jedarden cab7f8bf34 docs(pdftract-1zhu): add verification note for /Prev chain handler

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>

2026-05-22 19:15:47 -04:00

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pdftract-1zhu: Incremental Update Chain Handler Implementation

Summary

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.

Implementation Details

Core Function: `load_xref_with_prev_chain`

Located at xref.rs:2154-2269, this function:

Loads the trailing xref (auto-detects traditional vs stream vs hybrid)
Recursively follows /Prev pointers in trailers
Merges revisions with override semantics (newest wins for each object number)
Returns the latest revision's trailer

Key Features Implemented

Cycle detection: HashSet<u64> tracks visited offsets, emits STRUCT_CIRCULAR_REF
Depth limit: MAX_PREV_DEPTH = 32, emits STRUCT_DEPTH_EXCEEDED on exceed
Override semantics: For each ObjRef, LATER revision (loaded first) wins
Trailer handling: Latest revision's trailer returned (newest /Root, /Info, /ID)
Edge cases:
- /Prev <= 0 treated as absent (no previous revision)
- /Prev > file_size emits STRUCT_INVALID_PREV_OFFSET, ignores /Prev
- Hybrid files: each revision calls load_single_xref which handles hybrid merging

Hybrid Support

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.

Test Results

All `/Prev` Chain Tests PASS (12/12)

test parser::xref::tests::test_prev_chain_negative_prev_is_absent ... ok
test parser::xref::tests::test_prev_chain_object_added_only_in_latest ... ok
test parser::xref::tests::test_prev_chain_trailer_from_latest ... ok
test parser::xref::tests::test_prev_chain_three_revisions_latest_wins ... ok
test parser::xref::tests::test_prev_chain_zero_prev_is_absent ... ok
test parser::xref::tests::test_prev_chain_cycle_detection ... ok
test parser::xref::tests::test_prev_chain_depth_limit ... ok
test parser::xref::tests::test_prev_chain_invalid_offset ... ok
test parser::xref::tests::test_prev_chain_object_add_modify_free ... ok
test parser::xref::tests::test_prev_chain_hybrid_file ... ok
test parser::xref::tests::proptest_prev_chain_tests::prop_prev_chain_random_no_panic ... ok
test parser::xref::tests::proptest_prev_chain_tests::prop_prev_chain_random_offsets_no_panic ... ok

Acceptance Criteria Status

Criterion	Status	Notes
3-revision chain, latest wins	✅ PASS	test_prev_chain_three_revisions_latest_wins
Object 7: add/modify/free lifecycle	✅ PASS	test_prev_chain_object_add_modify_free
Object added only in latest	✅ PASS	test_prev_chain_object_added_only_in_latest
Trailer from latest revision	✅ PASS	test_prev_chain_trailer_from_latest
/Prev cycle detection	✅ PASS	test_prev_chain_cycle_detection
Depth limit (32 revisions)	✅ PASS	test_prev_chain_depth_limit
proptest: random configs	✅ PASS	prop_prev_chain_random_no_panic
INV-8 maintained	✅ PASS	No changes to xref module structure

Pre-existing Issues

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.

Files

Implementation: crates/pdftract-core/src/parser/xref.rs:2154-2269
Tests: crates/pdftract-core/src/parser/xref.rs:3826-4338
Constants: MAX_PREV_DEPTH = 32 at line 2113

Verification

cargo test -p pdftract-core prev_chain --lib
# Result: 12 passed; 0 failed

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