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nth_prime: fixed version

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This commit is contained in:
Dmitry Kokorin 2016-04-01 14:15:53 +03:00
parent 14bf87c43f
commit a8a482b106
1 changed files with 46 additions and 10 deletions
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56
cpp/nth-prime/nth_prime.cpp
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@ -1,5 +1,6 @@
#include "nth_prime.h"
#include <algorithm>
#include <cmath>
#include <cstring>
#include <stdexcept>
@ -18,7 +19,7 @@ number_t nth(size_t n, size_t sieve_size)
if (n == 1)
return 2;
vector<number_t> primes = {3, 5, 7, 11, 13};
vector<number_t> primes = {3, 5, 7, 11, 13, 17, 19, 23, 29, 31};
if (n - 2 < primes.size())
return primes[n - 2];
@ -32,24 +33,50 @@ number_t nth(size_t n, size_t sieve_size)
}
vector<char> sieve(sieve_size, false);
vector<number_t> next(n - 1, 0);
sieve[0] = true;
size_t first_number = 1;
number_t first_number = 1;
while (true) {
auto create_next = [&first_number](number_t p) -> number_t {
for (auto p : primes) {
return (p*p - first_number)/2;
};
size_t reminder = (first_number % (2*p));
size_t begin = reminder > p ? 3*p - reminder : p - reminder;
transform(primes.cbegin(), primes.cend(), next.begin(), create_next);
begin /= 2;
for (auto p : primes)
sieve[(p - first_number) / 2] = true;
for (size_t i = begin; i < sieve_size; i += p) {
auto process_prime = [sieve_size,&sieve](const number_t &p, number_t &next) {
number_t i = next;
for (; i < sieve_size; i += p) {
sieve[i] = true;
}
next = i - sieve_size;
};
while (true) {
for (size_t pn = 0; pn < primes.size(); ++pn) {
const number_t &p = primes[pn];
number_t i = next[pn];
for (; i < sieve_size; i += p) {
sieve[i] = true;
}
next[pn] = i - sieve_size;
// process_prime(primes[pn], next[pn]);
}
for (size_t i = 0; i < sieve_size; ++i) {
@ -58,18 +85,27 @@ number_t nth(size_t n, size_t sieve_size)
continue;
number_t p = first_number + 2*i;
primes.push_back(p);
if (primes.size() == n - 1)
return p;
for (size_t j = i; j < sieve_size; j += p) {
next[primes.size() - 1] = create_next(p);//(p*p - first_number)/2;
sieve[i] = true;
number_t j = next[primes.size() - 1];
for (; j < sieve_size; j += p) {
sieve[j] = true;
}
next[primes.size() - 1] = j - sieve_size;
}
size_t next_first_number = first_number + 2*sieve_size;
number_t next_first_number = first_number + 2*sieve_size;
if (next_first_number < first_number)
throw runtime_error("failed to reach prime number");