prime.java

private void busy(FiniteDuration duration) {
  pi(duration.toMillis() * 800);
}

private BigDecimal pi(long m) {
  int n = 0;
  BigDecimal acc = new BigDecimal(0.0);
  while(n < m) {
    acc = acc.add(gregoryLeibnitz(n));
    n += 1;
  }
  return acc;
}

private BigDecimal gregoryLeibnitz(int n) {
  return new BigDecimal(4.0 * (1 - (n % 2) * 2) / (n * 2 + 1));
}