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feat(pdftract-8n270): implement code block detection

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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>
This commit is contained in:
jedarden 2026-05-24 10:04:22 -04:00
parent e25a4fc78d
commit d3c4ecd268
3 changed files with 652 additions and 0 deletions
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558
crates/pdftract-core/src/layout/code.rs Normal file
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//! Code block classifier (Phase 4).
//!
//! This module implements classification of blocks as code based on:
//! 1. All spans use a monospace font
//! 2. The block is indented ≥ 2em relative to the surrounding body text
//!
//! Code blocks are typically distinguished by:
//! - Monospace font (Courier, Monaco, Consolas, etc.)
//! - Indentation from the main text column
//! - Consistent font throughout the block
use crate::font::strip_subset_prefix;
/// Check if a font name indicates a monospace font.
///
/// A font is considered monospace if its name (with subset prefix stripped)
/// contains any of the following case-insensitive substrings:
/// - "Mono"
/// - "Courier"
/// - "Code"
/// - "Fixed"
/// - "Console"
///
/// # Arguments
///
/// * `font_name` - The font name from the PDF (may include subset prefix)
///
/// # Returns
///
/// `true` if the font name indicates a monospace font, `false` otherwise.
///
/// # Examples
///
/// ```
/// use pdftract_core::layout::code::is_monospace_font_name;
///
/// assert!(is_monospace_font_name("Courier"));
/// assert!(is_monospace_font_name("Courier-New"));
/// assert!(is_monospace_font_name("Mono"));
/// assert!(is_monospace_font_name("SourceCodePro"));
/// assert!(is_monospace_font_name("Consolas"));
/// assert!(is_monospace_font_name("ABCDEF+Courier")); // Subset prefix
///
/// assert!(!is_monospace_font_name("Times-Roman"));
/// assert!(!is_monospace_font_name("Helvetica"));
/// ```
pub fn is_monospace_font_name(font_name: &str) -> bool {
let stripped = strip_subset_prefix(font_name).to_lowercase();
let monospace_indicators = ["mono", "courier", "code", "fixed", "console"];
monospace_indicators
.iter()
.any(|&indicator| stripped.contains(indicator))
}
/// Check if the FixedPitch flag (bit 0) is set in font descriptor flags.
///
/// PDF font descriptor flags use bit 0 to indicate fixed-pitch (monospace) fonts.
///
/// # Arguments
///
/// * `flags` - Optional flags value from the FontDescriptor
///
/// # Returns
///
/// `true` if the FixedPitch flag (bit 0) is set, `false` otherwise.
/// Returns `false` if flags is None.
///
/// # Examples
///
/// ```
/// use pdftract_core::layout::code::is_fixed_pitch_flag;
///
/// assert!(is_fixed_pitch_flag(Some(1))); // Bit 0 set
/// assert!(is_fixed_pitch_flag(Some(0b00000001)));
/// assert!(!is_fixed_pitch_flag(Some(0))); // Bit 0 not set
/// assert!(!is_fixed_pitch_flag(Some(2))); // Bit 1 set, not bit 0
/// assert!(!is_fixed_pitch_flag(None)); // No flags
/// ```
pub fn is_fixed_pitch_flag(flags: Option<u32>) -> bool {
match flags {
Some(f) => (f & 0x1) == 1,
None => false,
}
}
/// Check if a span uses a monospace font.
///
/// A span is considered monospace if EITHER:
/// 1. The font name indicates monospace (via `is_monospace_font_name`)
/// 2. The FixedPitch flag is set in the font descriptor
///
/// # Arguments
///
/// * `font_name` - The font name from the PDF
/// * `flags` - Optional flags value from the FontDescriptor
///
/// # Returns
///
/// `true` if the span uses a monospace font, `false` otherwise.
pub fn is_monospace_span(font_name: &str, flags: Option<u32>) -> bool {
is_monospace_font_name(font_name) || is_fixed_pitch_flag(flags)
}
/// Classify a block as code based on monospace and indentation criteria.
///
/// A block is classified as code if ALL of the following are true:
/// 1. All spans in the block use a monospace font
/// 2. The block is indented ≥ 2em relative to the column baseline
///
/// # Arguments
///
/// * `block` - The block to classify
/// * `column_baseline_x0` - The median x0 of non-code paragraph blocks in the column
/// * `font_size` - The font size in points (used to compute em width)
///
/// # Returns
///
/// `true` if the block should be classified as code, `false` otherwise.
