Initial data commit

Task/Smarandache-prime-digital-sequence/Java/smarandache-prime-digital-sequence.java

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+public class SmarandachePrimeDigitalSequence {
+
+    public static void main(String[] args) {
+        long s = getNextSmarandache(7);
+        System.out.printf("First 25 Smarandache prime-digital sequence numbers:%n2 3 5 7 ");
+        for ( int count = 1 ; count <= 21 ; s = getNextSmarandache(s) ) {
+            if ( isPrime(s) ) {
+                System.out.printf("%d ", s);
+                count++;
+            }
+        }
+        System.out.printf("%n%n");
+        for (int i = 2 ; i <=5 ; i++ ) {
+            long n = (long) Math.pow(10, i);
+            System.out.printf("%,dth Smarandache prime-digital sequence number = %d%n", n, getSmarandachePrime(n));
+        }
+    }
+
+    private static final long getSmarandachePrime(long n) {
+        if ( n < 10 ) {
+            switch ((int) n) {
+            case 1:  return 2;
+            case 2:  return 3;
+            case 3:  return 5;
+            case 4:  return 7;
+            }
+        }
+        long s = getNextSmarandache(7);
+        long result = 0;
+        for ( int count = 1 ; count <= n-4 ; s = getNextSmarandache(s) ) {
+            if ( isPrime(s) ) {
+                count++;
+                result = s;
+            }
+        }
+        return result;
+    }
+
+    private static final boolean isPrime(long test) {
+        if ( test % 2 == 0 ) return false;
+        for ( long i = 3 ; i <= Math.sqrt(test) ; i += 2 ) {
+            if ( test % i == 0 ) {
+                return false;
+            }
+        }
+        return true;
+    }
+
+    private static long getNextSmarandache(long n) {
+        //  If 3, next is 7
+        if ( n % 10 == 3 ) {
+            return n+4;
+        }
+        long retVal = n-4;
+
+        //  Last digit 7.  k = largest position from right where we have a 7.
+        int k = 0;
+        while ( n % 10 == 7 ) {
+            k++;
+            n /= 10;
+        }
+
+        //  Determine first digit from right where digit != 7.
+        long digit = n % 10;
+
+        //  Digit is 2, 3, or 5.  3-2 = 1, 5-3 = 2, 7-5 = 2, so digit = 2, coefficient = 1, otherwise 2.
+        long coeff = (digit == 2 ? 1 : 2);
+
+        //  Compute next value
+        retVal += coeff * Math.pow(10, k);
+
+        //  Subtract values for digit = 7.
+        while ( k > 1 ) {
+            retVal -= 5 * Math.pow(10, k-1);
+            k--;
+        }
+
+        //  Even works for 777..777 --> 2222...223
+        return retVal;
+    }
+
+}