Function pellMul

test/compute-engine/diophantine.test.ts:367–373

`(x1,y1)·(x2,y2)` in `ℤ[√D]`.

(
  [x1, y1]: [bigint, bigint],
  [x2, y2]: [bigint, bigint],
  D: bigint
)

/** `(x1,y1)·(x2,y2)` in `ℤ[√D]`. */
function pellMul(
  [x1, y1]: [bigint, bigint],
  [x2, y2]: [bigint, bigint],
  D: bigint
): [bigint, bigint] {
  return [x1 * x2 + D * y1 * y2, x1 * y2 + y1 * x2];
}

function pairKey([x, y]: [bigint, bigint]): string {
  return `${x},${y}`;