///
/// # Font Information
///
/// This function assumes that the block's spans have font information
/// accessible via a `font()` method that returns the font name, and
/// optionally a `flags()` method for FontDescriptor flags.
///
/// # Indentation Calculation
///
/// The indentation threshold is 2em, where:
/// - em_width = font_size (in points)
/// - threshold = 2.0 * font_size
///
/// A block is considered indented if its x0 position is at least
/// `column_baseline_x0 + 2 * font_size` points from the left.
///
/// # Examples
///
/// ```ignore
/// use pdftract_core::layout::code::classify_code;
///
/// // All-Courier block indented 24pt with font_size 12pt (2em=24pt)
/// let is_code = classify_code(&block, 72.0, 12.0);
/// assert!(is_code);
///
/// // Monospace block not indented
/// let is_code = classify_code(&block, 100.0, 12.0);
/// assert!(!is_code);
/// ```
pub fn classify_code<S>(
block: &crate::layout::line::Block<S>,
column_baseline_x0: f32,
font_size: f32,
) -> bool
where
S: MonospaceSpan,
{
// Criterion 1: All spans must use monospace font
for line in &block.lines {
for span in &line.spans {
if !span.is_monospace() {
return false;
}
}
}
// Criterion 2: Block must be indented ≥ 2em from column baseline
let em_width = font_size;
let indent_threshold = 2.0 * em_width;
let block_x0 = block.bbox[0];
block_x0 >= column_baseline_x0 + indent_threshold
}
/// Trait for spans that can report monospace status.
///
/// This trait allows the code classification logic to work with different
/// span representations while abstracting over font information access.
pub trait MonospaceSpan {
/// Check if this span uses a monospace font.
fn is_monospace(&self) -> bool;
}
/// Compute the column baseline x0 from a set of blocks.
///
/// The column baseline is the median x0 of all non-code paragraph blocks
/// in the column. This represents the typical left edge of body text.
///
/// # Arguments
///
/// * `blocks` - Blocks in the column
///
/// # Returns
///
/// The median x0 coordinate of non-code paragraph blocks, or 0.0 if no such blocks exist.
///
/// # Examples
///
/// ```ignore
/// use pdftract_core::layout::code::compute_column_baseline;
///
/// let blocks = vec![
/// make_paragraph_block(72.0), // x0 = 72
/// make_paragraph_block(72.0), // x0 = 72
/// make_paragraph_block(100.0), // x0 = 100 (indented)
/// ];
///
/// let baseline = compute_column_baseline(&blocks);
/// assert_eq!(baseline, 72.0); // Median of [72, 72, 100]
/// ```
pub fn compute_column_baseline<S>(blocks: &[crate::layout::line::Block<S>]) -> f32
where
S: MonospaceSpan,
{
// Collect x0 values from non-code paragraph blocks
let mut x0_values: Vec<f32> = blocks
.iter()
.filter(|b| b.kind == "paragraph")
.map(|b| b.bbox[0])
.collect();
if x0_values.is_empty() {
return 0.0;
}
// Compute median
x0_values.sort_by(|a, b| a.partial_cmp(b).unwrap_or(std::cmp::Ordering::Equal));
x0_values[x0_values.len() / 2]
}
/// Classify all blocks on a page, updating their kinds to "code" where appropriate.
///
/// This function processes blocks in column order and classifies each block
/// based on monospace font usage and indentation.
///
/// # Arguments
///
/// * `blocks` - Mutable slice of blocks to classify
///
/// # Algorithm
///
/// 1. Compute column baseline x0 from non-code paragraph blocks
/// 2. For each block, check if it meets code criteria
/// 3. Update block.kind to "code" if criteria are met
pub fn classify_page_code_blocks<S>(blocks: &mut [crate::layout::line::Block<S>])
where
S: MonospaceSpan,
{
// Compute column baseline x0 (median of non-code paragraph blocks)
let column_baseline_x0 = compute_column_baseline(blocks);
// Classify each block
for block in blocks.iter_mut() {
if block.kind == "paragraph" {
let font_size = block.median_font_size;
if classify_code(block, column_baseline_x0, font_size) {
block.kind = "code".to_string();
}
}
}
}
#[cfg(test)]
mod tests {
use super::*;
use crate::layout::line::{Block, Line};
/// Test helper: create a mock span with monospace info.
