SudokuGen

A fast sudoku puzzle generator.

Installation

npm install sudoku-gen

Usage

You can generate a sudoku using the getSudoku command, passing an optional difficulty level:

import { getSudoku } from 'sudoku-gen';

// Get a sudoku of specific difficulty (easy, medium, hard, expert)
const sudoku = getSudoku('easy');

// Get a sudoku of random difficulty
const sudoku = getSudoku();

getSudoku returns an object:

{
  puzzle: '41--75-----53--7--2-36-81--7-9--25-1-3--9-47--2-1-7---6587--9-----26-8--1925---47',
  solution: '416975238985321764273648159769432581531896472824157396658714923347269815192583647',
  difficulty: 'easy',
};

puzzle and solution are both 81 character strings representing the 81 cells in a sudoku grid:

puzzle contains placeholder dashes (-) for spaces which need to be filled in by the player. solution contains the entire grid so you can check the player's progress. The example code above would map to the following puzzle/solution:

How it works

Most sudoku generators start with a completed sudoku grid and remove numbers one at a time, using a backtracking algorithm to stop once the puzzle becomes unsolvable. This process is too slow to be performed in real-time, so usually requires a background task and database for generating and storing puzzles as they're created.

SudokuGen works differently. It starts with a known, solvable "seed" puzzle and performs various transformations to turn it into a brand new puzzle. This makes it extremely fast, with no requirement for a back end, whilst maintaining quality.

Each seed gives over 2.4 trillion unique puzzles. To put that in context, if you played sudoku 24/7 and took 3 minutes to solve each puzzle, it would take until your 13,915,534th birthday to exhaust a single seed :birthday:

Transformations

The following transformations are used ("!" = factorial):

• Rotate board - 4 permutations (0°, 90°, 180°, 270°).
• Shuffle column groups ("stacks") - 6 permutations (3!).
• Shuffle row groups ("bands") - 6 permutations (3!).
• Shuffle columns - 216 permutations (3! x 3! x 3!).
• Shuffle rows - 216 permutations (3! x 3! x 3!).
• Swap numbers - 362,880 permutations (9!).

Total permutations per seed = 4 x 6 x 6 x 216 x 216 x 362,880 = 2,437,996,216,320.