Implement O'Gorman 1993 Docstrum algorithm for reading order detection
on irregular layouts (magazines with sidebars) where XY-cut produces
fragmented regions.
Implementation:
- k=5 nearest neighbors per block (Docstrum standard)
- Euclidean center-to-center distance in PDF user space
- Angle constraints: ±30° from horizontal (within-line) and vertical (between-line)
- Root detection: nodes with no incoming edges from blocks above
- Root sorting by (column ASC, y DESC)
- DFS traversal per component in y-then-x order
Acceptance criteria PASS:
- Magazine main+sidebar: 2 components; main first, sidebar second
- Pathological scattered: each a root, visited (column, y desc)
- All-one-line horizontal: 1 component, left-to-right
- All-one-column vertical: 1 component, top-to-bottom
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
