depends on the runtime's integer conversion behaviour.

Variants: CRC32 (aliases ISO-HDLC, zlib, gzip, zip, png), CRC32C (aliases Castagnoli, iSCSI, snappy), JAMCRC, BZIP2 and MPEG2. Names match case-insensitively, ignoring -, /, _ and spaces.

Options:

  • SnappyMask : A logical (true/false). Apply the snappy framing-format mask to the result (default is false).

Examples​

Example #1​

The zlib/gzip/zip/PNG polynomial, which is what variant defaults to.

Crc32.Compute(File.Contents("C:\data\payload.bin"))

Example #2​

Castagnoli, as used by snappy and iSCSI.

Crc32.Compute(data, "CRC32C")

Example #3​

A masked CRC-32C, as it appears in a snappy framing-format chunk header.

Crc32.Compute(data, "CRC32C", [SnappyMask = true])

Example #4​

Verify a gzip file against the CRC in its own trailer.

let
    Gz       = File.Contents("C:\data\archive.gz"),
    Plain    = Binary.Decompress(Gz, Compression.GZip),
    Stored   = BinaryFormat.ByteOrder(BinaryFormat.UnsignedInteger32, ByteOrder.LittleEndian)(
                   Binary.Range(Gz, Binary.Length(Gz) - 8, 4)),
    Computed = Crc32.Compute(Plain)
in
    Stored = Computed

Category​

Driverless connectors