pdftract/notes/pdftract-5og4.md

pdftract-5og4: Hybrid Xref Handler Implementation

Summary

Implemented hybrid xref handler that merges traditional table + xref stream with traditional priority. The implementation was already present in the codebase; this verification confirms all acceptance criteria are met.

Implementation Status

Core Components (Already Implemented)

1. merge_hybrid function (xref.rs:197-248)

   • Merges traditional xref table (authoritative) with xref stream (supplementary)
   • Traditional entries always win for overlapping object numbers
   • Stream entries fill gaps not covered by traditional table
   • Sets is_hybrid: true on merged result
   • Emits STRUCT_HYBRID_CONFLICT diagnostic for Free/InUse conflicts

2. is_hybrid_trailer function (xref.rs:260-265)

   • Detects hybrid files by checking for /XRefStm key in trailer dict

3. Integration in load_single_xref (xref.rs:1964-1984)

   • Checks for hybrid trailer after parsing traditional xref
   • Extracts /XRefStm offset
   • Loads supplementary xref stream
   • Merges with traditional taking priority

Acceptance Criteria

Criterion	Status	Test
Critical test: traditional entries override stream	✅ PASS	test_merge_hybrid_traditional_priority
Stream-only type-2 entries added to merged map	✅ PASS	test_merge_hybrid_gap_fill
Free/InUse conflict emits STRUCT_HYBRID_CONFLICT	✅ PASS	test_merge_hybrid_free_inuse_conflict
Non-hybrid trailer skips merge (pure traditional)	✅ PASS	test_is_hybrid_trailer_detection + load_single_xref integration
proptest: random combinations never panic	✅ PASS	proptest_merge_hybrid_no_panic (added)

Changes Made

Added Comprehensive Proptest

Added proptest_merge_hybrid_no_panic to the proptest_tests module (xref.rs:2416-2447):

• Tests random combinations of traditional and stream entries
• Uses prop::collection::hash_map to generate random entry sets
• Verifies merge_hybrid never panics on any input
• Covers all entry types (InUse, Free, Compressed)

Test Results

test parser::xref::tests::test_merge_hybrid_stream_only ... ok
test parser::xref::tests::test_merge_hybrid_free_inuse_conflict ... ok
test parser::xref::tests::test_merge_hybrid_proptest_simple ... ok
test parser::xref::tests::test_merge_hybrid_gap_fill ... ok
test parser::xref::tests::test_merge_hybrid_empty_sections ... ok
test parser::xref::tests::test_merge_hybrid_traditional_only ... ok
test parser::xref::tests::test_merge_hybrid_traditional_priority ... ok
test parser::xref::tests::test_merge_hybrid_trailer_xrefstm_removed ... ok
test parser::xref::tests::proptest_tests::proptest_merge_hybrid_no_panic ... ok

test result: ok. 9 passed; 0 failed

INV-8 Status

INV-8 (invariant preservation) is maintained:

• Traditional entries are never modified or removed
• Stream entries only added when traditional doesn't have the object number
• Trailer is taken from traditional with /XRefStm removed
• All diagnostics from both sections preserved

References

• Plan section: Phase 1.3 line 1090 (hybrid files)
• PDF spec 7.5.8.4 (Hybrid-Reference Files)