59 lines
1.1 KiB
C++
59 lines
1.1 KiB
C++
#include "nth_prime.h"
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#include <cstddef>
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#include <cstring>
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#include <iostream>
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#include <stdexcept>
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#include <vector>
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namespace prime {
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using namespace std;
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number_t nth(size_t n)
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{
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if (n < 1)
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throw std::domain_error("invalid prime index");
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vector<number_t> primes = {2, 3, 5, 7, 11, 13};
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primes.reserve(n - 1);
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static const size_t sieve_size = 32*1024;
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bool sieve[sieve_size] = {0};
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sieve[0] = sieve[1] = true;
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size_t page = 0;
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while (primes.size() < n) {
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for (auto p : primes) {
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size_t start = page*sieve_size % p;
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for (size_t i = start; i < sieve_size; i += p) {
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sieve[i] = true;
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}
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}
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for (auto i : sieve) {
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if (!sieve[i]) {
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number_t new_prime = page*sieve_size + i;
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//cout << page << " " << i << " " << new_prime << endl;
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primes.push_back(new_prime);
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break;
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}
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}
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//if (
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//memset(sieve, 0, sieve_size);
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}
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return primes[n - 1];
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}
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}
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