Data commit

Task/Cuban-primes/Java/cuban-primes.java

@@ -0,0 +1,57 @@
+public class CubanPrimes {
+
+    private static int MAX = 1_400_000;
+    private static boolean[] primes = new boolean[MAX];
+
+    public static void main(String[] args) {
+        preCompute();
+        cubanPrime(200, true);
+        for ( int i = 1 ; i <= 5 ; i++ ) {
+            int max = (int) Math.pow(10, i);
+            System.out.printf("%,d-th cuban prime = %,d%n", max, cubanPrime(max, false));
+        }
+    }
+
+    private static long cubanPrime(int n, boolean display) {
+        int count = 0;
+        long result = 0;
+        for ( long i = 0 ; count < n ; i++ ) {
+            long test = 1l + 3 * i * (i+1);
+            if ( isPrime(test) ) {
+                count++;
+                result = test;
+                if ( display ) {
+                    System.out.printf("%10s%s", String.format("%,d", test), count % 10 == 0 ? "\n" : "");
+                }
+            }
+        }
+        return result;
+    }
+
+    private static boolean isPrime(long n) {
+        if ( n < MAX ) {
+            return primes[(int)n];
+        }
+        int max = (int) Math.sqrt(n);
+        for ( int i = 3 ; i <= max ; i++ ) {
+            if ( primes[i] && n % i == 0 ) {
+                return false;
+            }
+        }
+        return true;
+    }
+
+    private static final void preCompute() {
+        //  primes
+        for ( int i = 2 ; i < MAX ; i++ ) {
+            primes[i] = true;
+        }
+        for ( int i = 2 ; i < MAX ; i++ ) {
+            if ( primes[i] ) {
+                for ( int j = 2*i ; j < MAX ; j += i ) {
+                    primes[j] = false;
+                }
+            }
+        }
+    }
+}