0020: Decode the filters documents actually use

Status: implemented.

Context

Support\PdfStream decoded two filters, /FlateDecode and /ASCIIHexDecode, each written as a bare name, and ignored /DecodeParms entirely.

That covers what this package writes. It does not cover what it is handed, and the way it fails is the worst available: an object that cannot be read is an object the signer will not sign around, so a document nothing is wrong with comes back as unsignable.

Three things were missing and all three are ordinary:

• a filter chain, /Filter [/ASCII85Decode /FlateDecode], applied in order;
• a predictor, /DecodeParms <</Predictor 12 /Columns 5>>. Every modern generator compresses its cross-reference stream this way, because consecutive rows of a cross-reference table differ from each other by very little;
• /LZWDecode, which predates Flate and still appears from older tooling.

The predictor is the one that matters. Measured on a three-object PDF 1.5 document whose cross-reference stream uses PNG-Up, the old decoder inflated the stream correctly and then read the differences as the values:

xref = {"2": 16908288}

One entry, pointing 16 MB into a 379-byte file, with objects 1, 3 and 4 absent entirely. The catalog is object 1, and signing rewrites the catalog, so the document was unsignable and the error said so about the wrong thing.

Decision

Decode the filter chain, and undo the predictor.

Support\PdfFilters takes the whole /Filter and /DecodeParms pair and applies them in order. Enums\StreamFilter names the five it implements.

Null, not the raw bytes

A filter this package does not implement makes the payload null rather than returning it undecoded. Handing back something compressed that happens to contain << is how a caller ends up parsing noise as objects, and a wrong object is worse than a missing one everywhere else in this codebase too.

Predictors only where they mean something

StreamFilter::takesPredictor() is true only for Flate and LZW. A /Predictor beside an ASCII filter is not illegal, it is meaningless, and applying one anyway would corrupt a payload that had decoded correctly.

The PNG filter type is read per row, which is what RFC 2083 specifies and what /Predictor 15 means: not "filter 15", but "any of them, row by row". The declared value therefore decides only that a PNG predictor is in play.

What is deliberately not implemented

• /DCTDecode and /JPXDecode. Streams are read here to find objects, and an image is never one. Decoding a JPEG to look for a dictionary in it would be work spent to find nothing.
• /Crypt. Encrypted documents are refused before any of this (0014).
• A TIFF predictor over sub-byte components. It needs the samples unpacked and repacked, and a producer pairing 4-bit components with a predictor is rare enough that guessing is worse than answering null.

Checked against the standard, not against itself

The LZW test decodes the worked example from ISO 32000-1 §7.4.4.2 table 10, and gets back -----A---B. Round-tripping through an encoder written in the same hour would only establish that two pieces of code agree with each other.

/EarlyChange is honoured for the same reason it exists: it decides whether the code width grows one code before the dictionary is full, and a decoder ignoring it produces plausible bytes that are wrong from the first width change onward.

Consequences

• Support\PdfStream::decode() moves to Support\PdfFilters, which PdfStream composes. Both are @internal.
• Documents from Word, Chrome's print-to-PDF and LaTeX with compression are read where the predictor previously made them unreadable. tests/Resources/xref-stream-predictor.pdf is committed as the case, and it is signed and validated in the suite.
• Nothing about what the package writes changes. XrefStreamWriter still emits no filter at all, because a revision indexes a handful of objects and zlib's header and checksum would make the stream larger than the bytes they compress (0009).

Alternatives rejected

	Why not
Return the raw bytes for an unimplemented filter	A caller parses compressed noise as objects, confidently
Read /Predictor as a per-stream filter type	It is per row, and /Predictor 15 is not a type at all
Skip LZW as obsolete	It costs one method and the documents exist
Decode /DCTDecode for completeness	An image never holds an object worth finding
Round-trip LZW through an encoder written here	Establishes agreement, not correctness