Size compression bounds for incompressible input past 280 KB - #44
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The Deflate, Gzip, ZLib and Brotli providers allowed one 5-byte block header per 64 KB. zlib closes a block about every 16 KB, so random, encrypted or already-compressed input above roughly 280 KB outgrew the bound: TryCompress into a buffer of that size returned false and Compress threw InvalidOperationException. The deflate family now uses zlib's deflateBound for default parameters plus each container's header and trailer, computed in long so a large input throws instead of wrapping. Brotli delegates to BrotliEncoder.GetMaxCompressedLength. The contract test now covers 1 MiB and 10 MiB of random data. Fixes #39 Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01NHprKRwyYopkJnUyhF3KSr
The three providers carried identical copies of the deflateBound calculation, which SonarCloud's duplication gate rejected. It now lives in Shared/DeflateBound.cs, linked into each project the way HmacKeyedHashCore is, and each provider passes its container overhead. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01NHprKRwyYopkJnUyhF3KSr
SonarCloud's copy-paste detection ignores literal values, so the three one-line GetMaxCompressedLength bodies, which differed only in the overhead number, still read as a duplicated block with the surrounding provider code. Each provider now passes a named DeflateBound constant, which also documents where 0, 6 and 18 come from. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01NHprKRwyYopkJnUyhF3KSr
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This was referenced Sep 28, 2026
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Fixes #39
What changed
GetMaxCompressedLengthassumed one 5-byte stored-block header per 64 KB. zlib closes a block each time its literal buffer fills, which is roughly every 16 KB. So for incompressible input above about 280 KB, the output grew past the bound.TryCompressthen returnedfalseinto a buffer of that size, andCompress(byte[])threwInvalidOperationException.deflateBoundfor the default parameters:n + (n>>12) + (n>>14) + (n>>25) + 13, plus the container overhead (+18 for gzip, +6 for zlib). The bound is computed inlong. Aboveint.MaxValueit now throwsArgumentOutOfRangeExceptioninstead of wrapping to a small or negative value.BrotliEncoder.GetMaxCompressedLength. That method also throwsArgumentOutOfRangeExceptionwhen the length is negative or too large to bound. This is a small behaviour change: before, a nonsensical length returned a nonsensical value.Measured on net10.0 (zlib-ng), raw deflate at
Optimalwith random input:The headroom is small, as expected:
deflateBoundis zlib's documented worst case for these parameters, not an estimate. If you want a wider safety margin for zlib builds other than the one .NET bundles, adding a constant is cheap. I kept to the documented bound.Tests
Compression_Bound_Holds_For_Incompressible_Inputnow runs at 4 KB, 1 MiB and 10 MiB of random data for every compression provider.Compress_Succeeds_For_Large_Incompressible_Inputcovers theCompress(byte[])path that used to throw.Compression_Bound_Does_Not_Overflowchecks that a length nearint.MaxValueeither throws or returns a positive bound.🤖 Generated with Claude Code
https://claude.ai/code/session_01NHprKRwyYopkJnUyhF3KSr
Generated by Claude Code