starting new structure

src_old/sudoku/sudoku.ts

@@ -0,0 +1,54 @@
+/**
+ * Generate a fully valid Sudoku solution using a base pattern and structural shuffles.
+ * Returns a string of 81 digits (row-major order).
+ */
+export function generateSolvedBoard(): string {
+  const size = 9;
+  const box = 3;
+
+  // --- 1️⃣ Base pattern function ---
+  const pattern = (r: number, c: number) =>
+    (box * (r % box) + Math.floor(r / box) + c) % size;
+
+  // --- 2️⃣ Create base board ---
+  const baseBoard: number[][] = Array.from({ length: size }, (_, r) =>
+    Array.from({ length: size }, (_, c) => pattern(r, c) + 1)
+  );
+
+  // --- 3️⃣ Helper to shuffle an array in place ---
+  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;
+  };
+
+  // --- 4️⃣ Build shuffled row/col indices preserving 3×3 structure ---
+  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]));
+
+  // --- 5️⃣ Shuffle entire bands and stacks (groups of 3) ---
+  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));
+
+  // --- 6️⃣ Digit permutation (1–9) ---
+  const digits = shuffle([1, 2, 3, 4, 5, 6, 7, 8, 9]);
+
+  // --- 7️⃣ Build the final shuffled board ---
+  const board: number[][] = shuffledRows.map(r =>
+    shuffledCols.map(c => digits[baseBoard[r][c] - 1])
+  );
+
+  // --- 8️⃣ Flatten to 81-char string ---
+  return board.flat().join('');
+}
+