Instruction file imported from Akshay20051027/C- (.github/instructions/compression.instructions.md). Copyright stays with the author.

System.IO.Compression — Folder-Specific Guidance

Format Specification Correctness (D12)

• ZIP64 extensions must be used for files over 4GB — extra field sizes, offsets, and header values must use 64-bit fields when the 32-bit range is exceeded
• Compression levels must align with native library semantics — verify enum-to-native mapping is correct
• New compression format support (e.g., zstd in ZIP) must include a feature switch for trimming/AOT and an explicit opt-in mechanism
• Decompression must handle concatenated payloads and partial reads — the decompressor must not assume a single contiguous compressed stream
• Breaking changes to format handling must be documented and include migration guidance

Security

• Maximum decompressed size limits must be configurable to prevent zip-bomb attacks, following the existing deflate size limit pattern
• Archive extraction must validate entry paths to prevent path traversal attacks (entries with ../ segments)

Performance & Allocation (D5)

• Use ArrayPool<byte> for variable-size compression/decompression buffers — return buffers in finally blocks
• Avoid allocating excessively large fixed buffers per operation (100KB+ per compression operation is expensive)
• Pin buffers for the duration of native I/O operations
• Hot paths must avoid per-operation allocations — prefer pooled buffers and cached delegates
• Closures that capture state on hot paths must be eliminated — use static lambdas with explicit state

Async Operations

• Async compression/decompression must not perform the actual compression work synchronously before the first await
• Sync and async code paths must share non-trivial logic through common helpers to prevent divergence

Cross-Platform Metadata (D19)

• Archive extraction must preserve or correctly translate platform-specific metadata — Unix execute permissions, symlinks, and hidden file attributes
• File path operations within archives must use forward slashes as the archive-internal separator per the ZIP specification
• Tests must verify metadata round-trip on both Windows and Unix platforms

Native Interop

• Native library updates (brotli, zlib, zstd) must be tracked and the managed wrapper updated accordingly
• Use LibraryImport (source-generated) for new P/Invoke declarations
• SafeHandle-derived types must be used for native compression handles — never store raw IntPtr
• Native error codes must be mapped to appropriate .NET exceptions with the native error code preserved

Error Handling (D9)

• Exceptions must be the most specific applicable type — InvalidDataException for corrupt archives, IOException for I/O failures, with actionable context (entry name, expected vs actual values)
• Operations on streams that may not support Length/Seek must be guarded appropriately

Interoperability Testing (D10)

• Tests must use archive files created by external tools — not just round-trip tests with the same .NET implementation
• Test with archives from multiple platforms and compression libraries to verify cross-tool compatibility
• Cover edge cases: empty archives, many small entries, entries at size boundaries (4GB, uint.MaxValue)
• Dispose behavior must be tested — verify resources are released and post-disposal operations throw