Data update

Task/Twin-primes/Java/twin-primes-2.java

@@ -0,0 +1,39 @@
+import java.util.BitSet;
+
+public final class TwinPrimes {
+
+	public static void main(String[] args) {
+		final int limit = 1_000_000_000;
+		sievePrimes(limit);
+		
+		int target = 10;
+		int count = 0;
+	    boolean last = false;
+	    boolean first = true;
+	
+	    for ( int index = 5; index <= limit; index += 2 ) {
+	        last = first;
+	        first = primes.get(index);
+	        if ( last && first ) {
+	            count += 1;
+	        }
+	        if ( index + 1 == target ) {
+	        	System.out.println(String.format("%8d%s%d", count, " twin primes below ", index + 1));
+	        	target *= 10;
+	        }
+	    }		
+	}	
+	
+	private static void sievePrimes(int aLimit) {
+		primes = new BitSet(aLimit + 1);
+		primes.set(2, aLimit + 1);
+		for ( int i = 2; i <= Math.sqrt(aLimit); i = primes.nextSetBit(i + 1) ) {
+			for ( int j = i * i; j <= aLimit; j += i ) {
+				primes.clear(j);
+			}
+		}		
+	}
+	
+	private static BitSet primes;	
+
+}