vim-sudoku/src/useBoard/SudokuGenerator.ts

export type Difficulty = 'easy' | 'medium' | 'hard';

const CELLS_TO_REMOVE = {
  easy: 25,
  medium: 40,
  hard: 50,
} as const;

export class SudokuGenerator {
  private solution: string = '';
  private puzzle: string = '';

  constructor() {
    this.generate('medium');
  }

  generate(difficulty: Difficulty = 'medium', customRemovedCells?: number): void {
    this.solution = this.generateSolvedBoard();
    
    if (!this.validateFullBoard(this.solution)) {
      throw new Error('Generated invalid solved board');
    }
    
    this.puzzle = this.makePuzzle(this.solution, difficulty, customRemovedCells);
  }

  getSolution(): string {
    return this.solution;
  }

  getPuzzle(): string {
    return this.puzzle;
  }

  private generateSolvedBoard(): string {
    const size = 9;
    const box = 3;

    const pattern = (r: number, c: number) =>
      (box * (r % box) + Math.floor(r / box) + c) % size;

    const baseBoard: number[][] = Array.from({ length: size }, (_, r) =>
      Array.from({ length: size }, (_, c) => pattern(r, c) + 1)
    );

    const shuffle = <T>(arr: T[]): T[] => {
      const copy = [...arr];
      for (let i = copy.length - 1; i > 0; i--) {
        const j = Math.floor(Math.random() * (i + 1));
        [copy[i], copy[j]] = [copy[j], copy[i]];
      }
      return copy;
    };

    const rows = shuffle([0, 1, 2])
      .concat(shuffle([3, 4, 5]))
      .concat(shuffle([6, 7, 8]));
    const cols = shuffle([0, 1, 2])
      .concat(shuffle([3, 4, 5]))
      .concat(shuffle([6, 7, 8]));

    const rowBands = shuffle([0, 1, 2]);
    const colStacks = shuffle([0, 1, 2]);

    const shuffledRows = rowBands.flatMap(band => rows.slice(band * 3, band * 3 + 3));
    const shuffledCols = colStacks.flatMap(stack => cols.slice(stack * 3, stack * 3 + 3));

    const digits = shuffle([1, 2, 3, 4, 5, 6, 7, 8, 9]);

    const board: number[][] = shuffledRows.map(r =>
      shuffledCols.map(c => digits[baseBoard[r][c] - 1])
    );

    return board.flat().join('');
  }

  private makePuzzle(solution: string, difficulty: Difficulty, customRemovedCells?: number): string {
    if (solution.length !== 81 || !/^[1-9]+$/.test(solution)) {
      throw new Error('Invalid solved board: must be 81 digits 1–9');
    }

    const removeCount = customRemovedCells ?? CELLS_TO_REMOVE[difficulty];
    const board = solution.split('');

    const indices = Array.from({ length: 81 }, (_, i) => i);
    for (let i = indices.length - 1; i > 0; i--) {
      const j = Math.floor(Math.random() * (i + 1));
      [indices[i], indices[j]] = [indices[j], indices[i]];
    }

    for (let i = 0; i < removeCount; i++) {
      const idx = indices[i];
      board[idx] = '.';
    }

    return board.join('');
  }

  private validateFullBoard(board: string): boolean {
    if (board.length !== 81) return false;
    if (!/^[1-9]+$/.test(board)) return false;

    const getCell = (r: number, c: number) => Number(board[r * 9 + c]);

    for (let r = 0; r < 9; r++) {
      const sum = Array.from({ length: 9 }, (_, c) => getCell(r, c))
        .reduce((a, b) => a + b, 0);
      if (sum !== 45) return false;
    }

    for (let c = 0; c < 9; c++) {
      const sum = Array.from({ length: 9 }, (_, r) => getCell(r, c))
        .reduce((a, b) => a + b, 0);
      if (sum !== 45) return false;
    }

    for (let br = 0; br < 3; br++) {
      for (let bc = 0; bc < 3; bc++) {
        const sum = Array.from({ length: 9 }, (_, i) => {
          const r = br * 3 + Math.floor(i / 3);
          const c = bc * 3 + (i % 3);
          return getCell(r, c);
        }).reduce((a, b) => a + b, 0);
        if (sum !== 45) return false;
      }
    }

    return true;
  }
}