#[derive(Debug, Clone)]
struct TestSpan {
font_name: String,
flags: Option<u32>,
}
impl MonospaceSpan for TestSpan {
fn is_monospace(&self) -> bool {
is_monospace_span(&self.font_name, self.flags)
}
}
/// Test helper: create a mock line.
fn make_test_line(spans: Vec<TestSpan>) -> Line<TestSpan> {
Line {
spans,
bbox: [0.0, 0.0, 100.0, 12.0],
baseline: 2.4,
direction: crate::layout::line::LineDirection::Ltr,
page_relative_y: 0.5,
median_font_size: 12.0,
rendering_mode: None,
column: Some(0),
}
}
/// Test helper: create a mock block.
fn make_test_block(spans: Vec<TestSpan>, x0: f32, kind: &str) -> Block<TestSpan> {
Block {
lines: vec![make_test_line(spans)],
kind: kind.to_string(),
text: String::new(),
bbox: [x0, 0.0, x0 + 100.0, 12.0],
median_font_size: 12.0,
column: 0,
}
}
#[test]
fn test_is_monospace_font_name_courier() {
assert!(is_monospace_font_name("Courier"));
assert!(is_monospace_font_name("Courier-New"));
assert!(is_monospace_font_name("ABCDEF+Courier")); // Subset prefix
}
#[test]
fn test_is_monospace_font_name_mono() {
assert!(is_monospace_font_name("Mono"));
assert!(is_monospace_font_name("SourceCodePro"));
assert!(is_monospace_font_name("LiberationMono"));
}
#[test]
fn test_is_monospace_font_name_code() {
assert!(is_monospace_font_name("Code"));
assert!(is_monospace_font_name("SourceCodePro"));
assert!(is_monospace_font_name("FiraCode"));
}
#[test]
fn test_is_monospace_font_name_fixed() {
assert!(is_monospace_font_name("Fixed"));
assert!(is_monospace_font_name("Fixedsys"));
}
#[test]
fn test_is_monospace_font_name_console() {
assert!(is_monospace_font_name("Console"));
}
#[test]
fn test_is_not_monospace_font_name() {
assert!(!is_monospace_font_name("Times-Roman"));
assert!(!is_monospace_font_name("Helvetica"));
assert!(!is_monospace_font_name("Arial"));
}
#[test]
fn test_is_fixed_pitch_flag() {
assert!(is_fixed_pitch_flag(Some(1))); // Bit 0 set
assert!(is_fixed_pitch_flag(Some(0b00000001)));
assert!(is_fixed_pitch_flag(Some(0b11111111))); // All bits set
assert!(!is_fixed_pitch_flag(Some(0))); // Bit 0 not set
assert!(!is_fixed_pitch_flag(Some(2))); // Bit 1 set, not bit 0
assert!(!is_fixed_pitch_flag(None)); // No flags
}
#[test]
fn test_is_monospace_span_name_only() {
// Font name indicates monospace, no flags
assert!(is_monospace_span("Courier", None));
assert!(is_monospace_span("Mono", None));
}
#[test]
fn test_is_monospace_span_flags_only() {
// FixedPitch flag set, non-monospace name
assert!(is_monospace_span("CustomFont", Some(1)));
}
#[test]
fn test_is_not_monospace_span() {
// Neither name nor flags indicate monospace
assert!(!is_monospace_span("Times-Roman", None));
assert!(!is_monospace_span("Times-Roman", Some(0)));
}
#[test]
fn test_classify_code_all_courier_indented() {
// All-Courier block indented 24pt with font_size 12pt (2em=24pt)
let spans = vec![TestSpan {
font_name: "Courier".to_string(),
flags: None,
}];
let block = make_test_block(spans, 96.0, "paragraph"); // x0 = 96
// Column baseline at 72, block at 96: indent = 24pt = 2em
assert!(classify_code(&block, 72.0, 12.0));
}
#[test]
fn test_classify_code_not_indented() {
// All-monospace block but not indented enough
let spans = vec![TestSpan {
font_name: "Courier".to_string(),
flags: None,
}];
let block = make_test_block(spans, 80.0, "paragraph"); // x0 = 80
// Column baseline at 72, block at 80: indent = 8pt < 2em (24pt)
assert!(!classify_code(&block, 72.0, 12.0));
}
#[test]
fn test_classify_code_mixed_font() {
// Mixed serif+monospace -> NOT code
let spans = vec![
TestSpan {
font_name: "Courier".to_string(),
flags: None,
},
TestSpan {
font_name: "Times-Roman".to_string(),
