d3c4ecd268
Implement Phase 4.4 code block classification for detecting indented monospace code blocks. Features: - is_monospace_font_name: Check font name for monospace indicators (mono, courier, code, fixed, console - case-insensitive) - is_fixed_pitch_flag: Check FontDescriptor bit 0 (FixedPitch) - classify_code: Classify block as code if all spans monospace AND indented ≥ 2em from column baseline - classify_page_code_blocks: Post-processing pass to upgrade paragraph blocks to code kind Acceptance criteria: - All-Courier, indented 24pt, font_size 12pt (2em=24): Code ✓ - All-monospace, not indented: NOT Code ✓ - Mixed serif+monospace: NOT Code ✓ - One serif span at end: NOT Code ✓ - FixedPitch flag set, no "Mono" in name: STILL Code ✓ Closes: pdftract-8n270 Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
88 lines
3.4 KiB
Markdown
88 lines
3.4 KiB
Markdown
# Code Block Detection (pdftract-8n270)
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## Summary
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Implemented code block classification (Phase 4.4) for detecting indented monospace code blocks.
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## Implementation
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Created new module `crates/pdftract-core/src/layout/code.rs` with:
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1. **`is_monospace_font_name(font_name: &str) -> bool`**
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- Checks if font name (with subset prefix stripped) contains monospace indicators
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- Indicators: "mono", "courier", "code", "fixed", "console" (case-insensitive)
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2. **`is_fixed_pitch_flag(flags: Option<u32>) -> bool`**
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- Checks if FixedPitch flag (bit 0) is set in FontDescriptor flags
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- Per PDF spec, bit 0 indicates fixed-pitch (monospace) fonts
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3. **`is_monospace_span(font_name: &str, flags: Option<u32>) -> bool`**
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- Combines both checks: monospace if name OR FixedPitch flag indicates it
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4. **`classify_code<S>(block, column_baseline_x0, font_size) -> bool`**
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- Classifies block as code if:
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- ALL spans use monospace font
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- Block is indented ≥ 2em from column baseline (2 × font_size)
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5. **`compute_column_baseline<S>(blocks) -> f32`**
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- Computes median x0 of non-code paragraph blocks in column
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- Represents typical left edge of body text for indentation comparison
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6. **`classify_page_code_blocks<S>(blocks)`**
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- Post-processing pass that upgrades paragraph blocks to "code" kind
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- Uses column baseline and monospace detection
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## Acceptance Criteria
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| Criterion | Status | Notes |
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|-----------|--------|-------|
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| All-Courier, indented 24pt, font_size 12pt (2em=24) | ✅ PASS | `classify_code` returns true |
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| All-monospace, not indented | ✅ PASS | `classify_code` returns false |
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| Mixed serif+monospace | ✅ PASS | `classify_code` returns false |
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| One serif span at end | ✅ PASS | `classify_code` returns false |
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| FixedPitch flag set, no "Mono" in name | ✅ PASS | Still classified as code |
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## Files Modified
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- `crates/pdftract-core/src/layout/code.rs` (new)
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- `crates/pdftract-core/src/layout/mod.rs` (exported code module)
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## Testing
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All unit tests pass (107 passed, 0 failed):
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```bash
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cargo test --package pdftract-core --lib code
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```
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Test coverage includes:
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- Font name matching (Courier, Mono, Code, Fixed, Console)
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- FixedPitch flag detection
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- Monospace span detection
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- Code block classification
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- Column baseline computation
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- Page-level code block upgrade
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## Design Notes
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1. **MonospaceSpan trait**: Allows code detection to work with different span representations
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2. **Font subset prefixes**: Correctly strips "ABCDEF+" prefixes before checking font names
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3. **2em threshold**: As specified in plan, uses 2 × font_size for indentation requirement
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4. **Post-processing approach**: Code detection runs after block formation (Phase 4.4)
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5. **Median baseline**: Uses median (not mean) for robustness against outliers
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## Integration
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The code module is now exported from `layout::mod` and ready for integration into the extraction pipeline. The post-processing pass `classify_page_code_blocks` can be called after `group_lines_into_blocks` to upgrade paragraph blocks to code blocks.
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## TODO
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Per plan line 1726: "Indent threshold may miss flush-left code; add TODO."
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- Flush-left code blocks (no indentation) are currently NOT detected as code
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- This is intentional per the acceptance criteria ("not indented: NOT Code")
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- Future enhancement could detect flush-left code via additional heuristics
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## References
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- Plan section: Phase 4.4 (line 1708)
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- Bead: pdftract-8n270
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- ISO 32000-1 Table 123 (FontDescriptor flags, bit 0 = FixedPitch)
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