Data commit

Task/Circular-primes/C/circular-primes.c

@@ -0,0 +1,98 @@
+#include <stdbool.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <gmp.h>
+
+bool is_prime(uint32_t n) {
+    if (n == 2)
+        return true;
+    if (n < 2 || n % 2 == 0)
+        return false;
+    for (uint32_t p = 3; p * p <= n; p += 2) {
+        if (n % p == 0)
+            return false;
+    }
+    return true;
+}
+
+// e.g. returns 2341 if n = 1234
+uint32_t cycle(uint32_t n) {
+    uint32_t m = n, p = 1;
+    while (m >= 10) {
+        p *= 10;
+        m /= 10;
+    }
+    return m + 10 * (n % p);
+}
+
+bool is_circular_prime(uint32_t p) {
+    if (!is_prime(p))
+        return false;
+    uint32_t p2 = cycle(p);
+    while (p2 != p) {
+        if (p2 < p || !is_prime(p2))
+            return false;
+        p2 = cycle(p2);
+    }
+    return true;
+}
+
+void test_repunit(uint32_t digits) {
+    char* str = malloc(digits + 1);
+    if (str == 0) {
+        fprintf(stderr, "Out of memory\n");
+        exit(1);
+    }
+    memset(str, '1', digits);
+    str[digits] = 0;
+    mpz_t bignum;
+    mpz_init_set_str(bignum, str, 10);
+    free(str);
+    if (mpz_probab_prime_p(bignum, 10))
+        printf("R(%u) is probably prime.\n", digits);
+    else
+        printf("R(%u) is not prime.\n", digits);
+    mpz_clear(bignum);
+}
+
+int main() {
+    uint32_t p = 2;
+    printf("First 19 circular primes:\n");
+    for (int count = 0; count < 19; ++p) {
+        if (is_circular_prime(p)) {
+            if (count > 0)
+                printf(", ");
+            printf("%u", p);
+            ++count;
+        }
+    }
+    printf("\n");
+    printf("Next 4 circular primes:\n");
+    uint32_t repunit = 1, digits = 1;
+    for (; repunit < p; ++digits)
+        repunit = 10 * repunit + 1;
+    mpz_t bignum;
+    mpz_init_set_ui(bignum, repunit);
+    for (int count = 0; count < 4; ) {
+        if (mpz_probab_prime_p(bignum, 15)) {
+            if (count > 0)
+                printf(", ");
+            printf("R(%u)", digits);
+            ++count;
+        }
+        ++digits;
+        mpz_mul_ui(bignum, bignum, 10);
+        mpz_add_ui(bignum, bignum, 1);
+    }
+    mpz_clear(bignum);
+    printf("\n");
+    test_repunit(5003);
+    test_repunit(9887);
+    test_repunit(15073);
+    test_repunit(25031);
+    test_repunit(35317);
+    test_repunit(49081);
+    return 0;
+}