flags: None,
},
];
let block = make_test_block(spans, 96.0, "paragraph");
assert!(!classify_code(&block, 72.0, 12.0));
}
#[test]
fn test_classify_code_one_serif_at_end() {
// One serif span at end -> NOT code
let spans = vec![
TestSpan {
font_name: "Courier".to_string(),
flags: None,
},
TestSpan {
font_name: "Courier".to_string(),
flags: None,
},
TestSpan {
font_name: "Times-Roman".to_string(),
flags: None,
},
];
let block = make_test_block(spans, 96.0, "paragraph");
assert!(!classify_code(&block, 72.0, 12.0));
}
#[test]
fn test_classify_code_fixed_pitch_flag() {
// FixedPitch flag set, no "Mono" in name -> STILL code
let spans = vec![TestSpan {
font_name: "CustomFont".to_string(),
flags: Some(1),
}];
let block = make_test_block(spans, 96.0, "paragraph");
assert!(classify_code(&block, 72.0, 12.0));
}
#[test]
fn test_compute_column_baseline() {
let blocks = vec![
make_test_block(
vec![TestSpan {
font_name: "Times-Roman".to_string(),
flags: None,
}],
72.0,
"paragraph",
),
make_test_block(
vec![TestSpan {
font_name: "Times-Roman".to_string(),
flags: None,
}],
72.0,
"paragraph",
),
make_test_block(
vec![TestSpan {
font_name: "Times-Roman".to_string(),
flags: None,
}],
100.0,
"paragraph",
),
];
let baseline = compute_column_baseline(&blocks);
assert_eq!(baseline, 72.0); // Median of [72, 72, 100]
}
#[test]
fn test_compute_column_baseline_empty() {
let blocks: Vec<Block<TestSpan>> = vec![];
let baseline = compute_column_baseline(&blocks);
assert_eq!(baseline, 0.0);
}
#[test]
fn test_compute_column_baseline_no_paragraphs() {
let blocks = vec![
make_test_block(
vec![TestSpan {
font_name: "Courier".to_string(),
flags: None,
}],
72.0,
"heading",
),
make_test_block(
vec![TestSpan {
font_name: "Courier".to_string(),
flags: None,
}],
72.0,
"list",
),
];
let baseline = compute_column_baseline(&blocks);
assert_eq!(baseline, 0.0); // No paragraph blocks
}
#[test]
fn test_classify_page_code_blocks() {
let mut blocks = vec![
// Regular paragraph at baseline
make_test_block(
vec![TestSpan {
font_name: "Times-Roman".to_string(),
flags: None,
}],
72.0,
"paragraph",
),
// Indented monospace block -> should become code
make_test_block(
vec![TestSpan {
font_name: "Courier".to_string(),
flags: None,
}],
96.0,
"paragraph",
),
// Non-indented monospace block -> should stay paragraph
make_test_block(
vec![TestSpan {
font_name: "Courier".to_string(),
flags: None,
}],
72.0,
"paragraph",
),
];
classify_page_code_blocks(&mut blocks);
assert_eq!(blocks[0].kind, "paragraph"); // Unchanged
assert_eq!(blocks[1].kind, "code"); // Upgraded to code
assert_eq!(blocks[2].kind, "paragraph"); // Not indented enough
}
}

6
crates/pdftract-core/src/layout/mod.rs
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@ -2,6 +2,7 @@
//!
//! This module implements block-level layout analysis including:
//! - Caption classification (caption.rs)
//! - Code block classification (code.rs)
//! - Line formation (line.rs)
//! - Readability aggregation (readability.rs)
//! - English wordlist for dict coverage scoring (wordlist.rs)
@ -10,11 +11,16 @@
//! headings, figures, captions, etc.) based on spatial and font metrics.
pub mod caption;
pub mod code;
pub mod line;
pub mod readability;
pub mod wordlist;
pub use caption::{classify_caption, classify_page_captions, Block, PageContext};
pub use code::{
classify_code, classify_page_code_blocks, is_fixed_pitch_flag, is_monospace_font_name,
is_monospace_span, MonospaceSpan,
};
pub use line::{
compute_baseline, group_lines_into_blocks, union_bboxes, BlockInput, HasBBox, Line,
LineDirection, LineMetadata,

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