Data commit

Task/Long-primes/C/long-primes.c

@@ -0,0 +1,81 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+#define TRUE 1
+#define FALSE 0
+
+typedef int bool;
+
+void sieve(int limit, int primes[], int *count) {
+    bool *c = calloc(limit + 1, sizeof(bool)); /* composite = TRUE */
+    /* no need to process even numbers */
+    int i, p = 3, p2, n = 0;
+    p2 = p * p;
+    while (p2 <= limit) {
+        for (i = p2; i <= limit; i += 2 * p)
+            c[i] = TRUE;
+        do {
+            p += 2;
+        } while (c[p]);
+        p2 = p * p;
+    }
+    for (i = 3; i <= limit; i += 2) {
+        if (!c[i]) primes[n++] = i;
+    }
+    *count = n;
+    free(c);
+}
+
+/* finds the period of the reciprocal of n */
+int findPeriod(int n) {
+    int i, r = 1, rr, period = 0;
+    for (i = 1; i <= n + 1; ++i) {
+        r = (10 * r) % n;
+    }
+    rr = r;
+    do {
+        r = (10 * r) % n;
+        period++;
+    } while (r != rr);
+    return period;
+}
+
+int main() {
+    int i, prime, count = 0, index = 0, primeCount, longCount = 0, numberCount;
+    int *primes, *longPrimes, *totals;
+    int numbers[] = {500, 1000, 2000, 4000, 8000, 16000, 32000, 64000};
+
+    primes = calloc(6500, sizeof(int));
+    numberCount = sizeof(numbers) / sizeof(int);
+    totals = calloc(numberCount, sizeof(int));
+    sieve(64000, primes, &primeCount);
+    longPrimes = calloc(primeCount, sizeof(int));
+    /* Surely longCount < primeCount */
+    for (i = 0; i < primeCount; ++i) {
+        prime = primes[i];
+        if (findPeriod(prime) == prime - 1) {
+            longPrimes[longCount++] = prime;
+        }
+    }
+    for (i = 0; i < longCount; ++i, ++count) {
+        if (longPrimes[i] > numbers[index]) {
+            totals[index++] = count;
+        }
+    }
+    totals[numberCount - 1] = count;
+    printf("The long primes up to %d are:\n", numbers[0]);
+    printf("[");
+    for (i = 0; i < totals[0]; ++i) {
+        printf("%d ", longPrimes[i]);
+    }
+    printf("\b]\n");
+
+    printf("\nThe number of long primes up to:\n");
+    for (i = 0; i < 8; ++i) {
+        printf("  %5d is %d\n", numbers[i], totals[i]);
+    }
+    free(totals);
+    free(longPrimes);
+    free(primes);
+    return 0;
+}