cab7f8bf34
The /Prev chain handler for incremental PDF updates was already fully implemented. All 12 acceptance criteria tests pass. Verification note added at notes/pdftract-1zhu.md covering: - load_xref_with_prev_chain implementation (xref.rs:2154-2269) - Cycle detection, depth limiting, override semantics - Hybrid file support via load_single_xref - All tests passing (3-revision chain, object lifecycle, trailer handling) Co-Authored-By: Claude Code <noreply@anthropic.com>
70 lines
3.5 KiB
Markdown
70 lines
3.5 KiB
Markdown
# pdftract-1zhu: Incremental Update Chain Handler Implementation
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## Summary
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The `/Prev` chain handler for incremental PDF updates was already fully implemented in `crates/pdftract-core/src/parser/xref.rs`. All acceptance criteria are met.
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## Implementation Details
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### Core Function: `load_xref_with_prev_chain`
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Located at `xref.rs:2154-2269`, this function:
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1. Loads the trailing xref (auto-detects traditional vs stream vs hybrid)
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2. Recursively follows `/Prev` pointers in trailers
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3. Merges revisions with override semantics (newest wins for each object number)
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4. Returns the latest revision's trailer
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### Key Features Implemented
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- **Cycle detection**: `HashSet<u64>` tracks visited offsets, emits `STRUCT_CIRCULAR_REF`
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- **Depth limit**: `MAX_PREV_DEPTH = 32`, emits `STRUCT_DEPTH_EXCEEDED` on exceed
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- **Override semantics**: For each ObjRef, LATER revision (loaded first) wins
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- **Trailer handling**: Latest revision's trailer returned (newest /Root, /Info, /ID)
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- **Edge cases**:
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- `/Prev <= 0` treated as absent (no previous revision)
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- `/Prev > file_size` emits `STRUCT_INVALID_PREV_OFFSET`, ignores /Prev
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- Hybrid files: each revision calls `load_single_xref` which handles hybrid merging
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### Hybrid Support
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`load_single_xref` (xref.rs:2071-2107) detects hybrid files via `is_hybrid_trailer` and calls `merge_hybrid` when `/XRefStm` is present. This is invoked at each level of the `/Prev` chain.
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## Test Results
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### All `/Prev` Chain Tests PASS (12/12)
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```
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test parser::xref::tests::test_prev_chain_negative_prev_is_absent ... ok
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test parser::xref::tests::test_prev_chain_object_added_only_in_latest ... ok
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test parser::xref::tests::test_prev_chain_trailer_from_latest ... ok
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test parser::xref::tests::test_prev_chain_three_revisions_latest_wins ... ok
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test parser::xref::tests::test_prev_chain_zero_prev_is_absent ... ok
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test parser::xref::tests::test_prev_chain_cycle_detection ... ok
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test parser::xref::tests::test_prev_chain_depth_limit ... ok
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test parser::xref::tests::test_prev_chain_invalid_offset ... ok
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test parser::xref::tests::test_prev_chain_object_add_modify_free ... ok
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test parser::xref::tests::test_prev_chain_hybrid_file ... ok
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test parser::xref::tests::proptest_prev_chain_tests::prop_prev_chain_random_no_panic ... ok
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test parser::xref::tests::proptest_prev_chain_tests::prop_prev_chain_random_offsets_no_panic ... ok
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```
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### Acceptance Criteria Status
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| Criterion | Status | Notes |
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|-----------|--------|-------|
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| 3-revision chain, latest wins | ✅ PASS | test_prev_chain_three_revisions_latest_wins |
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| Object 7: add/modify/free lifecycle | ✅ PASS | test_prev_chain_object_add_modify_free |
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| Object added only in latest | ✅ PASS | test_prev_chain_object_added_only_in_latest |
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| Trailer from latest revision | ✅ PASS | test_prev_chain_trailer_from_latest |
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| /Prev cycle detection | ✅ PASS | test_prev_chain_cycle_detection |
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| Depth limit (32 revisions) | ✅ PASS | test_prev_chain_depth_limit |
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| proptest: random configs | ✅ PASS | prop_prev_chain_random_no_panic |
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| INV-8 maintained | ✅ PASS | No changes to xref module structure |
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## Pre-existing Issues
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Some xref tests fail (forward_scan, multi-subsection parsing), but these are unrelated to the `/Prev` chain handler and represent pre-existing issues from earlier work.
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## Files
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- Implementation: `crates/pdftract-core/src/parser/xref.rs:2154-2269`
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- Tests: `crates/pdftract-core/src/parser/xref.rs:3826-4338`
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- Constants: `MAX_PREV_DEPTH = 32` at line 2113
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## Verification
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```bash
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cargo test -p pdftract-core prev_chain --lib
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# Result: 12 passed; 0 failed
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```
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