/**
 * Create a playable Sudoku puzzle by removing random digits
 * from a fully solved board string.
 *
 * Difficulty only determines how many cells are removed.
 * - "easy":   removes ~40 cells
 * - "medium": removes ~50 cells
 * - "hard":   removes ~60 cells
 *
 * The returned string uses '.' for blanks.
 */
export function makePuzzle(
  solution: string,
  difficulty: "easy" | "medium" | "hard" = "medium"
): string {
  if (solution.length !== 81 || !/^[1-9]+$/.test(solution)) {
    throw new Error("Invalid solved board: must be 81 digits 1–9");
  }

  // --- removal count based on difficulty ---
  const removeCount = {
    easy: 40,
    medium: 50,
    hard: 60,
  }[difficulty];

  // --- create a mutable copy ---
  const board = solution.split("");

  // --- build a shuffled list of indices 0–80 ---
  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]];
  }

  // --- remove the desired number of cells ---
  for (let i = 0; i < removeCount; i++) {
    const idx = indices[i];
    board[idx] = ".";
  }

  return board.join("");
}