pdftract/notes/pdftract-1ofnz.md

pdftract-1ofnz: RTL direction detection (unicode-bidi majority bidi class)

Summary

Implemented detect_line_direction(line_text) -> LineDirection function in crates/pdftract-core/src/layout/line.rs.

Implementation Details

Location: crates/pdftract-core/src/layout/line.rs:458-496

Algorithm:

1. Walk each character in the text
2. Count L (Left-to-Right) vs R/AL (Right-to-Left/Arabic Letter) using unicode_bidi::bidi_class
3. All other bidi classes (EN, ES, ET, AN, CS, NSM, BN, B, S, WS, ON, etc.) are ignored per INV
4. Return:
   • LineDirection::Ltr if LTR count > RTL count OR both counts are zero (empty/neutral-only)
   • LineDirection::Rtl if RTL count > LTR count
   • LineDirection::Mixed if counts are equal (and both > 0)

Key design decision: Empty strings and neutral-only text (digits, punctuation) default to Ltr per bead acceptance criteria.

Acceptance Criteria Status

Criterion	Status	Notes
"Hello, World!" -> Ltr	PASS	Test: test_detect_line_direction_latin_text
"مرحبا بالعالم" -> Rtl	PASS	Test: test_detect_line_direction_arabic_text
Mixed Latin+Arabic: Mixed or dominant	PASS	Tests: test_detect_line_direction_mixed_latin_arabic, test_detect_line_direction_latin_more_than_arabic, test_detect_line_direction_arabic_more_than_latin
"123 456" digits only: Ltr default	PASS	Test: test_detect_line_direction_digits_only
"" -> Ltr	PASS	Test: test_detect_line_direction_empty_string

Additional Test Coverage

• test_detect_line_direction_punctuation_only: Punctuation-only text -> Ltr
• test_detect_line_direction_latin_dominant: Latin with punctuation/digits -> Ltr
• test_detect_line_direction_arabic_dominant: Arabic with digits -> Rtl
• test_detect_line_direction_hebrew_text: Hebrew text -> Rtl
• test_detect_line_direction_cyrillic_text: Cyrillic text -> Ltr

Tests Executed

cargo nextest run --package pdftract-core --lib 'layout::line::tests::test_detect_line_direction'

Result: 12/12 tests passed (all RTL direction detection tests) Module tests: 44/44 tests passed (entire line module)

Code Changes

Files modified:

1. crates/pdftract-core/src/layout/line.rs: Added detect_line_direction function with comprehensive documentation and tests
2. crates/pdftract-core/src/layout/header_footer.rs: Fixed pre-existing compilation error (removed nonexistent reading_order_rank field from test helper)

Commit: 4ab89e1 feat(pdftract-1ofnz): implement detect_line_direction with unicode-bidi

INV Compliance

• Numerals are bidi-neutral and do not drive direction
• Punctuation is neutral
• Empty lines default to Ltr

References

• Plan section: Phase 4.2 RTL detection (line 1668)