starting new structure

src_old/sudoku/make-puzzle.ts

@@ -0,0 +1,45 @@
+/**
+ * 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("");
+}
+