Initial data commit

Task/Circular-primes/Java/circular-primes.java

@@ -0,0 +1,91 @@
+import java.math.BigInteger;
+import java.util.Arrays;
+
+public class CircularPrimes {
+    public static void main(String[] args) {
+        System.out.println("First 19 circular primes:");
+        int p = 2;
+        for (int count = 0; count < 19; ++p) {
+            if (isCircularPrime(p)) {
+                if (count > 0)
+                    System.out.print(", ");
+                System.out.print(p);
+                ++count;
+            }
+        }
+        System.out.println();
+        System.out.println("Next 4 circular primes:");
+        int repunit = 1, digits = 1;
+        for (; repunit < p; ++digits)
+            repunit = 10 * repunit + 1;
+        BigInteger bignum = BigInteger.valueOf(repunit);
+        for (int count = 0; count < 4; ) {
+            if (bignum.isProbablePrime(15)) {
+                if (count > 0)
+                    System.out.print(", ");
+                System.out.printf("R(%d)", digits);
+                ++count;
+            }
+            ++digits;
+            bignum = bignum.multiply(BigInteger.TEN);
+            bignum = bignum.add(BigInteger.ONE);
+        }
+        System.out.println();
+        testRepunit(5003);
+        testRepunit(9887);
+        testRepunit(15073);
+        testRepunit(25031);
+    }
+
+    private static boolean isPrime(int n) {
+        if (n < 2)
+            return false;
+        if (n % 2 == 0)
+            return n == 2;
+        if (n % 3 == 0)
+            return n == 3;
+        for (int p = 5; p * p <= n; p += 4) {
+            if (n % p == 0)
+                return false;
+            p += 2;
+            if (n % p == 0)
+                return false;
+        }
+        return true;
+    }
+
+    private static int cycle(int n) {
+        int m = n, p = 1;
+        while (m >= 10) {
+            p *= 10;
+            m /= 10;
+        }
+        return m + 10 * (n % p);
+    }
+
+    private static boolean isCircularPrime(int p) {
+        if (!isPrime(p))
+            return false;
+        int p2 = cycle(p);
+        while (p2 != p) {
+            if (p2 < p || !isPrime(p2))
+                return false;
+            p2 = cycle(p2);
+        }
+        return true;
+    }
+
+    private static void testRepunit(int digits) {
+        BigInteger repunit = repunit(digits);
+        if (repunit.isProbablePrime(15))
+            System.out.printf("R(%d) is probably prime.\n", digits);
+        else
+            System.out.printf("R(%d) is not prime.\n", digits);
+    }
+
+    private static BigInteger repunit(int digits) {
+        char[] ch = new char[digits];
+        Arrays.fill(ch, '1');
+        return new BigInteger(new String(ch));
+    }
+}