Cabinet reader

Normative notes for CabReader in RecursiveAI.OneArchive.Cab: read only. Source: Microsoft's [MS-CAB] (Cabinet File Format) 2.1 to 2.4 and 3.1; LZX is in lzx-decoder.md. Gaps were closed with cabextract 1.11 and gcab 1.6 run as black boxes; findings are marked [tool].

Layout read

All integers are little-endian.

• CFHEADER (36 bytes): MSCF, reserved, cbCabinet, reserved, coffFiles, reserved, version minor and major, cFolders, cFiles, flags, setID, iCabinet. Then, if the reserve flag (4) is set, cbCFHeader (u16, at most 60000), cbCFFolder (u8), cbCFData (u8) and cbCFHeader reserved bytes; then the NUL-terminated szCabinetPrev, szDiskPrev (flag 1) and szCabinetNext, szDiskNext (flag 2), each at most 255 bytes. Reserved fields are not checked; major version must be 1 (minor is exposed, not checked); unknown flag bits are ignored. coffFiles must lie after the folder table and inside the file.
• CFFOLDER (8 bytes + cbCFFolder): coffCabStart, cCFData, typeCompress. Method = typeCompress & 15 (0 none, 1 MSZIP, 2 Quantum, 3 LZX); LZX window = typeCompress >> 8 & 31, which must be 15 to 21 (other values raise ArchiveUnsupportedException).
• CFFILE (16 bytes + name): cbFile, uoffFolderStart, iFolder, DOS date, DOS time, attribs, NUL-terminated name of at most 256 bytes. iFolder 0xFFFD, 0xFFFE and 0xFFFF mark files continued from the previous cabinet, into the next one, or both; they are listed (CabEntry.Continuation) but cannot be opened (ArchiveUnsupportedException naming the neighbouring cabinet). A file that is not continued must satisfy uoffFolderStart + cbFile <= min(0x7FFF8000, cCFData * 32768).
• CFDATA: csum (u32), cbData (u16), cbUncomp (u16), cbCFData reserved bytes, cbData bytes. Blocks of a folder are contiguous from coffCabStart. cbUncomp 0 means the block continues in the next cabinet (ArchiveUnsupportedException); more than 32768 is an error.

Names, times, attributes

• A name with _A_NAME_IS_UTF (0x80) is UTF-8 (invalid sequences become U+FFFD); otherwise it is ISO 8859-1 unless CabReaderOptions.LegacyNameEncoding says otherwise. \ becomes / in ArchiveEntry.Name; the stored bytes are in RawName.
• DOS date and time are read as UTC, like zip's. A date or time that is not valid gives no ModifiedTime.
• Mode is 0444, plus owner write unless _A_RDONLY, plus execute for everyone with _A_EXEC. Hidden and system are only in Attributes.

Data blocks

• Checksum ([MS-CAB] 3.1): XOR of the little-endian 32-bit words of the data, then the same function continued over cbData and cbUncomp (four bytes), where a trailing partial word is built from the last byte down. [tool] The data reserve is not included, although the specification calls the second part the "header area": with reserve bytes present, only this reading extracts under cabextract. A zero csum is not checked. A mismatch raises CompressionFormatException at the block's offset unless VerifyChecksum is false.
• Stored: cbData must equal cbUncomp.
• MSZIP: the block is CK then a raw deflate stream that ends with a final block and produces exactly cbUncomp bytes. Matches may reach into the previous 32 KiB of the folder's output (which is the history of the folder, not of the block); the reader keeps that history across blocks and uses the deflate decoder's preset dictionary for it. [tool] Reference tools assume every block but the last has 32768 bytes; this reader accepts any size.
• LZX: one frame per block with cbUncomp 32768 (the last block may be shorter; any other short block is an error); a cabinet is decoded with one LzxDecoder per folder.

Access and limits

• The source must be seekable (Archive copies a forward-only source into memory first, subject to MaxTotalSize).
• A folder is decoded from its first block. Reading files in table order costs one pass; going back decodes from the start again. At most two idle decoders are kept. Entries are listed in table order.
• Limits: MaxEntryCount, MaxPathLength, MaxPathDepth, MaxTotalSize (declared sizes, checked while listing), MaxExpansionRatio (compressed bytes counted per block including headers, output per block), MaxWindowSize (the LZX window, checked when an entry is opened). Errors name the format cab and the offset of the field or block at fault; errors from the deflate and LZX decoders are re-raised with the cabinet offset of the block and their own description.
• Not supported: Quantum folders (ArchiveUnsupportedException), files and blocks that span cabinets, LZX windows beyond 2^21, writing.