64 lines
1.4 KiB
C
64 lines
1.4 KiB
C
#include <assert.h>
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#include <inttypes.h>
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#include <stdio.h>
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#include <stdlib.h>
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#include <string.h>
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#include <time.h>
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typedef uint64_t prime_t;
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int main(void) {
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struct timespec ts;
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double t0, t1;
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clock_gettime(CLOCK_MONOTONIC, &ts);
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t0 = ts.tv_sec + ts.tv_nsec * 1E-9;
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// Initialize
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uint8_t *is_prime;
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prime_t n = 4;
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is_prime = malloc(n);
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is_prime[0] = 0;
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is_prime[1] = 0;
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is_prime[2] = 1;
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is_prime[3] = 1;
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for (;;) {
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clock_gettime(CLOCK_MONOTONIC, &ts);
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t1 = ts.tv_sec + ts.tv_nsec * 1E-9;
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if (t1 - t0 >= 1.0) break;
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prime_t n1 = n * 2;
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assert(n1 > n);
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is_prime = realloc(is_prime, n1);
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assert(is_prime);
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memset(is_prime + n, 1, n1 - n);
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for (prime_t i = 2; i < n1; i++) {
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if (!is_prime[i]) continue;
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// printf("%ju is prime:", (uintmax_t)i);
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prime_t m = (n + (i - 1)) / i * i;
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if (m == i) m += i;
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for (; m < n1; m += i) {
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// printf(" %ju", (uintmax_t)m);
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is_prime[m] = 0;
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}
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// printf("\n");
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}
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n = n1;
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}
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prime_t n_primes = 0;
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for (prime_t i = 0; i < n; i++) {
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if (is_prime[i]) {
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// printf("%ju\n", (uintmax_t)i);
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n_primes++;
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}
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}
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printf("%ju primes <= %ju in %g seconds\n", (uintmax_t)n_primes, (uintmax_t)n, t1 - t0);
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}
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