jedarden f7e6ff4173 docs(pdftract-5cqy): add xref stream parser verification note

The xref stream parser implementation was already complete in
crates/pdftract-core/src/parser/xref.rs. All acceptance criteria pass:

- Simple test /W [1 4 2] /Index [0 6]: 6 entries decoded correctly
- Type-2 compressed entries: route through ObjStm correctly
- Multi-subsection /Index [0 3 100 2]: produces correct entries
- Predictor support: FlateDecode + PNG predictor handled
- Zero-width field /W [1 4 0]: generation defaults to 0
- proptest: random byte sequences never panic
- INV-8 maintained: no production panics

All 11 xref stream tests pass.

Co-Authored-By: Claude Code <noreply@anthropic.com>

2026-05-22 15:30:02 -04:00

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pdftract-5cqy: Xref Stream Parser Implementation

Summary

Implemented xref stream parser for PDF 1.5+ cross-reference streams with full support for:

/W field widths (type_w, obj_w, gen_w)
Type 0 (free), Type 1 (in-use), Type 2 (compressed in ObjStm) entries
/Index subsection boundaries with default [0 /Size]
Big-endian multi-byte field decoding
Zero-width field handling
FlateDecode decompression with PNG predictor support
Proper error handling and diagnostics (INV-8 compliant)

Implementation Location

File: crates/pdftract-core/src/parser/xref.rs
Function: parse_xref_stream(source: &dyn PdfSource, stream_obj_offset: u64) -> XrefSection
Lines: 1252-1569

Key Features

Indirect object parsing: Uses Phase 1.2's ObjectParser::parse_indirect_object() to read the xref stream object
Stream decompression: Uses Phase 1.5's decode_stream() for FlateDecode with predictor support
Field width handling: Supports any /W [type_w obj_w gen_w] configuration including zero-width fields
Multi-subsection support: Handles /Index [first_1 count_1 first_2 count_2 ...] arrays
Big-endian decoding: read_big_endian_field() helper for 1-8 byte fields
Trailer dict extraction: Copies relevant keys (Root, Info, ID, Encrypt, Prev) from stream dict

Test Results

All xref stream tests pass:

test_parse_xref_stream_simple: PASS - /W [1 4 2] /Index [0 6] with 6 entries
test_parse_xref_stream_multi_subsection: PASS - /Index [0 3 100 2] produces correct entries
test_parse_xref_stream_type2_compressed: PASS - Type-2 entries route through ObjStm
test_parse_xref_stream_field_width_zero_gen: PASS - /W [1 4 0] (gen always 0)
test_parse_xref_stream_with_predictor: PASS - FlateDecode + PNG predictor
test_parse_xref_stream_invalid_entry_type: PASS - Unknown types emit diagnostics
test_parse_xref_stream_missing_size: PASS - Emits appropriate diagnostic
test_parse_xref_stream_invalid_w_array: PASS - Emits appropriate diagnostic
proptest_parse_xref_stream_no_panic: PASS - Random bytes never panic
proptest_parse_xref_stream_random_offset_no_panic: PASS - Random offsets never panic
test_debug_xref_stream_parsing: PASS - Debug helper test

INV-8 Compliance

Verified: No unwrap(), expect(), or panic!() in production xref stream parsing code.

Line 206: .unwrap_or(false) is safe (handles poisoned lock gracefully)
All other unwrap()/panic! calls are in #[cfg(test)] modules (allowed per INV-8)

Acceptance Criteria Status

Criterion	Status	Notes
Simple test /W [1 4 2] /Index [0 6]	✅ PASS	6 entries decoded correctly
Type-2 ObjStm routing	✅ PASS	Compressed entries parse correctly
Multi-subsection /Index [0 3 100 2]	✅ PASS	Entries at 0,1,2,100,101
Predictor (FlateDecode + PNG)	✅ PASS	Stream decoder handles transparently
Field width /W [1 4 0]	✅ PASS	Zero-width gen field defaults to 0
proptest random bytes	✅ PASS	No panics on random input
INV-8 maintained	✅ PASS	No production panics

Integration Points

Phase 1.2: Uses ObjectParser::parse_indirect_object() for reading the xref stream object
Phase 1.5: Uses decode_stream() for decompression with filter/predictor support
Object resolution: Type-2 entries return XrefEntry::Compressed { obj_stm_nr, index } for ObjStm resolver

References

Plan section: Phase 1.3 line 1089-1123 (xref streams)
PDF spec 7.5.8 (Cross-Reference Streams)
Bead: pdftract-5cqy

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