isPrime function

bool isPrime(
  1. int n
)

Is prime (small numbers), prime factors. Roadmap #126–127. Audited: 2026-06-12 11:26 EDT

Implementation

bool isPrime(int n) {
  // Primes are >= 2; handle 2 (the only even prime) and reject other evens so
  // the trial-division loop can step by 2 over odd candidates only.
  if (n < 2) return false;
  if (n == 2) return true;
  if (n % 2 == 0) return false;
  // A composite n has a factor <= sqrt(n), so testing odd divisors up to the
  // integer square root is sufficient (and far cheaper than testing to n).
  final int limit = _isqrt(n);
  // ignore: saropa_lints/prefer_correct_for_loop_increment -- steps by 2 to skip even candidates
  for (int d = 3; d <= limit; d += 2) {
    if (n % d == 0) return false;
  }
  return true;
}