uint64_t PrimeSieve::nthPrime(int64_t n, uint64_t start)
{
setStart(start);
double t1 = getWallTime();
if (n == 0)
n = 1; // Like Mathematica
else if (n > 0)
start = add_overflow_safe(start, 1);
else if (n < 0)
start = sub_underflow_safe(start, 1);
uint64_t stop = start;
uint64_t dist = nthPrimeDist(n, 0, start);
uint64_t nthPrimeGuess = add_overflow_safe(start, dist);
int64_t count = 0;
int64_t tinyN = 10000;
tinyN = max(tinyN, pix(isqrt(nthPrimeGuess)));
while ((n - count) > tinyN ||
sieveBackwards(n, count, stop))
{
if (count < n)
{
checkLimit(start);
dist = nthPrimeDist(n, count, start);
stop = add_overflow_safe(start, dist);
count += countPrimes(start, stop);
start = add_overflow_safe(stop, 1);
}
if (sieveBackwards(n, count, stop))
{
checkLowerLimit(stop);
dist = nthPrimeDist(n, count, stop);
start = sub_underflow_safe(start, dist);
count -= countPrimes(start, stop);
stop = sub_underflow_safe(start, 1);
}
}
if (n < 0)
count -= 1;
checkLimit(start);
uint64_t overValue = 3;
dist = nthPrimeDist(n, count, start) * overValue;
stop = add_overflow_safe(start, dist);
NthPrime np;
np.findNthPrime(n - count, start, stop);
seconds_ = getWallTime() - t1;
return np.getNthPrime();
}
void iterator::generate_next_primes()
{
primes_.clear();
while (primes_.empty())
{
start_ = add_overflow_safe(stop_, 1);
stop_ = add_overflow_safe(start_, get_interval_size(start_));
if (start_ <= stop_hint_ && stop_ >= stop_hint_)
stop_ = add_overflow_safe(stop_hint_, max_prime_gap(stop_hint_));
primesieve::generate_primes(start_, stop_, &primes_);
if (primes_.empty() && stop_ >= get_max_stop())
throw primesieve_error("next_prime() > " + PrimeFinder::getMaxStopString());
}
last_idx_ = primes_.size() - 1;
i_ = 0;
}
