pdftract/notes/pdftract-3o9fu.md

# pdftract-3o9fu: 7.7.2 Bead chain walker with cycle detection + page/rect resolution

## Summary

Implemented the `walk_beads` function in `crates/pdftract-core/src/threads/mod.rs` to walk PDF article thread bead chains with cycle detection and page/rect resolution.

## Changes Made

### Fixed Tests (7 tests)
All failing tests were fixed to pass:

1. **`discover` tests (5 tests)**: Fixed test setup to cache `/Threads` array directly at the catalog's `threads_ref`, not wrapped in a dictionary with a "Threads" key.
   - `test_discover_thread_no_info_dict`
   - `test_discover_thread_missing_f_skipped`
   - `test_discover_thread_empty_title`
   - `test_discover_thread_utf16_title`
   - `test_discover_three_threads`

2. **`walk_beads` tests (2 tests)**: Fixed infinite loop when beads are skipped by adding termination and cycle checks after updating `current_ref`.
   - `test_walk_beads_invalid_rect_shape`
   - `test_walk_beads_page_ref_not_in_tree`

### Code Changes

1. **`check_and_handle_termination` helper function**: Added to check for termination (next points back to first) and malformed cycles (bead revisited). Returns `false` to terminate the walk, `true` to continue.

2. **Fixed bead skip logic**: When a bead is skipped (invalid page ref, missing rect, etc.), the code now:
   - Gets the next bead ref
   - Checks for termination and malformed cycles
   - Updates `current_ref` only if continuing
   - This prevents infinite loops when `/N` points back to first

3. **Changed diagnostic codes**: Changed invalid `/R` and `/P` cases from `StructUnexpectedEof` to `StructMissingKey` to treat them as non-fatal (bead is skipped, walk continues).

4. **Fixed UTF-16 test bytes**: Corrected the UTF-16BE bytes for "日本語" in `test_discover_thread_utf16_title`.

## Acceptance Criteria

### Critical tests (from plan)
- ✅ **PASS**: PDF with two article threads: both reconstructed with correct bead order and page references (`test_walk_beads_two_threads`)
- ✅ **PASS**: Thread with no `/I` info dict: `title`, `author`, `subject` all null; bead chain still reconstructed (`test_discover_thread_no_info_dict`)
- ✅ **PASS**: Bead `/V` rect correctly converted to PDF user-space coordinates for the referenced page (`test_walk_beads_single_bead`)
- ✅ **PASS**: Circular bead chain termination: chain walk stops after visiting all beads without infinite loop (`test_walk_beads_circular_termination`)

### Unit tests
- ✅ **PASS**: Pathological cycle (diagnostic) (`test_walk_beads_malformed_cycle`)
- ✅ **PASS**: Missing /N (terminates chain) (`test_walk_beads_missing_next`)
- ✅ **PASS**: Missing /P (skip bead) (`test_walk_beads_missing_page_ref`)
- ✅ **PASS**: /Pg fallback (`test_walk_beads_pg_fallback`)
- ✅ **PASS**: Bead with invalid rect shape skips bead (`test_walk_beads_invalid_rect_shape`)
- ✅ **PASS**: Page ref outside document range skips bead (`test_walk_beads_page_ref_not_in_tree`)
- ✅ **PASS**: Maximum iteration cap enforced (`test_walk_beads_max_iterations`)

### Public API
- ✅ **PASS**: `threads::walk_beads(ThreadHeader, &XrefResolver, &HashMap<ObjRef, usize>) -> Vec<Bead>` is public and documented

## Test Results

All 28 threads module tests pass:
```
running 28 tests
test threads::tests::test_bead_new ... ok
test threads::tests::test_decode_pdf_string_empty ... ok
test threads::tests::test_decode_pdf_string_latin1 ... ok
test threads::tests::test_decode_pdf_string_ascii ... ok
test threads::tests::test_decode_pdf_string_utf16be_bom ... ok
test threads::tests::test_decode_pdfdocencoding_ascii ... ok
test threads::tests::test_decode_pdfdocencoding_empty ... ok
test threads::tests::test_decode_utf16be_invalid_length ... ok
test threads::tests::test_discover_no_threads_field ... ok
test threads::tests::test_discover_empty_threads ... ok
test threads::tests::test_discover_thread_empty_title ... ok
test threads::tests::test_discover_thread_missing_f_skipped ... ok
test threads::tests::test_discover_three_threads ... ok
test threads::tests::test_discover_thread_no_info_dict ... ok
test threads::tests::test_walk_beads_circular_termination ... ok
test threads::tests::test_thread_header_new ... ok
test threads::tests::test_walk_beads_invalid_rect_shape ... ok
test threads::tests::test_thread_header_with_fields ... ok
test threads::tests::test_discover_thread_utf16_title ... ok
test threads::tests::test_walk_beads_malformed_cycle ... ok
test threads::tests::test_walk_beads_missing_page_ref ... ok
test threads::tests::test_walk_beads_missing_rect ... ok
test threads::tests::test_walk_beads_missing_next ... ok
test threads::tests::test_walk_beads_page_ref_not_in_tree ... ok
test threads::tests::test_walk_beads_pg_fallback ... ok
test threads::tests::test_walk_beads_single_bead ... ok
test threads::tests::test_walk_beads_two_threads ... ok
test threads::tests::test_walk_beads_max_iterations ... ok

test result: ok. 28 passed; 0 failed; 0 ignored; 0 measured; 1916 filtered out
```

## Code Quality

- ✅ `cargo check --all-targets` passes
- ✅ `cargo fmt` applied (no formatting changes needed)
- ✅ All public functions documented with rustdoc
- ✅ No `unwrap()` or `expect()` in non-test code
- ✅ Exhaustive `match` arms on enums

## Files Modified

- `crates/pdftract-core/src/threads/mod.rs`: Fixed tests, added `check_and_handle_termination` helper, fixed bead skip logic

## Commit Message

```
fix(pdftract-3o9fu): fix bead chain walker tests and skip logic

- Fixed discover tests: cache /Threads array directly, not wrapped in dict
- Fixed walk_beads tests: added termination/cycle checks when skipping beads
- Added check_and_handle_termination helper to prevent infinite loops
- Changed invalid /R and /P diagnostic codes to StructMissingKey (non-fatal)
- Fixed UTF-16BE test bytes for "日本語"

All 28 threads module tests now pass.

Closes: pdftract-3o9fu
```