dweiss commented on PR #811:
URL: https://github.com/apache/commons-compress/pull/811#issuecomment-5461200306
> Commons Compress already depends on Commons Codec where you'll find
`org.apache.commons.codec.digest.PureJavaCrc32` and
`org.apache.commons.codec.digest.PureJavaCrc32C`.
Good to know, thank you. Unfortunately this won't work here because bzip2
uses a different byte/bit ordering from standard crc32 (or crc32c)
implementations. So it's very specific to this algorithm. Here's the LLM
explaining the details:
- PureJavaCrc32C: wrong polynomial (Castagnoli). It can't produce bzip2's
CRC under any transformation. Unusable.
- PureJavaCrc32: right polynomial, wrong bit orientation
(reflected/LSB-first; bzip2 is MSB-first). Usable only via
reverse32(crc(reverse8(M))), which costs a
bit-reversal pass per byte — and since it's plain Java slicing-by-8 with
no intrinsic behind it, you'd pay that pass on top of essentially the same table
computation the tree already does. Net effect ≈ 1.6× slower CRC than the
current class, for the sake of deleting ~60 lines.
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