Fast PRNG & Vector Graphics Architecture

Fast PRNG Engine (FastRand)

Undoku uses a custom Xorshift64 pseudo-random number generator designed for maximum throughput, reproducibility, and deterministic seeding:

class FastRand {
  constructor(seed = null) {
    if (seed === null || seed === undefined) {
      this.state = (BigInt(Date.now()) ^ 0x9E3779B97F4A7C15n) & 0xFFFFFFFFFFFFFFFFn;
    } else {
      this.state = BigInt(seed) & 0xFFFFFFFFFFFFFFFFn;
    }
    if (this.state === 0n) this.state = 1n;
  }

  nextUint64() {
    let x = this.state;
    x ^= (x << 13n) & 0xFFFFFFFFFFFFFFFFn;
    x ^= (x >> 7n) & 0xFFFFFFFFFFFFFFFFn;
    x ^= (x << 17n) & 0xFFFFFFFFFFFFFFFFn;
    this.state = x & 0xFFFFFFFFFFFFFFFFn;
    return this.state;
  }
}

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SVG Vector Renderer (server/src/render/svg.js)

Undoku includes a zero-dependency SVG graphics rendering engine for Sudoku boards, elimination heatmaps, and difficulty step curves.

1. Board Vector Graphic (renderBoardSVG)

• Renders responsive vector SVG graphics (image/svg+xml).
• Supports dark mode palette with customizable cell dimensions and subgrid borders.

2. Elimination Heatmap (renderHeatmapSVG)

• Maps candidate elimination intensity per cell across the $9 \times 9$ matrix.
• Color gradient maps values from cool indigo (#818cf8) to warm pink (#f472b6).

3. Trajectory Line Chart (renderTrajectorySVG)

• Generates step-by-step difficulty curves.
• Visually flags step-to-step suddenness spikes along the solution path.

4. Interactive SVG Puzzle Player (renderInteractivePlayerSVG)

• Renders a fully self-contained, playable Sudoku app inside a single SVG vector graphic.
• Embedded JavaScript and CSS handle cell focus, peer row/col/box highlighting, digit matching, keyboard navigation (1-9, Backspace, Arrow keys), SVG keypad input, hints (💡), reset (↺), and victory detection (🎉 PUZZLE SOLVED!).
• Generated on the fly via GET /api/puzzles/:id/player.svg.