import { randomInt } from "node:crypto";

/**
 * The number-matching challenge.
 *
 * The laptop shows ONE number; the phone asks for it. This defeats blind
 * approval — an attacker who can trigger a prompt but cannot see the victim's
 * screen has to guess.
 *
 * How much it buys is arithmetic, and worth stating because "number matching"
 * sounds stronger than the weak variant is:
 *
 *   mode "type"   two digits, entered on a keypad   log2(90) = 6.5 bits
 *   mode "choice" one of three buttons              log2(3)  = 1.6 bits
 *
 * With a single attempt and a cap of 3 sessions per source, "choice" leaves an
 * attacker at 1 - (2/3)^3 = 70%, which is not a control. "type" leaves them at
 * 1 - (89/90)^3 = 3.3%. Hence `type` is the default; `choice` exists because it
 * is the friendlier UX and some deployments will want it with eyes open.
 */
export type NumberMode = "type" | "choice";

/** Two digits. Never 0-9: a leading zero reads ambiguously across fonts. */
export function pickNumber(): number {
  return randomInt(10, 100);
}

/**
 * Two decoys for `choice` mode, distinct from each other and from the answer.
 * Returned already shuffled, so the correct one is not positionally biased.
 */
export function pickDecoys(answer: number): number[] {
  const chosen = new Set<number>([answer]);
  while (chosen.size < 3) chosen.add(pickNumber());

  const all = [...chosen];
  // Fisher-Yates with a CSPRNG. Math.random would be fine for display order,
  // but using it here invites someone to reuse it where it is not.
  for (let i = all.length - 1; i > 0; i--) {
    const j = randomInt(0, i + 1);
    [all[i], all[j]] = [all[j], all[i]];
  }
  return all;
}