Add coordinate-value-reverse function

src/util.ts

@@ -8,4 +8,14 @@ export function v(x: number, y: number, w: number): number {
   return x + 9 * (y - 1) + 81 * (w - 1);
 }
 
+export function getXYW(v: number): { x: number; y: number; w: number } {
+  assert(1 <= v && v <= 729);
+
+  const w = Math.floor((v - 1) / 81) + 1;
+  const y = (Math.floor((v - 1) / 9) % 9) + 1;
+  const x = ((v - 1) % 9) + 1;
+
+  return { x, y, w };
+}
+
 export type N = 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9;