40 lines
1021 B
C++
40 lines
1021 B
C++
#include <iomanip>
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#include <iostream>
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#include <gmpxx.h>
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using big_int = mpz_class;
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std::string to_string(const big_int& num, size_t n) {
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std::string str = num.get_str();
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size_t len = str.size();
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if (len > n) {
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str = str.substr(0, n / 2) + "..." + str.substr(len - n / 2);
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str += " (";
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str += std::to_string(len);
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str += " digits)";
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}
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return str;
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}
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bool is_probably_prime(const big_int& n) {
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return mpz_probab_prime_p(n.get_mpz_t(), 25) != 0;
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}
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int main() {
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big_int f = 1;
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for (int i = 0, n = 1; i < 31; ++n) {
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f *= n;
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if (is_probably_prime(f - 1)) {
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++i;
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std::cout << std::setw(2) << i << ": " << std::setw(3) << n
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<< "! - 1 = " << to_string(f - 1, 40) << '\n';
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}
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if (is_probably_prime(f + 1)) {
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++i;
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std::cout << std::setw(2) << i << ": " << std::setw(3) << n
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<< "! + 1 = " << to_string(f + 1, 40) << '\n';
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}
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}
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}
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