Data commit

Task/Long-primes/Java/long-primes.java

@@ -0,0 +1,69 @@
+import java.util.LinkedList;
+import java.util.List;
+
+public class LongPrimes
+{
+    private static void sieve(int limit, List<Integer> primes)
+    {
+        boolean[] c = new boolean[limit];
+        for (int i = 0; i < limit; i++)
+            c[i] = false;
+        // No need to process even numbers
+        int p = 3, n = 0;
+        int p2 = p * p;
+        while (p2 <= limit)
+        {
+            for (int i = p2; i <= limit; i += 2 * p)
+                c[i] = true;
+            do
+                p += 2;
+            while (c[p]);
+            p2 = p * p;
+        }
+        for (int i = 3; i <= limit; i += 2)
+            if (!c[i])
+                primes.add(i);
+    }
+
+    // Finds the period of the reciprocal of n
+    private static int findPeriod(int n)
+    {
+        int r = 1, period = 0;
+        for (int i = 1; i < n; i++)
+            r = (10 * r) % n;
+        int rr = r;
+        do
+        {
+            r = (10 * r) % n;
+            ++period;
+        }
+        while (r != rr);
+        return period;
+    }
+
+    public static void main(String[] args)
+    {
+        int[] numbers = new int[]{500, 1000, 2000, 4000, 8000, 16000, 32000, 64000};
+        int[] totals = new int[numbers.length];
+        List<Integer> primes = new LinkedList<Integer>();
+        List<Integer> longPrimes = new LinkedList<Integer>();
+        sieve(64000, primes);
+        for (int prime : primes)
+            if (findPeriod(prime) == prime - 1)
+                longPrimes.add(prime);
+        int count = 0, index = 0;
+        for (int longPrime : longPrimes)
+        {
+            if (longPrime > numbers[index])
+                totals[index++] = count;
+            ++count;
+        }
+        totals[numbers.length - 1] = count;
+        System.out.println("The long primes up to " + numbers[0] + " are:");
+        System.out.println(longPrimes.subList(0, totals[0]));
+        System.out.println();
+        System.out.println("The number of long primes up to:");
+        for (int i = 0; i <= 7; i++)
+            System.out.printf("  %5d is %d\n", numbers[i], totals[i]);
+    }
+}