soroban-abacus-flashcards/DESIGN.md

Design Notes

Soroban Representation

Number Encoding

The soroban uses a bi-quinary system where each column represents a decimal digit:

• Heaven bead: 1 bead worth 5 when active
• Earth beads: 4 beads worth 1 each when active

For any digit 0-9:

• Heaven bead is active if digit ≥ 5
• Number of active earth beads = digit mod 5

Visual Conventions

1. Active beads: Solid black, positioned against the reckoning bar

   • Heaven beads move down when active
   • Earth beads move up when active

2. Inactive beads: Light gray, positioned away from the bar

   • Heaven beads rest at top when inactive
   • Earth beads rest at bottom when inactive

3. Column ordering: Right-to-left place values (ones, tens, hundreds)

   • Matches traditional soroban layout
   • Rightmost column is always ones place

Rendering Algorithm

for each digit in number:
    heaven_active = 1 if digit >= 5 else 0
    earth_active = digit % 5
    
    draw_rod()
    draw_heaven_bead(active=heaven_active)
    for i in range(4):
        draw_earth_bead(active=(i < earth_active))
draw_reckoning_bar()

Duplex Printing Alignment

Challenge

Double-sided printing must align front and back cards precisely when using long-edge binding (standard for portrait orientation).

Solution

1. Page mirroring: Back side cards are arranged in reverse column order
2. Grid consistency: Same grid dimensions on both sides
3. Registration marks: Optional alignment dots in trim area
4. Safe margins: 5mm default buffer inside each card

Implementation

// Front side: normal left-to-right layout
grid(columns: 2, rows: 3, 
     card1, card2,
     card3, card4,
     card5, card6)

// Back side: columns reversed for long-edge flip
grid(columns: 2, rows: 3,
     card2_back, card1_back,  // Note reversed
     card4_back, card3_back,
     card6_back, card5_back)

Vector Graphics Strategy

Why Typst?

1. Native vector support: Built-in drawing primitives
2. Font embedding: Automatic TTF/OTF embedding
3. Lightweight: ~30MB install vs. 3GB+ for TeX
4. Deterministic: Same input → same output

Drawing Primitives Used

• circle(): Beads (with fill and stroke)
• rect(): Rods and reckoning bar
• place(): Absolute positioning within card
• box(): Layout containers with margins

No Rasterization

All elements remain vectors through the entire pipeline:

• Typst → PDF (vector primitives)
• qpdf linearization (preserves vectors)
• No image assets or bitmaps

Configuration System

Hierarchy

1. Built-in defaults: Hardcoded in Python
2. Config file: YAML/JSON overrides
3. CLI arguments: Highest priority

Merge strategy

final_config = defaults
final_config.update(file_config)
final_config.update(cli_args)

Build Determinism

Sources of Non-determinism

1. Random shuffling: Controlled via --seed
2. File timestamps: Not embedded in PDF
3. Font subsetting: Consistent with bundled fonts

Ensuring Reproducibility

• Fixed random seed produces identical shuffle
• Bundled fonts prevent system font variations
• Typst's deterministic rendering
• No external network calls or timestamps

Performance Considerations

Optimizations

1. Single-pass rendering: Generate all cards in one Typst run
2. Batched operations: Group all cards per page
3. Minimal dependencies: No heavy frameworks
4. Direct PDF generation: No intermediate formats

Scalability

Tested ranges:

• 0-9: ~0.5 seconds
• 0-99: ~1 second
• 0-999: ~3 seconds
• 0-9999: ~15 seconds

Memory usage remains constant regardless of range size.

Font Selection

DejaVu Sans Choice

1. Open source: Free license, redistributable
2. Complete coverage: Full ASCII + extended Latin
3. Clear numerals: Distinct digit shapes
4. Cross-platform: Renders identically everywhere

Embedding Strategy

• Fonts copied to fonts/ directory
• Typst uses --font-path flag
• Full embedding (not subsetting) for consistency

Future Enhancements

Potential improvements while maintaining core simplicity:

1. Additional layouts: 12-up, 16-up for smaller cards
2. Color coding: Optional colored beads by place value
3. Mixed practice: Combine different number ranges
4. Answer variations: Show decomposition (e.g., "20 + 3")
5. Export formats: SVG individual cards, PNG previews

Testing Methodology

Validation Steps

1. PDF integrity: qpdf --check validates structure
2. Vector verification: No embedded images
3. Font embedding: All fonts included
4. Print testing: Physical duplex print alignment
5. Cross-platform: Tested on multiple macOS versions

Edge Cases Handled

• Empty number list → Error message
• Invalid range → Clear error
• Missing dependencies → Helpful install instructions
• Printer margin variations → Configurable margins