// Original teaching example, 2026-10-10. No external packages or network calls.
// Contract: both inputs are JavaScript numbers, amountMinor is an integer
// in [0, 1_000_000_000], and ratePercent is an integer in [0, 100].
// Result: round the final payable amount to the nearest minor unit,
// with exact half units rounded up. This is a chosen teaching rule,
// not a statement of any merchant's policy or jurisdiction's law.

export const MAX_AMOUNT_MINOR = 1_000_000_000;

function validate(amountMinor, ratePercent) {
  if (!Number.isSafeInteger(amountMinor)) {
    throw new TypeError('amountMinor must be an integer number');
  }
  if (!Number.isSafeInteger(ratePercent)) {
    throw new TypeError('ratePercent must be an integer number');
  }
  if (amountMinor < 0 || amountMinor > MAX_AMOUNT_MINOR) {
    throw new RangeError('amountMinor must be between 0 and 1000000000');
  }
  if (ratePercent < 0 || ratePercent > 100) {
    throw new RangeError('ratePercent must be between 0 and 100');
  }
  return [BigInt(amountMinor), BigInt(ratePercent)];
}

export function payableCorrect(amountMinor, ratePercent) {
  const [amount, rate] = validate(amountMinor, ratePercent);
  // All multiplication, addition and division here are exact integer
  // operations. For nonnegative inputs, +50 before /100 is half-up.
  // Conversion back to Number is safe because the result <= the input cap.
  return Number((amount * (100n - rate) + 50n) / 100n);
}

export function payableFaulty(amountMinor, ratePercent) {
  const [amount, rate] = validate(amountMinor, ratePercent);
  // DELIBERATE FAULT relative to the contract: round the discount first.
  return Number(amount - (amount * rate + 50n) / 100n);
}