f7e6ff4173
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>
72 lines
3.5 KiB
Markdown
72 lines
3.5 KiB
Markdown
# pdftract-5cqy: Xref Stream Parser Implementation
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## Summary
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Implemented xref stream parser for PDF 1.5+ cross-reference streams with full support for:
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- `/W` field widths (type_w, obj_w, gen_w)
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- Type 0 (free), Type 1 (in-use), Type 2 (compressed in ObjStm) entries
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- `/Index` subsection boundaries with default `[0 /Size]`
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- Big-endian multi-byte field decoding
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- Zero-width field handling
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- FlateDecode decompression with PNG predictor support
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- Proper error handling and diagnostics (INV-8 compliant)
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## Implementation Location
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- File: `crates/pdftract-core/src/parser/xref.rs`
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- Function: `parse_xref_stream(source: &dyn PdfSource, stream_obj_offset: u64) -> XrefSection`
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- Lines: 1252-1569
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## Key Features
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1. **Indirect object parsing**: Uses Phase 1.2's `ObjectParser::parse_indirect_object()` to read the xref stream object
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2. **Stream decompression**: Uses Phase 1.5's `decode_stream()` for FlateDecode with predictor support
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3. **Field width handling**: Supports any `/W [type_w obj_w gen_w]` configuration including zero-width fields
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4. **Multi-subsection support**: Handles `/Index [first_1 count_1 first_2 count_2 ...]` arrays
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5. **Big-endian decoding**: `read_big_endian_field()` helper for 1-8 byte fields
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6. **Trailer dict extraction**: Copies relevant keys (Root, Info, ID, Encrypt, Prev) from stream dict
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## Test Results
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All xref stream tests pass:
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- `test_parse_xref_stream_simple`: PASS - /W [1 4 2] /Index [0 6] with 6 entries
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- `test_parse_xref_stream_multi_subsection`: PASS - /Index [0 3 100 2] produces correct entries
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- `test_parse_xref_stream_type2_compressed`: PASS - Type-2 entries route through ObjStm
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- `test_parse_xref_stream_field_width_zero_gen`: PASS - /W [1 4 0] (gen always 0)
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- `test_parse_xref_stream_with_predictor`: PASS - FlateDecode + PNG predictor
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- `test_parse_xref_stream_invalid_entry_type`: PASS - Unknown types emit diagnostics
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- `test_parse_xref_stream_missing_size`: PASS - Emits appropriate diagnostic
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- `test_parse_xref_stream_invalid_w_array`: PASS - Emits appropriate diagnostic
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- `proptest_parse_xref_stream_no_panic`: PASS - Random bytes never panic
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- `proptest_parse_xref_stream_random_offset_no_panic`: PASS - Random offsets never panic
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- `test_debug_xref_stream_parsing`: PASS - Debug helper test
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## INV-8 Compliance
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Verified: No `unwrap()`, `expect()`, or `panic!()` in production xref stream parsing code.
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- Line 206: `.unwrap_or(false)` is safe (handles poisoned lock gracefully)
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- All other `unwrap()`/`panic!` calls are in `#[cfg(test)]` modules (allowed per INV-8)
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## Acceptance Criteria Status
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| Criterion | Status | Notes |
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|-----------|--------|-------|
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| Simple test /W [1 4 2] /Index [0 6] | ✅ PASS | 6 entries decoded correctly |
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| Type-2 ObjStm routing | ✅ PASS | Compressed entries parse correctly |
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| Multi-subsection /Index [0 3 100 2] | ✅ PASS | Entries at 0,1,2,100,101 |
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| Predictor (FlateDecode + PNG) | ✅ PASS | Stream decoder handles transparently |
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| Field width /W [1 4 0] | ✅ PASS | Zero-width gen field defaults to 0 |
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| proptest random bytes | ✅ PASS | No panics on random input |
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| INV-8 maintained | ✅ PASS | No production panics |
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## Integration Points
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- **Phase 1.2**: Uses `ObjectParser::parse_indirect_object()` for reading the xref stream object
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- **Phase 1.5**: Uses `decode_stream()` for decompression with filter/predictor support
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- **Object resolution**: Type-2 entries return `XrefEntry::Compressed { obj_stm_nr, index }` for ObjStm resolver
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## References
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- Plan section: Phase 1.3 line 1089-1123 (xref streams)
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- PDF spec 7.5.8 (Cross-Reference Streams)
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- Bead: pdftract-5cqy
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